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Structure and functional properties of
heteropolyoxomolybdates supported on silica
SBA-15
vorgelegt von
Dipl.-Chem.
Rafael Zubrzycki
geb. in Berent
von der Fakultät II - Mathematik und Naturwissenschaften
der Technischen Universität Berlin
zur Erlangung des akademischem Grades
Doktor der Naturwissenschaften
-Dr. rer. nat.-
genehmigte Dissertation
Promotionsausschuss
Vorsitzender: Prof. Dr. rer. nat Thomas Friedrich
Berichter/Gutachter: Prof. Dr. rer. nat. Thorsten Ressler
Berichter Gutachter: Prof. Dr. rer. nat. Malte Behrens
Tag der wissenschaftlichen Aussprache: 20. März 2015
Berlin 2015
Abstract
Heteropolyoxomolybdates with Keggin structure (HPOM) were supported on SBA-15 and
introduced as model catalysts for investigating structure-property correlations during
selective propene oxidation. The chemical composition of the HPOM was varied by
substituting molybdenum with vanadium or tungsten. Subsequently, the various
heteropolyoxomolybdates were supported on nanostructured silica SBA-15. Additionally,
unsubstituted HPOM were deposited on SBA-15 with different pore radii. Unsupported
and supported heteropolyoxomolybdates were characterized by ex situ techniques yielding
a detailed knowledge about structure and chemical composition of the model catalysts.
Afterwards, the unsupported and supported heteropolyoxomolybdates were characterized
by in situ techniques and tested for their catalytic properties in the partial oxidation of
propene. HPOM supported on SBA-15 were investigated to elucidate the influence of
addenda atoms, the silanol groups of SBA-15, the pore radii of SBA-15, and the HPOM
loading on the resulting structures forming during propene oxidation conditions.
The initial Keggin structure was retained after supporting HPOM on SBA-15. The removal
of adsorbed water and a following dehydroxylation of silanol groups of SBA-15 lead to a
destabilizing effect on the Keggin ion during propene oxidation conditions. Subsequently,
the HPOM supported on SBA-15 formed a mixture of [MoOx] and [(V,W)Ox] species on
the support material under catalytic conditions. The [MoO6] units were influenced by the
structural evolution of neighboring [VO6] and [WO6] units of the initial Keggin ion
structure. The structural evolution of the [MoOx] and [(V,W)Ox] species lead to
predominantly tetrahedral [MoO4] and [VO4] units in vanadium substituted HPOM and to
predominantly octahedral [MoO6] and [WO6] units in tungsten substituted HPOM. The
formation of [MoO4] units or [MoO6] depended on the degree of vanadium or tungsten
substitution. The resulted [MOx] (M = V, W) units were in close vicinity to the [MoOx]
species. The various structures resulting for supported HPOM exhibited an influence on
the catalytic activity. The reaction rates at similar propene conversions for supported
HPOM decreased with higher [MoO4]/[MoO6] ratio. The higher reaction rate resulted in an
increased formation of total oxidation products. Hence, samples with an increased
[MoO4]/[MoO6] ratio exhibited an increased selectivity towards C3 oxidation products.
Zusammenfassung
Heteropolyoxomolybdate mit Keggin Struktur (HPOM) geträgert auf SBA-15 wurden als
Modellkatalysatoren für die selektive Propenoxidation verwendet und hinsichtlich ihrer
Struktur-Eigenschafts-Beziehungen untersucht. Die chemische Zusammensetzung der
HPOM wurde durch Substitution von Molybdän mit den sog. Addenda-Atomen Vanadium
oder Wolfram variiert. Anschließend wurden die verschiedenen Heteropolyoxomolybdate
auf SBA-15 geträgert. Zusätzlich wurden unsubstituierte HPOM auf SBA-15 mit
unterschiedlichen Porenradien geträgert. Die ungeträgerten und geträgerten HPOM wurden
charakterisiert, um detaillierte Informationen über die Struktur und die chemische
Zusammensetzung der Modellkatalysatoren zu erhalten. Danach wurden die ungeträgerten
und geträgerten HPOM unter Reaktionsbedingungen charakterisiert und auf ihre
katalytischen Eigenschaften bei der partiellen Oxidation von Propen getestet. Die auf SBA-
15 geträgerten HPOM wurden untersucht, um den Einfluss der Addenda-Atome, der
Silanolgruppen des SBA-15, der unterschiedlichen Porenradien des SBA- 15 und der
HPOM-Beladung auf die sich unter Propenoxidationsbedingungen bildenden Strukturen
aufzuklären.
Die Kegginstruktur blieb nach der Trägerung der HPOM auf SBA-15 erhalten. Die
Entfernung von adsorbiertem Wasser und eine folgende Dehydroxylierung der
Silanolgruppen des SBA-15 führten zu einer Destabilisierung der Keggin-Ionen unter
Propenoxidationsbedingungen. Anschließend bildeten die geträgerten HPOM unter
katalytischen Bedingungen eine Mischung aus [MoOx]- und [(V,W)Ox]-Spezies auf dem
Trägermaterial. Die [MoO6]-Einheiten wurden durch die strukturelle Entwicklung der
benachbarten [VO6]- und [WO6]-Einheiten aus der ursprünglichen Kegginstruktur
beeinflusst. Die Strukturentwicklung der [MoOx]- und [(V,W)Ox]-Spezies führte zu
überwiegend tetraedrischen [MoO4]- und [VO4]-Einheiten in den vanadiumsubstituierten
HPOM und zu überwiegend oktaedrischen [MoO6]- und [WO6]-Einheiten in den
wolframsubstitutierten HPOM. Die Bildung der [MoO4]- oder [MoO6]-Einheiten waren
von der Anzahl der Addenda-Atome pro Keggin-Ion abhängig. Die [MOx]-Einheiten
(M = V, W) befanden sich in unmittelbarer Nähe zu den [MoOx]-Einheiten. Die
verschiedenen Strukturen, die sich aus den geträgerten HPOM bildeten, zeigten einen
Einfluss auf die katalytische Aktivität. Die Reaktionsrate bei ähnlichen Propenumsätzen
nahm für die geträgerte HPOM mit höherem [MoO4]/[MoO6] Verhältnis zu. Die höhere
Reaktionsgrate führten zu einer erhöhten Bildung von Totaloxidationsprodukten. Die
Proben mit einem erhöhten [MoO4]/[MoO6] Verhältnis zeigte eine erhöhte Selektivität
gegen C3 Oxidationsprodukten.
VII
Contents
Abstract ................................................................................................................................ III
Zusammenfassung ................................................................................................................ V
Contents .............................................................................................................................. VII
Abbreviations ....................................................................................................................... X
1 Introduction ................................................................................................................ 1
1.1 Motivation .................................................................................................................. 1
1.2 Heteropolyoxomolybdates in partial oxidation reactions .......................................... 3
1.3 Supported heteropolyoxomolybdates partial oxidation reactions .............................. 5
1.4 Outline of the work .................................................................................................... 7
2 Characterization Methods .......................................................................................... 8
2.1 Structural Characterization ........................................................................................ 8
2.1.1 Powder X-ray diffraction ........................................................................................... 8
2.1.2 Vibrational spectroscopy ........................................................................................... 9
2.1.3 Physisorption ........................................................................................................... 10
2.1.4 X-ray absorption spectroscopy ................................................................................ 11
2.1.5 Nuclear magnetic resonance spectroscopy .............................................................. 13
2.2 Element Analysis ..................................................................................................... 14
2.2.1 X-ray fluorescence (XRF) spectroscopy.................................................................. 14
2.2.2 Atomic absorption spectroscopy (AAS) .................................................................. 15
2.3 Thermal analysis ...................................................................................................... 15
2.4 Catalytic Characterization ....................................................................................... 15
3 Charaterization of bulk P(V,W)xMo12-x (x = 0, 1 ,2) ............................................... 17
3.1 Sample Preparation .................................................................................................. 17
3.2 Sample characterization ........................................................................................... 19
3.3 Ex situ characterization of P(V,W)xMo12-x (x = 0, 1, 2) .......................................... 24
3.3.1 Quantification of metal loading by XRF ................................................................. 24
3.3.2 Long-range structure of as-prepared P(V,W)xMo12-x (x = 0, 1, 2) .......................... 24
3.4 Short-range order structural characterization of P(V,W)xMo12-x (x = 0, 1, 2) ......... 26
3.5 In situ Characterization of bulk heteropolyacids ..................................................... 32
3.5.1 In situ XRD of PMo12-x(V,W)x x = 0, 1, 2 during oxidation conditions .................. 32
3.5.2 Functional characterization of bulk HPOM ............................................................. 36
VIII
3.6 Summary.................................................................................................................. 41
4 Characterization of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) (10 wt.% Mo) ............... 42
4.1 Sample Preparation .................................................................................................. 42
4.2 Sample characterization ........................................................................................... 43
4.3 Results of the Characterization ................................................................................ 45
4.3.1 Long-range structure of as-prepared P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) ............ 45
4.3.2 Short-range order structural characterization of as-prepared P(V,W)xMo12-x- ...........
SBA-15 (x = 0, 1, 2) ................................................................................................ 48
4.4 Conclusion ............................................................................................................... 55
5 Characterization of PVxMo12-x-SBA-15 (x = 1, 2) under catalytic conditions ........ 56
5.1 Experimental............................................................................................................ 56
5.1.1 Sample Characterization .......................................................................................... 56
5.1.2 Sample preparation .................................................................................................. 59
5.2 Structural characterization of PVxMo12-x-SBA-15 (x = 1, 2) under ...........................
catalytic conditions .................................................................................................. 59
5.2.1 Local structure in activated PVxMo12-x-SBA-15 (x = 0, 1, 2) and a ...........................
reference V2Mo10Ox-SBA-15 under catalytic conditions ........................................ 62
5.2.2 Local structure of P in activated PV2Mo10SBA-15 under catalytic conditions ....... 68
5.2.3 Structure directing effects of vanadium and the support material on the structure ....
of activated PV2Mo10-SBA-15 under catalytic conditions ..................................... 70
5.3 Functional characterization of PVxMo12-x-SBA-15 (x = 0, 1, 2) ............................. 71
5.3.1 Reducibility ............................................................................................................. 71
5.3.2 Catalytic performance ............................................................................................. 72
5.3.3 Influence of phosphorus species on catalytic activity ............................................. 74
5.4 Summary.................................................................................................................. 76
6 Characterization of PWxMo12-x-SBA-15 (x = 1, 2) under catalytic conditions ....... 77
6.1 Experimental............................................................................................................ 77
6.1.1 Sample Characterization .......................................................................................... 77
6.1.2 Sample preparation .................................................................................................. 80
6.2 Structural evolution of PWxMo12-x-SBA-15 (x = 1, 2) under catalytic conditions . 80
6.2.1 Local structure in activated PWxMo12-x-SBA-15 (x = 0, 1, 2) and a ..........................
reference W2Mo10Ox-SBA-15 under catalytic conditions ....................................... 86
6.2.2 Comparison of the local structure around Mo centers in act. PW2Mo10-SBA-15 ......
and a reference act. W2Mo10Ox-SBA-15 under catalytic conditions ....................... 90
IX
6.3 Functional characterization of PWxMo12-x-SBA-15 (x= 1, 2) ................................. 94
6.3.1 Reducibility .............................................................................................................. 94
6.3.2 Catalytic performance .............................................................................................. 95
6.4 Summary .................................................................................................................. 98
7 Characterization of PMo12 supported on SBA-15 with tailored pore radii ............. 99
7.1 Experimental .......................................................................................................... 100
7.2 Structure of the support materials .......................................................................... 104
7.3 Characterization of PMo12-SBA-15 (10, 14, 19 nm) ............................................. 106
7.4 Structural evolution of PMo12- SBA-15 (10, 14, 19 nm) under catalytic ...................
conditions ............................................................................................................... 108
7.5 Functional characterization of PVxMo12-x-SBA-15 (x= 1, 2) ................................ 113
7.5.1 Influence of the resulting structures to catalytic activity ....................................... 113
7.6 Summary ................................................................................................................ 115
8 Characterization of PVMo11 supported on SBA-15 with different metal loading . 116
8.1 Experimental .......................................................................................................... 117
8.2 Characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and .......................
1 wt.% Mo) ............................................................................................................ 120
8.3 Structural evolution of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and .................
1 wt.% Mo) under catalytic conditions .................................................................. 123
8.4 Functional characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, ............
and 1 wt.% Mo) ..................................................................................................... 128
8.4.1 Reducibility ............................................................................................................ 128
8.4.2 Influence of the resulting structure on catalytic activity........................................ 129
8.5 Summary ................................................................................................................ 132
9 General discussion and Summary .......................................................................... 133
9.1 Structure directing effect of the support material .................................................. 133
9.2 Structure directing effects of the addenda atoms ................................................... 135
9.3 Structure activity relationships .............................................................................. 137
10 Conclusions ............................................................................................................ 142
11 References .............................................................................................................. 148
12 Appendix ................................................................................................................ 164
Danksagung ........................................................................................................................ XII
X
Abbreviations
act. activated
AHM ammonium heptamolybdate
BET Brunauer-Emmet-Teller
BJH Barrett-Joyner-Halenda
cf. compare (Latin "confer")
DTG derivative thermogravimetric
e.g. or example (Latin "exempli gratia")
eq. equation
et al. and others (Latin "et alii")
EXAFS extended X-ray absorption fine structure
exp. experimental
FID flame ionization detector
FT Fourier transformed
HASYLAB Hamburg Synchrotron Radiation Laboratory
HPOM heteropolyoxomolybdate with Keggin structure
i.e. that is (Latin "id est")
IR infrared
IUPAC International Union of Pure and Applied Chemistry
m/e mass-charge ratio
MoO3-SBA-15 molybdenum oxides supported on SBA-15
nom. nominal
Norm. normalized
PMo12 H3[PMo12O40
PVMo11 H4[PVMo11O40]
PV2Mo10 H5[PV2Mo10O40]
PWMo11 H3[PWMo11O40]
PW2Mo10 H3[PW2Mo10O40]
PZC point of zero charge
RT room temperature
SBA-15 mesoporous silica (Santa Barbara amorphous type material No. 15)
SDA structure-directing agent
TA thermal analysis
XI
TCD thermal conductivity detector
TG thermogravimetry
V2Mo10Ox-SBA-15 vanadium and molybdenum oxides supported on SBA-15
W2Mo10Ox-SBA-15 tungsten and molybdenum oxides supported on SBA-15
wt.% weight percent
XAFS X-ray absorption fine structure
XANES X-ray absorption near edge structure
XAS X-ray absorption spectroscopy
XRD X-ray diffraction
XRF X-ray fluorescence
1
1 Introduction
1.1 Motivation
Heteregenous catalyzed reactions play a fundamental role in the production of industrial
organic chemicals and intermediates. Selective catalytic oxidation processes generate
approximately one quarter of the value produced world wide by catalytic processes.[1] The
products include such intermediates as acrolein, acrylic acid, acrylonitrile, methacrylic
acid, MTBE, maleic anhydride, phthalic anhydride, ethylene and propylene oxide.[2] One
important industrial process is the selective oxidation of propene towards acrolein and
acrylic acid.[3,4] Acrylic acid is an important raw material in the fine chemicals industry.
The acid and its esters are important monomers for the preparation of polymers and are
used in the manufacture of paints and adhesives, in the treatment of paper and textile, as
well as superabsorbent.[5] The industrial production of acrylic acid is a two step process.
In the first reaction, propene is oxidized using a bismuth molybdate based catalyst resulting
in the production of acrolein. In the second reaction, acrolein is oxidized using a bismuth
molybdate based catalyst mixed with additional metal oxides with transitions metals such
as vanadium or tungsten[3]:
propene
acrolein
acrylic acid
The product yield of acrylic acid in this process is about 90%.[6] Increasing the product
yield using new or improved catalyst is of particular interest, because the industrial
processes can be made more economical and sustainable. Molybdenum based catalyst are
of particular interest as they are often used in partial oxidation catalysts for industrial
application.[7] The catalyst may be improved by varying the chemical complexity.
Additional metals such as W, Nb, or V stabilize characteristic crystallographic structures
which lead to oxidation catalysts with improved activity and selectivity.[8,9] However, the
influence of structural variety and chemical complexity in the mixed oxide systems on
catalytic performance is difficult to distinguish. Moreover, the functionality of individual
metal centers or particular structural motifs of these highly active mixed oxide catalysts
2
can hardly be determined. Hence, model systems are required which combine structural
invariance with compositional variety or vice versa.[10–12] Heteropolyoxomolybdates
(HPOM) with Keggin structure exhibit a broad compositional range while maintaining
their characteristic structural motif.[13–16] Therefore, substituting Mo atoms with addenda
atoms (i.e. V, W, Nb) make Keggin type HPOM suitable model system to study structure
activity relationships. A challenge within the study of structure-activity relationships of
those catalysts is to distinguish between the bulk and surface structures of this materials.
Catalytic reaction occurs on the crystalline surface of the bulk compounds. Therefore, the
majority of the bulk compound is not involved in the catalytic reaction. This leads to an
analytical problem, because the average of the bulk compound is measured with most
analytical methods. Therefore, a study of structure-activity relationships of surface
structures corresponding to the "real" catalyst is not feasible. An approach to solve this
analytic problem is the use of supported metal oxide catalysts.[17,18] Supported catalytic
species possesses high dispersions and an improved surface to bulk ratio. Hence,
differentiating between bulk and surface structures is no longer necessary. Therefore,
structure activity relationships can be readily deduced from the characteristic oxide species
observed on the support material under catalytic reaction conditions. Suitable support
materials for catalysts posseses a large surface area and a homogenous internal pore
structure with sufficiently large pores. The precursors are ideally highly dispersed on the
support material. Hence, all the centers are accessible on the surface and are involved in
the catalytic reaction. Furthermore, the support may interact with the precursor to stabilize
particular structural motifs without affecting the catalytic reaction.[19–21] Nanostructured
SiO2 materials such as SBA-15 represent suitable support systems for metal oxide
catalysts.[22–25] Additionally, the mechanical, thermal, and hydrothermal stability
improves SBA-15 as support material during catalytic conditions like high temperatures or
in the presence of steam.[26] The deposition of vanadium or tungsten substituted HPOM
on SBA-15 lead to well dispersed HPOM Keggin ions. HPOM supported on SBA-15 can
be used to vary the chemical composition while maintaining good accessibility of the
supported molybdenum based catalyst.[27] Therefore, a study of structure-activity
relationships of surface structures corresponding to the "real" catalyst was possible.
3
1.2 Heteropolyoxomolybdates in partial oxidation reactions
The first characterization of the Keggin structure was performed by J.F- Keggin at 1934
using XRD.[28] Fig. 1-1 depicts the Keggin ion structure typically represented by the
formula [XM12O40]x-8
where X is the central atom (Si4+
, P5+
, etc.), x is the oxidation state
and M is the metal ion (Mo6+
or W6+
). The smallest structural units of the Keggin structure
are metal-oxygen octahedra (MO6) and are called primary structure. The octahedra are
arrranged in four M3O13 (triad) groups surrounding the central tetrahedron XO4. The
Keggin ion structure is called secondary structure and the arrangement of the Keggin ion in
a crystal structure represents the tertiary structure.[29]
Heteropolyoxomolybdates with Keggin structure are able to catalyze a variety of reactions
as homogeneous or heterogeneous catalyst (Table 1-1). Thus, HPOM are part of current
investigations in catalysis research.[30–33] Substituted HPOM were intensely investigated
in the past with regard to partial oxidation reactions.[13–16,34,35] Li et al. showed, that
introducing vanadium in the Keggin ion structure of HPOM lead to an enhanced
P
Mo
O
Keggin structure triad
Fig. 1-1: (left) Triad of the Keggin structure (Mo3O13); (right) Keggin structure (secondary
structure).
4
Table 1-1: Summary of reactions, which are catalyzed by Heteropolyoxomolybdates with Keggin
structure.
reactions catalysing by HPOM[34–38]
isomerisation of alkanes polymerisation of THF
MeOH to olefins Diels-Alder Reaction
alkylation of paraffins oxidation of alkanes
oligomerisation of alkenes oxidation of alkenes
Friedel-Crafts Acylation hydrogenation of alkenes
Beckmann rearrangement methacrolein to methacrylic acid
catalytic activity in partial oxidation of propane.[36] Bondavera et al. investigated the
influence of vanadium on the catalytic activity in ammoxidation of methylpyrazine.[35]
The vanadium substituted HPOM showed an enhanced catalytic activity depending on the
degree of substitution. Comparable correlations between catalytic activity and the degree
of vanadium substitution were found for the oxidation of acrolein, isobutylene, and
isobutane.[37–39] Ressler et al. investigated in various studies the structural evolution of
vanadium substituted HPOM during propene oxidation.[13–16] H4[PVMo11O40] (PVMo11)
loses crystal water during treatment under propene oxidation conditions in the temperature
range from 373 to 573 K.[15] The release of crystal water is followed by partial
decomposition, reduction of the average Mo valence, and formation of cubic HPOM
(Mox[PVMo11-xO40]) at 573 K. The formation of cubic Mox[PVMo11-xO40] with Mo centers
outside the Keggin ion structure and V centers remaining in a lacunary Keggin ion
coincides with the onset of catalytic activity.[15] Niobium substituted HPOM
(H4[PNbMo11O40]) formed the characteristic cubic HPOM structure, similar to the
structural evolution of H3[PMo12O40], H4[PVMo11O40], and H5[PV2Mo10O40].[14] In
contrast to H3+x[PVxMo12-xO40] (x = 0, 1, 2) the lacunary Keggin ion decompose rapidly
towards MoO3 at about 673 K. The decomposition process correlated with a decrease in
catalytic activity.[14] Mestl et al. investigated the thermal induced decomposition of
PVMo11.[40] Upon loss of crystal water vanadyl and molybdenyl species are expelled from
the Keggin ion structure forming the lacunary Keggin ion. This defective structure further
disintegrated to triads and finally condensed to the thermodinamically stable MoO3.[40]
5
Therefore, it may be assumed, that the vanadium and niobium substitution in HPOM have
a structure directing effect, stabilizing a structure active in oxidation of propene. However,
the role of addenda atoms is still under discussion, because addenda atoms may have both
a structure directing effect and/ or a functional effect during propene oxidation conditions.
1.3 Supported heteropolyoxomolybdates in partial oxidation reactions
Support material
Mesoporous material SBA-15 was fist synthesized in 1998.[22,23] The SBA-15 structure
composed of hexagonal channels has a high surface area and a narrow pore size
distribution. Supramolecular aggregates are used for the synthesis of mesoporous systems
as structure-directing agents (SDAs).[42,43] In the synthesis of SBA-15 a block copolymer
is used as SDA.[22,23] The self-organization of the block copolymer promotes the
formation of a silica based inorganic network around the organic aggregates. Afterwards,
the organic template is removed through a calcination process. Fig. 1-2 illustrates a
schematic representation of the preparation of SBA-15.
The surface area and pore width are tailored by the preparation procedure. The typical
synthesis of SBA-15 leads to surface areas between 600 and 1000 m2/g and pore diameters
between 5 and 10 nm.[22] The pore radius is tunable with swelling agents resulting in pore
radii up to 50 nm.[46] The swelling agents are for example benzene, 1,3,5,-
Fig. 1-2: Schematic representation of the preparation of SBA-15 (adapted from [45]).
Globe
Micelle
Rod-Shaped
Micelle
Liquid-Crystalline
Phase
Organic-Inorganic
Composite
Mesoporous
Material
Calcination
6
trimethybenzene, decane, and gelatin.[47–50] The swelling agents enrich in the
hydrophobic chains of the surfactants in the micelles and expand the micelles resulting in a
larger diameter. The important conditions during the synthesis of SBA-15 with larger pores
are the initial synthesis temperature, the amount of swelling agent, and the hydrothermal
treatment time and temperature.[51] Generally, the pore diameter increases with lower
initial synthesis temperature influencing the formation of the micelles. The expansion of
the micelles is limited, because mesocellular foams with spherical mesopores are formed,
when higher relative amounts of swelling agents are used.[49] Further parameters for
adjusting the pore radius of SBA-15 are the hydrothermal treatment time and temperature.
The increase in the hydrothermal treatment temperature allows to achieve larger pore sizes
in a shorter period of time.[22,46] Disadvantages in changing the hydrothermal conditions
are long hydrothermal treatment times and high temperatures (e.g. 2 days at 130 C) that
lead to merging of adjacent cylindrical mesopores.[51]
Supported heteropolyoxomolybdates in partial oxidation reactions
Industrial applications and investigations of supported tungstate or molybdate heteropoly
acids with Keggin ion structure have been recently reviewed.[52,53] Various authors
reported, that the Keggin ion structure of supported tungstate or molybdate heteropoly
acids retained intact after deposition on silica, titania or zirconia based support
materials.[54–58,27] For H3[PMo12O40] supported on ZrO2 (PMo12-ZrO2) Devassy et al.
investigated the nature of the phosphorous species depending on Keggin loading and
calcination temperature.[39] They, reported a decomposition of the HPOM to oxide
species at temperatures above 723 K. The thermal stability of H3[PW12O40] supported on
ZrO2 (PW12-ZrO2) was investigated by López-Salinas et al.. The structural behaviour of
PW12-ZrO2 during calcination was comparable to that of PMo12-ZrO2. PW12-ZrO2
decomposed at temperatures above 773 K to form the corresponding supported oxides.[46]
Ressler et al. reported for H4[PVMo11O40] supported on SBA-15 (PVMo11-SBA-15) a
decomposition under propene oxidation conditions above 573 K resulting in Mo oxide
species.[27] The resulting molybdenum oxide species are comparable to that of
molybdenum oxide species synthesized from an ammonium hepta molybdate (AHM)
precursor. Both molybdenum oxide species on SBA-15 revealed comparable structural
motifs during treatment in propene oxidation conditions.[59] Results of the Mo K edge
7
XANES analysis of activated PVMo11-SBA-15 indicated tetrahedrally coordinated MoOx
species. A Comparison with references afforded about 50% of tetrahedrally coordinated
MoOx species on SBA-15.[27] Ressler et al. assumed that the [MoO6] units exhibited a
connectivity similar to that of the building blocks of MoO3. The MoO4 units may be
isolated or connected to other MoOx species on the surface of SBA-15.[27] Therefore, no
stable HPOM supported on SBA-15 could be obtained on SBA-15 under reactions
conditions. The role and structural evolution of V and P in PVMo11-SBA-15 under
catalytic conditions remained largely unknown.
1.4 Outline of the work
The objective of this work is to elucidate the role and structural evolution of Mo, V, W and
P in HPOM supported on SBA-15 under catalytic conditions. Additionally, the influence of
the support material SBA-15 on the stability of the supported HPOM is investigated with
respect to the role of dehyrdation processes of the SBA-15, the pore radii of SBA-15 and
HPOM loading. Therefore, the chemical composition of HPOM is varied by substituting
molybdenum with vanadium or tungsten. Subsequently, the various
heteropolyoxomolybdates are supported with different loading on nanostructured silica
SBA-15. Additionally, unsubstituted HPOM are deposited on SBA-15 with different pore
radii. The unsupported and supported heteropolyoxomolybdates are characterized by ex
situ techniques ensuring a detailed knowledge about structure and chemical composition of
the model catalysts. Afterwards, the unsupported and supported heteropolyoxomolybdates
are characterized by in situ techniques and tested for their catalytic properties in the partial
oxidation of propene. HPOM supported on SBA-15 are intensively investigated to clarify
the influence of the silanol groups of SBA-15, the pore radii of SBA-15, and the HPOM
loading on SBA-15 on the structures forming during propene oxidation conditions.
Additionally, the influence of the addenda atoms of the substituted HPOM supported on
SBA-15 on the resulting structures is investigated during propene oxidation conditions.
The characterization of the structural evolution especially of the addenda atoms (V, W) and
the heteroatom (P) focus on identifying the metal oxide structure and correlating the
structure with catalytic activity.
8
2 Characterization Methods
2.1 Structural Characterization
2.1.1 Powder X-ray diffraction
X-ray diffraction (XRD) is used for determining the long-range order structures of the
synthesized bulk samples in this work. For that, powder samples are irradiated with
monochromatic X-ray photons. The X-ray photons are inelastically and elastically
scattered by the electrons of atoms arranged in a periodic structure.[60] The X-ray photons
that are elastically scattered by the electrons of atoms are used for determining the crystal
structure. The scattered X-ray wave interfere constructively or destructively depending on
the distance between the lattice planes and the angle (θ), between the incident X-rays and
the lattice planes. The Bragg equation described the detectable X-ray photons resulting
from constructive interference as a relationship between the lattice spacing (d), and the
angle (θ), between incident X-rays and lattice plane.
nλ = 2d sin θ n = 1, 2, ... (2.1)
with:
n diffraction order
λ wavelength of the X-ray photons
d lattice spacing
θ angle between incident X-rays and lattice planes
Fig. 2-1 depicts a schematic representation of the scatted X-ray waves and interfere
constructively. Constructive interference takes place only when sum of the path lengths of
+ is an integer factor of the wavelength, λ. Diffraction peaks can be characterized by
the Miller indices, which describe the corresponding lattice planes. Detailed information
about data analyzing and structure refinement can be found elsewhere.[61,62]
9
2.1.2 Vibrational spectroscopy
The advantage of Infrared (IR) spectroscopy is that the method can be used to analyze
amorphous compounds. Amorphous compounds could not be analyzed by XRD.
Therefore, IR spectroscopy is chosen for excluding additional amorphous compounds
besides the crystalline compounds in the synthesized bulk samples. In solid states the
atoms in a crystal structure vibrate internally by changing the interatomic distances. This
vibrations lead to transitions that are within the range of the infrared radiation (0.7μm-
1000μm). The wavelength of the IR radiation is varied and the decrease of intensity is
measured.[63,64] A prerequisite for the absorption of IR radiation is a change in the dipole
moment in molecules or structural motifs in solid state compounds. Thus, the electric field
of the electromagnetic radiation couples to the dipole moment in the structure, resulting in
an absorption of the electromagnetic radiation. The observed vibration are called modes.
Detailed information can be found elsewhere.[68,69]
Similar to IR spectroscopy different vibrational modes are also observed in Raman
spectroscopy. In contrast to IR spectroscopy that uses the absorption resulting by various
vibrational modes at different irradiation energies, another method of excitation is selected
in Raman spectroscopy. In Raman spectroscopy the sample is irradiated with
monochromatic radiation, usually with a laser.[64,65] The wavelength of the radiation
should be chosen outside the absorption range of electronic transitions of the analyte
lattice planes
d A B
C
X-rays
Fig. 2-1: X-ray photons strikes the ordered lattice at an angle . X-ray photons are scattered and
interfere constructively in direction given by the Bragg equation (equation 2.6).
10
excluding electronic absorption effects at the excitation wavelength. In Raman
spectroscopy, the electromagnetic radiation induces a dipole moment in the molecule
achieving higher vibration levels. Hence, linear and homoatomic structural motifs can be
studied. Thus, Raman spectroscopy is complementary to IR spectroscopy. There are three
possible types of interactions subdivided into two categories, the Raman and Rayleigh
scattering. In the Rayleigh scattering, a photon with the energy causes a transition to a
higher virtual energy state of the structural motif and relaxes to a vibrational level of the
ground electronic state. The absorbed and emitted energy is equal in this process resulting
in an elastic scattering interaction. The excitation occur from any vibrational states
depending on the Boltzmann distribution. At room temperature, a excitation from the
vibrational ground state and the first excited vibrational state of the electronic ground state
is usual. In Raman scattering, the incident energy is different from the given energy
. Thus, Raman scattering is an inelastic scattering interaction. The energy
difference is located above (anti-Stokes radiation) and below (Stokes radiation) of the
incident monochromatic electromagnetic radiation.[63,69]
2.1.3 Physisorption
Physisorption measurements are used for determining the surface area and pore structure of
the used support material SBA-15. Physisorption (physical adsorption) denoted an
attaching gas to a surface, which was bound by van der Waals interactions at this. The gas
molecules are denoted as adsorbate and the surface as an adsorbent.[66,67] A dynamic
equilibrium is established between the adsorbent and adsorbate from the gas phase. In this
case, under isothermal conditions, the number of adsorbed molecules n depends on the
equilibrium pressure p of the gas. This relationship can be expressed by the following
equation.The physisorption isotherm can be obtained by plotting the relationship shown in
equation 2.2. The isotherms are divided into six classes by IUPAC.[68] The relevant type
of isotherms for this work is the type IV isotherm. The type IV isotherm has a specific
hysteresis curve, which is characteristic for mesoporous materials. The adsorption of the
monolayer and multi-layer on the walls of the mesopores is following by capillary
condensation in the mesopores of the adsorbent.
11
with:
number of adsorbed molecules
equilibrium pressure
saturation pressure of the adsorptive
The specific surface area of the investigated materials can be determined by the
Brunauer-Emmer-Teller (BET) method from the measurements of the isotherms.[69] In
addition to the specific surface area, the pore size distribution can be determined in the
mesoporous material from the capillary condensation. This method is the Barret-Joyner-
Halenda (BJH) method and is derived from the Kelvin equation.[70] In this case, the
emptying of pores is considered by stepwise lowering of the relative pressure , in
which the thinning of the multilayer film is taken into account. Detailed informations can
be found elsewhere.[66–70]
2.1.4 X-ray absorption spectroscopy
The X-ray absorption fine structure was first discovered in the 1920s. X-ray absorption
fine structure spectrocopy was interesting for the analysis of atoms in biological molecules,
catalysts, and amorphous materials since the presence of particle accelerators (synchrotron)
in the 1970`s.[71,72] X-Ray Absorption Spectroscopy (XAS) is an analytical method for
elucidating electronic properties and the local structure of the absorber atoms.[73,74] The
particular advantage of XAS is the applicability of the method even in the absence of long-
range order. These property is especially relevant for supported catalysts. XAS is used in
this work to determine the structural evolution of the Mo, V, and W centers in supported
HPOM during propene oxidation conditions.
Absorption of X-rays at the core-level binding energy leads to excitation of an electron
from a core level. Excitation of the electron at the core-level binding energy lead to a sharp
rise in absorption, which is denoted as absorption edge. The energy for the excitation is
specific for the absorber atom. XAS can be subdivided in two regions, X-ray near edge
structure (XANES) and the extended X-ray absorption fine structure (EXAFS).
12
The XANES region is typically located in a 50 eV range around the absorption edge. This
region contains information about the element specific oxidation states and coordination
geometries around the absorber atom.[75,76] The XANES pre-edge region resulted
through the excitation of an electron from a core level (K, L, M, depending on the
observed absorption edge) to an unoccupied state. The resulting dipole transitions obey the
following selection rules: the spin quantum number cannot be changed during the
transition Δs = 0, the orbital quantum number l and the total spin j have to change (Δl,
Δj) = ± 1. Therefore, electronic transitions from the s orbital of the K shell to a higher p
orbital and from the p orbital of the L shell to higher s or d orbitals are typical. The final
state of the transition can be also a hybridized orbital. Therfore, transitions from the s
orbital of the K shell to pd hybridized orbitals are possible due to the partially different
19.9 20.0 20.1 20.2 20.3 20.4 20.5 20.6
0
1
E [keV]
Ab
sorp
tion
[μd
]
EXAFS XANES
absorber atom
scattering atom
dipol transitions
Fig. 2-2: X-ray absorption spectrum with a schematic representation of the processes at the
absorption edge,. XANES region: Absorption of an X-ray photon and excitation to a higher
unoccupied level (dipole transition). EXAFS region: Representation of the fine structure of the
absorption edge resulting from the interference of the outgoing photoelectron wave from the
absorber atom with the incoming photoelectron wave resulting by backscaterring of neighbored
atoms.
13
orbital character. Transitions, that are forbidden in principle in quantum mechanics, occur
also in a non-centrosymmetric structure geometry, as in the tetrahedral geometry.
Additionally, transitions take place in non centrosymmetric structures such as a distorted
octahedral structure. Thus, the shape of the XANES region is caused by the local electronic
and geometric structure of the absorbing atom. All quantum mechanically forbidden but
occurring transitions are characterized by a pre-edge peak in the K edge/LI edge for the
investigated elements in this work (Mo, V, W). In octahedrons no pre-edge peak is
expected. The highest intensity of the pre-edge peak are observed for a tetrahedral
geometry. Therefore, a mixed coordination geometry (tetrahedral and octahedral) can be
quantified by a linear combination of reference spectra of tetrahedral and octahedral
coordinated references.
In the EXAFS region the generated photoelectron interact with the electron density of
adjacent atoms. The outgoing electron wave from the absorber atom reaches the
neighboring atoms and will be scattered back.[76] The incoming spherical electron waves
interferes with the outgoing photoelectron wave resulting in an oscillation of the absorption
coefficient and in the fine structure in the absorption spectrum (Fig. 2-2). The absorption
coefficient can be extracted from the EXAFS region of the absorption spectrum. The
oscillatory part can be separated from the atomic absorption of a free atom and is denoted
as EXAFS function . The EXAFS function contains information about the
coordination number, element specific backscattering amplitude, and the mean-squared
displacement of the neighboring atoms around the absorber atom. The EXAFS function
χ(k)can be transferred to a pseudo radial distribution function FT (χ(k)) through a Fourier
transformation. Detailed explanations can be found in Ref. [76].
2.1.5 Nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance (NMR) spectroscopy is an analytical method to elucidate
structures in solid states and molecules. NMR spectroscopy is used in this work for
determining the local structure of phosphorus in the activated supported HPOM after
treatment under propene oxidation conditions. NMR spectroscopy could be used for atoms
with a nuclear spin I ≠ 0. If such a nucleus is in an external magnetic field, the nuclear
level split as function of the nuclear spin. The lower nuclear level can be excited with
14
electromagnetic radiation resulting in transition between the energy levels. The transition
corresponds to the Larmor frequency and the chemical and electronic environment of the
atom, resulting in the resonant frequency . Thereby, the resonant frequencies change
depending on the local structure of the atom. The resulting NMR spectrum is normalized in
the abscissa to a standard substance. The normalization results in a device-nonspecific
scale, the chemical shift . NMR spectra of liquid or gas phase exhibit narrow signals with
characteristic splitting signals caused by coupling to other nuclei in the environment. This
results from the rapid rotation of the molecules in all directions and leads to averaging of
all orientation-dependent spin interactions. In the solid-state NMR spectroscopy rather
broad signals are obtained that show a characteristic shape. This broad signals result due to
the different orientation of the structural motifs in space, caused by an orientation-
dependent spin interactions. The MAS technology (Magic Angle Spinning) averages these
anisotropic spin interactions. Therefore, the sample is placed at an angle of = 54.7° to the
magnetic field and rotates around its own axis. With this construction, anisotropic
interactions are averaged and discrete signals obtained. These signals are comparable to
signals in liquid or gaseous phase.
2.2 Element Analysis
2.2.1 X-ray fluorescence (XRF) spectroscopy
XRF spectroscopy is particularly suitable for the quantitative analysis of metals in metal
oxides such as used in this work. Irradiation of the sample with X-ray photons results in
removal of an electron from a core shell. The resulted core hole is filled by an electron
from a higher level, which results in the emission of an X-ray photon. The energy of the
resulting fluorescence photons is described by Mosley`s law. The energy of the emitted
fluorescence photons is characteristic for each element and can be used to identify the
elemental composition of the sample. Detailed explanation can be found in Ref. [65].
15
2.2.2 Atomic absorption spectroscopy (AAS)
AAS is used in this work for the quantitative analysis of phosphorus. XRF spectroscopy is
due to overlapping of the main P peak and a Mo peak of limited use. For the
measurements, the samples are dissolved in a suitable solvent. Afterwards, the solution is
transferred in the atomizer.[77] Flames or electrothermal (graphite tube) atomizer are
typical atomizer using in AAS. In this work a pyrocoated graphite tube was used as
atomizer. A matrix modifier and the sample were injected into the graphite tube. The
graphite tube was heated up to 2873 K resulting in atomization of the phosphorus species.
Phosphorus atoms are irradadiated with a Hollow Cathode Lamp (HCL) containg small
amounts of phosphorus The HCL emmited the line spectrum of the element of interest
(here phosphorus). This lead to an excitation of the valence electrons in the atoms of the
element of interest resulting in an absorption of the line spectrum of the HCL. The
absorption is proportional to the amount of the element of interest and could be used for
quantitative analysis. Detailed information can be found elsewhere.[64]
2.3 Thermal analysis
Thermal analysis consist of analytical methods, which detect physical or chemical
properties of a substance or substance mixture as a function of temperature or time, while
the sample undergoes a controlled temperature program.[78] Thermogravimetry (TG) is
used mainly for the investigation of decomposition processes, thermal stability, or
dehydration processes.[79] The sample weight is monitored using a sensitive
thermobalance as a function of time or temperature. The first derivative of the TG signal
corresponds to the differentiated thermogravimetric curve, the DTG signal. The DTG
signal facilitates the identification of mass decreases due to the resulting maxima.
2.4 Catalytic Characterization
Quantitative catalysis measurements were performed using a fixed bed laboratory reactor
connected to an online gas chromatography system. Gas chromatography is a suitable
16
chromatographic method for the separation of analyte mixtures in gas phase.[80] A
detailed description of various reactor types and the individual assets of each setup can be
found elsewhere.[81,82] The used fixed-bed reactor consisted of a SiO2 tube. Reactants are
passed through the catalyst bed. At the outlet of the reactor, reaction products are analyzed
by gas chromatography. Therefore, the sample is injected in the inert gaseous mobile phase
(carrier gas). The sample in the carrier gas stream are passed through the stationary phase.
The interaction between the sample and the mobile or stationary phase strongly depends on
their physical and chemical properties. The larger the interaction with the stationary phase,
the slower the migration velocity of the sample components. A low interaction with the
stationary phase shorts retention time of the sample components. Therefore, the adsorption
on the stationary phase or dissolving in the mobile phase depends on whether and how a
sample component interacts with the stationary phase. The different migration velocities of
the current sample components resulted in various times at which the individual
compounds passed through the stationary phase. The various components can finally be
analyzed as discrete bands with suitable detectors. The retention time is the time each
component requires to move from the point of injection to the detector. The resulting bands
are detected as a function of retention time. Thus, individual sample components can be
qualitatively and quantitatively analyzed in this way.[83,84]
17
3 Charaterization of bulk P(V,W)xMo12-x (x = 0, 1 ,2)
Heteropolyoxomolybdates (HPOM) of the Keggin type (e.g., H3[PMo12O40]) are active
catalysts for the partial oxidation of alkanes and alkenes.[7,85–88] HPOM with Keggin
structure exhibit a broad compositional range while maintaining their characteristic
structural motifs.[14–16] Substituting Mo atoms with addenda atoms (i.e. V, W, Nb) make
Keggin type HPOM suitable model systems to study structure-activity relationships. Thus,
HPOM have been frequently studied as active catalysts for selective oxidation
reactions.[89] Therefore, H3[PMo12O40] (PMo12), H4[PVMo11O40] (PVMo11),
H5[PV2Mo10O40] (PV2Mo10), H3[PWMo11O40] (PWMo11), and H3[PW2Mo10O40]
(PW2Mo10) were synthesized as model catalysts in selective propene oxidation. For
elucidating structure activity correlations of model systems for catalytic investigations, a
detailed knowledge about structure and chemical composition is indispensable. Therefore,
various characterization methods, such as XRF, AAS, XRD, IR, Raman and XAS are
necessary for a sufficient characterization of the used catalyst systems. In this chapter, in
addition to ex situ characterization, in situ XRD at oxidizing conditions P(V,W)xMo12-x
(x = 0, 1 ,2) was performed. Catalytic testing revealed the functional properties of
P(V,W)xMo12-x (x = 0, 1 ,2) under propene oxidation conditions.
3.1 Sample Preparation
Preparation of H3[PMo12O40]
19.72 g MoO3 (Sigma Aldrich) was dissolved in 650 ml water and heated under reflux. 95
ml of 0.12 M phosphoric acid were added dropwise to the reaction mixture. The resulting
suspension was heated for 3 h and kept at 298 K for 24 h until a clear yellow solution was
obtained. The remainder was filtered of and the volume of the resulting yellow solution
was reduced to ~30 ml using an evaporator. H3[PMo12O40] crystallized during storage at
277 K for several days.
18
Preparation of H3+x[PVxMo12-x] (x=1,2)
H4[PVMo11O40] was prepared as follows. 17.85 g MoO3 (Sigma Aldrich) and 1.02 g V2O5
(Sigma Aldrich) were dissolved in 650 ml water and heated under reflux. 95 ml of 0.12 M
phosphoric acid were added dropwise to the reaction mixture. The resulting suspension
was heated for 3 h and kept at 298 K for 24 h until a clear orange solution was obtained.
The remainder was filtered of and the volume of the resulting orange solution was reduced
to ~30 ml using an evaporator. H4[PVMo11O40] crystallized during storage at 277 K for
several days.
H5[PV2Mo12O40] was prepared as follows. 16.89 g MoO3 (Sigma Aldrich) and 2.13 g V2O5
(Sigma Aldrich) were dissolved in 675 ml water and heated under reflux. 98 ml of 0.12 M
phosphoric acid were added dropwise to the reaction mixture. The resulting suspension
was heated for 3 h and kept at 298 K for 24 h until a red solution was obtained. The
remainder was filtered of and the volume of the resulting red solution was reduced to ~30
ml using an evaporator. H5[PV5Mo12O40] crystallized during storage at 277 K for several
days.
Preparation of H3[PWxMo12-x] (x=1,2)
H3[PWMo11O40] was prepared as follows. 3.12 g Na2WO4 · H2O (Merck) were dissolved
in 50 ml water. The aqueous solution of Na2WO4 was passed through an ion-exchange
resin (Ion exchanger I (Merck)).[90] The resulted sol of tungsten acid and 15 g MoO3
(Sigma Aldrich) were dissolved in 500 ml water and heated under reflux. 79 ml of 0.12 M
phosphoric acid were added dropwise to the reaction mixture. The resulting suspension
was heated for 3 h and kept at 298 K for 24 h until a clear yellow solution was obtained.
The remainder was filtered of and the volume of the resulting yellow solution was reduced
to ~30 ml using an evaporator. H3[PWMo11O40] crystallized during storage at 277 K for
several days.
H3[PW2Mo10O40] was prepared as follows. 3.97 g Na2WO4 · H2O (Merck) were dissolved
in 50 ml water. The aqueous solution of Na2WO4 was passed through an ion-exchange
resin (Ion exchanger I (Merck)).[90] The resulted sol of tungsten acid and to 13 g MoO3
(Sigma Aldrich) were dissolved in 455 ml water and heated under reflux. 75 ml of 0.12 M
phosphoric acid were added dropwise to the reaction mixture. The resulting suspension
19
was heated for 3 h and kept at 298 K for 24 h until a clear yellow solution was obtained.
The remainder was filtered of and the volume of the resulting yellow solution was reduced
to ~30 ml using an evaporator. H3[PW2Mo11O40] crystallized during storage at 277 K for
several days.
3.2 Sample characterization
X-Ray Fluorescence Analysis
Elemental analysis by X-ray fluorescence spectroscopy was performed on an X-ray
spectrometer (AXIOS, 2.4 kW model, PANalytical) equipped with a Rh K alpha source, a
gas flow detector and a scintillation detector. 60-80 mg of the samples were diluted with
wax (Hoechst wax C micropowder, Merck) at a ratio of 1:1 and pressed into 13 mm
pellets. Quantification was performed by standardless analysis with the SuperQ 5 software
package (PANalytical).
Atom Absorption Spectroscopy (AAS)
45 mg of P(V,W)xMo12-x (x = 0, 2) obtained before and after treatment under propene
oxidation condition (5% propene + 5% O2 in He; 723 K; 0 h and 12 h time on stream) were
diluted with aqueous ammonia to 10 ml. Measurements were performed with the Perkin
Elmer 1100B AAS-spectrometer equipped with pyrocoated graphite tubes, including a
platform, and an AS700 autosampler. Extinction was measured for 4 s (Table 3-1) at
213.6 nm using a super hollow cathode lamp (HCL) of Photron at 35 mA. Background
correction was achieved by alteration of D2 lamp radiation and HCL radiation,
respectively. Background was subtracted from peak area of extinction. Each sample was
measured 3 times. Therefore, 10 µl of LaCl3 solution as matrix modifier (Roth ≥ 99.9 %,
100 mg/l) were injected, and subsequently 90 µl of sample solution were added.[77] The
applied temperature program is presented in Table 3-1. The, syringe of autosampler was
purged before and after each step with diluted nitric acid.
20
Table 3-1: AAS measurement program with applied temperatures [K], heating durations [s],
dwelling times [s], and internal gas purges [ml min-1
(Ar)].
step temperature
setpoint [K]
time to reach
setpoint [s]
dwelling
time [s]
internal gas purge
[ml min-1
(Ar])
drying 363 7 5 300
drying 393 3 30 300
pyrolysis 573 3 20 300
pyrolysis 1673 5 40 300
pyrolysis 1673 1 5 0
atomization/measurement 2873 0 4 0
cleaning 2873 1 8 200
cooling RT fast - 300
X-ray absorption spectroscopy (XAS)
Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at
beamline X, V K edge (5.465 keV) and W LIII edge (10.204 keV) at beamline C at the
Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a Si(311) double crystal
monochromator at beamline X and a Si(111) double crystal monochromator at beamline C.
X-ray absorption fine structure (XAFS) analysis was performed using the software
package WinXAS v3.2..[91] Background subtraction and normalization were carried out
by fitting linear polynomials and 3rd degree polynomials to the pre-edge and to the post-
edge region of an absorption spectrum, respectively. The extended X-ray absorption fine
structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic
background μ0(k). The FT(χ(k)·k3), often referred to as pseudo radial distribution function,
was calculated by Fourier transforming the k3-weighted experimental χ(k) function,
multiplied by a Bessel window, into the R space. EXAFS data analysis was performed
using theoretical backscattering phases and amplitudes calculated with the ab-initio
multiple-scattering code FEFF7.[92] Structural data employed in the analyses were taken
from the Inorganic Crystal Structure Database (ICSD).
The modified H3[PMo12O40] (ICSD 209 [14,93]) model structure was modified by
substituting a Mo for either V or W (V for PVxMo12-x; W for PWxMo12 (x = 1, 2)). Single
scattering and multiple scattering paths of the model structure were calculated up to 6.0 Å
21
with a lower limit of 4.0% in amplitude with respect to the strongest backscattering path.
EXAFS refinements were performed in R space simultaneously to magnitude and
imaginary part of a Fourier transformed k3-weighted and k
1-weighted experimental χ(k)
using the standard EXAFS formula.[94] This procedure reduces the correlation between
the various XAFS fitting parameters. Structural parameters allowed to vary in the
refinement were (i) disorder parameter σ2
of selected single-scattering paths assuming a
symmetrical pair-distribution function and (ii) distances of selected single-scattering paths.
The statistical significance of the fitting procedure employed was carefully evaluated in
three steps as outlined in.[95] The procedures accounts for recommendations of the
International X-ray Absorption Society on criteria and error reports.[96] First, the number
of independent parameters (Nind) was calculated according to the Nyquist theorem Nind =
2/Π · ΔR· Δk + 2. In all cases, the number of free running parameters in the refinements
was well below Nind. Second, confidence limits were calculated for each individual
parameter. In the corresponding procedure, one parameter was successively varied by a
certain percentage (i.e. 0.05% for R and 5% for σ2) and the refinement was restarted with
this parameter kept invariant. The parameter was repeatedly increased or decreased until
the fit residual exceed the original fit residual by more than 5%. Eventually, the confidence
limit of the parameter was obtained from linear interpolation between the last and second
last increment for an increase in fit residual of 5%. This procedure was consecutively
performed for each fitting parameter. Third, a F test was performed to assess the
significance of the effect of additional parameters on the fit residual.[97]
Powder X-ray diffraction (XRD)
Ex situ XRD measurements were conducted on an X’Pert PRO MPD diffractometer
(Panalytical, θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel
PIXcel detector. Wide-angle scans (5-90° 2θ, variable slits) were measured in reflection
mode using a silicon sample holder.
In situ XRD measurements were conducted on a STOE diffractometer (θ-θ Mode) using an
Anton Paar in situ cell. Thermal stability tests were conducted in 20 % O2 in He (total flow
100 ml/min) in a temperature range from 323 K to 723 K. The gas phase composition at
the cell outlet was continuously monitored using a non-calibrated mass spectrometer in a
22
multiple ion detection mode (Pfeiffer Omnistar). Phase analysis was performed using the
X´Pert Highscore Plus software package (Panalytical).
IR Spectroscopy
IR spectra were recorded on a Magna System 750 of Nicolet in a wavenumber range of
400 – 4000 cm-1
. Samples were pressed into pellets of 13 mm in diameter after diluting
with KBr.
Raman Spectroscopy
Raman spectra were recorded on a FT-RAMAN spectrometer (RFS 100, Bruker). A
Nd:YAG laser with a wavelength of 1064 nm was used for excitation. Samples were
measured in a glass sample holder with a resolution of 1 cm-1
. The laser power at the
sample position was adjusted to 150 mW. All measurement consisted of 200 scans for each
sample. Scans were averaged for improvement of the signal-to-noise ratio.
Catalytic testing - selective propene oxidation
Quantitative catalysis measurements were performed using a fixed bed laboratory reactor
connected to an online gas chromatography system (Varian CP-3800) and a non-calibrated
mass spectrometer (Pfeiffer Omnistar). The fixed-bed reactor consisted of a SiO2 tube (30
cm length, 9 mm inner diameter) placed vertically in a tube furnace. In order to achieve a
constant volume and to exclude thermal effects, catalysts samples (~ 38 mg) were diluted
with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375 mg.
Additionally, samples were prepared for XRD annd AAS measurements. Therefore, pure
PMo12 (98 mg), PV2Mo10 (55 mg), and PW2Mo10 (55 mg) was placed in a SiO2 tube (30
cm length, 3 mm inner diameter) and fixed between two layers of quartz wool and treated
under catalytic conditions
For catalytic testing in selective propene oxidation a mixture of 5% propene (Linde Gas,
10% propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in
helium (Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas flow
rates of oxygen, propene, and helium were adjusted with separate mass flow controllers
23
(Bronhorst) to a total flow of 40 ml/min. All gas lines and valves were preheated to 473 K.
Hydrocarbons and oxygenated reaction products were analyzed using a Carbowax capillary
column connected to an AL2O3/MAPD column or a fused silica restriction (25 m x
0.32 mm) each connected to a flame ionization detector. O2, CO, and CO2 were separated
using a Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x 1/8``) as
precolumns combined with a back flush. For separation, a Hayesep Q packed column (0.5
m x 1/8``) was connected via a molsieve (1.5 m x 1/8``) to a thermal conductivity detector
(TCD).
Conversion, product selectivity, and reaction rate were calculated by the following
equations:
Conversion:
(3.1)
Selectivity:
(3.2)
reaction rate:
(3.3)
with:
volume fraction
stoichiometric factor
desired product
volume flow
mass of molybdenum
24
3.3 Ex situ characterization of P(V,W)xMo12-x (x = 0, 1, 2)
3.3.1 Quantification of metal loading by XRF
Quantitative analysis of P(V,W)xMo12-x (x = 0, 1, 2) was performed to verify the synthesis
process. Results of quantitative XRF measurements and nominal composition of
P(V,W)xMo12-x (x = 0, 1, 2) are summarized in Table 3-2. Experimental composition
corresponded very well to the nominal composition and confirmed a successful synthesis.
Table 3-2: Results of quantitative XRF measurements and nominal composition of P(V,W)xMo12-x
(x = 0, 1, 2).
elements
H P Mo V W O
PVMo11 nom.. wt.% 0.23 1.75 59.25 2.86 - 35.93
PVMo11 exp. wt.% - 0.75 62.07 2.24 - 34.48
PV2Mo10 nom. wt.% 0.29 1.78 55.22 5.86 - 36.84
PV2Mo10 exp.. wt.% - 0.89 58.62 4.91 - 35.14
PWMo11 nom. wt.% 0.16 1.62 55.16 - 9.61 33.45
PWMo11 exp.wt.% - 0.81 56.60 - 9.98 32.60
PW2Mo10 nom. wt.% 0.15 1.55 47.94 - 18.38 31.98
PW2Mo10 exp.wt.% - 0.66 50.73 - 17.35 31.27
3.3.2 Long-range structure of as-prepared P(V,W)xMo12-x (x = 0, 1, 2)
Long-range order structural analysis of P(V,W)xMo12-x (x = 0, 1, 2) was performed using
X-ray powder diffraction (XRD). Fig. 3-1 shows the XRD powder pattern together with
the theoretical pattern from structure refinement. H3[PMo12O40]·13H2O (ICSD 31128) was
used as model for the refinement.[98] The samples P(V,W)xMo12-x (x = 0, 1, 2) were
synthesized as 13 hydrate. Vanadyl or molybdenyl salts of the corresponding
heteropolyacids typically crystallize in the cubic crystal systems.[99,100] Other Hydrates,
for example 6 H2O, 30 H2O of the heteropolyacids, lead also to a cubic crystal
system.[101,102]
25
Therefore, the 13 hydrate, crystallizing in a triclinic crystal system, was chosen as desired
compound to ensure the incorporation of the addenda atoms (V, W) in the Keggin ion and
excluding the formation of the corresponding salt compounds. For the refinement, the
molybdenum atoms were statistically replaced by the addenda atoms (V, W) depending on
the degree of substitution in the model structure. Atomic coordinates were kept invariant.
All samples showed the typical pattern of the 13 hydrate of HPOM.[98] The observed
deviations are explained by different crystal water content due to grinding of the sample.
Table 3-3 summarized the lattice parameters of the refinements. The volumes of the unit
cells of the corresponding PVxMo12-x (x = 1, 2) samples indicated slightly decreased
Inte
nsity
Inte
nsity
Inte
nsity
Inte
nsity
Inte
nsity
10 20 30 40 50 60 70 80
Diffraction angle 2θ [°]
PV2Mo10
PMo12
PVMo11
PWMo11
PW2Mo10
experiment refinement difference
Fig. 3-1: XRD pattern of PMo12, PVMo11, PV2Mo10, PWMo11, and PW2Mo10 together with the
XRD structure refinement of the 13 hydrate phase.
26
volumes with higher degrees of vanadium substitution. The volumes of the unit cells of the
corresponding PWxMo12-x (x = 1, 2) samples were nearly independent of the tungsten
degree of substitution. A possible simplified explanation for the different behaviour of V
and W centers substituting for Mo in Keggin type HPOM may be the considerably
different ion radii of V (68 pm) and W (74 pm) in a six-fold coordination compared to Mo
(74 pm).[103] The smaller ionic radius of V resulted in a decreased unit cell in contrast to
W with identical ionic radius compared to Mo.
Table 3-3: Lattice parameters resulting from a refinement for PMo12, PVMo11, PV2Mo10, PWMo11,
and PW2Mo10.
Sample PMo12 PVMo11 PV2Mo10 PWMo11 PW2Mo10
Space group P P P P P
Lattice parameters
a (Å) 13.546 13.551 13.566 13.531 13.577
b (Å) 14.088 14.058 14.022 14.047 14.051
c (Å) 14.134 14.145 14.126 14.109 14.135
alpha (°) 60.616 60.542 60.584 60.784 60.656
beta (°) 67.871 67.619 67.524 67.919 67.752
gamma (°) 70.213 70.143 70.189 70.346 70.210
V (Å3) 2137.213 2130.313 2124.298 2129.766 2136.110
GOF 2.52 2.98 2.79 2.97 3.38
3.4 Short-range order structural characterization of P(V,W)xMo12-x (x = 0, 1,
2)
IR-/RAMAN-Spectroscopy
IR and Raman spectra of P(V,W)xMo12-x (x = 0, 1, 2) are shown in Fig. 3-2. All samples
exhibited identical IR and Raman bands and were comparable to bands known from the
literature for H3[PMo12O40].[104–106] Therefore, it may be assumed that the Keggin ion
structure existed for all samples without significant influence of the addenda atoms (V, W).
27
The IR and Raman signals are summarized in the Appendix (Table A 1; Table A 2).
H3+x[PVxMo12-xO40] (x = 1, 2) showed two shoulders in the IR spectra at ~1080 cm-1
(νas (P-Oi)) and ~980 cm-1
(νas (V-Ot)) which could be assigned to V incorporated in the
Keggin ion.[107,108] Additionally, the IR signals for νas (P-Oi) (~1064 cm
-1) and νas (M-
Ot) (~977 cm-1
) shifted to lower wavenumbers for vanadium substituted and to higher
wavenumbers for tungsten substituted HPOM. This confirmed the assumption of
incorporated addenda atoms (V, W) in the HPOM.[108–110] The absence of a band in the
Raman spectra at 1034 cm-1
related to a vanadyl cation suggested the V in the secondary
structure.[111–113]. Therefore, it may be assumed that no further structural motifs except
of the Keggin ion was formed independent of the degree of substitution.
X-ray Absorption Spectroscopy
Mo K edge analysis of P(V,W)xMo12-x (x = 0, 1, 2)
Fig. 3-3 depicts the theoretical and experimental Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x
(x = 0, 1, 2). The very similar shape of Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x
1400 1200 1000 800 600 400
Tra
nsm
issio
n
Wavenumber [cm-1]
1000 800 600 400 200
Inte
nsity
Wavenumber [cm-1]
Fig. 3-2: IR (left) and Raman spectra (right) of PMo12, PVMo11, PV2Mo10, PWMo11, and PW2Mo10.
PV2Mo10
PMo12
PVMo11
PWMo11
PW2Mo10
PV2Mo10
PMo12
PVMo11
PWMo11
PW2Mo10
28
(x = 0, 1, 2) indicated a similar local structure of the Mo centers in P(V,W)xMo12-x
(x = 0, 1, 2). For a more detailed analysis, theoretical phase and amplitudes were calculated
for Mo-O and Mo-Mo distances and used for EXAFS refinement. The results of the
refinements of the Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x (x = 0, 1, 2) are summarized
in Table 3-4. The shapes of Mo K edge FT(χ(k)·k3) of P(V,W)xMo12-x (x = 0, 1, 2) were
comparable to that of PVxMo12-x (x = 0, 1, 2) investigated by Ressler et al..[13,14,16]
Substituting PMo12 lead to decreasing amplitudes in the FT(χ(k)·k3) with higher amount of
addenda atoms in the Keggin structures. The diminished amplitudes were accounted for by
an increased disorder parameters σ2 depending on the degree of substitution. Probably the
decreased amplitudes resulted from a distortion of the [(V,W)O]6 units in substituted
Keggin ions based on PMo12. The M-O and Mo-Mo distances were comparable for all
P(V,W)xMo12-x (x = 0, 1, 2) samples. This indicated an incorporation of addenda V and W
atoms into the Mo based Keggin ion without influencing the structure. Hence, V and W
were suitable elements for substituting Mo atoms in HPOM.
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0 1 2 3 4 5 6 R [Ǻ]
FT
(χ(k
)·k
3)
Fig. 3-3: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of as prepared
PMo12, PVMo11, PV2Mo10, PWMo11, and PW2Mo10.
PV2Mo10
PMo12
PVMo11
PWMo11
PW2Mo10
29
Table 3-4: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in as prepared P(V,W)xMo12-x (x = 0, 1, 2). Experimental parameters were obtained
from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental Mo K
edge XAFS χ(k) of P(V,W)xMo12-x (x = 0, 1, 2) (k range from 3.0-13.7.0 Å-1
, R range from 0.9 to
4.0 Å, E0 = ~1.7, residuals ~11.3-20.0 Nind = 22, Nfree = 9). Subscript c indicates parameters that
were correlated in the refinement.
Keggin model PMo12 PVMo11 PV2Mo10
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 1 1.68 1.64 0.0024 1.65 0.0039 1.66 0.0052
Mo-O 2 1.91 1.78 0.0030c 1.78 0.0043c 1.77 0.0059c
Mo-O 2 1.92 1.95 0.0030c 1.95 0.0043c 1.94 0.0059c
Mo-O 1 2.43 2.40 0.0006 2.40 0.0011 2.40 0.0051
Mo-Mo 2 3.42 3.42 0.0054c 3.43 0.0068c 3.43 0.0076c
Mo-Mo 2 3.71 3.73 0.0054c 3.73 0.0068c 3.72 0.0076c
Keggin model PMo12 PWMo11 PW2Mo10
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 1 1.68 1.64 0.0024 1.63 0.0036 1.62 0.0026
Mo-O 2 1.91 1.78 0.0030c 1.77 0.0039c 1.77 0.0033c
Mo-O 2 1.92 1.95 0.0030c 1.94 0.0039c 1.94 0.0033c
Mo-O 1 2.43 2.40 0.0006 2.40 0.0020 2.41 0.0019
Mo-Mo 2 3.42 3.42 0.0054c 3.42 0.0082c 3.43 0.0094c
Mo-Mo 2 3.71 3.73 0.0054c 3.73 0.0082c 3.74 0.0094c
V K edge analysis of PVxMo12-x (x = 1, 2)
Fig. 3-4 depicts the theoretical and experimental V K edge FT(χ(k)·k3) of PVMo11 and
PV2Mo10. The very similar shape of the FT(χ(k)·k3) indicated similar local structures
around the V centers in the unsupported HPOM Keggin structure. A detailed structure
analysis was performed by EXAFS. The shapes of the V K edge FT(χ(k)·k3) of PVMo11
and PV2Mo10 were comparable to the V K edge FT(χ(k)·k3) of PVMo11 and PV2Mo10
30
investigated by Ressler et al..[16,14] The results of the refinement of the V K edge
FT(χ(k)·k3) of PVMo11 and PV2Mo10 are shown in Table 3-5.. All V-O and V-Mo distances
were comparable for PVxMo12-x (x = 1, 2). The disorder parameters (σ2) for all V-O and
Table 3-5: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the V atoms in as prepared PVMo11 and PV2Mo10. Experimental parameters were obtained from a
refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental V K edge
XAFS χ(k) of PV2Mo12-SBA-15 (k range from 3.0-11.0 Å-1
, R range from 0.9 to 4.0 Å, E0 = 0.6,
residuals 9.1-11.7; Nind = 16, Nfree = 9). Subscript c indicates parameters that were correlated in the
refinement.
Keggin model PVMo11 PV2Mo10
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2)
V-O 1 1.68 1.63 0.0078 1.62 0.0041
V-O 2 1.91 1.94 0.0154c 1.95 0.0120c
V-O 2 1.92 1.95 0.0154c 1.95 0.0120c
V-O 1 2.43 2.38 0.0178 2.38 0.0093
V-Mo 2 3.42 3.36 0.0173c 3.34 0.0151c
V-Mo 2 3.71 3.78 0.0173c 3.74 0.0151c
-0.05
0.00
0.05
0.10
0 1 2 3 4 5 6
FT
(χ(k
)·k
3)
R [Ǻ]
PV2Mo10
PVMo11
Fig. 3-4: Theoretical (dotted) and experimental (solid) V K edge FT(χ(k)·k3) of as prepared
PVMo11 and PV2Mo10.x.
31
V-Mo distances were decreased for PV2Mo10 compared PVMo11. This decreased disorder
parameters (σ2) indicated a lower degree of distortion of the [VO6] units incorporated in the
Keggin ion for PVMo11.
W K edge analysis of PWxMo12-x (x = 1, 2)
EXAFS analysis of PWxMo12-x (x = 1, 2) rarely have been reported in the literature.
Therefore, W LIII edge FT(χ(k)·k3) of PWxMo12-x (x = 1, 2) were compared to H3[PW12O40]
(PW12) as reference to exclude the formation of PW12. The results confirmed the
assumption, that tungsten was incorporated in the HPOM. Fig. 3-5 (left) depicts the
theoretical and experimental W LIII edge FT(χ(k)·k3) of PWxMo12-x (x = 1, 2) and the
experimental W LIII edge FT(χ(k)·k3) of PW12. All EXAFS spectra showed comparable
shapes, indicating a comparable structure around the W centers The W LIII edge χ(k)·k3 of
PW12 exhibited distinct differences compared to the W LIII edge χ(k)·k
3 of PWxMo12-x
(x = 1, 2) in the range above 9 Å-1
. This indicated a slightly different structure or other
backscattering atoms around the W centers. W LIII edge FT(χ(k)·k3) of PWxMo12-x
R [Ǻ]
0 1 2 3 4 5 6
Fig. 3-5: (left) Theoretical (dotted) and experimental (solid) W K edge FT(χ(k)·k3) and (right)
χ(k)·k3 of as prepared PW12, PWMo11, and PW2Mo10.
FT
(χ(k
)·k
3)
PW2Mo10
PWMo11
0.00
0.05
0.10
0.15
0.20
0.25 PW12
4 6 8 10
χ(k
)·k
3
-4
-2
0
2
4
6
8
10
12
14
k [Ǻ-1]
32
(x = 1, 2) could be sufficiently simulated using only W-Mo distances (Table 3-6). Hence,
the formation of predominantly PW12 was excluded. All disorder parameters (σ2) in the
EXAFS refinement for PWxMo12-x (x = 1, 2) were nearly identical indicating a comparable
degree of distortion.
Table 3-6: Type and number (N), and XAFS disorder paramters (σ2) of atoms at distance R from
the Mo atoms in as prepared PWMo11 and PW2Mo10. Experimental parameters were obtained from
a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental W LIII edge
XAFS χ(k) of PWxMo12-x (x = 1, 2) (k range from 3.4-11.5 Å-1
, R range from 0.9 Å to 3.8 Å, E0 =
3.2, residuals 9.1-13.2 Nind = 8, Nfree = 16)
Keggin model PWMo11-SBA-15 PW2Mo10-SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2)
W-O 1 1.68 1.69 0.0024 1.72 0.0025
W-O 2 1.91 1.82 0.0025 1.83 0.0026
W-O 2 1.92 1.94 0.0025 1.95 0.0026
W-O 1 2.43 2.32 0.0028 2.29 0.0028
W-Mo 2 3.42 3.46 0.0047 3.46 0.0045
W-Mo 2 3.71 3.69 0.0047 3.69 0.0045
3.5 In situ Characterization of bulk heteropolyacids
3.5.1 In situ XRD of PMo12-x(V,W)x x = 0, 1, 2 during oxidation conditions
For elucidating structure-activity relationships, an analysis of the structural evolution
during oxidizing conditions was useful. Fig. 3-6 depicts selected in situ powder pattern of
PMo12 during treatment in 20% oxygen He (323 K to 723 K). At 323 K the typical pattern
peaks for H3[PMo12O40]·3-8 H2O were identified.[114,115] Between 373 K and 473 K a
dehydration of the HPOM lead to H3[PMo12O40] without constitutional water.[116,117]
The dehydration process continued until 673 K, where the anhydrous [PMo12O38.5] and β-
MoO3 were found.[117,34,118] Appendix Fig. A 1depicts the ion current m/e = 18 during
temperature programmed oxidation representing the amount of water. The ion current
increased between 498 K and 648 K confirming the loss of constitutional water. At
33
723 K a mixture of α-MoO3 + β-MoO3 with a higher concentration of the metastabile β-
MoO3 was identified.[118] Fig. 3-7 shows the in situ powder pattern of PVMo11 and
PV2Mo10 during treatment in 20% oxygen in He (323 K to 723 K). The powder pattern at
323 K shows for both PVMo11 and PV2Mo11 a hydrated structure (3-8 hydrate).[18,19]
Subsequently a dehydration of the water of crystallization resulted in PVxMo12-x (x = 1, 2)
without water of crystallization. Between 473 K and 573 K for PVMo11 and 523 K for
PV2Mo10 a mixture of H3-x[PVxMo12-xO40] (x = 1, 2) and the anhydrous [PVxMo12-xO38.5-
0.5x] (x = 1, 2) was found. Above 623 K (for PVMo11) and 573 K (for PV2Mo10) only the
[PVxMo12-xO38.5-0.5x] (x = 1, 2) phase was identified.[117] At 673 K a mixture of
[PV2Mo10O37.5], α-MoO3, and β-MoO3 could be detected for PV2Mo10. At 723 K only
MoO3 was found for both PVMo11 and PV2Mo10. The phase composition of the two MoO3
phase was different to that of PMo12 at 723 K. The amount of β-MoO3 decreased with
higher degree of vanadium substitution resulting in mainly α-MoO3 for PV2Mo10.
Fig. 3-8 shows the in situ powder pattern of PWMo11 and PW2Mo10 during treatment in
20% oxygen in He (323 K to 723 K). The powder pattern at 323 K shows for both PVMo11
and PV2Mo11 a hydrated structure (3-8 hydrate) comparable to H3-x[PVxMo12-xO40]
(x = 0, 1, 2).[18,19] Subsequently dehydration of water of crystallization resulted in
10 15 20 25 30 35 40 45
723 K
673 K
623 K
573 K
523 K
473 K
423 K
373 K
323 K
α-MoO3 + β-MoO3
[PMo12O38.5] + β-MoO3
H3[PMo12O40]+ [PMo12O38.5]
H3[PMo12O40]+ [PMo12O38.5]
H3[PMo12O40]+ [PMo12O38.5]
H3[PMo12O40]
H3[PMo12O40]
H3[PMo12O40]
H3[PMo12O40]*3-8 H2O
Diffraction angle 2θ [°]
Inte
nsitiy
Fig. 3-6: Selected in situ powder pattern during treatment in 20% oxygen in He (temperature
range 323 K to 723 K) of H3[PMo12O40] · n H2O.
34
H3[PWxMo12-xO40] (x = 1, 2) without water of crystallization. Between 473 K and 623 K
for PWMo11 and PW2Mo10 a mixture of H3[PWxMo12-xO40] (x = 1, 2) and the anhydrous
[PWxMo12-xO38.5] (x = 1, 2) was detectable. In contrast to H3-x[PVxMo12-xO40] (x = 0, 1, 2)
no separated anhydrous [PWxMo12-xO38.5] (x = 1, 2) phase was found. At 673 K a mixture
of [PWxMo12-xO38.5] (x = 1, 2) and at 623 K (for PVMo11) and β-MoO3 was found. At
723 K only MoO3 was found for both PWMo11 and PW2Mo10. This phase composition of
the MoO3 phase were different to that of PMo12, PVMo11, and PV2Mo10. The amount of β-
MoO3 increased with higher degree of tungsten substitution resulting in mainly β-MoO3
for PWMo11 and PW2Mo10.
Both MoO3 modifications (α-MoO3, and β-MoO3) possess slight different structures. The
smallest structural motif in α-MoO3, and β-MoO3 is a distorted MoO6 octahedron.[119]
α-MoO3 possesses a characteristic layer structure, with edges MoO6 octahedra sharing
10 15 20 25 30 35 40 45
Diffraction angle 2θ [°]
Inte
nsitiy
Inte
nsitiy
10 15 20 25 30 35 40 45
Diffraction angle 2θ [°]
H3+x[PVxMo12-xO40] · 3-8 H2O (x = 1, 2)
H3+x[PVxMo12-xO40] (x = 1, 2)
H3+x[PVxMo12-xO40]+ [PVxMo12-xO40-1.5·x]
(x = 1, 2)
[PVxMo12-xO40-1.5·x] (x = 1, 2)
[PV2Mo10O37.5] + MoO3
MoO3
723 K
673 K
623 K
573 K
523 K
473 K
423 K
373 K
323 K
723 K
673 K
623 K
573 K
523 K
473 K
423 K
373 K
323 K
Fig. 3-7: Selected in situ powder pattern during treatment in 20% oxygen in (temperature range
323 K to 723 K) of H4[PVMo11O40] · n H2O (left) and H5[PV2Mo10O40] · n H2O (right).
PVMo11 PV2Mo10
35
within the layer. The layers in α-MoO3 are connected via van der Waals interaction of
corner-sharing MoO6 octahedra.[120] Similar to ReO3 the MoO6 octahedra in β-MoO3 are
exclusively corner-shared.[121] An explanation for the favored formation of α-MoO3
depending on the degree of vanadium substitution in PVxMo12-x (x = 1, 2) could be
Pauling`s 3rd rule.[122] This rule states, that the stability of an ionic structure decreases
with higher amount of corner-sharing polyhedra. The distance between the neighboring
MoO6 octahedra in α-MoO3 is smaller than that of edge-sharing MoO6 octahedra in
β-MoO3.[123] Therefore, the energy loss due to the incorporation of V5+
compared to Mo6+
was compensated by the formation of predominantly edge-sharing α-MoO3. W6+
and Mo6+
have an identical charge and the consideration of a lower charge was irrelevant.
That is the reason why tungsten substituted HPOM lead to the formation of predominantly
β-MoO3. Additionally, many tertiary Mo/V or rather Mo/W as well as V/W/Mo mixed
oxides are known in the literature.[124–126]The high miscibility of the mixed metal oxides
Inte
nsitiy
Inte
nsitiy
10 15 20 25 30 35 40 45
Diffraction angle 2θ [°]
10 15 20 25 30 35 40 45
Diffraction angle 2θ [°]
H3[PWxMo12-xO40] · 3-8 H2O (x = 1, 2)
H3 [PWxMo12-xO40] (x = 1, 2)
H3[PWxMo12-xO40]+ [PWxMo12-xO38.5] (x = 1, 2)
[PWxMo10O38.5] (x = 1, 2)+ MoO3
MoO3
723 K
673 K
623 K
573 K
523 K
473 K
423 K
373 K
323 K
723 K
673 K
623 K
573 K
523 K
473 K
423 K
373 K
323 K
Fig. 3-8: Selected in situ powder pattern during treatment in 20% oxygen in (temperature range
323 K to 723 K) of H3[PWMo11O40] · n H2O (left) and H3[PW2Mo10O40] · n H2O (right).
PWMo11 PW2Mo10
36
may be explained by the comparable ion radii in a six fold coordination and the preferred
formation of MO6 (x = V, W, Mo) octahedra.[125,126] A further structure stabilizing
effect of addenda atoms (V, W, Nb) in the synthesis of mixed molybdenum oxides with
Mo5O14-type structure has been shown.[127] Therefore, it may be assumed that the
addenda atoms (V, W) possessed a structure directing effect during decomposition of
HPOM under oxidizing conditions. The resulting structures depended on the addenda
atom. Hence, vanadium substituted PVxMo12-x (x = 1, 2) favored the formation of α-MoO3
with edge-shared MO6 octahedra (Mo, V). Tungsten substituted PWxMo12-x (x = 1, 2) lead
to the formation of β -MoO3 with corner-shared MO6 octahedra (Mo, W).
3.5.2 Functional characterization of bulk HPOM
Catalytic testing of P(V, W)xMo12-x (x = 0, 1, 2)
Reaction rates and selectivities of P(V,W)xMo12-x (x = 0, 1, 2) in propene oxidation at
723 K are shown in Fig. 3-9. Reaction rates for of P(V,W)xMo12-x (x = 0, 1, 2) were
calculated for similar propene oxidation conditions (4-5 % propene conversion). Reaction
rates for PVxMo12-x (x = 0, 1, 2) were different depending of the degree of vanadium and
tungsten substitution. The reaction rates for PVxMo12-x (x = 0, 1, 2) increased with higher
0
2
4
6
8
10
12
14
0
20
40
60
80
100
a b c e d
acrylic acid
acetic acid acrolein
acetone
acetaldehyde
CO
CO2 propionaldehyde
Se
lectivity [%
]
Re
actio
n r
ate
[µ
mo
l(p
roen
e)g
-1(M
o)s
-1]
Fig. 3-9: Reaction rate (µmol(propene)·g-1
(Mo)·s-1
) and selectivity of (a) PV2Mo10, (b) PVMo11,
(c) PMo12, (d) PWMo11, and (e) PW2Mo10 in 5% propene and 5% oxygen in He at 723 K.
37
degree of vanadium substitution from 5.8 µmol(propene)g-1
(Mo)s-1
(PMo12) to 12.3
µmol(propene)g-1
(Mo)s-1
(PV2Mo10). The tungsten substituted samples showed an
increased reaction rate to 7.4 µmol(propene)g-1
(Mo)s-1
too compared to PMo12. The
influence on the catalytic activity was higher for the vanadium substituted samples
PVxMo12-x (x = 1, 2) in contrast to the tungsten substituted samples PWxMo12-x (x = 1, 2).
Selectivities towards acrolein decreased with higher degree of vanadium substitution from
29% (PMo12) to 22% (PV2Mo10). In contrast to the selectivities of PVxMo12-x (x =' 1, 2),
the selectivities towards acrolein for both tungsten substituted samples (27%) were
comparable to that of unsubstituted PMo12 (29%). Selectivities towards propionaldehyde
and acetaldehyde decreased with higher degree of substitution independent of the addenda
atoms (V or W). Additionally, the selectivities towards acetic acid increased with higher
degree of vanadium substitution. In contrast to that PWxMo12-x (x = 1, 2) showed not
significant selectivities towards acetic acid (below 1%). The amount of total oxidation
products (CO; CO2) increased with higher degree of substitution. The distribution of the
total oxidation products was different depending to the nature and degree of substitution.
Compared to the product distribution of unsubstituted PMo12, PVxMo12-x (x = 1, 2) and
PWxMo12-x (x = 1, 2) showed an increased production of CO and CO2, respectively.
Influence of phosphorus and structure to the catalytic activity
In situ XRD characterization (chapter 3.5.1) showed an influence of the addenda atoms in
HPOM on the resulting structures during thermal treatment under oxidizing conditions. A
structural characterization during propene oxidation at temperatures above 698 K was not
feasible due to coke formation in the in situ cell of the XRD. Therefore, the reactor for
catalytic testing is used for catalytic treatment (5% propene and 5% O2 at 723 K; 0 h and
12h time on stream) of pure PMo12, PV2Mo12 and PW2Mo10. Subsequently, the samples
were investigated with XRD after treatment (0 h and 12 h time on stream) to obtain
structural information. Additional, the samples were investigated with AAS to quantify the
content of phosphorus. Fig. 3-10 depicts the evolution of the propene conversion for
PMo12, PV2Mo10, and PW2Mo10 up to 12 h time on stream at 723 K. The sample weights of
PMo12, PV2Mo10, and PW2Mo10 were chosen to reach a comparable propene conversion
(11.8-13.5) at 723 K with time on stream 0 h (c.f. chapter 3.2) All three samples exhibited
a decreased activity with longer time on stream. The degree of deactivation was higher for
38
PMo12 than for PV2Mo10 which again was higher than that of PW2Mo10. One explanation
for the deactivation process is an enrichment of phosphorous on the surface of bulk
HPOM. Phosphorus containing catalysts (i.e. VPO, FePO, MoPO) play a crucial role as
oxidation catalysts.[128] Millet et al. showed that adding small amounts of phosphoric acid
to the feed showed positive effects on long-term stability and catalytic performance of
FePO catalysts during ODH of isobutyric acid into methacrylic acid.[129] The phosphorus
source was needed to maintain a constant P/Fe ratio at the surface of the catalyst.[129]
Another example for the relevance of P in oxidation reactions are VPO catalysts.[128,130]
VPO catalysts showed migration of phosphorus species to the surface and a decreasing
amount of phosphorus in the catalyst during water vapor treatment. The excess of
phosphorus on the surface may suppress oxidation of VPO catalysts hindering formation of
active sites for oxidation reactions. Subsequently, hydrolysis of P-O-P or P-O-V groups
resulted in a removal of phosphate groups on the surface and an increasing
activity.[128,130] The quantification of phosphorus in all samples (P(V,W)xMo12-x
x = 0, 2) showed that the content of phosphorus was similar before and during treatment
under catalytic conditions (0 h and 12 h time on stream) (Appendix Fig. A 4). Therefore,
removal of phosphate groups from the HPOM was excluded. Hence, XRD structural
analysis of the treated (time on stream 0 h and 12 h) samples was used to elucidate
Fig. 3-10: propene conversion for PMo12 ( ), PV2Mo10 ( ), and PW2Mo10 ( ) during catalytic testing
(5% propene and 5% O2 at 723 K; 12h time on stream).
0 10 20 30 40
323
423
523
623
Cycles
T [
K]
0
2
4
6
8
10
12
14
pro
pe
ne
co
nvers
ion [%
]
723
0 h 12 h Time on stream
39
structural differences (Fig. 3-11). Structural differences may be also responsible for the
different deactivation processes and/or different reaction rates and selectivities (Fig. 3-9).
XRD powder pattern for each sample measured at 0 h and at 12 h were nearly identical.
XRD pattern peaks of PMo12 (0 h and 12 h time on stream) corresponded to that of α-
MoO3 (c.f. Appendix Fig. A 2).[41] α-MoO3 is the thermodynamically stable modification
of molybdenum oxides in their highest oxidation state (+6) and indicated an oxidizing
character of the catalytic gas composition.[131,132] Kühn et al. showed that the treatment
of α-MoO3 under propene oxidation conditions had no influence on the structure.[133]
Therefore, it may be assumed that PMo12 decomposed during propene oxidation conditions
at 723 K to the thermodynamically stable α-MoO3 which lead in a poor catalytic activity
(c.f. Fig. 3-9).[41]
The resulting structure for PV2Mo10 (0 h and 12 h time on stream) was probable a mixture
of lacunary Keggin ions and Keggin ions. Ressler et al. described for PV2Mo10 a dynamic
behaviour by isothermally switching from propene (reducing) to an oxidizing (propene and
oxgen) and back to propene (reducing) conditions at 723 K. This in situ XAS experiments
showed the formation of a short vanadium-molybdenum distance of about 2.8 Å for the
0h 12h
Fig. 3-11: XRD powder pattern of PMo12, PV2Mo10, and PW2Mo10 after treatment in 5% propene
and 5% oxygen in He at 723 K with time on stream 0 h (left) and 12 h (right).
Inte
nsitiy
10 20 30 40 50 60 70 80
Diffraction angle 2Ɵ [°]
PMo12
PV2Mo10
PW2Mo10
Inte
nsitiy
10 20 30 40 50 60 70 80
Diffraction angle 2Ɵ [°]
PMo12
PV2Mo10
PW2Mo10
40
reduced state. The oxidized state exhibits a longer distance of the vanadium center to an
extra-Keggin molybdenum center at 3.2 Å.[16] The results suggested a mixture of at least
two sites around two different V centers.[16] Therefore, it may be assumed, that the
resulted structure for PV2Mo10 treated during catalytic conditions (5% propene + 5%
oxygen in He at 723 K) corresponded to a mixture of lacunary Keggin ions and Keggin
ions. Hence, a final structural solution was not feasible. Nevertheless, the mixture of the
various structures may be responsible for the enhanced catalytic activity (c.f. Fig. 3-9).
XRD pattern of PW2Mo10 after catalytic treatment (0 h and 12 h time on stream) showed
broad peaks indicating an amorphous or less crystalline compound. A mixture of α-MoO3
and Mo17O47 could be indentified from the XRD pattern (Appendix Fig. A 3).
Molybdenum centers in Mo17O47 have an average valence of ~ +5.5. This indicated, that
the degree of reduction was higher for PW2Mo10 than for PMo12. Hence, tungsten lead to
an increased reducibility of PW2Mo10 during propene oxidation conditions.
The different structures formed during catalytic conditions (5% propene + 5% oxygen in
He at 723 K) depended on the substituted element (V, Mo). Therefore a structure directing
effect of the addenda atoms to structures formed during thermal treatment under catalytic
may be assumed. The different structures resulting during catalytic conditions (5% propene
+ 5% oxygen in He at 723 K) were an explanation for the different catalytic behaviours of
P(V,W)xMo12-x ( x = 0, 1, 2). α-MoO3 resulting from PMo12 showed the lowest catalytic
activity in propene oxidation. The mixture of lacunary Keggin ions and Keggin ions
resulting for PV2Mo10 and the mixture of α-MoO3 and Mo17O47 resulting for PW2Mo10
showed an increased catalytic activity in propene oxidation.
41
3.6 Summary
P(V,W)xMo12-x (x = 0, 1, 2) were examined by a combination of various characterization
techniques. The synthesis of P(V,W)xMo12-x (x = 0, 1, 2) lead to HPOM with the desired
Keggin type structure and chemical composition. P(V,W)xMo12-x (x = 0, 1, 2) crystallized
as 13 hydrate in a triclinic crystal system. The 13 hydrate structure of the HPOM indicated
the incorporation of the addenda atoms (V, W) in the Keggin ion. Hence, the formation of
the corresponding salt compounds was excluded. The volume of the unit cell decreased
with higher vanadium substitution because of the smaller ionic radius of V in a six-fold
coordination in contrast to W with an identical ionic radius compared to Mo. The results of
the IR and Raman measurements confirmed, that the addenda atoms (V, W) were
incorporated in the Keggin ion. Peaks in the IR and Raman spectra indicating additional
structure motifs except for the Keggin ion structure were not found. Results of the EXAFS
refinements indicated, that the addenda atoms (V, W) were incorporated in the Keggin ion
independent of the degree of substitution.
In situ XRD pattern of P(V,W)xMo12-x (x = 0, 1, 2) showed that the addenda atoms have an
structure directing effect during decomposition under oxidizing conditions. Vanadium
substitution in HPOM (PVxMo12-x (x = 1, 2)) lead to an increased formation of
predominantly α-MoO3 depending on the degree of vanadium substitution. Conversely,
tungsten substituted PWxMo12-x (x = 1, 2) exhibited an increased formation of β-MoO3
depending on the degree of tungsten substitution.
Catalytic tests revealed an increased activity for the substituted HPOM (P(V,W)xMo12-x
(x = 1, 2)) depending on the degree of substitution. The addenda atoms (V, W) had a
structure directing effect to the structures forming during propene oxidation conditions.
Unsubstituted PMo12 formed α-MoO3 with poor catalytic activity in propene oxidation.
Vanadium substitution probably lead to a mixture of lacunary Keggin ions and Keggin
ions, resulting in an enhanced catalytic activity. Tungsten substituted HPOM (P(W)xMo12-x
(x = 1, 2)) formed a mixture of α-MoO3 and Mo17O47 with an enhanced catalytic activity.
Hence, the different structures resulting during catalytic condition and the different
chemical compositions (P(V,W)xMo12-x (x = 0, 1, 2)) seemed to be responsible for the
different catalytic activities and selectivities during propene oxidation.
42
4 Characterization of P(V,W)xMo12-x-SBA-15 (x = 0, 1,
2) (10 wt.% Mo)
For elucidating structure activity correlations of model systems for catalytic investigations,
a detailed knowledge about structure and chemical composition is indispensable.
Therefore, various characterization methods, such as XRF, XRD, Nitrogen Physisorption,
and XAS are necessary for a sufficient characterization of the used catalyst systems. In this
chapter it will be shown that the initial Keggin structure was retained after supporting of
P(V,W)xMo12-x (x = 0, 1, 2) onto SBA-15. Furthermore it should be ensured, that the
supporting process lead to the desired metal loadings of 10 wt.% Mo for the current model
catalyst. Additionally, an analysis of the structure of the supported model catalyst was
carry out to confirm well dispersed Keggin ion structure motifs on the support material.
4.1 Sample Preparation
Preparation of the support material silica SBA-15
Silica SBA-15 was prepared according to Ref. [23]. 16.2 g of triblock copolymer (Aldrich,
P123) were dissolved in 294 g water and 8.8 g hydrochloric acid at 308 K and stirred for
24 h. After addition of 32 g tetraethyl orthosilicate for 24 h, the reaction mixture was
stirred for 24 h at 373 K. The resulting gel was transferred to a glass bottle and the closed
bottle was heated to 388 K for 24 h. Subsequently, the suspension was filtered and washed
with a mixture of H2O/EtOH (100:5). The resulting white powder was dried at 378 K for
3 h and calcined at 453 K for 3 h and at 823 K for 5 h. Three batches of silica SBA-15
were used for P(V, W)xMo12-x-SBA-15 (10 wt. % Mo).
Preparation of HPOM supported on silica SBA-15
HPOM (Chapter 3.1) were supported on SBA-15 via incipient wetness. The amount of
molybdenum was adjusted to 10 wt.%. Therefore an aqueous solution of HPOM was used.
43
4.2 Sample characterization
X-Ray Fluorescence Analysis
Elemental analysis by X-ray fluorescence spectroscopy was performed on an X-ray
spectrometer (AXIOS, 2.4 kW model, PANalytical) equipped with a Rh K alpha source, a
gas flow detector and a scintillation detector. 60-80 mg of the samples were diluted with
wax (Hoechst wax C micropowder, Merck) at a ratio of 1:1 and pressed into 13 mm
pellets. Quantification was performed by standardless analysis with the SuperQ 5 software
package (PANalytical).
Physisorption measurements
Nitrogen physisorption isotherms were measured at 77 K on a BEL Mini II volumetric
sorption analyzer (BEL Japan, Inc.). The silica SBA-15 sample was treated under vacuum
at 368 K for about 20 min and at 448 K for about 17 h before starting the measurement.
Data processing was performed using the BELMaster V.5.2.3.0 software package. The
specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method in
the relative pressure range of 0.03-0.24 assuming an area of 0.162 nm2
per N2
molecule.[69] The adsorption branch of the isotherm was used to calculate pore size
distribution and cumulative pore area according to the method of Barrett, Joyner, and
Halenda (BJH).[70]
Powder X-ray diffraction (XRD)
XRD measurements were conducted on an X’Pert PRO MPD diffractometer (Panalytical,
θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel PIXcel detector.
Wide-angle scans (5-90° 2θ, variable slits) were measured in reflection mode using a
silicon sample holder. Small-angle scans (0.4-6.0° 2θ, fixed slits) were collected in
transmission mode with the sample spread between two layers of Kapton foil.
44
X-ray absorption spectroscopy (XAS)
Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at
beamline X, V K edge (5.465 keV) and W LIII edge (10.204 keV) at beamline C at the
Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a Si(311) double crystal
monochromator at beamline X and a Si(111) double crystal monochromator at beamline C.
X-ray absorption fine structure (XAFS) analysis was performed using the software
package WinXAS v3.2..[91] Background subtraction and normalization were carried out
by fitting linear polynomials and 3rd degree polynomials to the pre-edge and to the post-
edge region of an absorption spectrum, respectively. The extended X-ray absorption fine
structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic
background μ0(k). The FT(χ(k)·k3), often referred to as pseudo radial distribution function,
was calculated by Fourier transforming the k3-weighted experimental χ(k) function,
multiplied by a Bessel window, into the R space. EXAFS data analysis was performed
using theoretical backscattering phases and amplitudes calculated with the ab-initio
multiple-scattering code FEFF7.[92] Structural data employed in the analyses were taken
from the Inorganic Crystal Structure Database (ICSD).
The modified H3[PMo12O40] (ICSD 209 [14,93]) model structure was modified by
substituting a Mo for either V or W (V for PVxMo12-x; W for PWxMo12 (x = 1, 2)) .Single
scattering and multiple scattering paths of the model structure were calculated up to 6.0 Å
with a lower limit of 4.0% in amplitude with respect to the strongest backscattering path.
EXAFS refinements were performed in R space simultaneously to magnitude and
imaginary part of a Fourier transformed k3-weighted and k
1-weighted experimental χ(k)
using the standard EXAFS formula.[94] This procedure reduces the correlation between
the various XAFS fitting parameters. Structural parameters allowed to vary in the
refinement were (i) disorder parameter σ2
of selected single-scattering paths assuming a
symmetrical pair-distribution function and (ii) distances of selected single-scattering paths.
Detailed information about the fitting procedure are described in chapter 3.2.
45
4.3 Results of the Characterization
Quantification of metal loading XRF
Quantitative analysis of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) was performed to verify the
supporting process. Results of quantitative XRF measurements and nominal composition
of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) was summarized in Table 4-1. Experimental
composition corresponded very well to the nominal composition and confirmed a
successful supporting process. For simplification of the nomenclature the samples were
still denoted as P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) (10 wt.% Mo).
Table 4-1: Results of quantitative XRF measurements and nominal composition of P(V,W)xMo12-x-
SBA-15 (x = 0, 1, 2).
Elements
H P Mo V W O Si
PMo12-SBA-15 nom. wt.% 0.03 0.27 10.01 - - 50.37 39.32
PMo12-SBA-15 exp. wt.% - 0.33 9.89 - - 50.30 39.41
PVMo11-SBA-15 nom. wt.% 0.04 0.29 10.00 0.48 - 50.33 38.85
PVMo11-SBA-15 exp. wt.% - 0.42 9.15 0.42 - 50.44 39.46
PV2Mo10-SBA-15 nom. wt.% 0.05 0.32 9.99 1.06 - 48.99 37.15
PV2Mo10-SBA-15 exp. wt.% - 0.50 9.86 1.1 - 50.13 38.13
PWMo11-SBA-15 nom. wt.% 0.03 0.29 9.99 - 1.74 49.67 38.28
PWMo11-SBA-15 exp. wt.% - 0.43 9.92 - 1.83 49.57 38.24
PW2Mo10-SBA-15 nom. wt.% 0.03 0.32 10.02 - 3.84 48.81 36.97
PW2Mo10-SBA-15 exp. wt.% - 0.43 9.80 - 3.94 48.74 37.09
4.3.1 Long-range structure of as-prepared P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2)
The long-range structure of as-prepared SBA-15 and P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2)
were investigated by low-angle and wide-angle X-ray diffraction (Fig. 4-1). At small
angles SBA-15 and P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) exhibited the characteristic
patterns ((10l, (11l) and (20l)) representing the hexagonal pore structure of nanostructured
46
SBA-15. The lattice constant of the hexagonal unit cell of a = 12.1 nm of SBA-15 was
determined from the (10l) peak (Fig. 4-1; left). Lattice constants of the hexagonal unit cell
resulting for P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) were nearly identical to unsupported
SBA-15. Supporting P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) on SBA-15 showed no influence
to the pore structure of the support material. Wide-angle X-ray diffraction patterns of a
SBA-15 and P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) with a loading of 10 wt.% Mo, showed
no long-ranged ordered molybdenum oxide species (Fig. 4-1; right). This indicated
sufficiently dispersed Keggin ions without formation of extended crystalline HPOM
structures.
1 2 3
10 20 30 40 50 60 70 80
Diffraction angle 2Ɵ [°] Diffraction angle 2Ɵ [°]
Inte
nsity
Inte
nsity
SBA-15
PMo12-SBA-15
PVMo11-SBA-15
PV2Mo10-SBA-15
(10l)
(11l)
(20l)
PWMo11-SBA-15
PW2Mo10-SBA-15
SBA-15
PMo12-SBA-15
PVMo11-SBA-15
PV2Mo10-SBA-15
PWMo11-SBA-15
PW2Mo10-SBA-15
Fig. 4-1: Low-angle (left) and wide-angle (right) X-ray diffraction patterns of SBA-15, PMo12-
SBA-15, PVMo11-SBA-15, PV2Mo10-SBA-15, PWMo11-SBA-15, PW2Mo10-SBA-15. (10l), (11l),
and (20l) reflections are indicated.
47
Nitrogen physisorption
Nitrogen adsorption/desorption isotherms and pore distributions (BJH) of SBA-15 and
PMo12-SBA-15 are shown in Fig. 4-2. The isotherms of SBA-15 and PMo12-SBA-15 were
of type IV indicative of mesoporous materials. Adsorption and desorption branches were
nearly parallel in SBA-15 and PMo12-SBA-15 as expected for regularly shaped pores. N2
physisorption isotherms of PMo12-SBA-15 resembled that of the original SBA-15.
Constrictions of the pores due to deposition of PMo12 on SBA-15 could be excluded. Pore
radius of the supported material decreased slightly. Therefore, the results of nitrogen
physisorption measurements indicated, that PMo12 was sufficient dispersed on the support
material without influence on the pore structure.
SBA-15 possessed a BET surface area of 843 m2/g. BJH calculations of pore size
distributions resulted in a pore diameter of dBJH = 10.6 nm. Given the pore diameter and
the lattice constant (a = 12.1 nm), a wall thickness of the SBA-15 material used amounted
to ~2 nm. PMo12-SBA-15 was chosen exemplary to elucidated an influence on pore
diameter and surface area of the support material after deposition. PVMo11-SBA-15,
PV2Mo10-SBA-15, PWMo11-SBA-15 and, PW2Mo10-SBA-15 were not analyzed by
0.0 0.2 0.4 0.6 0.8 1.0
0
200
400
600
800
Vo
lum
e [m
l g
-1]
Relative Pressure p/p0
5 7 9 11 13 15
0
0.1
0.2
0.3
0.4
Pore Diameter [nm]
dV
/dp [m
l nm
-1g
-1]
Fig. 4-2: Nitrogen physisorption isotherms of silica SBA-15 (square) and PMo12-SBA-15 (circle),
and pore distributions of silica SBA-15 (square) and PMo12-SBA-15 (circle), (inset).
48
nitrogen physisorption because of the required pretreatment (0.5 h at 450°C in He) which
leads to a dehydration and dehydroxylation process concomitant with structural changes of
the orginate Keggin structure. Therefore the result was used as indicator to exclude a
significant influence on the pore structure.
Table 4-2 summarizes specific surface area aBET, external surface area aEXT, area
corresponding to the mesopores aMeso, pore diameter dpore, and mesopore volume VMeso of
SBA-15 and PMo12-SBA-15
Table 4-2: Specific surface area aBET (calculated by BET method), external surface area aEXT
(calculated as the difference between aBET and aMeso), area corresponding to the mesopores aMeso,
pore diameter dBJH (calculated by BJH method), mesopore volume VMeso, of SBA-15 and PMo12-
SBA-15.
aBET (m2/g) aExt (m
2/g) aMeso (m
2/g) dBJH (nm) VMeso (cm
3/g)
SBA-15 843 145 698 10.6 1.233
PMo12-SBA-15 603 67 536 9.2 0.940
4.3.2 Short-range order structural characterization of as-prepared P(V,W)xMo12-x-SBA-
15 (x = 0, 1, 2)
X-Ray Absorption Spectroscopy
XAS was chosen as suitable spectroscopic method to analyze the structure of the Keggin
ion motif on the support material. XRD was unsuitable for a structural analysis, because no
long-ranged ordered molybdenum oxide species (Fig. 4-1; right) were expected. IR- and
RAMAN-spectroscopy were not chosen as analyzing method as well. The IR- and
RAMAN spectra of P(V,W)xMo12-x (x = 0, 1, 2) were superimposed with the excess of
SBA-15. Additionally, XAS is an element specific method and suitable for analyzing
substituted compounds.
49
Mo K edge analysis of P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2)
Fig. 4-3 (left) depicts the Mo K edge XANES spectra of P(V,W)xMo12-x-SBA-15
(x = 0, 1, 2) and (right) the theoretical and experimental Mo K edge FT(χ(k)·k3) of
P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2). A comparison of the Mo K edge XANES spectra
confirmed identical structural motifs in P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2). Analysis of
the Mo K edge position (Fig. 4-3, left; broken line) yielded an average valence of ~6 (cf.
Appendix Fig. A 6) of the substituted supported HPOM.[134] Therefore, substitution and
the supporting process did not have an influence on the average valence of Mo in
P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2). The very similar shape of Mo K edge FT(χ(k)·k3) of
P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) indicated a similar local structure around the Mo
centers in the unsupported and supported HPOM Keggin structure. P(V,W)xMo12-x-SBA-
15 (x = 0, 1, 2). The results of the refinements of the Mo K edge FT(χ(k)·k3) of
0 1 2 3 4 5 6 -0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
PMo12-SBA-15
PVMo11-SBA-15
PV2Mo10-SBA-15
PWMo11-SBA-15
PW2Mo10-SBA-15
R [Ǻ]
20.0 20.1 20.2
PMo12-SBA-15
PVMo11-SBA-15
PV2Mo10-SBA-15
PWMo11-SBA-15
PW2Mo10-SBA-15
No
rma
lize
d a
bsorp
tion
Photon energy [keV]
FT
(χ(k)·k
3)
Fig. 4-3: (left) Mo K edge XANES with indicated Mo K position (broken line) and (right)
theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of as prepared PMo12-SBA-15,
PVMo11-SBA-15, PV2Mo10-SBA-15, PWMo11-SBA-15, PW2Mo10-SBA-15.
50
P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) are shown in Table 4-3. Substituting PMo12 lead to
decreasing amplitudes in the FT(χ(k)·k3) with higher amount of addenda atoms in the
Keggin structures. The diminished amplitudes were discernible in all increased disorder
parameters σ2 with substitution degree. Probably the decreased amplitudes resulted from a
distortion of the [(V,W)O]6 units in substituted Keggin ions based on PMo12. Comparable
results were found in the unsupported P(V,W)xMo12-x (x = 0, 1, 2) (cf. chapter 3)
Table 4-3: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in as prepared P(V, W)xMo12-x-SBA-15 (x= 0, 1, 2). Experimental parameters were
obtained from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the
experimental Mo K edge XAFS χ(k) of P(W,V)xMo12-x-SBA-15 (x = 0, 1, 2) (k range from 3.0-13.7
Å-1
, R range from 0.9 to 4.0 Å, E0 = ~1.7, residuals ~11.3-20.0 Nind = 22, Nfree = 9). Subscript c
indicates parameters that were correlated in the refinement.
Keggin model PMo12-SBA-15 PVMo11-SBA-15 PV2Mo10-SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 1 1.68 1.64 0.0025 1.64 0.0035 1.66 0.0053
Mo-O 2 1.91 1.78 0.0033c 1.78 0.0035c 1.77 0.0067c
Mo-O 2 1.92 1.95 0.0033c 1.95 0.0035c 1.94 0.0067c
Mo-O 1 2.43 2.40 0.0008 2.40 0.0006 2.39 0.0008
Mo-Mo 2 3.42 3.42 0.0052c 3.43 0.0057c 3.43 0.0076c
Mo-Mo 2 3.71 3.74 0.0052c 3.73 0.0057c 3.72 0.0076c
Keggin model PMo12-SBA-15 PWMo11-SBA-15 PW2Mo10-SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 1 1.68 1.64 0.0025 1.64 0.0028 1.62 0.0026
Mo-O 2 1.91 1.78 0.0033c 1.77 0.0038c 1.77 0.0033c
Mo-O 2 1.92 1.95 0.0033c 1.94 0.0038c 1.94 0.0033c
Mo-O 1 2.43 2.40 0.0008 2.40 0.0013 2.41 0.0019
Mo-Mo 2 3.42 3.42 0.0052c 3.42 0.0073c 3.43 0.0094c
Mo-Mo 2 3.71 3.74 0.0052c 3.74 0.0073c 3.73 0.0094c
51
V K edge analysis of PV2Mo10-SBA-15 (x = 1, 2)
Fig. 4-4 (left) depicts the V K edge XANES spectra of PV2Mo10 and PV2Mo10-SBA-15
and (right) the theoretical and experimental V K edge FT(χ(k)·k3) of PV2Mo10 and
PV2Mo10-SBA-15. XAS analysis at the V K edge for PVMo11-SBA-15 was hardly feasible
due to the low content of V (~0.5 wt.%) beside Mo (~10 wt.%). Hence only an XAS
analysis at V K edge for PV2Mo10-SBA-15 was performed and compared to unsupported
PV2Mo10. V K edge XANES spectra of PV2Mo10 -SBA-15 were identical with V K edge
XANES spectra of PV2Mo10 (Fig. 4-4, left). Comparing the pre-edge peak at the V K edge
of PV2Mo10SBA-15 and vanadium oxide as reference compound indicated an average V
valence between 4 and 5 (Appendix Fig. A 5). Results of the Mo and V K edge XANES
spectra indicate a successful deposition of heteropolyoxomolybdates on the support
material. The very similar shape of the FT(χ(k)·k3) indicated similar local structure around
the V and Mo centers in the unsupported and supported HPOM Keggin structure. A
detailed structure analysis was performed by EXAFS. The results of the refinement of the
V K edge FT(χ(k)·k3) of PV2Mo10-SBA-15 are shown in Table 4-4. Supporting PV2Mo10
No
rma
lize
d a
bsorp
tion
0.00
0.05
0.10
FT
(χ(k)·k
3)
R [Ǻ]
0 1 2 3 4 5 6 5.60
0.0
0.5
1.0
1.5
5.45 5.50 5.55
Photon energy [keV]
PV2Mo10
PV2Mo10-SBA-15
PV2Mo10
PV2Mo10-SBA-15
Fig. 4-4: (left) V K edge XANES of PV2Mo10-SBA-15 and PV2Mo10; (right) Theoretical (dotted)
and experimental (solid) V K edge FT(χ(k)·k3) of as prepared PV2Mo10-SBA-15 and PV2Mo10.
52
on SBA-15 resulted in a decreased amplitude of the corresponding V K edge FT(χ(k)·k3).
Conversely, the Mo K edge FT(χ(k)·k3) of PVxMo12-x-SBA-15 (x = 0, 1, 2) showed a minor
increasing amplitude in the range between 2.5-3.8 Å. This increase in amplitude of the
FT(χ(k)·k3) was previously reported for PVMo11 and PVMo11-SBA-15.[27] Eventually, the
Mo and V K edge FT(χ(k)·k3) confirmed that the Keggin ion structure motifs were
maintained upon supporting PVxMo12-x on SBA-15.
Table 4-4: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the V atoms in as prepared PV2Mo12-SBA-15. Experimental parameters were obtained from a
refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental V K edge
XAFS χ(k) of PV2Mo12-SBA-15 (k range from 3.0-11.0 Å-1
, R range from 0.9 to 4.0 Å, E0 = 0.6,
residuals 13.8; Nind = 16, Nfree = 9). Subscript c indicates parameters that were correlated in the
refinement.
Keggin model PV2Mo10-SBA-15
N R(Å) R(Å) σ2(Å
2)
V-O 1 1.68 1.60 0.0053
V-O 2 1.91 1.94 0.0017c
V-O 2 1.92 1.95 0.0017c
V-O 1 2.43 2.43 0.0156
V-Mo 2 3.42 3.32 0.0178c
V-Mo 2 3.71 3.71 0.0178c
53
W LIII edge analysis of PWxMo12-x-SBA-15 (x = 1, 2)
Fig. 4-5 (left) depicts the W LIII edge XANES spectra of PWMo11-SBA-15 and PW2Mo10-
SBA-15 and (right) the theoretical and experimental W LIII K edge FT(χ(k)·k3) of
PWMo11-SBA-15 and PW2Mo10-SBA-15. XANES spectra of PWxMo12-x-SBA-15 (x= 1,
2) exhibited increased white lines comparing to bulk PWxMo12-x (x= 1, 2) (cf. chapter 0)
The change of white line intensities indicated a change of the absorption properties.
Therefore, it could be assumed, that the supported Keggin ions were well dispersed on the
SBA-15 causing an increased whiteline. The very similar shape of W LIII edge EXAFS
spectra of PWxMo12-x-SBA-15 (x = 1, 2) FT(χ(k)·k3) indicated a similar local structure of
the W centers in the unsupported and supported HPOM Keggin structure. Comparable to
the white line height, the amplitudes representing the W-Mo distances were increased.
Thus, a dispersion effect could be determined from the amplitudes or rather from decreased
disorder parameters σ2 representing the heights of the amplitudes compared to bulk
PWxMo12-x (x = 1, 2). The results confirmed the results of the vanadium substituted
R [Ǻ] Photon energy [keV]
No
rma
lize
d a
bsorp
tion
FT
(χ(k
)·k
3)
10.2 10.3 10.4
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 1 2 3 4 5 6
0.00
0.05
0.10
0.15
PW2Mo10-SBA-15
PWMo11-SBA-15 PW2Mo10-SBA-15
PWMo11-SBA-15
Fig. 4-5: (left) W LIII edge XANES of PW2Mo10-SBA-15 and PWMo11-SBA-15; (right)
Theoretical (dotted) and experimental (solid) W K edge FT(χ(k)·k3) of as prepared PW2Mo10-SBA-
15 and PWMo11-SBA-15.
54
PVxMo12-x (x = 1, 2). The supporting process did not have an influence on the Keggin ion
structure on the support material.
Table 4-5: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in as prepared PWxMo11-x-SBA-15 (x=1, 2). Experimental parameters were obtained
from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the experimental W LIII
edge XAFS χ(k) of PWxMo12-x-SBA-15 (x = 1, 2) (k range from 3.4-11.5 Å-1
, R range from 0.9 Å to
3.8 Å, E0= ~ 3.2, residuals ~9.1-13.5 Nind = 8, Nfree = 16). Subscript c indicates parameters that
were correlated in the refinement.
Keggin model PWMo11-SBA-15 PW2Mo10-SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2)
W-O 1 1.68 1.71 0.0018 1.72 0.0017
W-O 2 1.91 1.82 0.0019 1.83 0.0018
W-O 2 1.92 1.95 0.0019 1.95 0.0018
W-O 1 2.43 2.31 0.0020 2.29 0.0020
W-Mo 2 3.42 3.46 0.0049 3.47 0.0035
W-Mo 2 3.71 3.70 0.0049 3.69 0.0036
55
4.4 Conclusion
P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) were examined by a combination of various
characterization techniques. The supporting process of P(V,W)xMo12-x (x = 0, 1, 2) on
SBA-15 via incipient wetness lead to the desired metal loadings of 10 wt.% Mo for the
current model catalyst. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) were sufficiently dispersed on
the support material without any influence on the pore structure of the support material.
Supporting P(V,W)xMo12-x on SBA-15 resulted in regular and well dispersed Keggin ions
on the support material. The formation of extended crystalline HPOM structures could be
excluded. Therefore, substituting Mo atoms with addenda atoms (i.e. V, W) make Keggin
type heteropolyoxomolybdates suitable model systems to study structure activity
relationships. Supported catalytic species posses high dispersions and an improved surface
to bulk ratio. Therefore, structure activity relationships can be readily deduced from the
characteristic oxide species observed on the support material under catalytic conditions.
Hence, in the following chapters (5-6) P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) with a loading
of 10 wt.% Mo were investigated under catalytic conditions and structure activity
relationships presented.
56
5 Characterization of PVxMo12-x-SBA-15 (x = 1, 2)
under catalytic conditions
Keggin type H4[PVMo11O40] has been reported to exhibit a pronounced interaction effect
with SBA-15 as support material.[27] This effect resulted in a further decreased thermal
stability of the supported Keggin ions compared to the bulk materials. Under catalytic
conditions PVMo11-SBA-15 formed a mixture of tetrahedrally and octahedrally
coordinated [MoO4] and [MoO6] units.[27] However, the role and structural evolution of V
and P in PVxMo12-x-SBA-15 (x = 0, 1, 2) under catalytic conditions remained largely
unknown. In this chapter in situ X-ray absorption spectroscopy investigations at the Mo K
edge of PVxMo12-x-SBA-15 (x = 0, 1, 2) and at the V K edge of PV2Mo10-SBA-15 during
propene oxidation conditions were performed. Moreover, 31
P MAS NMR measurement of
PV2Mo10-SBA-15 and the reference H3PO4 supported on SBA-15 (denoted as H3PO4-
SBA-15) after catalytic oxidation with propene are described. Correlations between
structural evolution of [MoOx] and [VOx] units and performance under catalytic conditions
will be described. Additionally, the obtained structures and catalytic performances were
compared to a suitable supported reference material.
5.1 Experimental
5.1.1 Sample Characterization
31
P-NMR measurement
31P MAS NMR spectra were recorded on a Bruker Avance 400 spectrometer (
31P: 161.92
MHz) using a 4 mm double resonance HX MAS probe. Data collection used a 90° pulse
with a relaxation delay of 60 s under a MAS rotation of 12 kHz. Spectra were referenced to
85% H3PO4 in aqueous solution using solid NH4H2PO4 (δ = 0.81 ppm) as a secondary
reference.
57
X-ray absorption spectroscopy (XAS)
Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at
beamline X at the Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a
Si(311) double crystal monochromator. Transmission XAS experiments at the V K edge
(5.465 keV) were also performed at HASYLAB, using a Si(111) double crystal
monochromator at beamline C. In situ experiments were conducted in a flow reactor at
atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min, temperature range
from 303 to 723 K, heating rate 4 K/min). The gas phase composition at the cell outlet was
continuously monitored using a non-calibrated mass spectrometer in a multiple ion
detection mode (Omnistar from Pfeiffer).
X-ray absorption fine structure (XAFS) analysis was performed using the software
package WinXAS v3.2..[91] Background subtraction and normalization were carried out
by fitting linear polynomials and 3rd degree polynomials to the pre-edge and to the post-
edge region of an absorption spectrum, respectively. The extended X-ray absorption fine
structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic
background μ0(k). The FT(χ(k)·k3), often referred to as pseudo radial distribution function,
was calculated by Fourier transforming the k3-weighted experimental χ(k) function,
multiplied by a Bessel window, into the R space. EXAFS data analysis was performed
using theoretical backscattering phases and amplitudes calculated with the ab-initio
multiple-scattering code FEFF7.[92] Structural data employed in the analyses were taken
from the Inorganic Crystal Structure Database (ICSD).
Single scattering paths in the hexagonal MoO3 model structure (ICSD 75417 [135]) and a
modified Na2MoO4 structure (ICSD 24312 [136]) were calculated up to 6.0 Å with a lower
limit of 4.0% in amplitude with respect to the strongest backscattering path. EXAFS
refinements were performed in R space simultaneously to magnitude and imaginary part of
a Fourier transformed k3-weighted and k
1-weighted experimental χ(k) using the standard
EXAFS formula.[94] This procedure reduces the correlation between the various XAFS
fitting parameters. Structural parameters allowed to vary in the refinement were (i)
disorder parameter σ2
of selected single-scattering paths assuming a symmetrical pair-
distribution function and (ii) distances of selected single-scattering paths.
Detailed information about the fitting procedure are described in chapter 3.2.
58
Powder X-ray diffraction (XRD)
XRD measurements were conducted on an X’Pert PRO MPD diffractometer (Panalytical,
θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel PIXcel detector.
Wide-angle scans (5-90° 2θ, variable slits) were measured in reflection mode using a
silicon sample holder.
Temperature programmed reduction
Temperature programmed reduction (TPR) was performed with a catalysts analyzer from
BEL Japan Inc. equipped with a silica glass tube reactor. Samples were placed on silica
wool inside the reactor next to a thermocouple. A gas flow (5 % H2 in Ar) of 60 ml/min
was adjusted during reaction. A heating rate of 8 K / min to 973 K was used while H2
consumption was measured with a TCD unit. All samples were pretreated with a gas flow
of 60 ml/min Ar at 393 K for about 45 min before starting the measurement. For
measurements 37.2 mg PMo12-SBA-15, 40.4 mg PVMo11-SBA-15, and 39.8 PV2Mo10-
SBA-15 were used.
Catalytic testing - selective propene oxidation
Quantitative catalysis measurements were performed using a fixed bed laboratory reactor
connected to an online gas chromatography system (Varian CP-3800) and a non-calibrated
mass spectrometer (Pfeiffer Omnistar). The fixed-bed reactor consisted of a SiO2 tube (30
cm length, 9 mm inner diameter) placed vertically in a tube furnace. In order to achieve a
constant volume and to exclude thermal effects, catalysts samples (~ 38 mg) were diluted
with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375 mg. For
catalytic testing in selective propene oxidation a mixture of 5% propene (Linde Gas, 10%
propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in helium
(Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas flow rates of
oxygen, propene, and helium were adjusted with separate mass flow controllers
(Bronhorst) to a total flow of 40 ml/min. All gas lines and valves were preheated to 473 K.
Hydrocarbons and oxygenated reaction products were analyzed using a Carbowax capillary
column connected to an AL2O3/MAPD column or a fused silica restriction (25 m·0.32 mm
59
each) connected to a flame ionization detector. O2, CO, and CO2 were separated using a
Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x 1/8``) as precolumns
combined with a back flush. For separation, a Hayesep Q packed column (0.5 m x 1/8``)
was connected via a molsieve (1.5 m x 1/8``) to a thermal conductivity detector (TCD).
Details about the calculation of conversion, selectivity, and reaction rate are described in
chapter 3.2.
5.1.2 Sample preparation
PVxMo12-x (x = 0, 1, 2) supported SBA-15 were prepared as described in chapter 4.1.
A reference material (denoted as V2Mo10Ox-SBA-15) was prepared as follows.
232.1 mg (NH4)6Mo7O24·4H2O and 29.3 mg (NH4)6V10O28·6H2O were dissolved in water
and were deposited via incipient wetness on 1 g SBA-15 to obtain metal loading of 10
wt.% Mo and 1 wt.% V. The sample was dried for 18 h at room temperature and calcined
for 3 h at 773 K. H3PO4-SBA-15 was prepared by depositing 1.1 ml of 0.12 M phosphoric
acid on 1 g of silica SBA-15.
5.2 Structural characterization of PVxMo12-x-SBA-15 (x = 1, 2) under
catalytic conditions
In situ XANES analysis
PVxMo12-x-SBA-15 (x = 1, 2) was investigated by in situ XAS in propene oxidation
conditions. Fig. 5-1 depicts the evolution of molybdenum XANES spectra of PVMo11-
SBA-15 during temperature-programmed treatment in 5% propene and 5% oxygen. XAS
analysis at V K edge for PVMo11-SBA-15 was hardly feasible due to the low content of V
(~0.5 wt.%) beside Mo (~10 wt.%).
Fig. 5-2 shows the evolution of vanadium (a) and molybdenum (b) XANES spectra of
PV2Mo10-SBA-15 during temperature-programmed treatment in 5% propene and 5%
oxygen. The pre-edge peak features can be employed to elucidate the local structure
around the metal centers. Using the pre-edge peak height sufficed to quantify the
contribution of tetrahedral [MO4] and distorted [MO6] (M = V, Mo) units present during
60
thermal treatment of the catalysts. The pre-edge peak heights of in situ V and Mo K edge
XANES spectra at 298 K (Fig. 5-2) were attributed to the distorted [MO6] (M = V, Mo)
building units of the Keggin ion with the metal centers in their highest oxidation
states.[14,133] Mo K edge XANES spectra of PVxMo12-x-SBA-15 (x = 1, 2) and V K edge
XANES spectra of PV2Mo10-SBA-15 remained unchanged within the temperature range
from 298 K through 473 K. Hence, the Keggin structure appeared to be stable up to 473 K.
Between 473-648 K the pre-edge peak height increased with temperature. This indicated
5.46 5.48
5.5 5.52
5.54
373
473
673
T [K]
573
Photon energy
[keV]
V K edge
No
rma
lize
d
abso
rptio
n
No
rma
lize
d
abso
rptio
n
20.20
20.00
00
20.05 20.10
20.15
373
473
673
573
Photon energy
[keV]
T [K]
Mo K edge
Fig. 5-2: (left) in situ V K edge XANES spectra and (right) in situ Mo K edge XANES spectra of
PV2Mo10-SBA-15 during temperature-programmed treatment in 5% propene and 5% oxygen in
helium in a temperature range between 300 K and 723 K.
20.00 20.05
20.10 20.15
20.20
373 473
573 673
Photon energy
[keV]
No
rma
lize
d
abso
rptio
n
T [K]
Fig. 5-1: in situ Mo K edge XANES spectra of PVMo11-SBA-15 during temperature-programmed
treatment in 5% propene and 5% oxygen in helium in a temperature range between 300 K and
723 K.
61
structural changes from octahedral to tetrahedral [MOx] (M = V, Mo) units during thermal
treatment under catalytic conditions comparable to unsubstituted PMo12-SBA-15 (chapter
7.4).[95,59] Evolution of the normalized pre-edge peak height together with the ion
currents of H2O, CO, CO2, acrolein, and acetone during oxidation of propene are shown in
Fig. 5-3, (right). In situ Mo K edge FT(χ(k)·k3) (Fig. 5-3, left) indicated that the Keggin
structure was intact up to 473 K. Structural changes between 473 K and 648 K as observed
in the Mo K edge FT(χ(k)·k3) coincided with the evolution of pre-edge height of V and Mo
K edge of PV2Mo10-SBA-15. No structural changes in the FT(χ(k)·k3) of Mo could be
determined above 648 K. Moreover, changes in pre-edge heights of V and Mo occurred on
the same time scale thereby confirming the incorporation of V centers in the Keggin
structure. The onset of catalytic activity and the formation of various selective oxidation
products coincided with the detected structural changes. Apparently, the catalytically
active species formed during thermal activation from the original Keggin structure under
reaction conditions. These mainly consisted of V and Mo species centers in a particular
tetrahedral coordination. The evolution of the [MoO4]/[MoO6] ratio of PVxMo12-x-SBA-15
(x = 0, 1, 2) based on a linear combination of bulk MoO3 and bulk Na2MoO4 (cf. chapter
No
rm. p
re e
dg
e p
ea
k
heig
ht
heig
ht
373 473 573 673
No
rm. io
n c
urr
ent
m/e=18 (H2O) m/e=28 (CO) m/e=44 (CO2) m/e=56 (acroleine) m/e=58 (acetone)
pre edge peak height V
pre edge peak height Mo
T [K]
T [K]
0.04
0.06
6 0 2 4 373 473
573 673
R [Ǻ]
FT
(χ(k
)·k
3)
0.02
Fig. 5-3: (left) Evolution of Mo K FT(χ(k)·k3) of PV2Mo10-SBA-15 during thermal treatment in 5%
propene and 5% oxygen in helium in the temperature range from 303 to 723 K (4 K/min); (right)
evolution of normalized ion current of H2O (m/e 18), CO (m/e 28), CO2 (m/e 44), acroleine (m/e
56), and acetone (m/e 58), and normalized pre-edge height of V and Mo K edge of PV2Mo10-SBA-
15 during thermal treatment in 5% propene and 5% oxygen in helium in the temperature range from
303 to 723 K (4 K/min).
62
7.4) during propene oxidation conditions was shown in Fig. 5-4. A comparison of the
structural changes of PVxMo12-x-SBA-15 (x = 0, 1, 2) indicated identical onset
temperatures. An increase of tetrahedral [MoO4] units with V substitution was determined.
5.2.1 Local structure in activated PVxMo12-x-SBA-15 (x = 0, 1, 2) and a reference
V2Mo10Ox-SBA-15 under catalytic conditions
Local structure of Mo centers in act. PVxMo12-x-SBA-15 (x = 0, 1, 2)
Fig. 5-5 (left) shows the Mo K edge FT(χ(k)·k3) of act. PMo12-SBA-15, act. PVMo11-SBA-
15, and act. PV2Mo10-SBA-15 after thermal treatment under catalytic conditions at 723 K.
The Mo K edge FT(χ(k)·k3) were nearly similar for all three PVxMo12-x-SBA-15
(x = 0, 1, 2) and exhibited features similar to that of previously reported dehydrated
molybdenum oxides and HPOM supported on SBA-15.[27,59] Minor differences between
the three PVxMo12-x-SBA-15 (x = 0, 1, 2) are marked in the Mo K edge FT(χ(k)·k3) and the
Mo K edge χ(k)·k3. For a more detailed analysis hexagonal MoO3 was used as structural
model. Theoretical XAFS phases and amplitudes were calculated for Mo-O and Mo-Mo
distances and used for EXAFS refinement. The results of the refinement are shown in
Table 5-1. The first coordination sphere of the Mo K edge FT(χ(k)·k3) of as prepared
300 400 500 600 700
0
10
20
30
40
50
60
[MoO
4]/[M
oO
6] ra
tio
[%
]
Fig. 5-4: Quantification of the [MoO4]/[MoO6] ratio of PMo12-SBA-15, PVMo11-SBA-15, and
PV2Mo10-SBA-15 during thermal treatment under propene oxidation conditions.
T [K]
PMo12-SBA-15 PVMo11-SBA-15 PV2Mo10-SBA-15
63
PVxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 4.3) exhibited differences compared to act.
PVxMo12-x-SBA-15 (x = 1, 2). The first peak in the FT(χ(k)·k3) originated mainly from the
tetrahedral species on the SBA-15 support and could be sufficiently simulated using four
Mo-O distances. These four distances sufficiently accounted for the minor amount
octahedral [MoO6] species. Confirming the results of the XANES analysis 1st and 2nd
disorder parameters (1st-σ2, 2nd-σ
2) were higher for act. PMo12-SBA-15 and indicated a
decreasing amount of tetrahedral structural motifs compared to act. PVxMo12-x-SBA-15 (x
= 1, 2). In addition, the 4th disorder parameter (4th-σ2) was smaller than the disorder
parameters for act. PMo12-SBA-15. This disorder parameter mainly represented the
fraction of octahedral [MoO6] species. Hence, the reduced disorder parameter indicated an
increasing amount of octahedral structural motifs in act. PMo12-SBA-15 compared to act.
PVxMo12-x-SBA-15 (x = 1, 2). Therefore a structure directing effect of addenda vanadium
resulting in an increased concentration of tetrahedral [MoO4] units under propene
oxidation conditions was determined. A distinct peak at ~3 Å in the FT(χ(k)·k3) indicated
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 1 2 3 4 5 6
PVMo10-SBA-15
PV2Mo10-SBA-15
PMo12-SBA-15
R [Ǻ]
FT
(χ(k
)·k
3)
4 6 8 10 12 14 16
-4
0
4
8
12
k [Ǻ]-1
χ(k)·
k3
Fig. 5-5: (left) Mo K edge FT(χ(k)·k3) and (right) Mo K edge χ(k)·k
3of act. PMo12-SBA-15, act.
PVMo11-SBA-15, and act. PV2Mo10-SBA-15 after thermal treatment under propene oxidation
conditions at 723 K.
64
the formation of dimeric or oligomeric [MoxOy] units on SBA-15. Hence, only isolated
tetrahedral [MoO4] units can be excluded as major molybdenum oxide species.[137] The
disorder parameters σ2
of the Mo-Mo distances for act. PVxMo12-x-SBA-15 (x = 0, 1, 2)
were nearly identical indicating a comparable degree of oligomerization of Mo species on
silica SBA-15 independent of the V substitution in contrast to PMo12 supported on SBA-15
with larger pore radii (chapter 7.4).
Table 5-1: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in act. PVxMo12-x -SBA-15 (x = 0, 1, 2). Experimental parameters were obtained
from a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the experimental
Mo K edge XAFS χ(k) of act PVxMo12-x -SBA-15 (x = 0, 1, 2) (k range from 3.4-16.0 Å-1
, R range
from 0.9 to 4.0 Å, E0 = -5.2, residuals ~12.3-12.8 Nind = 26, Nfree = 12). Subscript c indicates
parameters that were correlated in the refinement.
hex-MoO3
model
act. PMo12-
SBA-15
act. PVMo11-
SBA-15
act. PV2Mo11-
SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 2 1.67 1.67 0.0015 1.67 0.0012 1.67 0.0013
Mo-O 2 1.96 1.89 0.0038c 1.89 0.0034c 1.88 0.0034c
Mo-O 1 2.20 2.19 0.0038c 2.18 0.0034c 2.18 0.0034c
Mo-O 1 2.38 2.35 0.0011 2.36 0.0014 2.34 0.0017
Mo-Mo 2 3.31 3.49 0.0068c 3.50 0.0066c 3.49 0.0061c
Mo-Mo 2 3.73 3.63 0.0068c 3.63 0.0066c 3.63 0.0061c
Mo-Mo 2 4.03 3.73 0.0100 3.75 0.0100 3.75 0.0100
Comparison of the local structure around the Mo centers in act. PV2Mo10-SBA-15
and a reference act. V2Mo10Ox-SBA-15 under catalytic conditions
Fig. 5-6 shows the Mo K edge FT(χ(k)·k3) of act. PV2Mo10-SBA-15 and activated
V2Mo10Ox-SBA-15 after thermal treatment under catalytic conditions. Linear combinations
of the XANES spectra of Na2MoO4 and MoO3 references were used to determine the
amount of tetrahedral [MoO4] and octahedral [MoO6] units in act. PV2Mo10-SBA-15 and
act. V2Mo10Ox-SBA-15. Apparently, act. PV2Mo10-SBA-15 consisted of a mixture of
tetrahedral [MoO4] and octahedral [MoO6] units in a ratio of 1:1. For act. V2Mo10Ox-SBA-
65
15 a ratio of 1:3 was found. A comparison of the pseudo radial distribution function of act.
V2Mo10Ox-SBA-15 and act. PV2Mo10-SBA-15 confirmed the results of the XANES
analysis. The first peak of the Mo K edge FT(χ(k)·k3) of act. V2Mo10Ox-SBA-15 exhibited
differences compared to that of act. PV2Mo10-SBA-15. The first peak in both FT(χ(k)·k3)
originated from the tetrahedral and octahedral species on the SBA-15 support and could be
sufficiently simulated using four Mo-O distances accounting for the amount of octahedral
[MoO6] species (Table 5-2). The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ
2) were
higher for act. V2Mo10Ox-SBA-15 and indicated a decreasing concentration of tetrahedral
[MoO4] units. The third Mo-O distance is considerably shorter than the distance in act.
PV2Mo10-SBA-15. In addition, the 4th disorder parameter (4th-σ2) is smaller than the
disorder parameter for act. PV2Mo10-SBA-15. This disorder parameter mainly represented
the fraction of octahedral MoO6 units. Hence, the reduced disorder parameter indicated an
increasing amount of octahedral structural motifs in act. V2Mo10Ox-SBA-15 compared to
act. PV2Mo10-SBA-15. Furthermore, the amplitude in the Mo K edge FT(χ(k)·k3) of act.
V2Mo10Ox-SBA-15 at higher Mo-Mo shells resembled the shape of α-MoO3. The Mo-Mo
distance at ~3.3 Ǻ is characteristic for α-MoO3. These results confirmed the existence of
crystalline α-MoO3 which was also identified by XRD before thermal treatment under
catalytic conditions (Fig. 5-6, right). Estimating the amount of α-MoO3 from the amplitude
Fig. 5-6: (left) Mo K edge FT(χ(k)·k3) of activated PV2Mo10-SBA-15 and activated V2Mo10Ox-
SBA-15 after thermal treatment in 5% propene and 5% oxygen in helium at 723 K; (right) XRD
of as prepared PV2Mo10-SBA-15, as prepared V2Mo10Ox-SBA-15, and simulated MoO3.
-0.04
0.00
0.04
0.08
0 1 2 3 4 5 6
R [Ǻ]
act. V2Mo10Ox -SBA-15 act. PV2Mo12-SBA-15
10 20 30 40 50 60 70
Inte
nsity
FT
(χ(k
)·k
3)
MoO3
PV2Mo10-SBA-15
V2Mo10Ox-SBA-15
Diffraction angle 2Ɵ [°]
66
at ~3.3 Ǻ (not phase corrected) in the pseudo radial distribution function yielded an
amount of about ~20%.
Table 5-2: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15. Experimental parameters were
obtained from a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the
experimental Mo K edge XAFS χ(k) of act. PV2Mo10-SBA-15 (k range from 3.6-16.0 Å-1
, R range
from 0.9 to 4.0 Å, E0v= ~ -5.2, residual ~12.8 Nind = 27, Nfree =12) and act. V2Mo10Ox-SBA-15 (k
range from 3.6-16.0 Å-1
, R range from 0.9 to 4.0 Å, E0 = ~ -0.4, residual ~ 10.9, Nind = 26, Nfree =14).
Subscript c indicates parameters that were correlated in the refinement.
Type act. PV2Mo11-SBA-15 Type act. V2Mo10Ox-SBA-15
N R(Ǻ) σ2(Ǻ
2) N R(Ǻ) σ
2(Ǻ
2)
Mo-O 2 1.67 0.0013 Mo-O 2 1.68 0.0023
Mo-O 2 1.88 0.0034c Mo-O 2 1.90c 0.0042c
Mo-O 1 2.18 0.0034c Mo-O 1 2.11c 0.0042c
Mo-O 1 2.34 0.0017 Mo-O 1 2.34 0.0001
Mo-Mo - - - Mo-Mo 1 3.14 0.0061c
Mo-Mo 2 3.49 0.0061c Mo-Mo 1 3.28 0.0061c
Mo-Mo 2 3.63 0.0061c Mo-Mo 2 3.71 0.0057
Mo-Mo 2 3.75 0.0100 Mo-Mo 2 3.92 0.0113
Comparison of the local structure around V centers in act. PVxMo12-x-SBA-15
(x = 0, 1, 2) and a reference act. V2Mo10Ox-SBA-15 under catalytic conditions
The evolution of the local structure around V centers in the supported catalysts and
reference materials differed from that of the Mo centers. Fig. 5-7 shows the V K edge
FT(χ(k)·k3) of act. PV2Mo10-SBA-15 (left) and act. V2Mo10Ox-SBA-15 (right). The
amplitudes at distances between 3-4 Ǻ indicated different scattering atoms. Single
scattering paths of a Na2MoO4 structure (ICSD 24312 [136]) were used for the EXAFS
refinement of act. V2Mo10Ox-SBA-15 and act. PV2Mo10-SBA-15.The theoretical model for
act. PV2Mo10-SBA-15 based on Na2Mo2O7 with replaced Mo atoms with V atoms per
formula unit. The theoretical model structure for act. PV2Mo10-SBA-15 based on the
67
Na2Mo2O7 structure with two replaced Mo atoms by V atoms per formula unit. Results of
the refinements for the V K edge FT(χ(k)·k3) are given in Table 5-3. The distances between
1-2 Ǻ corresponded to a tetrahedral [VO4] unit. Distances R and disorder parameters σ2
were nearly identical for act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15 (Fig. 5-7). The
first Mo coordination sphere corresponded to a mixture of octahedral and
tetrahedral [MoOx] species. In contrast to the first Mo-O peak with six individual Mo-O
distances, the first V-O peak could be sufficiently simulated using two V-O distances. The
two V-O distances sufficiently accounted for the tetrahedral [VO4] species. Silica atoms
from the support were found at a distance of ~2.55 Ǻ. Additionally, a V-Mo distance was
identified in the V K edge FT(χ(k)·k3) of act. PV2Mo10-SBA-15. V-O and V-V distances in
act. V2Mo10Ox-SBA-15 were very similar to those in dehydrated VxOy-SBA-15
synthesized with a butylammonium decavanadate precursor.[95]
Assuming only V-V distances resulted in a sufficient agreement between experimental and
theoretical spectra in contrast to act. PV2Mo10-SBA-15. This indicated that [VOx] and
[MoOx] species were not present in close vicinity to each other. The results of Mo K and V
K edge analysis of act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15 confirmed that only
act. PV2Mo10-SBA-15 contained supported V-O-Mo mixed oxides structural motifs
forming under catalytic conditions.
Active sites of selective oxidation catalysts often consist of multiple metal atoms.[137]
Synthesis routes of supported ternary oxides with different metal oxide precursors rarely
have been reported. Vanadium substituted Keggin ions enabled the synthesis of connected
-0.02
0.00
0.02
FT
(χ(k
)·k
3)
0 1 2 3 4 5 6
Experiment Theory
R [Ǻ] 0 1 2 3 4 5 6
-0.02
0.00
0.02
0.04
FT
(χ(k
)·k
3)
R [Ǻ]
Experiment Theory
Fig. 5-7: V K edge FT(χ(k)·k3) of (left) act. PV2Mo10-SBA-15 and (right) act. V2Mo10Ox-SBA-15
after thermal treatment in 5% propene and 5% oxygen in helium at 723 K.
act. PV2Mo10-SBA-15 act. V2Mo10Ox-SBA-15
68
[VOx] and [MoOx] species not readily available from physically mixed precursors.
Apparently, the proximity of vanadium and molybdenum in the Keggin precursors is a
prerequisite for obtaining connected metal oxide species on a support material.
Table 5-3: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the V atoms in act. PV2Mo10-SBA-15 and act. V2Mo10Ox-SBA-15. Experimental paramters were
obtained from a refinement of a modified [Mo2O7]2-
model system (ICSD 24312 [136]) were Mo is
replaced by V and Si additional added compared to the experimental V K edge XAFS χ(k) of act.
PV2Mo10-SBA-15 (k range from 3.0-10.0 Å-1
, R range from 0.9 to 3.8 Å, E0 = ~8.8, residual ~ 6.0
Nind = 13, Nfree =8) and act. V2Mo10Ox -SBA-15 (k range from 3.0-10.0 Å-1
, R range from 0.9 to 3.8
Å, E0 = ~8.8, residual ~ 11.3, Nind = 13, Nfree =8). Subscript c indicates parameters that were
correlated and f that were fixed in the refinement.
Type act. PV2Mo11-SBA-15 Type act. V2Mo10Ox-SBA-15
N R(Ǻ) σ2(Ǻ
2) N R(Ǻ) σ
2(Ǻ
2)
V-O 2 1.83 0.0183 V-O 2 1.82 0.0160
V-O 2 1.83c 0.0183c V-O 2 1.82c 0.0160c
V-Si 1 2.54 0.0172 V-Si 1 2.55 0.0094
V-O 1 2.91 0.005f V-O 1 2.95 0.0046f
V-O 1 3.10 0.0246 V-V 1 3.30 0.0089
V-Mo 1 3.60 0.0221 V-V 1 3.58 0.0089c
5.2.2 Local structure of P in activated PV2Mo10SBA-15 under catalytic conditions
Fig. 5-8 shows 31
P-MAS-NMR measurements of as-prepared and act. PV2Mo10-SBA-15 in
comparison to as-prepared and activated reference H3PO4-SBA-15. The 31
P MAS NMR
spectrum of PV2Mo10-SBA-15 resembled that of bulk PVMo11.[138] This confirmed that
the majority of P centers was located in Keggin ions supported on SBA-15.[139,140] The
peak in the spectrum of H3PO4-SBA-15 at 0.8 ppm could be assigned to molecular H3PO4.
In the spectrum of act. H3PO4-SBA-15 four pronounced peaks can be seen at chemical
shifts of 0.8, -10.8, 22.8, and -35.9 ppm. Zhi-qiang Zhang et al. reported similar results for
SiO2 impregnated with H3PO4.[140,141] Accordingly, the peak at 0.1 ppm is characteristic
for H3PO4, while the peaks at -10.8 and -22.8 ppm were attributed to terminal and internal
69
phosphate groups of condensed phosphates, respectively.[141] Krawietz et al. assigned the
peak at -35.9 ppm to silicon hydrogen tripolyposphate (SiHP3O10, -35 ppm).[142] For
H3[PMo12O40] supported on ZrO2 (PMo12-ZrO2) Devassy et al. investigated the nature of
phosphorous species depending on Keggin loading and calcination temperature.[54]
PMo12-ZrO2 exhibited a comparable broadening of the peaks in the 31
P MAS NMR
spectrum with increasing calcination temperature. Decomposition of the HPOM to the
oxide species was observed at temperatures above 723 K. Thermal stability of
H3[PW12O40] supported on ZrO2 (PW12-ZrO2) was investigated by López-Salinas et al..
The structural behaviour of PW12-ZrO2 during calcination was comparable to that of
PMo12-ZrO2. PW12-ZrO2 decomposed at temperatures above 773 K to form the
corresponding supported oxides.[56] The authors assigned an additional peak at -30 ppm to
phosphorous oxides exhibiting P-O-P motifs. In the 31
P MAS NMR spectra of act.
PV2Mo10-SBA-15 studied here, a broad resonance indicated structural rearrangement and a
40
(ppm)
100 80 60 20 0 -20 -40 -60 -80 -100
act. H3PO4-SBA-15
H3PO4-SBA-15 n
orm
. in
ten
sity
100 80 60 40 20 0 -20 -40 -60 -80 -100
(ppm)
act. PV2Mo10-SBA-15
PV2Mo10-SBA-15
no
rm.
inte
nsity
Fig. 5-8: 31
P MAS NMR spectra of asprepared H3PO4-SBA-15, PV2Mo10-SBA-15, and thermal
treated under catalytic conditions (5% propene and 5% oxygen in He) at 723 K act. H3PO4-SBA-
15 and act. PV2Mo10-SBA-15.
70
partial decomposition of the Keggin ions during thermal treatment under catalytic
conditions. Moreover, the 31
P MAS NMR spectra of act. PV2Mo10-SBA-15 resembled that
of a VPO-SBA-15 sample treated under oxidative and reductive conditions.[143] A broad
resonance observed for the VPO sample between -12 and -38 ppm was attributed to
various vanadyl orthophosphates phases (-8.4 to 21.2 ppm) and phosphorus bound to the
SBA-15 support (ca. -38 ppm). Comparable structural motifs can be assumed for act.
PV2Mo10-SBA-15. However, formation of phosphorous oxide or SiHP3O10 exhibiting
linked P-O-P structures could be excluded. In total, the 31
P MAS NMR results indicated a
variety of structural motifs in the activated samples studied here. Apparently, phosphorus
remained connected to the molybdate- and/or vanadate-species of the [(Mo,V)Ox] units
during propene oxidation conditions.
5.2.3 Structure directing effects of vanadium and the support material on the structure of
activated PV2Mo10-SBA-15 under catalytic conditions
Characteristic differences were revealed by comparing the structural evolution of bulk
HPOM during thermal treatment to that of supported HPOM. In bulk HPOM the Keggin
ion partially decomposes under catalytic conditions to form a lacunary Keggin anion.[13]
In this process Mo cations migrate on extra Keggin sites while remaining coordinated to
the resulting lacunary Keggin anion.[13] Driving force for the formation of lacunary
Keggin anions may be the relaxation of the Keggin structure at elevated temperature upon
removal of structural water. Eventually, this leads to the formation of more extended oxide
structures. These structural changes at temperatures above 573 K are accompanied by
reduction of the metal centers.[16,13] Vanadium incorprated in bulk HPOM acts as a
structural promoter facilitating the formation of the active (Mo, V) oxide phase under
catalytic conditions. The incorporated V centers result in a pronounced destabilization and
accelerated decomposition of the Keggin ion at elevated temperatures.[14,117] The
structural characteristics of model systems like MoOx-SBA-15 and VOx-SBA-15 depend
on their hydration states.[144,95] A comparable effect could be responsible for the
structural evolution of HPOM supported on SBA-15. Adsorbed water and silanol groups
from the support may possess a structure stabilizing effect on the Keggin ion. This effect
would be comparable to that of water of crystallization and constitutional water in bulk
71
HPOM under ambient conditions.[117] Vansant et al. reported that amorphous silica
showed dehydroxylation of silanol groups between 473-673 K resulting in a decrease of a
silanol density from 4.6 OH/nm2 (473 K) to 2.3 OH/nm
2 (673 K).[145] TG measurement
of PVMo11-SBA-15 showed a mass loss of about 2 wt.% between 473 and 673 K which
correlated with the temperature range of structural rearrangement of PVMo11-SBA-15
under catalytic conditions (c.f. Chapter 8.2; Fig. 8-1).
Thus, desorption of water and dehydroxylation of silanol groups may be responsible for the
formation of act. PVxMo12-x-SBA-15 (x = 0, 1, 2). SBA-15 seemed to possess a directing
effect on the formation of activated (Mo, V, P)Ox structures depending on the treatment
conditions (i.e. temperature, gas composition). Apparently, the thermal stability of Keggin
ions supported on SBA-15 was significantly decreased. While vanadium had a minor
influence on the thermal stability, the interaction with the support material appeared to be
more important. Nevertheless, vanadium still had a distinct structure directing effect to
form V-O-Mo mixed structures under catalytic conditions. The presence of tetrahedral
[VO4] species lead to an increasing ratio of tetrahedral [MoO4] to octahedral [MoO6]
species. Compared to SBA-15 other support materials exhibit different structure directing
effects depending on the acidity of the surface.[25,146,147] For instance, mainly isolated
[MoO4] units existed on an alkaline MgO support in agreement with the behaviour of Mo
oxides in alkaline solution.[137] Here, the acidic surface of silica SBA-15 resulted in
mainly linked M-O-M (M = Mo, V) species again corresponding to the behaviour of
vanadates and molybdates in acidic solutions.[37,148]
5.3 Functional characterization of PVxMo12-x-SBA-15 (x = 0, 1, 2)
5.3.1 Reducibility
Fig. 5-9 shows the H2 TPR profiles of PVxMo12-x-SBA-15 (x = 0, 1, 2). The resulted
H2 TPR profiles revealed one sharp (~800 K) and a very broad (873- 973 K) reduction
peak. The shapes of the H2 TPR profiles were nearly identical, just the reduction
temperatures slightly increased with the vanadium content from 790 K for PMo12-SBA-15
to 808 K for PV2Mo10-SBA-15. The H2 TPR profiles at ~800 K were comparable to
molybdenum oxide supported on SBA-15 with Mo loadings between 9.5 wt.% and 13.3
72
wt.%.[149,150] Lou et al. assigned the sharp reduction peak from oligomeric MoOx
species or small MoOx clusters and the broaded signal above ~800 K to the reduction of
monomeric MoOx species.[150] Vanadium oxide supported on SBA-15 with a loading
between 1.0 wt.% V and 4.5 wt.% V reduced between 769 K and 799 K depended on the
dispersion degree.[151] Hence, the reducibility were comparable to molybdenum oxides
supported on SBA-15. A comparison of vanadium and molybdenum oxides supported on
Al2O3 with similar metal surface density showed that the temperature of the maximum of
H2-consumption were slightly higher (~10 K) for the supported vanadium oxides.[152] A
comparable intrinsic effect of the metals could be responsible for the shift to higher
reduction temperatures with higher vanadium loading. However, no significant changes in
the reducibility depending on the vanadium substitution degree were detected.
5.3.2 Catalytic performance
Reaction rates and selectivities of PMo12-SBA-15, PVMo11-SBA-15, PV2Mo10-SBA-15,
V2Mo10Ox-SBA-15, and bulk PV2Mo10 in propene oxidation at 723 K are shown in Fig.
5-10. Reaction rates for PVxMo12-x-SBA-15 (x = 0, 1, 2) were calculated for similar
propene oxidation conditions (~ 14-16% propene conversion). The propene conversion for
bulk PV2Mo10 (~ 3%) was lower due to the strongly decreased catalytic activity. Adjusting
H2 c
onsu
mp
tio
n
T [K]
373 473 573 673 773 873 973
790 K
800 K
808 K
PMo12-SBA-15
PVMo11-SBA-15
PV2Mo10-SBA-15
Fig. 5-9: Temperature.programmed reduction (H2-TPR) of PMo12-SBA-15, PVMo11-SBA-15, and
PV2Mo10-SBA-15 measured at a heating rate of 8 Kmin-1
5% H2 in Ar.
73
to similar propene oxidation conditions for the low active sample would lead to an large
volume and thermal effects. Hence, comparing of the catalytic performance of PVxMo12-x-
SBA-15 (x = 0, 1, 2) to that of the low active sample needs to be done carefully.
Reaction rates for PVxMo12-x-SBA-15 (x = 0, 1, 2) were similar and independent of the
degree of vanadium substitution. Selectivities for CO increased at the expense of
acetaldehyd with higher degree of vanadium substitution. The results of the catalytic
performance of bulk PVxMo12-x (x = 0, 1, 2) showed strong increased reaction
rates for vanadium substituted bulk HPOM. Selectivities towards oxygenates, especially
acetaldehyd decreased at the expense of CO with increased vanadium substitution degree.
PV2Mo10 was chosen as bulk HPOM for comparing the product distribution (i.e. acrylic
acid, acetic acid, acrolein, acetone, propionaldehyde, acetaldehyde, CO, and CO2) to
PV2Mo10-SBA-15. While, PV2Mo10 showed a slightly increased selectivity towards
acrolein, PV2Mo10-SBA-15 exhibited an increased selectivity towards acetic acid. Total
oxidation products CO and CO2 amounted to about ~55% in the resulting oxidation
products. Conversely, the reaction rates of PV2Mo10 and PV2Mo10-SBA-15 exhibited
considerable differences. The catalytic activity of PV2Mo10-SBA-15 was four times higher
than that of PV2Mo10. Apparently, higher dispersion and an improved surface to bulk ratio
of Keggin ions resulted in a much increased activity at comparable selectivity. Structural
0
20
40
60
80
100
0
10
20
30
40
50
60
70
a b c e d
acrylic acid
acetic acid acrolein
acetone
acetaldehyde
CO
CO2 propionaldehyde
Se
lectivity [%
]
Re
actio
n r
ate
[µ
mo
l(p
rope
ne
)g-1(M
o)s
-1]
Fig. 5-10: Reaction rate (µmol(propene)g-1
(Mo)s-1
) and selectivity of (a) PMo12-SBA-15, (b)
PVMo11-SBA-15, (c) PV2Mo10-SBA-15, (d) V2Mo10Ox-SBA-15, and (e) bulk PV2Mo10 in 5%
propene and 5% oxygen in He at 723 K.
74
analysis of act. PVxMo12-x-SBA-15 (x = 0, 1, 2) revealed an increased concentration of
tetrahedral [MoO4] units at comparable degree of oligomerization. Apparently, the
additional [VOx] species in act. PVxMo12-x-SBA-15 (x = 0, 1, 2) lead to new
multifunctional active sites, resulting in a different product distribution without influence
to the reaction rates.
In contrast to the HPOM samples, V2Mo10Ox-SBA-15 showed a decreasing activity and a
different product distribution compared to PV2Mo10-SBA-15. While, the amount of total
oxidation products in the gas phase was considerably lower, an increasing selectivity to
acetaldehyde was determined. The structural analysis indicated that act. V2Mo10Ox-SBA-
15 possessed an increased amount of higher oligomerized Mo species and a decreased
content of tetrahedral [MoO4] units. It was shown earlier, that higher oligomerized V and
Mo species showed an increased selectivity towards oxidations products.[137,153]
Additionally, the majority of [VOx] and [MoOx] species in act. V2Mo10Ox-SBA-15 did not
seem to be directly connected to each other. Local separation of the [VOx] and [MoOx]
species may be responsible for the increased concentration of acetaldehyde, which is
mainly formed by vanadium based catalysts in contrast to molybdenum based
catalysts.[1,31,50] Apparently, the new multifunctional active site consisting of connected
[VO4] and [MoOx] units lead to an increased amount of total oxidation products for act.
PV2Mo10-SBA-15 in contrast to not connected [VO4] and [MoOx] units in act. V2Mo10Ox-
SBA-15. Furthermore, availability of dimeric or oligomeric [(V,Mo)Ox] units increased the
selectivity towards oxygenates in contrast to isolated [MoO4] units [47,50]. Hence,
connected [VO4] and [MoOx] units and the general degree of oligomerization of
[(V,Mo)Ox] units influenced the catalytic activity and selectivity towards propen oxidation.
5.3.3 Influence of phosphorus species on catalytic activity
Phosphorus containing catalysts (i.e. VPO, FePO, MoPO) play a crucial role as oxidation
catalysts.[128] Adding small amounts of phosphoric acid to the feed showed positive
effects on long-term stability and catalytic performance of FePO catalysts during ODH of
isobutyric acid into methacrylic acid. The phosphorus source was needed to maintain a
constant P/Fe ratio at the surface of the catalyst.[129] VPO catalysts showed migration of
phosphorus species to the surface and a decreasing amount of phosphorus in the catalyst
75
during water vapor treatment. The excess of phosphorus on the surface suppressed
oxidation of VPO catalysts and hindered formation of active sites for oxidation reactions.
Subsequently, hydrolysis of P-O-P or P-O-V groups resulted in a removal of phosphate
groups on the surface and an increasing activity.[128,130] Moreover, adding V and P to
MoOx based catalysts for ODH of ethane afforded an increasing selectivity and conversion
towards ethane. Haddad et al. suggested synergistic effects between structurally related
oxides like (V,Mo)5O14 and (V, Mo)PO phases to be responsible for the enhancened
catalytic performance.[154] Here, the formation of water as byproduct during oxidation of
propene may have favored the migration of phosphorus species under catalytic conditions.
The different surface to bulk ratios of PV2Mo10 and PV2Mo10-SBA-15 could lead to a
different migration and hydrolysis of phosphate groups in the materials. The available
Keggins in PV2Mo10-SBA-15 were located on the surface of the support material.
Therefore, an enrichment of phosphate groups was not possible for PV2Mo10-SBA-15. An
enrichment of phosphate groups on the surface of bulk PV2Mo10 would results in a higher
P/M (M = Mo, V) with a possible influence on catalytic activity and selectivity. However,
the comparable selectivity of bulk PV2Mo10 and PV2Mo10-SBA-15 (Fig. 5-10) was
indicative of similar active centers despite different P/M (M = Mo, V) ratios. Therefore,
the increased catalytic activity of PV2Mo10-SBA-15 was attributed to a higher dispersion
and an improved surface to bulk ratio of supported Keggin ions.
76
5.4 Summary
The structural evolution of PVxMo12-x-SBA-15 (x = 0, 1, 2) and a mixture of V and Mo
oxides supported on SBA-15 during propene oxidation conditions was examined by in situ
X-ray absorption spectroscopy at the Mo K and V K edge. Additionally, 31
P MAS NMR
measurements of supported PV2Mo10-SBA-15 and H3PO4-SBA-15 after catalytic reaction
were performed. During thermal treatment under propene oxidation conditions PVxMo12-x-
SBA-15 (x = 0, 1, 2) formed a mixture of mainly tetrahedral [MoOx] and [VOx] units.
Changes in the local structure around the V centers coincided with structural changes of
the Mo centers and the onset of catalytic activity. The concentration of tetrahedral [MoO4]
units correlated with the degree of vanadium substitution without affecting to the degree of
oligomerization of the [MoxOy] and [VxOy] species. Apparently, the mainly tetrahedral
[MoOx] and [VOx] units were in close vicinity and able to interacted under catalytic
conditions. The new multifunctional active site consisting of connected [VO4] and [MoOx]
units lead to an increased amount of total oxidation products without influencing the
reaction rate. Conversely, structural analysis of activated reference V2Mo10Ox-SBA-15
synthesized with individual V and Mo precursors indicated that [VOx] and [MoOx] species
were mostly separated from each other on the surface of SBA-15. Moreover, activated
V2Mo10Ox-SBA-15 possessed an increased amount of higher oligomerized [MoxOy]
species and a decreased content of tetrahedral [MoO4] units. This may explain the observed
increased selectivity towards partial oxidations products. The structural environment of
phosphorus in PV2Mo10-SBA-15 under catalytic conditions corresponded to a mixture of
various species. Phosphorus was linked to both the SBA-15 support via P-O-Si bonds and
to the Mo or V centers of the [MoOx] or [VOx] units. In total, supported vanadium
substituted Keggin ions are suitable precursors to synthesize connected [VOx] and [MoOx]
species on SBA-15. Apparently, the proximity of vanadium and molybdenum in the
Keggin precursors a prerequisite for obtaining connected metal oxide species.
77
6 Characterization of PWxMo12-x-SBA-15 (x = 1, 2)
under catalytic conditions
Keggin type H4[PVMo11O40] has been reported to exhibit a pronounced interaction effect
with SBA-15 as support material.[14] This effect resulted in a further decreased thermal
stability of the supported Keggin ions compared to the bulk materials. PVMo11-SBA-15
formed a mixture of tetrahedrally and octahedrally coordinated [MoO4] and [MoO6] units
during propene oxidation.[14] The structural evolution and role of tungsten in PWxMo12-x-
SBA-15 (x = 1, 2) during propene oxidation were not part of previous investigations.
Therefore, a first structural and functional characterization were necessary to elucidated
structure activity of supported molybdenum oxide based model catalysts. In this chapter in
situ X-ray absorption spectroscopy investigations at the LIII-LI edges of PWxMo12-x-SBA-
15 (x = 1, 2) during propene oxidation conditions were performed. Correlations between
structural evolution of [MoOx] and [WOx] units and performance under catalytic conditions
will be described. Additionally, the obtained structures and catalytic performances were
compared to a suitable supported reference material.
6.1 Experimental
6.1.1 Sample Characterization
X-ray absorption spectroscopy (XAS)
Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at
beamline at X and W LIII-LI edges (10.204-12.098 keV) at beamline C at the Hamburg
Synchrotron Radiation Laboratory, HASYLAB. Using a Si(311) double crystal
monochromator at Beamline X for the Mo K edge and a Si(111) double crystal
monochromator at Beamline C for the W LIII-LI edges. In situ experiments were conducted
in a flow reactor at atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min,
temperature range from 303 to 723 K, heating rate 4 K/min). The gas phase composition at
the cell outlet was continuously monitored using a non-calibrated mass spectrometer in a
multiple ion detection mode (Omnistar from Pfeiffer).
78
X-ray absorption fine structure (XAFS) analysis was performed using the software
package WinXAS v3.2..[91] Background subtraction and normalization were carried out
by fitting linear polynomials and 3rd degree polynomials to the pre-edge and to the post-
edge region of an absorption spectrum, respectively. The extended X-ray absorption fine
structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic
background μ0(k). The FT(χ(k)·k3), often referred to as pseudo radial distribution function,
was calculated by Fourier transforming the k3-weighted experimental χ(k) function,
multiplied by a Bessel window, into the R space. EXAFS data analysis was performed
using theoretical backscattering phases and amplitudes calculated with the ab-initio
multiple-scattering code FEFF7.[92] Structural data employed in the analyses were taken
from the Inorganic Crystal Structure Database (ICSD).
Single scattering paths in the hexagonal MoO3 model structure (ICSD 75417 [135]) and a
modified H3[PMo12O40] structure (ICSD 209 [14,93]) were calculated up to 6.0 Å with a
lower limit of 4.0% in amplitude with respect to the strongest backscattering path. EXAFS
refinements were performed in R space simultaneously to magnitude and imaginary part of
a Fourier transformed k3-weighted and k
1-weighted experimental χ(k) using the standard
EXAFS formula.[94] This procedure reduces the correlation between the various XAFS
fitting parameters. Structural parameters allowed to vary in the refinement were (i)
disorder parameter σ2
of selected single-scattering paths assuming a symmetrical pair-
distribution function and (ii) distances of selected single-scattering paths.
Detailed information about the fitting procedure are described in chapter 3.2.
Powder X-ray diffraction (XRD)
XRD measurements were conducted on an X’Pert PRO MPD diffractometer (Panalytical,
θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel PIXcel detector.
Wide-angle scans (5-90° 2θ, variable slits) were collected in reflection mode using a
silicon sample holder. Small-angle scans (0.4-6.0° 2θ, fixed slits) were measured in
transmission mode with the sample spread between two layers of Kapton foil.
79
Temperature programmed reduction
Temperature programmed reduction (TPR) was performed with a catalysts analyzer from
BEL Japan Inc. equipped with a silica glass tube reactor. Samples were placed on silica
wool inside the reactor next to a thermocouple. A gas flow (5 % H2 in Ar) of 60 ml/min
was adjusted during reaction. A heating rate of 8 K / min to 973 K was used while H2
consumption was measured with a TCD unit. All samples were pretreated with a gas flow
of 60 ml/min Ar at 393 K for about 45 min before starting the measurement. For
measurements 37.2 mg PMo12-SBA-15, 33.4 mg PWMo11-SBA-15, and 34.3 mg
PW2Mo10-SBA-15 were used.
Catalytic testing - selective propene oxidation
Quantitative catalysis measurements were performed using a fixed bed laboratory reactor
connected to an online gas chromatography system (Varian CP-3800) and a non-calibrated
mass spectrometer (Pfeiffer Omnistar). The fixed-bed reactor consisted of a SiO2 tube (30
cm length, 9 mm inner diameter) placed vertically in a tube furnace. In order to achieve a
constant volume and to exclude thermal effects, catalysts samples (~ 38 mg) were diluted
with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375 mg. For
catalytic testing in selective propene oxidation a mixture of 5% propene (Linde Gas, 10%
propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in helium
(Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas flow rates of
oxygen, propene, and helium were adjusted with separate mass flow controllers
(Bronhorst) to a total flow of 40 ml/min. All gas lines and valves were preheated to 473 K.
Hydrocarbons and oxygenated reaction products were analyzed using a Carbowax capillary
column connected to an AL2O3/MAPD column or a fused silica restriction (25 m·0.32 mm
each) connected to a flame ionization detector. O2, CO, and CO2 were separated using a
Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x 1/8``) as precolumns
combined with a back flush. For separation, a Hayesep Q packed column (0.5 m x 1/8``)
was connected via a molsieve (1.5 m x 1/8``) to a thermal conductivity detector (TCD).
Details about the calculation of conversion, selectivity, and reaction rate are described in
chapter 3.2.
80
6.1.2 Sample preparation
PWxMo12-x (x=1,2) supported SBA-15 were prepared as described in chapter 4.1.
A reference material (denoted as W2Mo10Ox-SBA-15) was prepared as follows. 241.0 mg
(NH4)6Mo7O24·4H2O and 29.3 mg (NH4)6W12O39·xH2O were dissolved in water and were
deposited via incipient wetness on 1 g SBA-15 to obtain metal loading of 10 wt.% Mo and
3.8 wt.% W. The sample was dried for 18 h at room temperature and calcined for 3 h at
773 K.
6.2 Structural evolution of PWxMo12-x-SBA-15 (x = 1, 2) under catalytic
conditions
In situ XANES analysis
PWxMo12-x-SBA-15 (x = 1, 2) was investigated by in situ XAS in propene oxidation
conditions. Fig. 6-1 shows the evolution of molybdenum Mo K edge XANES spectra of
PW2Mo10-SBA-15 during temperature-programmed treatment in 5% propene and 5%
oxygen. Mo K edge XANES spectra of PWxMo12-x-SBA-15 (x = 1, 2) remained unchanged
within the temperature range from 298 K through 473 K comparable to PVxMo12-x-SBA-15
20.00
20.05
20.10
20.15
20.20
373
473
573 673
No
rma
lize
d
abso
rptio
n
T [K]
Photon energy [keV]
Fig. 6-1: in situ Mo K edge XANES spectra of PW2Mo10-SBA-15 during temperature-programmed
treatment in 5% propene and 5% oxygen in helium in a temperature range between 300 K and
723 K.
81
(x = 0, 1, 2) (cf. chapter 5.2). Hence, the Keggin ion appeared to be stable up to 473 K
independent of the degree of substitution. Between 473-648 K the pre-edge peak height
increased during thermal treatment under catalytic conditions.[15,23] This increasing pre-
edge peak corresponded to a structural rearrangement to tetrahedral [MoO4] species. Linear
combinations of the XANES spectra of MoO3 and bulk Na2MoO4 references were used to
determine the amount of tetrahedral [MoO4] and octahedral [MoO6] units of PWxMo12-x-
SBA-15 (x = 0, 1, 2) during propene oxidation conditions (cf. chapter 7.4). The evolution
of the tetrahedral [MoO4] to octahedral [MoO6] ratio of PWxMo12-x-SBA-15 (x = 0, 1, 2)
during propene oxidation conditions was shown in Fig. 6-2. In contrast to PVxMo12-x-
SBA-15 (x = 1, 2) the W substitution lead to decreased concentration of tetrahedral
[MoO4]
species. Hence, tungsten substitution lead to mostly octahedral [MoO6] species resulting
during propene oxidation conditions. Fig. 6-3 depicts the W LIII and LI edge XANES
spectra of PWxMo12-x-SBA-15 (x = 1, 2) during temperature-programmed treatment in 5%
propene and 5% oxygen. Compared to the onset temperature of 473 K of the structural
rearrangement of the [VOx] and [MoOx] units for PVxMo12-x-SBA-15 (x = 1, 2) (c.f. 5.2), a
delayed structural change of the initial octahedral [WO6] units was observed. The onset of
structural changes increased to 550 K for PW2Mo12-SBA-15 and to 623 K for PWMo11-
SBA-15. The X-ray absorption W LIII edge corresponds to electron transitions from 2p3/2
Fig. 6-2: Quantification of the [MoO4]/[MoO6] ratio of PMo12-SBA-15, PWMo11-SBA-15, and
PW2Mo10-SBA-15 during thermal treatment under propene oxidation conditions.
0
10
20
30
40
50
60
300 400 500 600 700
T [K]
[MoO
4]/[M
oO
6] ra
tio
[%
]
PMo12-SBA-15 PWMo11-SBA-15 PW2Mo10-SBA-15
82
orbital to vacant 5d orbitals and to the vacuum level.[155] The contribution of the possible
p-s transitions is ca. 50 times weaker.[156] Hence, especially the white line W LIII edge
reflects the electronic state of the vacant states of the absorbing atoms. The white line in
the W LIII edge is assigned to the 5d orbital split by the ligand field. The orbital split by an
Fig. 6-4: (a) W LIII edge XANES spectra of PW2Mo10-SBA-15 and (b) 2nd derivates of W LIII
edge XANES spectra of PW2Mo10-SBA-15; ( ) experiment, ( ) fitting function, and ( )fitting
peaks.
5d
10.18 10.20 10.22 10.24
No
rma
lize
d
abso
rptio
n
2nd
de
riva
tive
s
Photon energy [keV]
(a)
(b)
2p3/2
eg
t2g
d-orbital
splitting
Fig. 6-3: in situ W LIII edge (left) and W LI edge (right) XANES spectra of PW2Mo10-SBA-15
during temperature-programmed treatment in 5% propene and 5% oxygen in helium in a
temperature range between 300 K and 723 K.
T [K]
No
rma
lize
d
abso
rptio
n
12.05 12.15
12.25 12.35
373 473
573 673 10.35
10.15 10.20
10.25 10.30
373
473 573
673
T [K]
No
rma
lize
d
abso
rptio
n
Photon energy [keV]
Photon energy [keV]
83
octahedral ligand field is stronger than in a tetrahedral ligand field.[157,158] Therefore, an
analysis of the white line was suitable to elucidate the electronic structure of the possible
structural motifs. Fig. 6-4 shows an example for a white line analysis. The 2nd derivates of
the W LIII edge spectra resulted in a splitted peak representing the electron transitions from
2p3/2 to split 5d states (t2g and eg orbitals). The difference in energy position of the splitted
peaks corresponds to the energy difference between eg and t2g. For elucidating the energy
difference, the 2nd derivates of two Lorentz functions were used to simulated the 2nd
derivate of the W LIII edge spectra. The resulted positions of the two minima were used to
calculate the energy difference.
W LI edge XANES spectra represent the transition from the 2s orbital and have "K edge
character". Hence, the W LI edge XANES spectra may be interpreted comparable to a K
edge spectrum. A change in the pre edge peak height may correspond to a structural
rearrangement. Fig. 6-5 shows the results of the analysis of W LIII and LI edge XANES
spectra meausred during propene oxidation conditions. XAS analysis at W LI edge for
PWMo11-SBA-15 was hardly feasible due to the low content of W (~1.8 wt.%) beside Mo
(~10 wt.%). The structural changes of the [WO6] in PVxMo12-x-SBA-15 (x = 1, 2) were
delayed compared to [VOx] units in PVxMo12-x-SBA-15 (x = 1, 2) during propene
oxidation conditions (cf. chapter 5.2). [WO6] species in PWMo11-SBA-15 and in
PW2Mo10-SBA-15 changed their local structure above 620 K and 560 K, respectively. This
( )
W L
I pre
ed
ge p
ea
k h
eig
ht
T [K]
300 400 500 600 700
( )
En
erg
y G
ap [e
V]
2.5
2.6
2.7
2.8
2.9
3.0
3.1
3.2
0.360
0.365
0.370
0.375
0.380
0.385
0.390
0.395
0.400 PWMo11-SBA-15 PW2Mo10-SBA-15
Fig. 6-5: (square) Evolution of the energy gap of PWMo11-SBA-15 and PW2Mo10-SBA-15 during
thermal treatment under propene oxidation conditions; (cycle) evolution of the pre edge height of W
LI edge XANES spectra of PW2Mo10-SBA-15 during thermal treatment under propene oxidation
conditions.
84
temperatures corresponded to the temperatures where the major structural rearrangement of
the [MoOx] species were finished. For elucidating, the type of structural rearrangement, a
detailed analysis of the 2nd derivates of the W LIII edge XANES spectra was necessary.
Fig. 6-6 shows the 2nd derivates of the W LIII edge XANES spectra of PW2Mo10-SBA-15
before and after propene oxidation conditions and during propene oxidation at 723 K.
Additionally, the 2nd derivates of the W LIII edge XANES spectra of WO3 and Na2WO4
were used as references. Unexpectedly the 2nd derivates of the W LIII edge XANES
spectra of PW2Mo10-SBA-15 after propene oxidation conditions exhibited an increased
energy gap in contrast to the derivates of the W LIII edge XANES spectra of PW2Mo10-
SBA-15 during propene oxidation conditions at 723 K. An analysis of the ratio of the areas
of the Lorentz functions used for the refinement revealed that the ligand field under
propene oxidation conditions was octahedral. Comparable result was obtained for
PWMo11-SBA-15. An identification of the type of ligand field of the unknown structure
motif was possible, because the X-ray absorption intensity is t2g:eg = 3:2 for an octahedral
[WO6] and eg:t2g = 2:3 for a tetrahedral [WO4] unit.[157] Table 6-1 summarizes the results
of the detailed W LIII edge analysis. Hence, the initial octahedral [WO6] units persisted
Fig. 6-6: Second derivates of W LIII edge XANES spectra of (left) WO3 and Na2WO4 and (right)
PW2Mo10-SBA-15 before, at 723 K, and after propene oxidation; experiment ( ), fitting function
( ), and fitting peaks ( ).
10.19 10.20 10.21 10.22 10.23 10.24
2nd D
erivative
s
10.19 10.20 10.21 10.22 10.23 10.24
2nd D
erivative
s
WO3
Na2WO4
PW2Mo10SBA-15
before
at 723 K
after
Photon energy [keV] Photon energy [keV]
85
during propene oxidation conditions in contrast to the [MoO4] and [MoO6] units in
PWxMo12-x-SBA-15 (x = 0, 1, 2) and vanadium substituted PVxMo12-x-SBA-15 (x = 1, 2)
(cf. chapter 5.2). Nevertheless structural rearrangements for PWMo11-SBA-15 (620-723 K)
and PW2Mo10-SBA-15 (560-723K) were identified and could be assigned to a reversible
distortion of the octahedral [WO6] units under propene oxidation conditions. The reversible
distortion of the [WO6] units explained the delayed onset temperature of the structural
rearrangement as well. The octahedral [WO6] seemed to influenced the structural
rearrangements of the octahedral [MoO6] to tetrahedral [MoO4] units under catalytic
conditions. Therefore, a shift to increased onset temperatures of the distortion process
depending on the decreased degree of W substitution was detectable.
Table 6-1: Peak positions of the fitting Lorentz peaks, splitted peak energy (difference of the peak
positions of the fitting Lorentz peaks), quotient of the fitting peak areas (peak 1area/ peak 2area), and
the resulting ligand field of PW2Mo10-SBA-15 before and after propene oxidation conditions and at
723 K at propene oxidation conditions and the references WO3 and Na2WO4..
Peak 1 [keV] Peak 2 [keV] splitted peak
energy [eV]
peak 1area /
peak 2area
Resulted
ligand field
PW2Mo10-SBA-15
(before) 10.2151 10.2183 3.2 1.38 Oh
PW2Mo10-SBA-15
(723 K) 10.2153 10.2179 2.6 1.52 Oh
PW2Mo10-SBA-15
(after) 10.2152 10.2183 3.1 1.37 Oh
WO3 10.2147 10.2187 4.0 1.29 Oh
Na2WO4 10.2143 10.2167 2.4 0.68 Td
The interaction between the [MoOx] and [WO6] units resulted in a structure directing effect
to mostly octahedral [MoO6] units under propene oxidation conditions depending on the
degree of W substitution. The structure directing effect of the addenda tungsten atoms
differed from the structure directing of addenda vanadium in supported HPOM. In
PVxMo12-x-SBA-15 (x = 1, 2), the [MoO6] units were influenced by the neighboring [VO6]
86
units of the initial Keggin ion structure resulting in mostly tetrahedral [MoO4] and [VO4]
units depending on the degree of V substitution during thermal treatment under propene
oxidation conditions (cf. chapter 5.2).
6.2.1 Local structure in activated PWxMo12-x-SBA-15 (x = 0, 1, 2) and a reference
W2Mo10Ox-SBA-15 under catalytic conditions
Local structure around the Mo centers in act. PWxMo12-x-SBA-15 (x = 0, 1, 2)
Fig. 5-5 (left) shows the Mo K edge FT(χ(k)·k3) of act. PMo12-SBA-15, act. PWMo11-
SBA-15, and act. PW2Mo10-SBA-15 after thermal treatment under propene oxidation
conditions at 723 K. The resulted Mo K edge FT(χ(k)·k3) of PWxMo12-x-SBA-15 ( x = 0, 1,
2) were similar. Minor differences are marked in the Mo K edge FT(χ(k)·k3) and the Mo K
edge χ(k)·k3. For a more detailed analysis hexagonal MoO3 was used as structural model.
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 1 2 3 4 5 6
R [Ǻ]
FT
(χ(k
)·k
3)
4 6 8 10 12 14 16
-4
0
4
8
12
k [Ǻ]-1
χ(k)·
k3
PWMo10-SBA-15
PW2Mo10-SBA-15
PMo12-SBA-15
Fig. 6-7: (left) Mo K edge FT(χ(k)·k3) and (right) Mo K edge χ(k)·k
3 of act. PMo12-SBA-15, act.
PWMo11-SBA-15, and act. PW2Mo10-SBA-15 after thermal treatment under propene oxidation
conditions at 723 K.
87
XAFS phases and amplitudes were calculated for Mo-O and Mo-Mo distances and used for
EXAFS refinement. The results of the refinement are shown in Table 6-2. The first peak of
Mo K edge FT(χ(k)·k3) of as prepared PWxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 4.3)
exhibited differences compared to act. PWxMo12-x-SBA-15 (x = 1, 2). The first peak in the
FT(χ(k)·k3) originated from tetrahedral and octahedral [MoOx] species on the SBA-15
support and could be sufficiently simulated using four Mo-O distances. These four
distances sufficiently accounted for the minor amount of octahedral [MoO6] species.
Confirming the results of the XANES analysis 1st and 2nd disorder parameters (1st-σ2,
2nd-σ2) were higher for act. PWMo11-SBA-15 and PW2Mo10-SBA-15 indicating a
decreasing amount of tetrahedral structural motifs compared to act. PVxMo12-x-SBA-15
(x = 0, 1, 2). In addition, the 4th disorder parameters were smaller than the disorder
parameter for act. PMo12-SBA-15. This disorder parameter represented mainly the fraction
of octahedral [MoO6] species. Hence, the reduced disorder parameter indicated an
increasing amount of octahedral structural motifs in act. PWxMo12-x-SBA-15 (x = 1, 2)
compared to act. PMo12 -SBA-15. Therefore a structure directing effect of addenda
tungsten resulting in a decreased ratio of tetrahedral [MoO4] to octahedral [MoO6] under
Table 6-2: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in act. PWxMo12-x -SBA-15 (x = 0, 1, 2). Experimental parameters were obtained
from a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the experimental
Mo K edge XAFS χ(k) of act. PWxMo12-x -SBA-15 (x = 0, 1, 2) (k range from 3.4-16.0 Å-1
, R range
from 0.9 to 4.0 Å, E0= ~ -5.2, residuals ~12.1-14.7 Nind = 26, Nfree =12). Subscript c indicates
parameters that were correlated and f fixed in the refinement.
hex-MoO3
model
act. PMo12-
SBA-15
act. PWMo11-
SBA-15
act. PW2Mo10-
SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 2 1.67 1.67 0.0015 1.67 0.0019 1.68 0.0024
Mo-O 2 1.96 1.89 0.0038c 1.90 0.0042c 1.90 0.0045c
Mo-O 1 2.20 2.19 0.0038c 2.18 0.0042c 2.16 0.0045c
Mo-O 1 2.38 2.35 0.0011 2.35 0.0009 2.35 0.0008
Mo-Mo 2 3.31 3.49 0.0068c 3.49 0.0077c 3.49 0.0072c
Mo-Mo 2 3.73 3.63 0.0068c 3.63 0.0077c 3.63f 0.0072c
Mo-Mo 2 4.03 3.73 0.0100 3.72 0.0103 3.74 0.0095
88
propene oxidation conditions was determined. The distinct peak at ~3 Å (not phase
corrected) in the FT(χ(k)·k3) indicated the formation of dimeric or oligomeric [MoxOy]
units on SBA-15. Therefore, isolated octahedral [MoO6] and tetrahedral [MoO4] units can
be excluded as major molybdenum oxide species.
Local structure around the W centers in act. PWxMo12-x-SBA-15 (x = 1, 2)
Fig. 6-8 (left) shows the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15 at 723 K during
propene oxidation conditions. Fig. 6-8 (right) depicts the W LIII edge FT(χ(k)·k3) of act.
PWMo11-SBA-15, and act. PW2Mo10-SBA-15 after thermal treatment under propene
oxidation conditions at 723 K. Comparing the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-
SBA-15 at 723 K and after propene oxidation conditions resulted in significant differences.
For a detailed structure analysis theoretical XAFS phases and amplitudes were calculated
for W-O, W-Si and W-Mo distances and used for EXAFS refinement. The used theoretical
Fig. 6-8: Experimental (solid) and theoretical (dashed) W LIII edge FT(χ(k)·k3) of act. (left)
PW2Mo10-SBA-15 during thermal treatment in 5% propene and 5% oxygen in helium at 723 K;
(right) act. PWMo11-SBA-15 and act. PW2Mo10-SBA-15 after thermal treatment in 5% propene and
5% oxygen in helium at 723 K.
-0.02
0.00
0.02
0.04
0 1 2 3 4 5 6
0.00
0.05
0.10
FT
(χ(k
)·k
3)
R [Ǻ]
PW2Mo10-SBA-15
PWMo11-SBA-15
0 1 2 3 4 5 6
FT
(χ(k
)·k
3)
R [Ǻ]
89
model system based on modified H3[PMo12O40] (ICSD 209 [14,93]) where P was replaced
by Si. Thus, the model structure corresponded to a former triad of the Keggin ion with a Si
bond. The result of the refinements are summarized in Table 6-3. The shapes of W LIII
edge FT(χ(k)·k3) of as prepared PWxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 4.3) were
different to that of act. PWxMo12-x-SBA-15 (x = 1, 2). This results confirmed the
assumption of structural changes of [WOx] units in PWxMo12-x-SBA-15 (x = 1, 2) during
propene oxidation conditions. The first peak of W LIII edge FT(χ(k)·k3) of act. PWxMo12-x-
SBA-15 (x = 1, 2) could be sufficiently simulated using three W-O distances. These
distances sufficiently accounted for the amount of octahedral [WO6] species. The
refinement of the W LIII edge FT(χ(k)·k3) of PW2Mo10-SBA-15 during propene oxidation
at 723 K indicated two decreased disorder parameters (1st-σ2, 2nd-σ
2) and one increased
disorder parameter (3rd-σ2) compared to PW2Mo10-SBA-15 after catalytic conditions. The
resulting distances of the first shell of both W LIII edge FT(χ(k)·k3) were nearly identical.
Hence, the different disorder parameters corroborated a distorted arrangement of the
octahedral [WO6] units. Generally, disorder parameters will increase linear with
temperature, if a structural rearrangement can be excluded.[159,13] Hence the identified
Table 6-3: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in act. PWMo11-SBA-15, act. PW2Mo10-SBA-15 after and act. PW2Mo10-SBA-15
during thermal treatment in 5% propene and 5% oxygen in helium at 723 K. Experimental
paramters were obtained from a refinement of modified H3[PMo12O40] (ICSD 209 [14,93]) model
structure to the experimental W LIII edge XAFS χ(k) of act. PWxMo12-x-SBA-15 (x = 1, 2) (k range
from 3.0-13.6 Å-1
, R range from 1.0 to 3.8 Å, E0 = ~ -4.5 residual ~12.9 Nind = 10, Nfree = 20).
Subscript c indicates parameters that were correlated in the refinement.
act. PWMo11-
SBA-15
act. PW2Mo11-
SBA-15 (723 K)
act. PW2Mo11-
SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
W-O 2 1.68 1.79 0.0048c 1.76 0.0055c 1.77 0.0065c
W-O 2 1.91 1.69 0.0048c 1.69 0.0055c 1.71 0.0065c
W-O 2 1.92 1.89 0.0021 1.89 0.0032 1.89 0.0026
W-Mo 2 3.42 3.56 0.0117c 3.53 0.0145c 3.53 0.0087c
W-Si 1 3.10 3.08 0.0057 3.11 0.0063 3.06 0.0034
W-Mo 2 3.71 3.78 0.0117c 3.80 0.0145c 3.74 0.0087c
90
changes in the first disorder parameters (1st σ2, 2nd σ
2 and 3rd σ
2) were due to various
distorted arrangement of the octahedral [WO6] units. The 2nd W-Mo distance was
increased for act. PW2Mo10-SBA-15 at 723 K compared to act. PW2Mo10-SBA-15 after
thermal treatment. Thus, it may be assumed that a different binding state of the W-Mo
bonds existed at 723 K and after thermal treatment. A similar feature could be indentified
in the W-Si bond. The resulted structural motifs especially the shorter 3rd W-O distance,
indicated that the connection to phosphorus was broken as well. Nevertheless, the triad of
the original Keggin ion may persisted resulting in connected edge- and corner-sharing
octahedral [(W, Mo)O6] units. The [(W, Mo)O6] units were additionally connected to the
support material. Typical distances for edge-sharing tungsten oxide compounds and
corner-sharing H3[PW12O40], (NH4)10H2W12O42·4H2O, and WO3 were 3.4-3.6 Å and 3.7-
3.9 Å, respectively.[28,160,161] Therefore, the resulting W-Mo distances between 3.53-
3.80 Å in act. PW2Mo10-SBA-15 corresponded to both edge- and corner-sharing units.
Ross-Medgarden et al. found for WO3 supported on SiO2 a comparable structure motif.
The resulted structure after dehydration conditions corresponded to a Si containing Keggin
type cluster with corner- and edge-shared [WO6] units on the support material with an
interacting bond to Si.[162] Therefore, a comparable structural motif of W substituted
heteropolyoxomolybdates on SBA-15 resulting under propene oxidation conditions was
expected.
6.2.2 Comparison of the local structure around Mo centers in act. PW2Mo10-SBA-15 and
a reference act. W2Mo10Ox-SBA-15 under catalytic conditions
Fig. 6-9 shows the Mo K edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15 and act. W2Mo10Ox-
SBA-15 after thermal treatment under catalytic conditions. The first peak of the Mo K
edge FT(χ(k)·k3) of act. W2Mo10Ox-SBA-15 resembled that of act. PW2Mo10-SBA-15. In
contrast to the reference V2Mo10Ox-SBA-15 no Mo-Mo distance at ~3.3 Ǻ (not phase
corrected) indicating crystalline α-MoO3 could be detected. Hence the resulted [MoxOy]
species seemed to be very well dispersed on the support material SBA-15. The first peak in
both FT(χ(k)·k3) originated from tetrahedral [MoO4] and octahedral [MoO6] species on the
SBA-15 support and could be sufficiently simulated using four Mo-O distances. These four
distances sufficiently accounted for the amount of octahedral [MoO6] species. Linear
91
combinations of the XANES spectra of Na2MoO4 and MoO3 references were used to
determine the amount of tetrahedral [MoO4] and octahedral [MoO6] units in act.
PW2Mo10-SBA-15 and act. W2Mo10Ox-SBA-15. Apparently, act. W2Mo10Ox-SBA-15
consisted of a mixture of increased tetrahedral [MoO4] and decreased octahedral [MoO6]
units. For act. W2Mo10Ox-SBA-15 and for act. PW2Mo10-SBA-15 a [MoO4]:[MoO6] ratio
of 3:2 and 1:4. were found, respectively. A comparison of the pseudo radial distribution
function of act. W2Mo10Ox-SBA-15 and act. PW2Mo10-SBA-15 confirmed the results of
the XANES analysis (Table 6-4). The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ
2)
were higher for act. PW2Mo10-SBA-15 and indicated a decreasing amount of tetrahedral
MoO4 units. In addition, the 4th disorder parameter (4th-σ2) of is smaller than the disorder
parameter for act. W2Mo10Ox-SBA-15. This disorder parameter mainly represented the
fraction of octahedral MoO6 units. Therefore, the reduced disorder parameter indicated an
increasing amount of octahedral structural motifs in act. PW2Mo10-SBA-15 compared to
act. W2Mo10Ox-SBA-15. Furthermore, the Mo-Mo distances and disorder parameters were
comparable for act. PW2Mo10-SBA-15 and act. W2Mo10Ox-SBA-15 indicating well
dispersed [MoOx] units. A significant amount of crystalline MoO3 compared to the
reference act. V2Mo10Ox-SBA-15 (cf. chapter 5.2.1) could be excluded. The ratio of
0 1 2 3 4 5 6
-0.05
0.00
0.05
0.10
0.15
0.20
FT
(χ(k
)·k
3)
R [Ǻ]
Fig. 6-9: Mo K edge FT(χ(k)·k3) of act. PWMo11-SBA-15 and act. W2Mo10Ox-SBA-15 after
thermal treatment in 5% propene and 5% oxygen in helium at 723 K.
act. PW2Mo10-SBA-15
act. W2Mo10Ox -SBA-15
92
[MoO4]/[MoO4] units was increased indicating a favored interaction of the [MoxOy]
species with the support material SBA-15 in contrast to act. PW2Mo10-SBA-15.
Table 6-4: Type and number (N), and XAFS disorder paramters (σ2) of atoms at distance R from
the Mo atoms in act. PWxMo12-x -SBA-15 (x = 0, 1, 2). Experimental parameters were obtained
from a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the experimental
Mo K edge XAFS χ(k) of act. PWxMo12-x -SBA-15 (x = 0, 1, 2) (k range from 3.4-16.0 Å-1
, R range
from 0.9 to 4.0 Å, E0= ~ -5.2, residuals ~12.1-14.7 Nind = 26, Nfree = 12). Subscript c indicates
parameters that were correlated and f fixed in the refinement.
hex-MoO3
model
act. PW2Mo12-
SBA-15
act. W2Mo10Ox-
SBA-15
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2)
Mo-O 2 1.67 1.68 0.0024 1.67 0.0017
Mo-O 2 1.96 1.90 0.0045c 1.88 0.0041c
Mo-O 1 2.20 2.16 0.0045c 2.18 0.0041c
Mo-O 1 2.38 2.35 0.0008 2.35 0.0013
Mo-Mo 2 3.31 3.49 0.0072c 3.47 0.0072c
Mo-Mo 2 3.73 3.63f 0.0072c 3.61 0.0072c
Mo-Mo 2 4.03 3.74 0.0095 3.70 0.0099
Comparison of the local structure around the W centers in act. PW2Mo10-SBA-15 and a
reference act. W2Mo10Ox-SBA-15 under catalytic conditions
Fig. 6-10 (left) depicts the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15, act.
W2Mo10Ox -SBA-15 after thermal treatment under propene oxidation conditions at 723 K,
and monoclinic WO3. Different structural motifs could be assumed comparing the shapes
of the W LIII edge FT(χ(k)·k3) of act. PW2Mo10-SBA-15 and act. W2Mo10Ox -SBA-15. W
LIII edge FT(χ(k)·k3) and χ(k)·k
3 of act.W2Mo10Ox -SBA-15 were nearly identical to
monoclinic WO3. Thus, the predominant [WOx] species seemed to be crystalline
monoclinic WO3. XRD (Fig. 6-11) of the reference W2Mo10Ox-SBA-15 before thermal
treatment confirmed this assumption. Comparable to the reference V2Mo10Ox-SBA-15 (cf.
chapter 5.2.1) and in contrast to act. PW2Mo10-SBA-15 the [WOx] and [MoOx] units were
not in a close vicinity. Active sites of selective oxidation catalysts are often multifunctional
93
and consist of multiple metal centers.[28] Synthesis routes of supported ternary oxides
with different metal oxide precursors rarely have been reported. Hence, the synthesis of
supported tungsten substituted Keggin ions enabled the synthesis of connected [WOx] and
[MoOx] species on SBA-15 comparable to the vanadium substituted PVxMo12-x-SBA-15
10 20 30 40 50 60 70 80
Diffraction angle 2Ɵ[°]
Inte
nsity
WO3
W2Mo10Ox -SBA-15
PW2Mo10-SBA-15
Fig. 6-11: XRD of as prepared PW2Mo10-SBA-15, as prepared W2Mo10Ox-SBA-15, and
monoclinic WO3.
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0 1 2 3 4 5 6
R [Ǻ]
FT
(χ(k
)·k
3)
-5
0
5
10
15
20
25
30
χ(k)·
k3
k [Ǻ]-1
4 6 8 10 12
Fig. 6-10: (left) W LIII edge FT(χ(k)·k3) and (right) W LIII edge χ(k)·k
3 of act. PW2Mo10-SBA-15
(green), act. W2Mo10Ox-SBA-15 (red) after thermal treatment under propene oxidation conditions
at 723 K and monoclinic WO3 (blue) as reference.
WO3
act. W2Mo10Ox -SBA-15
act. PW2Mo10-SBA-15
94
(x = 1, 2). Therefore the results of the structural characterization showed that the proximity
of tungsten and molybdenum in the Keggin precursors was necessary to obtain connected
metal oxide species on a support material.
6.3 Functional characterization of PWxMo12-x-SBA-15 (x= 1, 2)
6.3.1 Reducibility
Fig. 6-12 shows the H2TPR profiles of PWxMo12-x-SBA-15 (x = 0, 1, 2). The resulted H2
TPR profiles revealed one sharp (~800 K) and a very broad (873- 973 K) reduction peak
comparable to PVxMo12-x-SBA-15 (x = 0, 1, 2) (cf. chapter 5.3.1). The shapes of the H2
TPR profiles were nearly identical. The reduction temperatures increased slightly
with the degree of tungsten substitution from 790 K for PMo12-SBA-15 to 815 K for
PW2Mo10-SBA-15. The H2 TPR profiles at ~800 K were comparable to molybdenum
oxides supported on SBA-15 with Mo loadings between 9.5 wt.% and 13.3 wt.%.[149,150]
Lou et al. assigned the sharp reduction peak to oligomeric [MoxOy] species or small MoOx
clusters and the broaded signal above ~800 K to the reduction of monomeric [MoOx]
species (cf. chapter 5.3.1).[150] Tungsten oxide reduced at temperatures between 1063-
1273 K. Tungsten oxide supported on SiO2 with a loading of 8.0 wt.% W had a reduction
H2 c
onsu
mp
tio
n
T [K]
373 473 573 673 773 873 973
790 K
808 K
815 K
PMo12-SBA-15
PWMo11-SBA-15
PW2Mo10-SBA-15
Fig. 6-12: Temperature programmed reduction (H2 TPR) of PMo12-SBA-15, PWMo11-SBA-15,
and PW2Mo10-SBA-15 measured at a heating rate of 8 Kmin-1
5% H2 in Ar.
95
temperature of 804 K and 1073 K .[163,164] The reduction peak at the higher temperature
(1073 K) was ascribed to the reduction of well dispersed tungsten species.[165] Comparing
the typical reduction temperatures of supported tungsten oxides to molybdenum oxides,
slightly higher reduction temperatures for supported tungsten oxides were determined.
Therefore, the slight increase of reduction temperature with the degree of tungsten
substitution degree may be interpreted as intrinsic effect of the addenda tungsten,
comparable to vanadium substituted PVxMo12-x-SBA-15 (x = 0, 1, 2) (cf. chapter 5.3.1).
However, slight changes in the reducibility depending on the degree of tungsten
substitution were detected.
6.3.2 Catalytic performance
Reaction rate and selectivity of PMo12-SBA-15, PWMo11-SBA-15, PW2Mo10-SBA-15,
W2Mo10Ox-SBA-15, and bulk PW2Mo10 in propene oxidation at 723 K are shown in Fig.
6-13. Reaction rates for PWxMo12-x-SBA-15 (x = 0, 1, 2) were calculated for similar
propene oxidation conditions (~ 14-17% propene conversion). The propene conversion for
bulk PW2Mo10 (~ 3%) and W2Mo10Ox-SBA-15 (~ 2%) were lower due to the strong
decreased catalytic activity. Adjusting to similar propene oxidation conditions for the low
active samples would lead to a large volume of the samples and thermal effects. Hence, the
comparison of the catalytic performance between PWxMo12-x-SBA-15 (x = 0, 1, 2) and the
low active samples has to be done carefully.
Reaction rates for PWxMo12-x-SBA-15 (x = 0, 1, 2) slightly increased with the degree of
tungsten substitution in contrast to constant reaction rates for vanadium substituted
PVxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 5.3.2). Selectivities for CO increased at the
expense of those of acetaldehyd with higher degree of tungsten substitution. Structural
analysis of act. PWxMo12-x-SBA-15 (x = 0, 1, 2) revealed a decreased concentration of
tetrahedral [MoO4] units as a function of tungsten substitution. The degree of
oligomerization of [MoxOy] seemed to be comparable in all act. PWxMo12-x-SBA-15 (x =
0, 1, 2) samples. Therefore, the additional [WO6] species in act. PWxMo12-x-SBA-15 (x =
0, 1, 2) may lead to new multifunctional active sites, resulting in a slightly different
product distribution and increased reaction rates. This [WO6] species in act. PWxMo12-x-
SBA-15 (x = 0, 1, 2) showed a structure directing effect towards formation of octahedral
96
[MoO6] units. Hence, due to the compositional and structural variety structure-activity
correlation remained vague.
The reaction rate of PW2Mo10-SBA-15 strongly increased due to the improved surface to
bulk ratio compared to bulk PW2Mo10. While, PW2Mo10 showed a slightly increased
selectivity towards acrolein, PW2Mo10-SBA-15 exhibited an increased selectivity towards
acetic acid. Total oxidation products CO and CO2 amounted to about ~55% in the resulting
oxidation products. Apparently, higher dispersion and an improved surface to bulk ratio of
Keggin ions resulted in a much increased activity at comparable selectivity similarly to
PVxMo12-x-SBA-15 (x = 1, 2) (cf. chapter 5.3.2).
In contrast to the supported HPOM samples, W2Mo10Ox-SBA-15 showed a strongly
decreased activity. The product distribution of W2Mo10Ox-SBA-15 was comparable to
unsubstituted act. PMo12-SBA-15. The amount of total oxidation products in the gas phase
was slightly lower and an increasing selectivity to acetaldehyde and acrolein was
determined compared to act. PWxMo12-x-SBA-15 (x = 1, 2). The structural analysis
indicated that act. W2Mo10Ox-SBA-15 possessed an increased amount of oligomerized
[MoxOy] species and an increased content of tetrahedral [MoO4] units. Additionally, the
predominant tungsten oxide species seemed to be crystalline monoclinic WO3 which may
not significantly participate in the catalytic reaction. Apparently, the new multifunctional
0
20
40
60
80
100
0
10
20
30
40
50
60
70
a b c e d
acrylic acid acetic acid acrolein
acetone
acetaldehyd
e
CO CO2 propionaldehyd
e
Se
lectivity [
%]
rea
ctio
n r
ate
[µ
mo
l(pro
pe
ne
)g-1
(Mo
)s-1
]
Fig. 6-13: Reaction rate (µmol(propene)·g-1
(Mo)·s-1
) and selectivity of (a) PMo12-SBA-15, (b)
PWMo11-SBA-15, (c) PW2Mo10-SBA-15, (d) W2Mo10Ox-SBA-15, and (e) bulk PW2Mo10 in 5%
propene and 5% oxygen in He at 723 K.
97
active site resulting from connected [WO6] and [MoxOy] units lead to an increasing amount
of total oxidation products for act. PW2Mo10-SBA-15 in contrast to not connected [WO6]
and [MoxOy] units in act. W2Mo10Ox-SBA-15.
Combining the results of the structural and functional characterization, the increase of
octahedral [MoO6] units resulting from tungsten substitution lead to an enhanced catalytic
activity and slightly different selectivity. The reference act. W2Mo10Ox-SBA-15 with
decreased octahedral [MoO6] units and crystalline WO3 exhibited a decreased catalytic
activity at comparable selectivity compared to unsubstituted act. PMo12-SBA-15.
Apparently, oligomerized octahedral [MoO6] species were the catalytically active sites in
propene oxidation. The various selectivities as a function of tungsten substitution in act.
PWxMo12-x-SBA-15 (x = 0, 1, 2) may be caused by new multifunctional active sites
consisting of connected [WO6] and [MoxOy] species. Apparently, tetrahedral [MoO4]
species were not involved in selective propene oxidation which confirmed the results of
previous studies.[137]
98
6.4 Summary
Structural evolution of PWxMo12-x-SBA-15 (x = 0, 1, 2) and a reference W2Mo10Ox-SBA-
15 during propene oxidation conditions were examined by in situ X-ray absorption
spectroscopy at the Mo K and W K edges. During thermal treatment under propene
oxidation conditions PWxMo12-x-SBA-15 (x = 1, 2) formed a mixture of mainly octahedral
[MoOx] and [WO6] units. Changes in the local structure around the W centers were
delayed compared to the structural changes of the Mo centers depending on the degree of
tungsten substitution. The delayed structural rearrangement corresponded to the
temperatures where the major structural rearrangement of the [MoxOy] species was
finished. The octahedral [WO6] units distorted during propene oxidation conditions.This
influenced the structural changes of the [MoxOy] species resulting in mainly octahedral
[MoO6] units depending on the degree of tungsten substitution. The degree of
oligomerization of the [MoxOy] species for all act. PWxMo12-x-SBA-15 (x = 0, 1, 2) was
comparable and independent of the degree of tungsten substitution. Apparently, the mainly
octahedral [MoOx] and [WO6] units were in close vicinity and able to interacted under
catalytic conditions. The new multifunctional active site resulting due to connected [WO6]
and [MoOx] units lead to an increased reaction rate and increased amount of total oxidation
products. Conversely, structural analysis of activated reference W2Mo10Ox-SBA-15
synthesized with individual W and Mo precursors indicated that [WO6] and [MoOx]
species were mostly separated from each other on the surface of SBA-15. Moreover,
activated W2Mo10Ox-SBA-15 possessed a decreased amount of oligomerized [MoxOy]
species and an increased content of tetrahedral [MoO4] units. Additionally, the
predominantly tungsten oxide species seemed to be crystalline monoclinic WO3 and may
not be involved in the catalytic reaction. This may explain the strongly decreased reaction
rates and similar selectivities towards partial oxidations products compared to
unsubstituted PMo12-SBA-15. In total, supported tungsten substituted Keggin ions are
suitable precursors to synthesize connected [WO6] and [MoOx] species on SBA-15.
Apparently, the proximity of tungsten and molybdenum in the Keggin precursors is a
prerequisite for obtaining connected metal oxide species.
99
7 Characterization of PMo12 supported on SBA-15 with
tailored pore radii
Nanostructured SiO2 materials such as SBA-15 represent suitable support systems for
oxide catalysts.[22-25] Studies on H4[PVMo11O40] supported on SBA-15 revealed a
structure directing effect of the silica support on the stability of the resulting Mo oxide
species.[27] H4[PVMo11O40] supported on SBA-15 formed a mixture of tetrahedrally and
octahedrally coordinated and linked [MoO4] and [MoO6] units under catalytic
conditions.[27] Previous studies have shown that catalytic activity and selectivity scales
with both the concentration and the degree of oligomerization of tetrahedral [MoO4] and
octahedral [MoO6] units at the surface.[137] Isolated [MoO4] units supported on MgO
were nearly inactive for propene oxidation. The catalytic activity and selectivity towards
oxygenates increased with increasing amount of [MoxOy] species.[137] Previously, the
degree of oligomerization was adjusted by either varying the metal loading or altering the
surface acidity of the support material.[25,153,166,167] In addition, only few other
characteristics of supported model systems are conceivable to alter the connectivity of
supported MoOx species. A complimentary approach may be varying the pore radii of the
support material. This could lead to modified structure directing effects on supported
HPOM at constant metal oxide loading and identical surface acidity. Subsequently, the
resulting [MoxOy] structures on tailored SBA-15 may be used to further elucidated
structure-activity relationships.
A study with tailored SBA-15 as support material was necessary elucidating the
catalytically active structural motifs. Therefore, H3[PMo12O40] was supported on SBA-15
with modified pore radii (10, 14, 19 nm). The Samples were prepared with a surface
coverage of 1 Keggin ion per 13 nm2 independent of the pore radii. PMo12-SBA-15 (10, 14,
19 nm) were treated under propene oxidation conditions. In situ X-ray absorption
spectroscopy investigations at the Mo K edge of PMo12 supported on SBA-15 with
different pore radii (10, 14, 19 nm) during catalytic conditions are presented. A detailed
analysis of the structures resulting under catalytic conditions is performed and correlated
with the catalytic activity and product distribution towards propene oxidation.
Additionall,y a comparison of the thermal stability of the supported samples was
established.
100
7.1 Experimental
Sample Characterization
X-Ray Fluorescence Analysis
Elemental analysis by X-ray fluorescence spectroscopy was performed on an X-ray
spectrometer (AXIOS, 2.4 kW model, PANalytical) equipped with a Rh K alpha source, a
gas flow detector and a scintillation detector. 60-80 mg of the samples were diluted with
wax (Hoechst wax C micropowder, Merck) at a ratio of 1:1 and pressed into 13 mm
pellets. Quantification was performed by standardless analysis with the SuperQ 5 software
package (PANalytical).
Physisorption measurements
Nitrogen physisorption isotherms were measured at 77 K on a BEL Mini II volumetric
sorption analyzer (BEL Japan, Inc.). Silica SBA-15 samples were treated under vacuum at
368 K for about 20 min and at 448 K for about 17 h before starting the measurement. Data
processing was performed using the BELMaster V.5.2.3.0 software package. The specific
surface area was calculated using the Brunauer–Emmett–Teller (BET) method in the
relative pressure range of 0.03-0.24 assuming an area of 0.162 nm2
per N2 molecule.[69]
The adsorption branch of the isotherm was used to calculate pore size distribution and
cumulative pore area according to the method of Barrett, Joyner, and Halenda (BJH).[70]
Powder X-ray diffraction (XRD)
XRD measurements were conducted on an X’Pert PRO MPD diffractometer (Panalytical,
θ-θ geometry), using Cu K alpha radiation and a solid-state multi-channel PIXcel detector.
Wide-angle scans (5-90° 2θ, variable slits) were collected in reflection mode using a
silicon sample holder. Small-angle scans (0.4-6.0° 2θ, fixed slits) were collected in
transmission mode with the sample spread between two layers of Kapton foil.
101
Thermal analysis
Thermogravimetric (TG) measurements were conducted using a SSC 5200 from Seiko
Instruments. The gas flow through the sample compartment was adjusted to 100 ml/min
(20% O2 and 80% He). Samples were measured with a rate of 2 K/min in the range from
298 K to 823 K.
X-ray absorption spectroscopy (XAS)
Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at
beamline X at the Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a
Si(311) double crystal monochromator. In situ experiments were conducted in a flow
reactor at atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min, temperature
range from 303 to 723 K, heating rate 4 K/min). The gas phase composition at the cell
outlet was continuously monitored using a non-calibrated mass spectrometer in a multiple
ion detection mode (Omnistar from Pfeiffer).
X-ray absorption fine structure (XAFS) analysis was performed using the software
package WinXAS v3.2..[91] Background subtraction and normalization were carried out
by fitting linear polynomials and 3rd
degree polynomials to the pre-edge and post-edge
region of an absorption spectrum, respectively. The extended X-ray absorption fine
structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic
background μ0(k) The FT(χ(k)·k3), often referred to as pseudo radial distribution function,
was calculated by Fourier transforming the k3-weighted experimental χ(k) function,
multiplied by a Bessel window, into the R space. EXAFS data analysis was performed
using theoretical backscattering phases and amplitudes calculated with the ab-initio
multiple-scattering code FEFF7.[92] Structural data employed in the analyses were taken
from the Inorganic Crystal Structure Database (ICSD).
Single scattering and multiple scattering paths in the H3[PMo12O40] (ICSD 209 [14,93])
and hexagonal MoO3 (ICSD 75417 [135]) model structure was calculated up to 6.0 Å with
a lower limit of 4.0% in amplitude with respect to the strongest backscattering path.
EXAFS refinements were performed in R space simultaneously to magnitude and
imaginary part of a Fourier transformed k3-weighted and k
1-weighted experimental χ(k)
using the standard EXAFS formula.[94] This procedure reduces the correlation between
102
the various XAFS fitting parameters. Structural parameters allowed to vary in the
refinement were (i) disorder parameter σ2
of selected single-scattering paths assuming a
symmetrical pair-distribution function and (ii) distances of selected single-scattering paths.
Detailed information about the fitting procedure are described in chapter 3.2.
Catalytic testing - selective propene oxidation
Quantitative catalysis measurements were performed using a fixed bed laboratory reactor
connected to an online gas chromatography system (Varian CP-3800) and a non-calibrated
mass spectrometer (Pfeiffer Omnistar). The fixed-bed reactor consisted of a SiO2 tube (30
cm length, 9 mm inner diameter) placed vertically in a tube furnance. In order to achieve a
constant volume and to exclude thermal effects, catalysts samples (~ 38-76 mg) were
diluted with boron nitride (Alfa Aesar, 99.5%) to result in an overall sample mass of 375
mg. For catalytic testing in selective propene oxidation a mixture of 5% propene (Linde
Gas, 10% propene (3.5) in He (5.0)) and 5% oxygen (Linde Gas, 20% O2 (5.0) in He (5.0))
in helium (Air Liquide, 6.0) was used in a temperature range of 293-723 K Reactant gas
flow rates of oxygen, propene, and helium were adjusted with separate mass flow
controllers (Bronhorst) to a total flow of 40 ml/min. All gas lines and valves were
preheated to 473 K. Hydrocarbons and oxygenated reaction products were analyzed using
a Carbowax capillary column connected to an AL2O3/MAPD column or a fused silica
restriction (25 m·0.32 mm each) connected to a flame ionization detector. O2, CO, and CO2
were separated using a Hayesep Q (2 m x 1/8``) and a Hayesep T packed column (0.5 m x
1/8``) as precolumns combined with a back flush. For separation, a Hayesep Q packed
column (0.5 m x 1/8``) was connected via a molsieve (1.5 m x 1/8``) to a thermal
conductivity detector (TCD). Details about the calculation of conversion, selectivity, and
reaction rate are described in chapter 3.2.
Sample preparation
Silica SBA-15 samples with a pore diameter of ~10 nm was prepared according to Ref.
[22]. 16.2 g of triblock copolymer (Aldrich, P123) were dissolved in 294 g water and 8.8 g
hydrochloric acid at 308 K and stirred for 24 h. After addition of 32 g tetraethyl
orthosilicate, the reaction mixture was stirred for 24 h at 373 K. The resulting gel was
103
transferred to a glass bottle and the closed bottle was heated to 388 K for 24 h.
Subsequently, the suspension was filtered by vacuum filtration and washed with a mixture
of H2O/EtOH (100:5).
Silica SBA-15 with large pores were prepared according to Refs. [46],[51]. Silica SBA-15
with a pore diameter ~14 nm (or ~19 nm) was prepared as follows. 9.6 g of triblock
copolymer (Aldrich, P123) were dissolved in 336 ml hydrochloric acid (1.3 M) at 288 K
(or 290 K). After addition of 0.108 g ammonium fluoride the solution was stirred for 16 h
(or 24 h). 20.7 g tetraethyl orthosilicate and 8.08 g 1,3,5-triisopropylbenzene were added to
the solution. The resulting gel was transferred to a glass bottle and the closed bottle was
heated to 393 K (or 373 K) for 29 h (or 48 h). Subsequently, the suspension was filtered by
vacuum filtration and washed with a mixture of EtOH/HCl/H2O (100:10:100). The
resulting white powders were dried at 378 K for 3 h and calcined at 453 K for 3 h and at
823 K for 5 h.
H3[PMo12O40] was prepared as described in chapter 3.1. H3[PMo12O40] was supported on
SBA-15 via incipient wetness. The amount of molybdenum was adjusted to 10 wt.%, 6.7
wt.%, and 5.2 wt.% on SBA-15 (10, 14, 19 nm).
104
7.2 Structure of the support materials
Pore size distributions and specific surface areas of the synthesized support materials were
calculated from N2 adsorption/desorption isotherms. SBA-15 (10 nm), SBA-15 (14 nm),
and SBA-15 (19 nm) showed typical type IV isotherms indicative of mesoporous
materials. Adsorption and desorption branches in hysteresis range were nearly parallel for
SBA-15 (10 nm) and SBA-15 (14 nm) indicating regular shaped pores Fig. 7-1. N2
isotherm for SBA-15 (19 nm) showed a slight broadening of the hysteresis loop, indicating
the development of minor constrictions. BET surface areas were calculated from
physisorption data. The tailored SBA-15 samples exhibited areas between 400 and
850 m2/g. Specific surface area aBET (calculated by BET method), external surface area aEXT
(calculated as the difference between aBET and aMeso), and area corresponding to the
mesopores aMeso are summarized in Table 7-1. Fig. 7-1 (inset) shows the pore size
distribution derived from BJH analysis resulting in three different pore diameters of ~10,
~14, and ~19 nm. Small-angle X-ray diffraction patterns of the tailored SBA-15 are
presented in Fig. 7-2 (left). The second derivates of small-angle X-ray diffraction patterns
are shown for clarity in Fig. 7-2 (right). SBA-15 (10 nm) and SBA-15 (14 nm) exhibited
Vo
lum
e [m
l g
-1]
0
200
400
600
800
5 10 15 20 25
dV
/dp [m
l nm
-1g
-1]
rrrd
p
dp [nm]
Relative Pressure p/p0
0.2 0.4 0.6 0.8 1.0 0.0
Fig. 7-1: Nitrogen physisorption isotherms of silica SBA-15 (10 nm) (square), SBA-15 (14 nm)
(circle), and SBA-15 (19 nm) (triangle) and pore distributions of of silica SBA-15 (10 nm)
(square), SBA-15 (14 nm) (circle), and SBA-15 (19 nm) (triangle)(inset).
105
the typical patterns with low-angle 10l, 11l, and 20l peaks corresponding to the two-
dimensional hexagonal symmetry.
Small-angle X-ray diffraction pattern of SBA-15 (19 nm) showed one peak at low values
of 2Ɵ. The lattice spacings d10l, derived from the Bragg equation (eq. 2.1.1), and unit cell
constants a0, corresponding to the hexagonal pore arrangement, are given in Table 7-1. The
received structural parameter of the tailored SBA-15 confirmed a successful synthesis of
mesoporous SiO2 materials with different pore size distributions, and high surface areas.
Table 7-1: Specific surface area aBET (calculated by BET method), external surface area aEXT
(calculated as the difference between aBET and aMeso), area corresponding to the mesopores aMeso,
pore diameter dpore (calculated by BJH method), mesopore volume VMeso, d10l-values (derived from
low-angle XRD), unit cell constants a0 (corresponding to the hexagonal pore arrangement) of SBA-
15 (10 nm), SBA-15 (14 nm), and SBA-15 (19 nm).
aBET
(m2/g)
aExt
m2/g)
aMeso
(m2/g)
dBJH
(nm) VMeso (cm
3/g)
d10l
( nm)
a0
(nm)
SBA-15 (10 nm) 843 145 698 10.3 1.233 10.52 12.14
SBA-15 (14 nm) 525 83 442 13.8 1.344 12.52 14.46
SBA-15 (19 nm) 395 50 345 18.5 0.957 14.77 17.05
-2.0 -1.5 -1.0 -0.5 0.5 1.0 1.5 2.0
SBA-15 (10nm) SBA-15 (14nm) SBA-15 (19nm)
No
rm. in
ten
sity
-1.0 -0.5 0.5 1.0
2nd
de
riva
te
norm
. in
tensity
Diffraction angle 2Ɵ [°] Diffraction angle 2Ɵ [°]
Fig. 7-2: (left) Low-angle X-ray diffraction patterns and (right) 2nd derivates of the low-angle X-
ray diffraction patterns SBA-15 (10 nm), SBA-15 (14 nm), and SBA-15 (19 nm).
106
7.3 Characterization of PMo12-SBA-15 (10, 14, 19 nm)
Local structure around theMo centers in PMo12-SBA-15 (10, 14, 19 nm)
Fig. 7-3 shows the theoretical and experimental Mo K edge FT(χ(k)·k3) of PMo12-SBA-15
(10, 14, 19 nm). The shapes of the FT(χ(k)·k3) resembled that of bulk PMo12 indicating a
similar local structure around the Mo centers in supported and unsupported HPOM Keggin
ions. For a more detailed structural analysis H3[PMo12O40] Keggin ions (ICSD 209
[14,93]) was chosen as model structure. Comparing the distances R and disorder
parameters σ2 of PMo12-SBA-15 (10, 14, 19 nm) supported on SBA-15 with different pore
radii exhibited no significant differences between the initial Keggin ion structure and
Keggin ions supported on SBA-15 (chapter 3) structure. The good agreement between
theory and experiment for PMo12-SBA-15 (10, 14, 19 nm) confirmed the Keggin ion
structure upon supporting PMo12 on SBA-15.[27]
R [Å]
FT
(χ(k)·k
3)
0 1 2 3 4 5 6 -0.05
0.00
0.05
0.10
0.15
0.20
0.25
PMo12-SBA-15 (19 nm)
PMo12-SBA-15 (14 nm)
PMo12-SBA-15 (10 nm)
Fig. 7-3: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of PMo12 supported
on SBA-15 (10 nm), SBA-15 (14 nm), and SBA-15 (19 nm).
107
Table 7-2: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in as prepared PMo12-SBA-15 (10, 14, 19 nm). Experimental parameters were
obtained from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93]) to the
experimental Mo K edge XAFS χ(k) of PMo12-SBA-15 (10, 14, 19 nm) (k range from 3.0-13.7 Å-1
,
R range from 0.9 to 4.0 Å, E0= ~ 2.3, residuals ~11.3-12.5 Nind = 22, Nfree = 9). Subscript c indicates
parameters that were correlated in the refinement.
Keggin model
PMo12-SBA-15
(10nm)
PMo12-SBA-15
(14nm)
PMo12-SBA-15
(20nm)
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 1 1.68 1.65 0.0025 1.65 0.0029 1.65
0.0021
Mo-O 2 1.91 1.78c 0.0033c 1.78c 0.0040c 1.79c 0.0032c
Mo-O 2 1.92 1.95c 0.0033c 1.95c 0.0040c 1.94c 0.0032c
Mo-O 1 2.43 2.40 0.0008 2.39 0.0010 2.40 0.0007
Mo-Mo 2 3.42 3.42 0.0052c 3.41 0.0056c 3.42 0.0054c
Mo-Mo 2 3.71 3.74 0.0052c 3.74 0.0056c 3.74 0.0054c
Thermal stability of PMo12 supported on SBA-15 with different pore radii
Fig. 7-4 depicts the measured thermogravimetric data of PMo12-SBA-15 (10, 14, 19 nm) in
20% O2 in He. The mass loss between 303 K and 373 K was ascribed to desorption of
physically adsorbed water on the surface of the materials. Relative mass loss decreased for
samples with large pore diameter. Afterwards a nearly constant mass in between 373 K and
448 K could be detected. The temperature range 448-523 K showed a mass loss of ~1%
(PMo12-SBA-15 (11 nm)), ~0.7% (PMo12-SBA-15 (14 nm)), and ~0.5% (PMo12-SBA-15
(19 nm)). Sample mass between 523 K and 823 K did not change for all samples.
Comparable behaviour was shown for pure silica samples in vacuum.[168] Silica
dehydrated between room temperature and 453 K followed by the dehydroxylation process
of silanol groups between 453 K and 673 K. This resulted in the formation of siloxane
groups and a decrease of silanol density from 4.6 OH/nm2 (473 K) to 2.3 OH/nm
2
(673 K).[145,168]
Comparing dehydration and dehydroxylation processes of supported HPOM and bulk
HPOM revealed a correlation between dehydration and thermal stability of the Keggin ion.
Bulk PMo12 loses 1.5 molecules of constitutional water between 673-713 K
108
under dry air conditions. This decomposition is accompanied by the formation of
MoO3.[117] Structures resulting for supported HPOM after thermal treatment under
oxidizing conditions were comparable to stabilized two dimensional hexagonal MoO3 on
SBA-15.[27,59] It has been shown that structural characteristics of supported model
systems like MoOx-SBA-15 and VOx-SBA-15 depended mainly on their hydration states
and previous calcination processes.[95,144] A comparable effect may be responsible for
the structural evolution of HPOM supported on SBA-15. Adsorbed water and silanol
groups from the support material may possess a structure stabilizing effect on the Keggin
ion. This effect would be comparable to that of water of crystallization and constitutional
water in bulk HPOM under ambient conditions.[117] Therefore, dehydroxylation of SBA-
15 may be the driving force for the structural decomposition of the Keggin ion resulting in
the formation of Mo oxide species on SBA-15.
7.4 Structural evolution of PMo12- SBA-15 (10, 14, 19 nm) under catalytic
conditions
PMo12-SBA-15 (10, 14, 19 nm) samples were investigated by in situ XAS under catalytic
conditions. Fig. 7-5 shows the evolution of molybdenum XANES spectra of PMo12-SBA-
15 during temperature-programmed treatment in 5% propene and 5% oxygen. The
T [K]
No
rma
lize
d M
ass [%
] N
orm
aliz
ed M
ass [%
]
373 473 573 673 773
90
92
94
96
98
100 PMo12-SBA-15 (10 nm) PMo12-SBA-15 (14 nm) PMo12-SBA-15 (19 nm)
Fig. 7-4: Thermograms of PMo12-SBA-15 (10 nm), PMo12-SBA-15 (14 nm), and PMo12-SBA-15
(19 nm) at 20% O2 in He.
109
resulting structures exhibited an increased concentration of tetrahedral [MoO4] units. The
pre-edge peak features in the Mo K edge XANES spectra can be employed to elucidate the
local structure around the Mo center. Using the pre-edge peak height sufficed to quantify
the contribution of tetrahedral [MoO4] and distorted [MoO6] units present under catalytic
conditions. Fig. 7-6 showed the Mo K edge XANES spectra of PMo12-SBA-15 (14 nm)
and a spectrum calculated from a linear combination of bulk MoO3 and bulk
20.0 20.1 20.2 0.00
0.25
0.50
0.75
1.00
Photon energy [keV]
Na2MoO4
MoO3
No
rma
lize
d a
bsorp
tion
Fig. 7-6: Refinement of the sum (dotted) of XANES spectra of references MoO3 and Na2MoO4
(dashed) to Mo K edge XANES spectrum of activated PMo12-SBA-15 (14 nm) after thermal
treatment under propene oxidation conditions at 723 K.
20.00
20.05
20.10 20.15
20.2
373
473
573
673
Photon energy
T [K]
No
rma
lize
d
abso
rptio
n
Fig. 7-5: in situ Mo K edge XANES spectra of PMo12-SBA-15 during temperature-programmed
treatment in 5% propene and 5% oxygen in helium in a temperature range between 300 K and
723 K.
110
Na2MoO4. The linear combination represented the amount of distorted [MoO6] and
tetrahedral [MoO4] units. Quantitative evolution of the structural units was used to
visualize changes in the structure of PMo12-SBA-15 (10, 14, 19 nm) during temperature
programmed treatment in 5% propene and 5% oxygen. Fig. 7-7 depicts the calculated
concentration of tetrahedral [MoO4] units during thermal treatment under catalytic
conditions. No significant structural changes of PMo12-SBA-15 (10, 14, 19 nm) could be
detected in the temperature range between 303 K and 448 K. Apparently, the Keggin
structure was stable on silica SBA-15 in the temperature range 303-448 K. Subsequently,
concentration of tetrahedral [MoO4] units considerably increased in the temperature range
between 448 K and 598 K for PMo12-SBA-15 (10, 14, 19 nm). The onset of structural
rearrangement was identical for all PMo12-SBA-15 (10, 14, 19 nm) samples and
independent of the pore radii of the support materials. The stability of the Keggin ion
seemed to depend only on the nature of the support material. The structural evolution in the
temperature range (448-598 K) correlated to the dehydration and dehydroxylation process
of SiO2 under oxidizing conditions (20% O2 in He). Apparently, dehydroxylation process
was also the driving force for the structural rearrangement of the PMo12-SBA-15 (10, 14,
19 nm) samples under catalytic conditions. At a temperature of about 598 K a higher
amount of the tetrahedral [MoO4] units for PMo12-SBA-15 (14, 19 nm) could be detected
compared to conventional PMo12-SBA-15 (10 nm) (Fig. 7-7). The concentration of
Fig. 7-7: Evolution of MoO4/MoO6 ratio of PMo12-SBA-15 (10 nm), PMo12-SBA-15 (14 nm), and
PMo12-SBA-15 (19 nm) during thermal treatment under propene oxidation conditions.
300 400 500 600 700
0
20
40
60
80
PMo12-SBA-15 (10nm) PMo12-SBA-15 (14nm) PMo12-SBA-15 (19nm)
[MoO
4]/[M
oO
6] ra
tio
[%
]
T [K]
111
tetrahedral [MoO4] units for PMo12-SBA-15 (14, 19 nm) amounted to ~65% compared to
~45% for PMo12.SBA-15 (10 nm). Hence, the concentration of tetrahedral [MoO4] units
increased with the larger pore radii of the support material. Quantification of tetrahedral
[MoO4] and octahedral [MoO6] units in the temperature range between 598 K and 723 K
confirmed this assumption. The [MoO4] concentration of PMo12-SBA-15 (14 nm) and
PMo12-SBA-15 (19 nm) were comparable and reached the highest concentration with 75%
[MoO4] units at 723 K. PMo12-SBA-15 (10 nm) reach a maximum of tetrahedral [MoO4]
units (~50%) at 657 K. Subsequently, a decreasing concentration of tetrahedral [MoO4]
units to 40% at 723 K was determined.
Influence of the pore radii to the resulting structure of [MoxOy] species
Fig. 7-8 shows the Mo K edge FT(χ(k)·k3) of activated PMo12-SBA-15 (10, 19 nm) after
thermal treatment under propene oxidation conditions. The FT(χ(k)·k3) of act. PMo12-SBA-
15 exhibited features similar to that of previously reported dehydrated molybdenum oxides
and HPOM supported on SBA-15.[27,59] For a more detailed structural analysis
hexagonal MoO3 was chosen as model structure. Theoretical XAFS phases and amplitudes
were calculated for Mo-O and Mo-Mo distances and used for EXAFS refinement. The
results of the refinement are given in Table 7-3. The first peak of Mo K edge FT(χ(k)·k3) of
act. PMo12-SBA-15 (10 nm) exhibited differences compared to act. PMo12-SBA-15 (14,
19 nm). The first peak in the FT(χ(k)·k3) originated mainly from the tetrahedral species on
-0.08
-0.04
0.00
0.04
0.08
FT
(χ(k
)·k
3
0 1 2 3 4 5 6
PMo12-SBA-15 (10 nm)
R [Å]
PMo12-SBA-15 (19 nm)
Fig. 7-8: Mo K edge FT(χ(k)·k3) of activated PMo12-SBA-15 (10 nm) and activated PMo12-SBA-
15 (19 nm) after thermal treatment under propene oxidation conditions at 723 K.
112
the SBA-15 support and could be sufficiently simulated using four Mo-O distances. These
four distances sufficiently accounted for the minor amount of octahedral [MoO6] species.
The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ
2) were higher for act. PMo12-SBA-15
(10 nm) and indicated a lower amount of tetrahedral structural units. Additionally, the 4th
disorder parameter (4th-σ2) was smaller than that of act. PMo12-SBA-15 (14, 19 nm) with
larger pores. This disorder parameter mainly represented the fraction of octahedral [MoO6]
species, and corresponded to increasing amount of octahedral structural motifs in act.
PMo12-SBA-15 (10 nm) compared to act. PMo12-SBA-15 (14, 19 nm).
A distinct peak at ~3 Å in the FT(χ(k)·k3) indicated a significant amount of dimeric or
oligomeric [MoxOy] units on SBA-15 (10, 14, 19 nm) independent of the pore radii. Hence,
isolated tetrahedral [MoO4] units can be excluded as major molybdenum oxide
species.[137] The obtained Mo-Mo distances were identical for act. PMo12-SBA-15 (10,
14, 19 nm) samples and, thus, were independent of the pore radii. The disorder parameters
σ2
of the Mo-Mo distances for act. PMo12-SBA-15 (10 nm) were slightly increased
compared to act. PMo12-SBA-15 (14, 19 nm). This indicated a decreased oligomerization
degree of Mo silica SBA-15 with larger pore diameters. Apparently, the
Table 7-3: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in act. PMo12-SBA-15 (10, 14, 19 nm). Experimental parameters were obtained from
a refinement of a hexagonal MoO3 model structure (ICSD 75417 [135]) to the experimental Mo K
edge XAFS χ(k) of act. PMo12-SBA-15 (10, 14, 19 nm) (k range from 3.4-16.0 Å-1
, R range from
0.9 to 4.0 Å, E0= ~ -5.2, residuals ~12.5 Nind = 26, Nfree =12). Subscript c indicates parameters that
were correlated in the refinement.
hex-MoO3
model
act. PMo12-
SBA-15 (10nm)
act. PMo12-SBA-
15 (14nm)
act. PMo12-SBA-
15 (20nm)
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 2 1.67 1.67 0.0015 1.67 0.0009 1.67 0.0009
Mo-O 2 1.96 1.89 0.0038c 1.88 0.0024c 1.88 0.0024c
Mo-O 1 2.20 2.19 0.0038c 2.17 0.0024c 2.17 0.0024c
Mo-O 1 2.38 2.35 0.0011 2.34 0.0030 2.34 0.0029
Mo-Mo 2 3.31 3.49 0.0068c 3.49 0.0054c 3.49 0.0058c
Mo-Mo 2 3.73 3.63 0.0068c 3.62 0.0054c 3.62 0.0058c
Mo-Mo 2 4.03 3.73 0.0100 3.73 0.0089 3.72 0.0096
113
concentration of the [MoxOy] species reached a minimum on the large pore samples
PMo12-SBA-15 (14, 19 nm). These large pore SBA-15 (dMeso = 14-19 nm) materials
possessed a larger angle of inclination because of the decreased curvature of the pores in
these samples. This may reduce the contact area between the decomposing Keggin ions
eventually resulting in a higher concentration of dispersed and isolated oxide species.
Conversely, thermal decomposition of Keggin ions on small pore support materials was
prone to result in connected [MoxOy] species. The Mo-O, Mo-Mo distances and disorder
parameters for act. PMo12-SBA-15 (14 nm) and act. PMo12-SBA-15 (19 nm) were nearly
identical and different from those of PMo12-SBA-15 (10 nm). This confirmed the results of
the quantification of tetrahedral [MoO4] (~75%) and distorted [MoO6] (~25%) units,
present on large pore materials under catalytic conditions. Apparently, the formation of
oligomeric [MoxOy] units mostly consisting of tetrahedral [MoO4] units depended on the
pore radius of the silica SBA-15. Hence, act. PMo12-SBA-15 (10 nm) with smaller pores
favored the formation of more extended structures on the support material.
7.5 Functional characterization of PVxMo12-x-SBA-15 (x= 1, 2)
7.5.1 Influence of the resulting structures to catalytic activity
PMo12-SBA-15 (10, 14, 19 nm) samples were tested under catalytic conditions for
selective propene oxidation. Fig. 7-9 shows a comparison of the selectivities towards the
oxidation products and reaction rates. The oxidation product distributions were comparable
for all three PMo12-SBA-15 (10, 14, 19 nm) samples. The oxidation product distributions
were comparable for all three PMo12-SBA-15 (10, 14, 19 nm) samples. The reaction rates
for PMo12-SBA-15 (14 nm) and PMo12-SBA-15 (19 nm) were also nearly identical. In
contrast to the samples with larger pores, PMo12-SBA-15 (10 nm) showed a ~14% higher
reaction rate during propene oxidation. The theoretical Mo coverage of 0.9 Mo/nm2 was
similar for all PMo12-SBA-15 (10, 14, 19 nm) samples. However, a significant difference
between all SBA-15 (10, 14, 19 nm) materials was the curvature of the surface in the
pores. The pore structure of mesoporous SBA-15 corresponds to that of hollow cylinders.
Thus, the curvature of the walls of these cylinders decreases with higher pore radius.
Therefore, arrangement of spherical Keggin ions on an area along the inner surface of
pores with different pore radii leads to a decreasing distance between the spheres at smaller
114
pore radius. Hence, assuming a volume of 1 nm3 per Keggin ion and considerating the
various curvatures for SBA-15 (10, 14, 19 nm) lead to an increased effective coverage at
smaller pore radius. Therefore, the increased effective distance of the Keggin ion on
PMo12-SBA-15 (14 nm) and PMo12-SBA-15 (19 nm) resulted in a lower concentration of
[MoxOy] units compared to PMo12-SBA-15 (10 nm). Hence, the catalytic activity in
propene oxidation increased with higher concentration of [MoxOy] units under catalytic
conditions. [MoxOy] units orginating from PMo12-SBA-15 (10 nm) resulted in an enhanced
catalytic activity without significant influence on the product distribution. Therefore, a
higher concentration of [MoxOy] units at similar loadings improved the catalytic activity
towards propene oxidation. Comparable results have been shown for PVMo11 supported on
SiO2 (Aerosil 300: 295 m2g
-1, Nippon Aerosil Co., Ltd.) with different loadings during
oxidation of methacrolein.[167] Selective propene oxidation requires the transfer of more
than two electrons. Therefore, [MoxOy] sites are necessary to selectively oxidize the
propene molecule.[37,169,170] Catalytic activity of supported vanadium oxide based
catalysts depended also on the concentration of [VxOy] units comparable to
[MoxOy].[153,171] Therefore, [MoxOy] units seemed to be necessary for catalytic activity
while their concentration increased with the effective coverage.
0
20
40
60
80
100
30
40
50
60
70
Se
lectivity [%
]
reactio
n r
ate
µm
ol(
pro
pen
e)g
-1(M
o)s
-1
acrylic acid
acetic acid acrolein
acetone
acetaldehyde
CO
CO2 propionaldehyde
10 nm 14 nm 20 nm
Fig. 7-9: Reaction rate (µmol(propene)g-1
(Mo)s-1
) and selectivity of PMo12-SBA-15 (10, 14,
20 nm) in 5% propene and 5% oxygen in He at 723 K.
115
7.6 Summary
Structural evolution of H3[PMo12O40] supported on SBA-15 (PMo12-SBA-15) with
different pore radii (10, 14, 19 nm) was examined by in situ X-ray absorption spectroscopy
investigations at the Mo K edge during propene oxidation conditions. Large pore SBA-15
was successfully used as support material for molybdenum based oxidation catalysts.
Supporting heteropolyoxo molybdates on large pore SBA-15 resulted in regular Keggin
ions on the support material. During thermal treatment in propene oxidation conditions
PMo12-SBA-15 (10, 14, 19 nm) formed a mixture of mostly tetrahedral [MoO4] and
octahedral [MoO6] units. The onset temperature of structural changes of PMo12-SBA-15
(10, 14, 19 nm) during thermal treatment in propene oxidation conditions was largely
independent of the pore size of SBA-15. The stability of the Keggin ions depended mostly
on the nature of the support. Apparently, the dehydroxylation of silanol groups of the
support material was the driving force for the structural instability of the Keggin ion. The
resulting [MoxOy] structures present under catalysis conditions depended on the pore size
of the support material. A higher concentration of octahedral [MoO6] units and higher
oligomerized [MoxOy] units was detected for act. PMo12-SBA-15 (10 nm) compared to act.
PMo12-SBA-15 (14, 19 nm). The higher concentration of [MoxOy] units present in act.
PMo12-SBA-15 (10 nm) resulted in an increased catalytic activity compared to to activated
PMo12-SBA-15 (14, 19 nm) with a lower concetration of [MoxOy] units. Selectivities
towards oxidation products during propene oxidation were comparable and largely
independent of the pore radii of act. PMo12-SBA-15 (10, 14, 19 nm). Apparently, tailoring
the pore radius of silica SBA-15 permitted to prepare Mo oxide model systems to
investigate correlations between activity and structure of characteristic oxide species at
similar loadings.
116
8 Characterization of PVMo11 supported on SBA-15
with different metal loading
H4[PVMo11O40] supported on SBA-15 forms a mixture of tetrahedrally and octahedrally
coordinated and linked [MoO4] and [MoO6] units under catalytic conditions.[27]
Supposingly, the catalytic activity and selectivity towards oxygenates increases with
increasing amount of linked [MoxOy] species.[27] Accordingly, isolated [MoO4] units
supported on MgO were nearly inactive for propene oxidation.[137] The degree of
oligomerization may be varied by increasing or decreasing the metal loading or by altering
the surface acidity of the support material.[25,148,153] Additionally, the variation of pore
radii of the support material showed various structure directing effects of supported HPOM
at constant metal oxide loading and identical surface acidity (cf. chapter 7). A higher
concentration of octahedral [MoO6] and [MoxOy] units for PMo12-SBA-15 with smaller
pore radius could be detected compared to PMo12-SBA-15 with larger pores. While, the
catalytic activity increased with the amount of [MoxOy], the selectivities towards oxidation
products during propene oxidation conditions were comparable and independent of the
pore radius. This indicated similar active sites in act. PMo12-SBA-15 with various pore
radii. For elucidating structure activity correlations a study with varied metal loading was
necessary to established the catalytic active structure motifs. Variation of metal loading of
PVMo11-SBA-15 could lead to different structure directing effects during propene
oxidation conditions. Therefore, H4[PVMo11O40] was supported on SBA-15 with different
Mo loading (1 wt.% Mo, 5 wt.% Mo, and 10 wt.% Mo) to elucidate the resulting structure
under propene oxidation conditions and the influence on the catalytic activity. Hence,
PVMo11-SBA-15 was treated under propene oxidation conditions. In situ X-ray absorption
spectroscopy investigations at the Mo K edge of PVMo11 supported on SBA-15 with
different Mo loading) under catalytic conditions were conducted. A detailed analysis of the
structures present under catalytic conditions was performed and correlated with the
catalytic activity and product distribution towards propene oxidation.
117
8.1 Experimental
Sample Characterization
X-Ray Fluorescence Analysis
Elemental analysis by X-ray fluorescence spectroscopy was performed on an X-ray
spectrometer (AXIOS, 2.4 kW model, PANalytical) equipped with a Rh K alpha source, a
gas flow detector and a scintillation detector. 60-80 mg of the samples were diluted with
wax (Hoechst wax C micropowder, Merck) at a ratio of 1:1 and pressed into 13 mm
pellets. Quantification was performed by standardless analysis with the SuperQ 5 software
package (PANalytical).
X-ray absorption spectroscopy (XAS)
Transmission XAS experiments were performed at the Mo K edge (19.999 keV) at
beamline X at the Hamburg Synchrotron Radiation Laboratory, HASYLAB, using a
Si(311) double crystal monochromator. In situ experiments were conducted in a flow
reactor at atmospheric pressure (5 vol% oxygen in He, total flow ~30 ml/min, temperature
range from 303 to 723 K, heating rate 4 K/min). The gas phase composition at the cell
outlet was continuously monitored using a non-calibrated mass spectrometer in a multiple
ion detection mode (Omnistar from Pfeiffer).
X-ray absorption fine structure (XAFS) analysis was performed using the software
package WinXAS v3.2..[91] Background subtraction and normalization were carried out
by fitting linear polynomials and 3rd degree polynomials to the pre-edge and post-edge
region of an absorption spectrum, respectively. The extended X-ray absorption fine
structure (EXAFS) χ(k) was extracted by using cubic splines to obtain a smooth atomic
background μ0(k) The FT(χ(k)·k3), often referred to as pseudo radial distribution function,
was calculated by Fourier transforming the k3-weighted experimental χ(k) function,
multiplied by a Bessel window, into the R space. EXAFS data analysis was performed
using theoretical backscattering phases and amplitudes calculated with the ab-initio
multiple-scattering code FEFF7.[92] Structural data employed in the analyses were taken
from the Inorganic Crystal Structure Database (ICSD).
118
Single scattering and multiple scattering paths in the H3[PMo12O40] (ICSD 209 [14,93])
and hexagonal MoO3 (ICSD 75417 [135]) model structures were calculated up to 6.0 Å
with a lower limit of 4.0% in amplitude with respect to the strongest backscattering path.
EXAFS refinements were performed in R space simultaneously to magnitude and
imaginary part of a Fourier transformed k3-weighted and k
1-weighted experimental χ(k)
using the standard EXAFS formula.[94] This procedure reduces the correlation between
the various XAFS fitting parameters. Structural parameters allowed to vary in the
refinement were (i) disorder parameter σ2
of selected single-scattering paths assuming a
symmetrical pair-distribution function and (ii) distances of selected single-scattering paths.
Detailed information about the fitting procedure are described in chapter 3.2.
Temperature programmed reduction
Temperature programmed reduction (TPR) was performed with a catalysts analyzer from
BEL Japan Inc. equipped with a silica glass tube reactor. Samples were placed on silica
wool inside the reactor next to a thermocouple. A gas flow (5 % H2 in Ar) of 60 ml/min
was adjusted during reaction. A heating rate of 8 K / min to 973 K was used while H2
consumption was measured with a TCD unit. All samples were treated with a gas flow of
60 ml/min Ar at 393 K for about 45 min before starting the measurement. For
measurements 37.2 mg PVMo11-SBA-15 (10 wt. % Mo), 35.0 mg PVMo11-SBA-15 (5 wt.
% Mo), and 34.5 mg PVMo11-SBA-15 (1 wt. % Mo), were used.
Catalytic testing - selective propene oxidation
Quantitative catalysis measurements were performed using a fixed bed laboratory reactor
connected to an online gas chromatography system (Varian CP-3800) and a non-calibrated
mass spectrometer (Pfeiffer Omnistar). The fixed-bed reactor consisted of a SiO2 tube (30
cm length, 9 mm inner diameter) placed vertically in a tube furnace. In order to achieve a
constant volume and to exclude thermal effects, catalysts samples were diluted with SBA-
15 and boron nitride (Alfa Aesar, 99.5%). For catalytic testing in selective propene
oxidation a mixture of 5% propene (Linde Gas, 10% propene (3.5) in He (5.0)) and 5%
oxygen (Linde Gas, 20% O2 (5.0) in He (5.0)) in helium (Air Liquide, 6.0) was used in a
temperature range of 293-723 K Reactant gas flow rates of oxygen, propene, and helium
119
were adjusted with separate mass flow controllers (Bronhorst) to a total flow of 40 ml/min.
All gas lines and valves were preheated to 473 K. Hydrocarbons and oxygenated reaction
products were analyzed using a Carbowax capillary column connected to an AL2O3/MAPD
column or a fused silica restriction (25 m·0.32 mm each) connected to a flame ionization
detector. O2, CO, and CO2 were separated using a Hayesep Q (2 m x 1/8``) and a Hayesep
T packed column (0.5 m x 1/8``) as precolumns combined with a back flush. For
separation, a Hayesep Q packed column (0.5 m x 1/8``) was connected via a molsieve (1.5
m x 1/8``) to a thermal conductivity detector (TCD).
Sample preparation
Silica SBA-15 was prepared according to [22,23] as described in chapter 4.1. PVMo11-
SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) was prepared via incipient wetness.
The amount of molybdenum was adjusted to 10 wt.%, 5 wt.% Mo, and 1 wt.% Mo.
Therefore an aqueous solution of HPOM was used.
120
8.2 Characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1
wt.% Mo)
Quantification of metal loading XRF
Quantitative analysis of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) were
performed to verify the supporting process. Results of quantitative XRF measurements and
nominal compositions of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo)
were summarized in Table 8-1.
Table 8-1: Results of quantitative XRF measurements and nominal composition of PVMo11-SBA-
15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo).
elements
H P Mo V O Si
PVMo11-SBA-15 nom. wt.% 0.04 0.29 10.00 0.48 50.33 38.85
PVMo11-SBA-15 exp. wt.% - 0.42 9.15 0.42 50.44 39.46
PVMo11-SBA-15 nom. wt.% 0.02 0.15 4.99 0.24 51.79 42.80
PVMo11-SBA-15 exp. wt.% - 0.19 4.37 0.24 51.93 43.28
PVMo11-SBA-15 nom. wt.% 0.00 0.03 1.00 0.05 52.96 45.96
PVMo11-SBA-15 exp.. wt.% - 0.03 0.83 0.04 53.00 46.10
Experimental composition corresponded well to the nominal composition and confirmed a
successful supporting process. For simplification of the nomenclature the samples were
still denoted as PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo).
Thermal stability of PVMo11 supported on SBA-15 with various metal loading (10
wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo)
Fig. 8-1 depicts the measured thermogravimetric data of PVMo11-SBA-15 (10 wt.% Mo, 5
wt.% Mo, and 1 wt.% Mo) in 20% O2 in He. The mass loss between 303 K and 373 K was
ascribed to desorption of physically adsorbed water on the surface of the supported
materials. The loss of adsorb water delayed with higher Mo loading. The absolute mass
121
loss in this temperature range was between 8-12 wt.% for all PVMo11-SBA-15 (10 wt.%
Mo, 5 wt.% Mo, and 1 wt.% Mo) samples. The samples were not pretreated. Thus, the
amount of physically adsorbed water on the surface of the samples may depend on the
storage conditions. Afterwards a nearly constant mass between 373 K and 448 K could be
detected. The mass showed a loss for PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo) in
temperature range 448-523 K. The mass of PVMo11-SBA-15 (1 wt.% Mo) was nearly
constant in this temperature range. Subsequently, between 523 K and 823 K the mass
slightly decreased for all samples. Pure silica showed a comparable behaviour in vacuum.
Silica dehydrated between room temperature and 453 K followed by the dehydroxylation
process of silanol groups. This resulted in the formation of siloxane groups.[145,168]
Adsorbed water and silanol groups from the support material may possess a structure
stabilizing effect on the Keggion ion. This effect would be comparable to that of water of
crystallization and constitutional water in bulk HPOM.[117] It has been shown that
structural characteristics of supported model systems like MoOx-SBA-15 and VOx-SBA-15
depended mainly on their hydration states and previous calcination processes.[59,95]
Therefore, the delayed desorption of physically adsorbed water at higher Mo loadings, may
indicative a variable stability of the Keggin ion.
T [K]
Norm
aliz
ed
Mass [
%]
Norm
aliz
ed
Mass [
%]
373 473 573 673 773
Fig. 8-1: Thermograms of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) at 20% O2
in He.
10 wt.% Mo
5 wt.% Mo
1 wt.% Mo
86
88
90
92
94
96
98
100
122
Structure of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo)
Fig. 8-2 shows the theoretical and experimental Mo K edge FT(χ(k)·k3) of PVMo11-SBA-
15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo). The shapes of the FT(χ(k)·k3) resembled
that of bulk PVMo11 indicating similar local structure around the Mo centers in supported
and unsupported HPOM Keggin structure. For a more detailed structural analysis, the
H3[PMo12O40] Keggin structure was chosen as model structure. A comparison of the
distances R and disorder parameters σ2 of PVMo11 supported on SBA-15 with different Mo
loadings (10 wt.% Mo, 5 wt.% Mo) exhibited no significant differences between the initial
Keggin ion structure and Keggin ions supported on SBA-15. PVMo11-SBA-15 (1 wt.%
Mo) exhibited increased 2nd and 3rd Mo-O distances compared to PVMo11-SBA-15 (10
wt.% Mo, 5 wt.% Mo).This distances corresponded to the edge- and corner-sharing
octahedral [MoO6] units. Additionally, the 2nd Mo-Mo distance was slightly increased
confirming the increased Mo-O distances. The various Mo-O and Mo-Mo distances may
indicate a slightly different binding state compared to PVMo11-SBA-15 (10 wt.% Mo, 5
wt.% Mo) possibly orginated from a stronger interaction of the Keggin ions with the
PVMo11-SBA-15 (1wt.% Mo)
R [Å]
FT
(χ(k)·k
3)
0 1 2 3 4 5 6 -0.05
0.00
0.05
0.10
0.15
0.20
0.25
Fig. 8-2: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of PVMo11
supported on SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.% Mo).
PVMo11-SBA-15 (10wt.% Mo)
PVMo11-SBA-15 (5wt.% Mo)
123
support material SBA-15. However, the good agreement between theory and experiment
for PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo) confirmed the maintained Keggin ion
structure upon supporting PVMo11 on SBA-15.[27]
Table 8-2: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in as prepared PVMo11-SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.% Mo). Experimental
parameters were obtained from a refinement of H3[PMo12O40] model structure (ICSD 209 [14,93])
to the experimental Mo K edge XAFS χ(k) of PVMo11-SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.%
Mo) (k range from 3.0-13.7 Å-1
, R range from 0.9 to 4.0 Å, E0 = ~ 1.7, residuals ~13.3-18.0 Nind =
22, Nfree = 9). Subscript c indicates parameters that were correlated in the refinement.
Keggin model
PVMo11-SBA-
15 (10wt.%
Mo) Mo)
PVMo11-SBA-15
(5wt.% Mo)
PVMo11-SBA-15
(1wt.%Mo)
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 1 1.68 1.64 0.0035 1.65 0.0036 1.66
0.0004
Mo-O 2 1.91 1.78 0.0035c 1.78 0.0051c 1.82c 0.0030c
Mo-O 2 1.92 1.95 0.0035c 1.96 0.0051c 1.99c 0.0030c
Mo-O 1 2.43 2.40 0.0006 2.39 0.0010 2.38 0.0014
Mo-Mo 2 3.42 3.43 0.0057c 3.43 0.0063c 3.42 0.0066c
Mo-Mo 2 3.71 3.73 0.0057c 3.72 0.0063c 3.74 0.0066c
8.3 Structural evolution of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and
1 wt.% Mo) under catalytic conditions
PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.% Mo) samples were investigated by
in situ XAS under catalytic conditions. Fig. 8-3 shows the evolution of molybdenum
XANES spectra of PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo) samples during
temperature-programmed treatment in 5% propene and 5% oxygen. The resulting spectra
exhibited an increasing pre-edge peak with higher temperatures. The pre-edge peak
features in the Mo K edge XANES spectra can be employed to elucidate the local structure
around the Mo center. This increasing pre-edge peak correspond to structural changes from
octahedral [MoO6] species to partial tetrahedral [MoO4] species. The evolution of the ratio
of tetrahedral [MoO4] to octahedral [MoO6] units of PVMo11-SBA-15 (10 wt.% Mo, 5
124
wt.% Mo, and 1 wt.% Mo) based on a linear combination of bulk MoO3 and bulk
Na2MoO4 (cf. chapter 7.4) during propene oxidation conditions was shown in Fig. 8-4. In
contrast to P(V,W)xMo12-x-SBA-15 (x = 1, 2) (cf. chapter 5.2, 6.2) with identical Mo
loading and to PMo12-SBA-15 (10, 14, 19 nm) (cf. chapter 7.4) with tailored pore radii, the
onset temperatures the structural changes decreased with lower Mo loading. Structural
changes of PVMo11-SBA-15 (1 wt.% Mo) could be detected above 400 K and of PVMo11-
SBA-15 (5 wt.% Mo) above 425 K. Apparently, the Keggin ions in PVMo11-SBA-15 (10
Fig. 8-4: Quantification of the tetrahedral MoO4 ratio of ( )PVMo11-SBA-15 (10 wt.% Mo),
( )PVMo11-SBA-15 (5 wt.% Mo), and ( ) PMo11-SBA-15 (1 wt.% Mo) during thermal
treatment under propene oxidation conditions.
300 400 500 600 700
0
20
40
60
80
[MoO
4]/[M
oO
6] ra
tio
[%
]
T [K]
20 20.05
20.10 20.15
Photon energy [keV]
No
rma
lize
d
abso
rptio
n
T [K] 373 473
573 673
20 20.05
20.10 20.15
Photon energy [keV]
No
rma
lize
d
abso
rptio
n
T [K] 373 473
573 673
Fig. 8-3: in situ Mo K edge XANES spectra of (left) PVMo11-SBA-15 (5 wt.% Mo) and (right)
PVMo11-SBA-15 (1 wt.% Mo) during temperature-programmed treatment in 5% propene and 5%
oxygen in helium in a temperature range between 300 K and 723 K.
125
wt.% Mo) were stable on silica SBA-15 in the temperature range 303-448 K. Hence, the
stability of the Keggin ion increased with loadings from 1 wt. Mo % to 10 wt. Mo %.
Subsequently, concentration of tetrahedral [MoO4] units for all PVMo11-SBA-15 (10 wt.%
Mo, 5 wt.% Mo, and 1 wt.% Mo) considerably increased with higher temperature.
However, the temperatures of the structural evolution to mainly tetrahedral [MoO4] units
correlated to the dehydration and dehydroxylation process of SiO2 under oxidizing
conditions (20% O2 in He). Therefore, dehydroxylation was the driving force for the
structural rearrangement of the PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, and 1 wt.%
Mo) samples under catalytic conditions. This stability seemed to depend only on the nature
of the support material comparable to PMo12-SBA-15 (10, 14, 19 nm) (cf. chapter 7.4).
The structural rearrangement finished at ~ 550 K (PVMo11-SBA-15 (1 wt.% Mo)) and 598
K (PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo). Hence, higher metal loadings resulted in
increased temperatures, where the structural change were finished. Subsequently, the
concentration of tetrahedral [MoO4] units for PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo,
1 wt.% Mo) was constant and reached the highest concentration with ~75% [MoO4] units
at 723 K. PVMo11-SBA-15 (10 wt.% Mo) reach the highest concentration with ~50%
[MoO4] units at 723 K. Therefore, an increasing metal loading may lead to a decreased
amount of tetrahedral [MoO4] units.
Influence of metal loading to the resulting structure of [MoxOy] species
Fig. 8-5 shows the Mo K edge FT(χ(k)·k3) of activated PVMo11-SBA-15 (10 wt.% Mo, 5
wt.% Mo, 1 wt.% Mo) after thermal treatment under propene oxidation conditions. The
FT(χ(k)·k3) of act. PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo) exhibited
features similar to that of dehydrated molybdenum oxides and HPOM supported on SBA-
15.[27,59] For a more detailed structural analysis hexagonal MoO3 was chosen as model
structure. Theoretical XAFS phases and amplitudes were calculated for Mo-O and Mo-Mo
distances and used for EXAFS refinement. The results of the refinement are shown in Fig.
8-5. The first peak in the Mo K edge FT(χ(k)·k3) of act. activated PVMo11-SBA-15 (10
wt.% Mo) exhibited differences compared to that of act. activated PVMo11-SBA-15 (5
wt.% Mo, 1 wt.% Mo). The first peak in the FT(χ(k)·k3) originated mainly from the
tetrahedral [MoO4] species on the SBA-15 support and could be sufficiently simulated
126
using four Mo-O distances. These four distances sufficiently accounted for the minor
amount of octahedral [MoO6] species. The 1st and 2nd disorder parameters (1st-σ2, 2nd-σ
2)
were higher for act. PVMo11-SBA-15 (10 wt.% Mo) and indicated a decreasing amount of
tetrahedral [MoO4] units. Additionally, the 4th disorder parameter (4th-σ2) was smaller
than the disorder parameters for act. activated PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo)
with lower metal loading. This disorder parameter mainly represented the fraction of
octahedral [MoO6] species. Hence, the reduced disorder parameter indicated an increasing
amount of octahedral structural motifs in act. activated PVMo11-SBA-15 (10 wt.% Mo)
compared to act. PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo) and comparable to act.
PMo12-SBA-15 (14, 19 nm) (cf. chapter 7.3). The distinct peak at ~3 Å in the FT(χ(k)·k3)
indicated the formation of dimeric or oligomeric [MoOx] units on SBA-15 independent of
metal loading. The obtained Mo-Mo distances were comparable for act. PVMo11-SBA-15
(10 wt.% Mo, 5 wt.% Mo) samples and largely independent of the metal loading.
Additionaly, the disorder parameters of the Mo-Mo distances were lower for act. PVMo11-
SBA-15 (5 wt.% Mo) compared to act. PVMo11-SBA-15 (10 wt.% Mo) indicating a lower
oligomerization degree for the [MoxOy] species in act. PVMo11-SBA-15 (5 wt.% Mo). The
third Mo-Mo distance for act. PVMo11-SBA-15 (1 wt.% Mo) was increased compared to
act. PVMo12-SBA-15 (1wt.% Mo)
act. PVMo12-SBA-15 (10wt.% Mo)
act. PVMo12-SBA-15 (5wt.% Mo)
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
FT
(χ(k)·k
3)
R [Å] 0 1 2 3 4 5 6
Fig. 8-5: Theoretical (dotted) and experimental (solid) Mo K edge FT(χ(k)·k3) of activated PVMo11
supported on SBA-15 (10wt.% Mo, 5wt.% Mo, 1wt.% Mo).
127
Table 8-3: Type and number (N), and XAFS disorder parameters (σ2) of atoms at distance R from
the Mo atoms in act. PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo). Experimental
parameters were obtained from a refinement of a hexagonal MoO3 model structure (ICSD 75417
[135]) to the experimental Mo K edge XAFS χ(k) of act. PMo12-SBA-15 (10, 14, 19 nm) (k range
from 3.6-16.0 Å-1
, R range from 0.9 to 4.0 Å, E0 = ~ -5.2, residuals ~12 Nind = 26, Nfree = 12).
Subscript c indicates parameters that were correlated in the refinement.
hex-MoO3
model
act. PVMo11-SBA-
15 (10wt.% Mo)
act. PVMo11-SBA-
15 (5wt.% Mo)
act. PVMo11-SBA-
15 (1wt.%Mo)
N R(Å) R(Å) σ2(Å
2) R(Å) σ
2(Å
2) R(Å) σ
2(Å
2)
Mo-O 2 1.67 1.67 0.0012 1.67 0.0007 1.66 0.0007
Mo-O 2 1.96 1.89 0.0034c 1.87 0.0024c 1.86 0.0029c
Mo-O 1 2.20 2.18 0.0034c 2.18 0.0024c 2.17 0.0029c
Mo-O 1 2.38 2.36 0.0014 2.34 0.0024 2.35 0.0026
Mo-Mo 2 3.31 3.50 0.0066c 3.49 0.0049c 3.49 0.0051c
Mo-Mo 2 3.73 3.63 0.0066c 3.62 0.0049c 3.64 0.0051c
Mo-Mo 2 4.03 3.75 0.0100 3.75 0.0087 3.79 0.0118
act. PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo). This indicated a slightly different binding
state of Mo species on silica SBA-15 with lower metal loading. Apparently, the degree of
oligomerization of Mo of the [MoxOy] species reached a minimum on the samples with
lower metal loadings. This was comparable to PMo12-SBA-15 (14, 19 nm) samples with
large pores (cf. chapter 7.3). Additionally, the act. PVMo11-SBA-15 (1 wt.% Mo) showed a
slight different binding state compared to samples with higher metal loading. Therefore,
the Mo-O distances and disorder parameters for act. PVMo11-SBA-15 (5 wt.% Mo) and
act. PVMo11-SBA-15 (1 wt.% Mo) were nearly identical and different from those of act.
PVMo11-SBA-15 (10 wt.% Mo). The comparable Mo-O distances and disorder parameters
for act. PVMo11-SBA-15 (5 wt.% Mo) and act. PVMo11-SBA-15 (1 wt.% Mo) confirmed
the results of the quantification of tetrahedral [MoO4] and distorted [MoO6] units. The
quantification resulted in ~70% tetrahedral [MoO4] for both act. PVMo11-SBA-15 (5 wt.%
Mo) and act. PVMo11-SBA-15 (1 wt.% Mo) present under catalytic conditions. A
comparable quantification was found for molybdenum oxide supported on SBA-15 with a
Mo loading of 5.5wt.%.[59] The slightly increased Mo-Mo distances in act. PVMo11-SBA-
15 (1 wt.% Mo) may indicate a further decreased degree of oligomerization. Therefore, the
128
formation of dimeric or oligomeric [MoxOy] units mostly consisting of tetrahedral [MoO4]
units depended on metal loading on silica SBA-15. Hence, act. PVMo11-SBA-15 (10 wt.%
Mo) favored the formation of more extended structures on the support material due to
higher metal loading.
8.4 Functional characterization of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.%
Mo, and 1 wt.% Mo)
8.4.1 Reducibility
Fig. 8-6 shows the H2 TPR profiles of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.%
Mo). The resulted H2 TPR profiles of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo)
revealed one sharp reduction peak (~800 K) and a very broad signal after the sharp peak
comparable to PVxMo12-x-SBA-15 (x = 0, 1, 2) (cf. chapter 5.3.1). The H2 consumption for
PVMo11-SBA-15 (5 wt.% Mo) was due to lower metal loading compared to PVMo11-SBA-
15 (10 wt.% Mo). Additionally, the peak height of the sharp peak decreased for PVMo11-
SBA-15 (5 wt.% Mo) compared to the broaded signal above ~800 K. The H2 TPR profile
of PVMo11-SBA-15 (1 wt.% Mo) exhibited only a broad signal above 748 K. The H2 TPR
profiles of PVMo11-SBA-15 (10 wt.% Mo) were comparable to molybdenum oxides
supported on SBA-15 with Mo loadings between 9.5 wt.% and 13.3 wt.%.[149,150] Lou et
al. assigned the sharp reduction peak to oligomeric MoOx species or small MoOx clusters.
H2 TPR of PVMo11-SBA-15 (10 wt.% Mo) with the reduced sharp peak compared to the
broaded signal above ~800 K indicated a decreased degree of oligomerization comparable
to molybdenum oxide supported on SBA-15 with a Mo loading of 6.6 wt.%.[149] H2 TPR
profile of PVMo11-SBA-15 (1 wt.% Mo) was different from H2 TPR profiles of supported
molybdenum oxides with low Mo loadings (~2 wt.%). Typically, molybdenum oxides with
Mo loadings below 5 wt.% exhibited a shift of the reduction temperature to higher
temperatures (> 1000 K) indicating the reduction of predominantly monomeric MoOx
species.[149,150] Therefore, the broad signal above 748 K may corresponded dimeric or
oligomeric [(V,Mo)xOy] species. This species were lower oligomerized than the PVMo11-
SBA-15 (10 wt.% Mo, 5 wt.% Mo) samples. Apparently, mostly of the triads ([(V,Mo)xOy]
species) of the initial Keggin ion persisted during temperature programmed reduction.
129
Comparing the typical reduction temperatures of supported molybdenum oxides and
PVMo11-SBA-15 with different metal loading, a slightly higher degree of oligomerization
for PVMo11-SBA-15 with higher metal loading was determined.[150] Therefore, the
decreased height of the sharp peak compared to the broad signal above ~800 K PVMo11-
SBA-15 (5 wt.% Mo) indicated a decreased oligomeric [(V,Mo)xOy] species. Apparently,
the degree of oligomerization reached a minimum for PVMo11-SBA-15 (1 wt.% Mo),
indicated oligomerized [(V,Mo)xOy] species. In contrast to that, supported molybdenum
oxide synthesized from a AHM precursor lead to predominantly monomeric MoOx species
with Mo loadings below 2wt.%.[149,150]
8.4.2 Influence of the resulting structure on catalytic activity
Reaction rates and selectivities of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo)
in propene oxidation at 723 K are shown in Fig. 8-7. The reaction rates of PVMo11-SBA-
15 (5 wt.% Mo, 1 wt.%) were measured under various propene conversion conditions. The
propene conversion was varied to achieve a constant sample volume and to exclude
thermal effects. Therefore, PVMo11-SBA-15 (10 wt.% Mo) was mechanically diluted with
H2 c
on
su
mptio
n
T [K]
373 473 573 673 773 873 973
10 wt.% Mo
5 wt.% Mo 1 wt.% Mo
Fig. 8-6: Temperature programmed reduction (H2 TPR) of PVMo11-SBA-15 (10 wt.% Mo),
PVMo11-SBA-15 (5 wt.% Mo), and PVMo11-SBA-15 (1 wt.% Mo) measured at a heating rate of 8
Kmin-1
5% H2 in Ar.
130
SBA-15 to concentrations of 5 wt.% Mo and 1 wt.% Mo to achieve sample volumes
comparable to PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo).
The reaction rate of PVMo11-SBA-15 (10 wt.% Mo) (35.6 µmol/g(Mo)s) at
isoconversional conditions was slightly increased compared to that of PVMo11-SBA-15 (5
wt.% Mo) (34.1 µmol/g(Mo)s). The oxidation product distributions were comparable for
PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo) samples. The reaction rate for PVMo11-SBA-
15 (10 wt.% Mo) (29.9 µmol/g(Mo)s) at isoconversional conditions was also slightly
increased compared to that of PVMo11-SBA-15 (1 wt.% Mo) (28.8 µmol/g(Mo)s). The
product distribution of PVMo11-SBA-15 (10 wt.% Mo) was different from that of PVMo11-
SBA-15 (1 wt.% Mo). Selectivities towards acrolein and propionaldehyd increased and the
amount of total oxidation products decreased for PVMo11-SBA-15 (1 wt.% Mo) compared
to PVMo11-SBA-15 (10 wt.% Mo). Hence, an influence of the Mo loading on the catalytic
properties may be assumed. Thus, samples with lower Mo loading exhibited a decreased
catalytic activity comparable to PMo12-SBA-15 (10 nm, 14 nm, 20 nm) (cf. chapter 7.5.1).
This indicated a lower degree of oligomerization for samples with lower Mo loadings
0
20
40
60
80
100
a b c e d
acrylic acid acetic acid acrolein
acetone
acetaldehyd
e
CO CO2 propionaldehyd
e
Se
lectivity [
%]
rea
ctio
n r
ate
[µ
mo
l(pro
pe
ne
)g-1
(Mo
)s-1
]
Fig. 8-7: Reaction rate (µmol(propene)g-1
(Mo)s-1
) and selectivity at different propene conversions
of (a) PVMo11-SBA-15 (10 wt.% Mo), (b) PVMo11-SBA-15 (10 wt.% Mo), (c) PVMo11-SBA-15
(5 wt.% Mo), (d) PVMo11-SBA-15 (10 wt.% Mo), and (e) PVMo11-SBA-15 (10 wt.% Mo) in 5%
propene and 5% oxygen in He at 723 K.
0 5 10 15 20 25 30 35 40 45 50 55
14.1% 7.7% 8.3% 5.2% 3.7% propene conversion
131
confirming the results of the XAS analysis and TPR of act. PVMo11-SBA-15 (10 wt.% Mo,
5 wt.% Mo, 1 wt.% Mo). Nevertheless the catalytic activity in propene oxidation increased
with higher degree of oligomerization of the [(V,Mo)xOy] units under catalytic conditions.
Compared to PVMo11-SBA-15 (5 wt.% Mo) [(V,Mo)xOy] units orginating from PVMo11-
SBA-15 (10 wt.% Mo) resulted in an enhanced catalytic activity without significant
influence on the product distribution. The different product distribution of PVMo11-SBA-
15 (1 wt.% Mo) may result from slightly different binding states of the [(V,Mo)xOy] units.
The [(V,Mo)xOy] species resulting from PVMo11-SBA-15 (1 wt.% Mo) lead to an
increased selectivity towards acrolein and propionaldehyd. Grasseli et al. discussed the
role of site isolation i.e. the spatial separation of active sites on the surface of a
heterogenous catalyst.[170,172] A comparable effect could be responsible for the enhanced
selectivity towards partial oxidation products in act. PVMo11-SBA-15 (1 wt.% Mo).
Hence, may be assumed, that the [(V,Mo)xOy] species resulting in act. PVMo11-SBA-15 (1
wt.% Mo) improved the favorable number of active oxygens. Apparently, this active
oxygen species seemed to be bridged M-O-M (M = V, Mo) oxygen. Isolated [VOx] and
[MoOx] species would result mostly in total combustion (~ 90%). [137,153]
132
8.5 Summary
Structural evolution of H4[PVMo11O40] supported on SBA-15 (PVMo11-SBA-15)
with various Mo loadings (10 wt.% Mo, 5 wt.% Mo, 1 wt.% Mo) was examined by in situ
X-ray absorption spectroscopy investigations at the Mo K edge during propene oxidation
conditions. Supporting heteropolyoxo molybdates on SBA-15 with different Mo loadings
resulted in regular Keggin ions on the support material. During thermal treatment in
propene oxidation conditions the molybdenum oxide species on SBA-15 formed a mixture
of octahedral [MoO6] and mostly tetrahedral [MoO4] units. The ratio of the tetrahedral
[MoO4] to octahedral [MoO6] units increased with lower Mo loading. The onset
temperature of structural changes of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo, 1 wt.%
Mo) during thermal treatment in propene oxidation conditions increased with higher Mo
loading. A delayed desorption of physically adsorbed water in PVMo11-SBA-15 with
higher Mo loading was indicative of a varied stability of the Keggin ion. Hence, desorption
of physically adsorbed water and dehydroxylation of silanol groups of the support material
were the driving forces for the structural instability of the Keggin ion. The instability lead
to the formation of act.PVMo11-SBA-15. The resulting [(V,Mo)xOy] structures present
under catalysis conditions depended on Mo loading. A higher concentration of octahedral
[MoO6] units together with higher oligomerized [(V,Mo)xOy] species could be detected for
higher Mo loading. The higher oligomerized [(V,Mo)xOy] species present in act. PVMo11-
SBA-15 (10 wt.% Mo) showed a slight increased catalytic activity compared to lower
oligomerized [(V,Mo)xOy] species in act. PVMo11-SBA-15 (5 wt.% Mo, 1 wt.% Mo). The
selectivities towards oxidation products during propene oxidation conditions were
comparable and independent of the Mo loading (10 wt.% Mo, 5 wt.% Mo) indicating the
same active sites in act. PVMo11-SBA-15. The product distribution for PVMo11-SBA-15
(1 wt.% Mo) was different from those of PVMo11-SBA-15 (10 wt.% Mo, 5 wt.% Mo).
Selectivities towards acrolein and propionaldehyd increased and total oxidation products
decreased for PVMo11-SBA-15 (1 wt.% Mo) compared to PVMo11-SBA-15 (10 wt.% Mo,
5 wt.% Mo). The different product distribution of PVMo11-SBA-15 (1 wt.% Mo) may
result from a slightly different binding state of the [(V,Mo)xOy] species. This different
binding state of the [(V,Mo)xOy] species may lead to an increased selectivity towards
partial oxidation products.
133
9 General discussion and Summary
9.1 Structure directing effect of the support material
SBA-15 as support material had a significant influence on the structures of supported
species that formed during thermal treatment. Generally, the structures of model systems
such as MoOx-SBA-15, VOx-SBA-15, and WOx depend on their hydration
states.[17,59,95,144,149,173,174] SBA-15 or rather SiO2 adsorbed water at ambient
conditions. This water is removed at temperatures above 423 K.[168,175] In this work
HPOM were supported via incipient wetness with an aqueous solution on the support
material (cf. Chapter 4.1). Therefore, it may be assumed that the HPOM were incorporated
in a matrix of adsorbed water comparable to higher hydrates of bulk HPOM or to an
aqueous solution. The removal of adsorbed water and the following dehydroxylation of
silanol groups during thermal treatment lead to a destabilizing effect on the Keggin
ion.[145] This effect would be comparable to that of the removal of water of crystallization
and constitutional water in bulk HPOM. The removal of constitutional water in bulk
HPOM leads to a decomposition resulting in MoO3 during thermal treatment.[116] TG
measurement of PVMo11-SBA-15 showed a mass loss of about 2 wt.% between 473 and
673 K during oxidizing conditions indicating the dehydroxylation of silanol groups (c.f.
Chapter 8.2; Fig. 8-1). The destabilizing effect on the Keggin ion was independent of the
addenda atoms (V,W) in P(V,W)x-SBA-15 (x = 0, 1, 2), the pore radii of the SBA-15, and
HPOM loading.
The structures forming during thermal treatment in propene oxidation conditions depended
on the nature of SiO2, pore radii of the SBA-15, and HPOM loading. According to Wachs
et al. the structure of supported metal oxides is correlated to the net pH at the point of zero
charge (pzc) of the oxide support.[166] SiO2 has a pH of 3.9 at pzc. Therefore, the low pH
at pzc of SiO2 leads to mainly linked M-O-M (M = Mo, V, W) species corresponding to
the behaviour of molybdates, vanadates, and wolframates in acidic solutions.[37,148,176]
Compared to SBA-15, other support materials exhibit different structure directing effects
depending on the acidity of the surface.[69–71] For instance, mainly isolated [MoO4] and
[VO4] units existed on an alkaline MgO support in agreement with the behaviour of
molybdates and vanadates in alkaline solution.[137,153]
134
The pore radius also had a significant influence on the structures formed during thermal
treatment under propene oxidation conditions. Fig. 9-1 depicts a two dimensional
schematic representation of pores with different radii. Squares in the pores represent the
Keggin ions. The radius (r1) of the smaller pore is half of the pore radius of the second
pore (r2 = 2r1). Doubling of the pore radius lead to a doubling of the circumference of the
pore and the number of squares representing the Keggin ions. Nevertheless, the effective
distance D between the squares is clearly smaller in the smaller pore (D1) than in the larger
pore (D2). Therefore, thermal treatment under propene oxidation conditions lead to lower
oligomerized [MoOx] species and an increased [MoO4]/[MoO6] ratio at smaller pore radii
as shown in Chapter 7.
The HPOM loading has a further influence on the structures formed during thermal
treatment under propene oxidation conditions (Chapter 8). This effect was comparable to
the effect of SBA-15 with different pore radii. The surface coverage for samples with
lower HPOM loading was decreased resulting in less extended species on the support
material. Hence, the degree of oligomerization of the [MoOx] species was decreased and
the [MoO4]/[MoO6] ratio was increased at lower HPOM loading. This decreasing degree of
oligomerization at lower metal loading has been shown for various supported metal
oxides.[59,95,137,149,153,177,178]
r1 2r1 = r2
D1
D2
D1 < D2
Fig. 9-1: Two dimensional schematic representation of pores with squares representing the Keggin
ions.
135
9.2 Structure directing effects of the addenda atoms
Both bulk HPOM and supported HPOM exhibited a structure directing effect of addenda
atoms. The structures forming during thermal treatment under oxidizing and propene
oxidation conditions for bulk HPOM depended on the degree of substitution and type of
addenda atoms (V, W). Bulk HPOM decompose during thermal treatment under oxidizing
conditions resulting in MoO3. The loss of constitutional water depended also on the degree
of substitution and type of addenda atoms (V, W). Vanadium substitution lead to decreased
decomposition temperatures of 573 K for PV2Mo10, 623 K for PVMo11, and 673 K for
unsubstituted PMo12 K according to the literature.[35,179] Tungsten substitution had no
influence on the decomposition temperatures resulting in the loss of constitutional water.
Subsequently, the HPOM without constitutional water decomposed to MoO3. The resulting
modifications were α-MoO3 and β-MoO3. Vanadium substituted HPOM (PVxMo12-x x = 1,
2) favored the formation of α-MoO3 and tungsten substituted HPOM (PWxMo12-x (x = 1,
2)) favored the formation of β-MoO3 (c.f. Chapter 3.5). An explanation of the structure
directing effect are the different charges and ion radii of V5+
, W6+
, and Mo6+
resulting in
the edge-shared structure of α-MoO3 for (PVxMo12-x (x = 1, 2)) and corner-shared structure
of β-MoO3 for PWxMo12-x x = 1, 2 according to Pauling`s rules.[122]
During thermal treatment under propene oxidation conditions, bulk HPOM (PVxMo12-x x =
0, 1, 2) decomposed to various structures depending on the type of addenda atoms (V, W).
PMo12 decomposed to α-MoO3 as the thermodynamically stable modification of
molybdenum oxides in their highest oxidation state (+ 6). PV2Mo10 decomposed during
thermal treatment in propene oxidation conditions at 723 K into a mixture of various
structures. Ressler et al. performed in situ XAS measurements of PVMo11 during propene
oxidation conditions.[14] In this process Mo cations migrate on extra Keggin sites while
remaining coordinated to the resulting lacunary Keggin anion.[13] Driving force for the
formation of lacunary Keggin anions may be the relaxation of the Keggin structure at
elevated temperature upon removal of structural water. These structural changes at
temperatures above 573 K are accompanied by reduction of the molybdenum
centers.[13,14] Subsequently, the reduced Mo centers reoxidized upon 723 K.[13,14] In
another study on PV2Mo10 Ressler et al. described a dynamic behaviour by isothermally
switching from propene (reducing) to oxidizing (propene and oxygen) and back to propene
(reducing) conditions at 723 K. The results of the in situ XAS experiment revealed for the
136
reduced state the formation of a short vanadium-molybdenum distance of about 2.8 Å. The
oxidized state exhibited a longer distance of the vanadium center to an extra-Keggin
molybdenum center at 3.2 Å.[16] The results suggested a mixture of at least two sites
around two different V centers.[16] Therefore, it may be assumed, that the structures
formed during treatment of PV2Mo10 under propene oxidation conditions corresponded to a
mixture of various structures comparable to lacunary Keggin ions or Keggin ions.
PW2Mo10 decomposed during thermal treatment in propene oxidation conditions at 723 K
to a mixture of α-MoO3 and Mo17O47.[41,44] Molybdenum in Mo17O47 has an average
valence of ~ +5.5 which indicated, that the degree of reduction was higher for PW2Mo10
than for PMo12. Hence, tungsten lead to an increased reducibility of PW2Mo10 during
propene oxidation conditions.
The addenda atoms (V, W) in substituted HPOM supported on SBA-15 (P(V,W)xMo12-x-
SBA-15 (x = 0, 1, 2)) exhibited also a structure directing effect on the structures forming
during thermal treatment in propene oxidation conditions at 723 K. Vanadium lead to an
increasing [MoO4]/[MoO6] ratio in contrast to tungsten substituted HPOM supported on
SBA-15 (Fig. 9-2). The typical structure resulting for dehydrated molybdenum oxides
supported on SiO2 was a mixture of tetrahedral [MoO4] and octahedral [MoO6]
units.[27,59,149] Dehydrated vanadium oxides supported on SiO2 lead to the formation of
predominantly [VO4] units.[17,25,95] Therefore, it may be assumed, that during the
decomposition process, neighboring [MoO6] units were influenced by [VO6] units of the
20
40
60
80 PV2Mo10-SBA-15 PVMo11-SBA-15 PMo12-SBA-15 PWMo11-SBA-15 PW2Mo10-SBA-15
[MoO
4]/[M
oO
6] ra
tio
[%
]
Fig. 9-2: [MoO4]/[MoO6] ratio of P(V,W)xMo12-SBA-15 (x = 0, 1, 2) during thermal treatment
under propene oxidation condition (5% propene + 5% O2 in He) at 723 K.
137
initial Keggin ion structure. This influence lead to tetrahedral [MoO4] and [VO4] units
during thermal treatment under propene oxidation conditions (c.f. Chapter 5.2). Hence, the
formation of [MoO4] units depended on the degree of vanadium substitution.
Dehydrated tungsten oxides or H3[PW12O40] supported on SiO2 corresponded to a Si
containing Keggin type cluster with corner- and edge-shared [WO6] units on the support
material.[58,162,180] Therefore, additional [WO6] species in PWxMo12-x-SBA-15 (x = 1,
2) influenced the Mo species of the initial Keggin ion structure resulting in predominantly
[MoO6] and [WO6] units during thermal treatment under propene oxidation conditions (c.f.
Chapter 6.2). Both in vanadium and in tungsten substituted supported HPOM
(P(V,W)xMo12-x-SBA-15 (x = 1, 2)) the resulting [MOx] (M = V, W) units were in close
vicinity to the [MoOx] species.
9.3 Structure activity relationships
The different structures forming under catalytic conditions (5% propene + 5% oxygen in
He at 723 K) for bulk P(V,W)xMo12-x (x = 0, 1, 2) depended on the substituted element
(V, W). The different structures forming during catalytic conditions correlated with the
different catalytic behaviours of P(V,W)xMo12-x ( x = 0, 1, 2). Fig. 9-3 depicts the resulting
reaction rates and selectivities towards C3 oxidation products and CO, CO2. Both
vanadium substituted PVxMo12-x (x = 1, 2) and tungsten substituted PWMo12-x (x = 1, 2)
showed increased reaction rates compared to unsubstituted PMo12. The significant
differences between the samples were the resulting structures. α-MoO3 resulting from
PMo12 showed the lowest catalytic activity in propene oxidation. The lacunary Keggin and
Keggin ion resulting from PV2Mo10 and the mixture of α-MoO3 and Mo17O47 resulting
from PW2Mo10 showed an increased catalytic activity during propene oxidation. Therefore,
it may be assumed that the different catalytic activities of the various structures depended
on the type of addenda atom (V, W) and the degree of substitution. The resulting structures
for the substituted HPOM indicated also a lower average valence of molybdenum for the
lacunary Keggin and Keggin ion resulting from PV2Mo10 and the mixture of α-MoO3 and
Mo17O47 resulting from PW2Mo10 (c.f. Chapter 3.5.2).
138
Vanadium centers in substituted HPOM (PVxMo12-x (x = 1, 2)) can reversibly change their
oxidation state from V5+
to V4+
without significant destabilization of the lacunary Keggin
or intact Keggin ion.[15,16] Additionally, reduced V4+
has an ion radius of 72 pm which is
comparable to that of Mo6+
(74 pm). The similar ion radius of V4+
may stabilize partially
reduced intermediates.[103,181] Ressler et al. described in various XANES studies, that
the Mo centers in bulk HPOM reduced between 573 K and 723 K to an average valence of
~ 5.85 during propene oxidation conditions. Subsequently, the Mo centers reoxidized
above 723 K.[13–16] The ion radii of reduced Mo5+
centers (75 pm) and W5+
centers
(76 pm) were larger than the ion radius of V4+
(72 pm) [103,181]. In particular at elevated
temperatures the larger ion radii of reduced Mo5+
and W5+
centers destabilized the Keggin
ion structure resulting in α-MoO3 or the corresponding partially reduced Mo17O47.
Therefore, the reduction process during propene oxidation conditions above 573 K may
lead to a destabilization of the lacunary Keggin ions or Keggin ions. Hence, the increased
catalytic activity may result from the significantly different structures of PVxMo12-x
(x = 1, 2) in contrast to PMo12 (Fig. 9-3). The partial reduced Mo17O47 phase resulting from
PWxMo12-x (x = 1, 2) was stabilized by tungsten. Tungsten centers are able to occupy up to
30% of the sites of molybdenum atoms on Mo17O47.[182] Therefore, the formation of a
mixture of α-MoO3 and Mo17O47 exhibited more reduced Mo centers than
4
5
6
7
8
9
10
11
12
13
PV2Mo10
PVMo11
PMo12
PWMo11
PW2Mo10
reactio
n r
ate
µm
ol(p
rop
ene)g
-1(M
o)s
-1
20 25 30 35 40 45 50 55 60 65 70 75 80
Se
lectivity [%
]
CO+CO2
C3 oxidation products
Fig. 9-3: (left) Reaction rates (µmol(propene)/g(Mo)) and (right) selectivities towards C3 oxidation
products and CO+CO2 of bulk P(V,W)xMo12-x (x = 0, 1, 2) in 5% propene and 5% oxygen in He at
723 K.
PV2Mo10
PVMo11
PMo12
PWMo11
PW2Mo10
139
α-MoO3 resulting from unsubstituted PMo12. This partially reduced mixture of α-MoO3
and Mo17O47 may lead to the enhanced catalytic activity.
The different structures and average valences of Mo resulting from P(V,W)xMo12-x
(x = 0, 1, 2) may also lead to different selectivities (Fig. 9-3). Both structure and average
valence of the resulting compounds have an influence on the product distribution.[2,4,183]
Therefore, explaining the various selectivities was hindered by simultaneously varying
composition and structure. Reduced metal centers may be particularly effective in
activating gas-phase oxygen, resulting in an oversupply of surface-bond electrophilic
oxygen. Generally, oxidation reactions proceed in two steps. The first step is the reduction
of the catalyst with propene at the expense of lattice oxygen. The second step is the
reoxidation of the catalyst with gas phase oxygen.[4] The reoxidation process is described
by a series of reactions starting with adsorption of gas phase oxygen and eventually leading
to the formation of lattice oxygen. If the reoxidation process is slow, propene may be
attacked by nonselective oxygen species.[4] The reduced metal centers indicated, that the
reaction rate of the reduction process was higher compared to that of the oxidation process.
Hence, surface-bond electrophilic oxygen is prone to further oxidize propene or acrolein to
CO2.[4,16,184] Therefore, it may be assumed, that reduced metal centers were responsible
for the decreased selectivity towards C3 oxidation products and increased formation of
total oxidation products.
Fig. 9-4 (left) depicts reaction rates as a function of the [MoO4]/[MoO6] ratio resulting for
act. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) and PMo12-SBA-15 (14 nm, 19 nm) (coverage of
1 Keggin ion per 13 nm2) during propene oxidation (5% propene + 5% O2 in He; 723 K).
The reaction rates were determined for similar propene conversions. The reaction rate
decreased with higher [MoO4]/[MoO6] ratio independent of the addenda atoms (V,W) or
pore radii of SBA-15. Additionally, the [MoO4]/[MoO6] ratio was an indicator for the
degree of oligomerization. In all samples with higher [MoO4]/[MoO6] ratio, a decreased
degree of oligomerization was assumed. This result confirmed the assumption, that
selective oxidation takes place only in the presence of bridging M-O-M bonds.[4] The
increased degree of oligomerization leads to a higher concentration of Mo-O-Mo bonds
resulting in an increased reaction rate. The increased reaction rate resulted in increased
total combustion (Fig. 9-4). The samples with a higher [MoO4]/[MoO6] ratio and a
decreased degree of oligomerization exhibited an increased selectivity towards C3
140
oxidation products. Hence, reaction rates and selectivity towards the desired C3 oxidation
products competed with each other. Therefore, it may be assumed that the excess of
bridging M-O-M (M = Mo, V, W) lead to overoxidation according to the
literature.[2,4,170]
Addenda atoms had an additional influence on the reaction rates and selectivities for act.
P(V,W)xMo12-x-SBA-15 (x = 1, 2). It was shown for both act PVxMo12-x-SBA-15 (x = 1, 2)
and act. PWxMo12-x-SBA-15 (x = 1, 2), that the reaction rates and selectivities differed
from that of references synthesized with individual metal (Mo, V, W) precursors (c.f.
Chapter 5.3.2, 6.3.2). Structural analysis revealed that the resulting [MOx] (M = V, W)
species in act. (P(V,W)xMo12-x-SBA-15 (x = 1, 2)) were in close vicinity to the [MoOx]
species. Conversely, the [MOx] (M = V, W) species in act. V2Mo10Ox-SBA-15 and act.
W2Mo12Ox-SBA-15 were mostly separated from the [MoOx] species Apparently, the
neighboring [MOx] (M = V, W) units and [MoOx] units in act. (P(V,W)xMo12-x-SBA-15 (x
= 1, 2)) resulted in an increased formation of CO and CO2. Various mixed metal oxides
exhibited an increased formation of CO and CO2 with higher chemical complexity [185–
187]. Probably, the neighboring [MOx] (M = V, W) units and [MoOx] units were able to
activate gas-phase oxygen, resulting in an oversupply of surface-bond electrophilic
oxygen. This electrophilic oxygen is prone to further oxidize propene or acrolein to CO2
20
25
30
35
40
45
50
55
60
10 20 30 40 50 60 70 80 90
reactio
n r
ate
µm
ol(p
rop
ene)g
-1(M
o)s
-1
Se
lectivity [
%]
40
45
50
55
60
65
70
20 30 40 50 60 70 80
[MoO4]/[MoO6] ratio [%]
[MoO4]/[MoO6] ratio [%]
CO+CO2
C3 oxidation products
2V V
2W
W
2W
2W
W
W
2V
2V V
V
Fig. 9-4: (left) Reaction rates and (right) selectivities towards C3 oxidation products and CO+CO2
as a function of the [MoO4]/[MoO6] ratio of act. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2) in 5%
propene and 5% oxygen in He at 723 K. The type of addenda atoms (V,W) and degree of
substitution (x = 1, 2) are marked.
141
comparable to the effect of particularly reduced metal centers in bulk oxides.[8,118,121]
The references act. V2Mo10Ox-SBA-15 and act. W2Mo12Ox-SBA-15 with mostly separated
[MOx] (M = V, W) and [MoOx] species exhibited an increased selectivity towards C3
oxidation products and a decreased formation of CO and CO2. The neighboring [MOx]
(M = V, W) units and [MoOx] units in act. P(V,W)xMo12-x-SBA-15 (x = 1, 2) with higher
chemical complexity exhibit an increased formation of CO2 and CO according to the
literature.[185–187]
142
10 Conclusions
Introduction
Understanding structure-activity correlations of functional materials is an important issue
in catalysis and materials science. Often model systems are investigated. For elucidating
structure activity correlations of model systems for catalytic investigations, a detailed
knowledge about structure and chemical composition is indispensable. Thus, various
characterization methods are necessary for a sufficient characterization of the catalyst
systems. Heteropolyoxomolybdates (HPOM) of the Keggin type exhibit a broad
compositional range while maintaining their characteristic structural motifs. Therefore,
H3[PMo12O40] (PMo12), H4[PVMo11O40] (PVMo11), H5[PV2Mo10O40] (PV2Mo10),
H3[PWMo11O40] (PWMo11), and H3[PW2Mo10O40] (PW2Mo10) were synthesized as model
catalysts in selective propene oxidation. P(V,W)xMo12-x (x = 0, 1 ,2) were supported on
SBA-15. The initial Keggin structure of P(V,W)xMo12-x (x = 0, 1 ,2) was retained after the
supporting process. In situ XAS investigations under propene oxidation conditions of
P(V,W)xMo12-x-SBA-15 (x = 0, 1 ,2) were conducted. Catalytic testing elucidated the
functional properties of P(V,W)xMo12-x-SBA-15 (x = 0, 1 ,2) during propene oxidation
conditions. A detailed analysis of the structures formed under catalytic conditions was
conducted and correlated with the catalytic activity and product distribution towards
propene oxidation.
Synthesis of bulk HPOM and supported HPOM
The synthesis of P(V,W)xMo12-x (x = 0, 1, 2) lead to HPOM with the desired Keggin type
structure and chemical composition. P(V,W)xMo12-x (x = 0, 1, 2) crystallized as 13 hydrate
in a triclinic crystal system. The 13 hydrate structure of the HPOM ensured the
incorporation of the addenda atoms (V, W) in the Keggin ion. The volume of the unit cell
decreased with higher vanadium substitution because of the smaller ionic radius of V in a
six-fold coordination. W with an identical ionic radius compared to Mo had no influence
on the volume of the unit cell in contrast to the vanadium substitution. The results of IR
and Raman measurements confirmed, that the addenda atoms (V, W) were incorporated in
143
the Keggin ion. The IR and Raman spectra exhibited the typical peaks of the Keggin ion
structure. The EXAFS refinements indicated, that the addenda atoms (V, W) were
incorporated in the Keggin ion independent of the degree of substitution.
XRD and physisorption measurements of the tailored SBA-15 (10 to 19 nm) confirmed a
successful synthesis of mesoporous SiO2 materials with different pore size distributions,
high specific areas, and the typical pore structure of SBA-15. Supporting P(V,W)xMo12-x
(x = 0, 1, 2) on SBA-15 (10 to 20 nm pore radius) via incipient wetness lead to the desired
metal loadings (1 to 10 wt. Mo or rather 1 Keggin ion per 130 to 13 nm2). P(V,W)xMo12-x-
SBA-15 (x = 0, 1, 2) were sufficiently dispersed on the support material without affecting
the pore structure of the support material. The formation of extended crystalline HPOM
structures could be excluded.
Structure directing effect of SBA-15
SBA-15 as support material had a significant influence on the structures formed during
thermal treatment. SBA-15 or rather SiO2 adsorbed water at ambient conditions. This water
is removed at temperatures above 423 K. The removal of adsorbed water and the following
dehydroxylation of silanol group lead to a destabilizing effect on the Keggin ion.
Therefore, it may be assumed that the HPOM were incorporated in a matrix of physisorbed
water comparable to higher hydrates of bulk HPOM or to an aqueous solution. This effect
would be comparable to that of the removal of water of crystallization and constitutional
water in bulk HPOM. The removal of constitutional water in bulk HPOM results in
decomposition and formation of MoO3 during thermal treatment. The destabilizing effect
on the Keggin ion was independent of the addenda atoms (V,W) in P(V,W)x-SBA-15
(x = 0, 1, 2), the pore radii of the SBA-15, and HPOM loading. Hence, the stability of
Keggin ions supported on SBA-15 was significantly decreased compared to bulk HPOM,
with decomposition temperatures between 623 K and 713 K.
The resulting structures forming during thermal treatment in propene oxidation conditions
depended on the nature of SiO2, pore radii of the SBA-15, and HPOM loading. The
structure of supported metal oxides is correlated to the net pH at the point of zero charge
(pzc) of the oxide support. SiO2 have a pH of 3.9 at pzc. Therefore, the low pH at pzc of
SiO2 lead to mainly linked M-O-M (M = Mo, V, W) species corresponding to the
behaviour of molybdates, vanadates and wolframates in acidic solutions.
144
The pore radius had also a significant effect on the structures formed during thermal
treatment under propene oxidation conditions. A higher concentration of octahedral
[MoO6] units and higher oligomerized [MoxOy] units resulted for act. PMo12-SBA-15
(10 nm) compared to act. PMo12-SBA-15 (14, 19 nm). Enlarging the pore radii lead to an
increased effective distances between the Keggin ions. The effective distances were clearly
smaller in the smaller pores than in the larger pores. Therefore, the structures forming
during thermal treatment under propene oxidation conditions consisted of lower
oligomerized [MoOx] species and an increased [MoO4]/[MoO6] ratio with smaller pore
radius. Apparently, tailoring the pore radius of silica SBA-15 permitted to prepare Mo
oxide model systems to investigate correlations between activity and structure of
characteristic oxide species at similar loadings.
The HPOM loading had a further influence on the structures forming during thermal
treatment under propene oxidation conditions. The surface coverage for samples with
lower HPOM loading was decreased resulting in less extended species on the support
material. Hence, the degree of oligomerization of the [MoOx] species was decreased and
the [MoO4]/[MoO6] ratio was increased with lower HPOM loading. This effect was
comparable to the effect resulting for SBA-15 with different pore radii.
Structure directing effect of the addenda atoms
The addenda atoms exhibited structure directing effect in both bulk HPOM and supported
HPOM. The structures forming during thermal treatment under oxidizing and propene
oxidation conditions for bulk HPOM depended on the degree of substitution and type of
addenda atoms (V, W). Bulk HPOM decomposed during thermal treatment under
oxidizing conditions resulting in MoO3. The release of constitutional water of the HPOM
depended also on the degree of substitution and type of addenda atoms (V, W). Vanadium
substitution lead to decreased decomposition temperatures of 573 K for PV2Mo10, of 623 K
for PVMo11, and of 673 K for unsubstituted PMo12. Tungsten substitution had no influence
on the release of constitutional water of the HPOM. Subsequently, the HPOM without
constitutional water decomposed to MoO3. Vanadium substitution in HPOM (PVxMo12-x (x
= 1, 2)) lead to an increased formation of predominantly α-MoO3 depending on the degree
of vanadium substitution. Conversely, tungsten substituted PWxMo12-x (x = 1, 2) resulted in
an increased formation of β-MoO3 depending on the degree of tungsten substitution. The
145
different charges and ion radii of V5+
, W6+
, and Mo6+
were responsible for the structure
directing effect resulting in the rather edge-shared structure α-MoO3 for (PVxMo12-x (x = 1,
2)) and corner-shared structure β-MoO3 for PWxMo12-x x = 1, 2 according to Pauling`s
rules.
Bulk HPOM (PVxMo12-x x = 0, 1, 2) decomposed during thermal treatment in propene
oxidation conditions to various structures depending on the type of addenda atoms (V, W).
The Mo centers in bulk HPOM partially reduced between 573 K and 723 K to an average
valence of ~ 5.85 and reoxidized above 723 K during propene oxidation conditions. The
ion radii of reduced Mo5+
centers (75 pm) and W5+
centers (76 pm) were larger than the ion
radius of V4+
(72 pm). In particular at elevated temperatures the larger ion radii of reduced
Mo5+
centers and W5+
centers destabilized the Keggin ion structure. Therefore, PMo12
decomposed to the thermodynamically stable modification of molybdenum oxides, α-
MoO3. PW2Mo10 decomposed during thermal treatment in propene oxidation conditions to
a mixture of α-MoO3 and Mo17O47. In contrast to PWxMo12-x (x = 0, 1, 2), PV2Mo10
decomposed during thermal treatment in propene oxidation conditions at 723 K to a
mixture of various structures. The ion radius of reduced V4+
(72 pm) was comparable to
the ion radius of Mo6+
(74 pm) stabilizing partially reduced intermediates of the initial
Keggin ion structure. Vanadium substitution lead probably to a mixture of lacunary Keggin
ion and Keggin ions.
The addenda atoms (V, W) in substituted HPOM supported on SBA-15 (P(V,W)xMo12-x-
SBA-15 (x = 0, 1, 2)) exhibited also a structure directing effect on the structures forming
during thermal treatment in propene oxidation conditions at 723 K. Vanadium substituted
HPOM lead to an increasing [MoO4]/[MoO6] ratio whereas tungsten substituted HPOM to
a decreasing [MoO4]/[MoO6] ratio of the resulting [MOx] (M = Mo, V, W) species
supported on SBA-15. The [MoO6] units were influenced by the neighboring [VO6] units
and [WO6] units of the initial Keggin ion structure resulting in tetrahedral [MoO4] and
[VO4] units and octahedral [MoO6] and [WO6] units during thermal treatment under
propene oxidation conditions. Hence, the formation of [MoO4] or [MoO6] units depended
on the degree of vanadium or tungsten substitution. Both in vanadium and in tungsten
substituted supported HPOM (P(V,W)xMo12-x-SBA-15 (x = 1, 2)) the resulting [MOx] (M
= V, W) units were in close vicinity to the [MoOx] species.
146
Structure-activity correlations
The different structures forming under catalytic conditions (5% propene + 5% oxygen in
He at 723 K) for bulk P(V,W)xMo12-x (x = 0, 1, 2) depended on the addenda atoms (V, W).
Both vanadium substituted PVxMo12-x (x = 1, 2) and tungsten substituted PWMo12-x
(x = 1, 2) showed increased reaction rates compared to unsubstituted PMo12. α-MoO3
resulting from PMo12 showed the lowest catalytic activity in propene oxidation. The
mixture of lacunary Keggin ions and Keggin ions resulting from PV2Mo10 and the mixture
of α-MoO3 and Mo17O47 resulting from PW2Mo10 showed an increased catalytic activity
during propene oxidation. The resulting structures for the substituted HPOM indicated also
a lower average valence of molybdenum compared to unsubstituted HPOM. Hence, both
the structure and the average valence of the metal centers were responsible for the different
catalytic behaviour. The various structures and average valences of Mo resulting from
P(V,W)xMo12-x (x = 0, 1, 2) may also lead to various selectivities. Both PVxMo12-x (x = 1,
2) and PWxMo12-x (x = 1, 2) lead to an increased formation of total oxidation products and
a decreased selectivity towards C3 oxidation products depending on the degree of
substitution. The reduced metal centers in the mixture of lacunary Keggin ions and Keggin
ions resulting from PV2Mo10 and the mixture of α-MoO3 and Mo17O47 resulting from
PW2Mo10 may be particularly effective in activating gas-phase oxygen, resulting in an
oversupply of surface-bond electrophilic oxygen. This electrophilic oxygen is prone to
further oxidize propene or acrolein to CO2. Therefore, it may be assumed, that the reduced
average valence in the structures resulting from P(V,W)xMo12-x (x = 1, 2) was responsible
for the decreased selectivity towards C3 oxidation products and higher formation of total
oxidation products.
The various structures resulting for supported HPOM exhibited also an influence on the
catalytic activity. The reaction rates at similar propene conversion for supported HPOM
decreased with higher [MoO4]/[MoO6] ratio for act. P(V,W)xMo12-x-SBA-15 (x = 0, 1, 2).
Therefore, the concentration of [MoO4] and [MoO6] correlated with the catalytic activity.
Additionally, the [MoO4]/[MoO6] ratio was an indicator for the degree of oligomerization.
In samples with higher [MoO4]/[MoO6] ratios a decreased degree of oligomerization is
assumed. These results confirmed the assumption, that selective oxidation takes place only
in the presence of bridging M-O-M bonds. The increased degree of oligomerization lead to
147
a higher concentration of Mo-O-Mo bonds resulting in an increased catalytic activity.
However, the higher reaction rate resulted in an increased formation of total oxidation
products. Samples with an increased [MoO4]/[MoO6] ratio exhibited in an increased
selectivity towards C3 oxidation products. Hence, reaction rate and selectivity towards the
desired C3 oxidation products competed with each other.
The addenda atoms had an additional influence on the reaction rates and selectivities for
act. P(V,W)xMo12-x-SBA-15 (x = 1, 2). The reaction rates and selectivities of both act
PVxMo12-x-SBA-15 (x = 1, 2) and act. PWxMo12-x-SBA-15 (x = 1, 2) were different from
the reaction rates and selectivities of the references synthesized with individual metal (Mo,
V, W) precursors. Structural analysis revealed that the resulting [MOx] (M = V, W) species
in act. (P(V,W)xMo12-x-SBA-15 (x = 1, 2)) were in close vicinity to the [MoOx] species.
Conversely, the [MOx] (M = V, W) species in the references act. V2Mo10Ox-SBA-15 and
act. W2Mo12Ox-SBA-15 were mostly separated from the [MoOx] species Apparently, the
neighboring [MOx] (M = V, W) and [MoOx] species resulted in an increased formation of
CO and CO2. Various mixed metal oxides exhibited an increased formation of CO and CO2
with higher chemical complexity. Probably, the new multifunctional active site was able to
activate gas-phase oxygen, resulting in an oversupply of surface-bond electrophilic
oxygen. This electrophilic oxygen is prone to further oxidize propene or acrolein to CO2 .
The references act. V2Mo10Ox-SBA-15 and act. W2Mo12Ox-SBA-15 with mostly separated
[MOx] (M = V, W) and [MoOx] species exhibited an increased selectivity towards C3
oxidation products and a decreased formation of CO and CO2.
148
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12 Appendix
Table A 1: Wave numbers [cm-1
] of the meausured IR vibration bands for P(V,W)xMo12-x
(x = 0, 1, 2)(Chapter 3.4).
PMo12 PVMo11 PV2Mo10 PWMo11 PW2Mo10 Assignment
1064 (s) 1062 (s) 1060 (s) 1066 (s) 1068 (s) νas (P-O), νas (Mo-Ot)
(asymmetric coupling)
961 (vs) 960 (vs) 959 (vs) 964 (vs) 964 (vs) νas (P-O), νas (Mo-Ot)
(symmetric coupling)
869 (m) 865 (m) 862 (m) 870 (m) 874 (m) νas (Mo-Oe-Mo)
781 (vs) 779 (vs) 779 (vs) 784 (vs) 783 (vs) νas (Mo-Oe-Mo)
Table A 2: Wave numbers [cm-1
] of the meausured Raman vibration bands for P(V,W)xMo12-x
(x = 0, 1, 2) )(Chapter 3.4).
PMo12 PVMo11 PV2Mo10 PWMo11 PW2Mo10
999 (vs) 997 (vs) 1001 (vs) 996 (vs) 1001 (vs) νas (Mo-Ot)
972 (sh) 973 (sh) 974 (sh) 982 (sh) 982 (sh) νas (Mo-Ot)
909 (vw) 904 (vw) 904 (vw) 888 (vw) 888 (vw) νas (Mo-Oe-Mo)
611 (w) 609 (w) 617 (w) 608 (w) 608 (w) νas (Mo-Oc-Mo),
δ (Mo-Oc-Mo)
- 497 (vw) 507 (vw) 497 (vw) 498 (vw) δ (Mo-Oc-Mo)
- 460 (vw) 454 (vw) 460 (vw) 463 (vw) δ (Mo-Oc-Mo)
368 (vw) 370 (vw) 372 (vw) 370 (vw) 366 (vw) δ (Mo-Oe-Mo)
250 (m) 252 (m) 256 (m) 252 (m) 248 (m) δ (Oc-Mo-Oc),
δ (Oe-Mo-Oe')
227 (m) 224 (m) 224 (m) 224 (m) 231 (m) δ (Mo-Oe-Mo)
156 (m) 160 (m) 161 (m) 160 (m) 159 (m) δ (Mo-O-Mo),
δ (O-Mo-O)
111 (m) 112 (m) 113 (m) 112 (m) 108 (m) δ (Oc-Mo-Ot)
84 (w) 85 (w) 85 (w) 85 (w) 83 (w) δ (Mo-O-Mo),
δ (O-Mo-O)
165
Fig. A 2: XRD powder pattern of PMo12 after thermal treatment during catalytic conditions
:conditions (5% propene + 5% oxygen in He at 723 K) and diffraction peaks of bulk reference
material α-MoO3 (ICSD 76365 [129]).
10 20 30 40 50 60 70 80
Inte
nsity
Diffraction angle [2Ɵ]
α-MoO3
No
rma
lize
d io
n c
urr
ent
m/e = 18 (H2O)
temperature
0 1000 2000 3000 4000 5000 6000 0
100
200
300
400
tem
pera
ture
[K
]
Cycle
Fig. A 1: Ion current m/e = 18 corresponding to water and temperature steps of in situ XRD
meausurements of PMo12 during oxidation conditions (20% O2 in He; RT-723 K).
166
0.15
0.20
0.25
0.30
0.35
0.40
0h 12h
0h 12h
PW2Mo10 PV2Mo10
Ab
sorp
tion
PMo12
0h 12h
Fig. A 4: AAS absorption of phosphorus in PMo12, PVMo11, and PV2Mo10 before and after
treatment during propene oxidation conditions (5% propene + 5% O2 in He; 723 K; 0h and 12h
time on stream).
10 20 30 40 50 60 70 80
α-MoO3
Mo17O47
Fig. A 3: XRD powder pattern of PW2Mo10 after thermal treatment during catalytic conditions
:conditions (5% propene + 5% oxygen in He at 723 K) and diffraction peaks for bulk reference
materials α-MoO3 (ICSD 76365 [41]) and Mo17O47 (ICSD 36098 [44]).
Inte
nsity
Diffraction angle [2Ɵ]
167
5.45 5.50 5.55 5.60
0.0
0.5
1.0
1.5
2.0
2.5
No
rma
lize
d a
bsorp
tion
Photon energy [ke'V]
PV2Mo10-SBA-15
PV2Mo10
V2O5
VO2
Fig. A 5: V K edge XANES of PV2Mo10-SBA-15, PV2Mo10 and the references V2O5, VO2.
0 5 10 15 20 25 30 35 40
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Mo
avera
ge
va
lence
Mo K edge, (eV)
Mo oxide references
PMo12-SBA-15
PVMo11-SBA-15
PV2Mo10-SBA-15
PWMo11-SBA-15
PW2Mo10-SBA-15
Fit Curve
Mo
MoO2
Mo4O11
MoO3
Fig. A 6: Mo average valence of the Mo oxide references and P(V,W)xMo12-x-SBA-15 ( x = 0, 1, 2)
as a function of the Mo K edge position.
XII
Danksagung
Ich bedanke mich bei Herrn Prof. Dr. Thorsten Ressler für die interessante
wissenschaftliche Fragestellung. Insbesondere danke ich ihm auch für die exzellente
fachliche Betreuung während der gesamten Zeit meiner Forschungstätigkeit in seinem
Arbeitskreis. Ich danke außerdem Herrn Prof. Dr. Malte Behrens für die Anfertigung des
Zweitgutachtens.
Mein besonderer Dank gilt der gesamten Arbeitsgruppe Ressler für die angenehme und
freundschaftliche Arbeitsatmosphäre. Ich danke vor allem Gregor Koch, Alexander Müller,
Sven Kühn und Dr. Juliane Scholz für ihre stete Diskussionsbereitschaft. Bei Alexander
Hahn und Dr. Thomas Christoph Rödel bedanke ich mich für die wissenschaftliche und
technische Unterstützung bei der Durchführung zahlreicher Experimente. Ich danke auch
Semiha Schwarz für die technische Hilfestellung bei der Durchführung von TG-
Messungen und die zahlreichen Ratschläge innerhalb- und außerhalb des
Forschungsthemas. Besonders will ich mich an dieser Stelle bei Dr. Anke Walter
bedanken, die mich als Forschungspraktikant und Diplomand in zahlreiche Methoden der
analytischen Chemie eingeführt und mich in dieser Zeit für die Arbeit in der
instrumentellen Analytik und Katalyseforschung exzellent vorbereitet hat. Darüber hinaus
bedanke ich mich bei Lars Eggers, Mario Willoweit, Tina Somnitz und Larissa Braun, die
mich im Rahmen ihrer Bachelorarbeiten unterstützt haben.
Dem gesamten Arbeitskreis Lerch danke ich für die Aufnahmen der
Weitwinkelbeugungsdaten. Ich bedanke mich bei den Arbeitskreisen Grohmann und Lerch
für die freundliche Atmosphäre und tatkräftige Unterstützung im Syntheselabor.
Ich bedanke mich bei allen Mitgliedern des Instituts für Chemie der TU Berlin, die mich
bei meiner Arbeit unterstützt haben.
Dem DESY und dem HASYLAB in Hamburg sei für die Bereitstellung zahlreicher
Messzeiten gedankt. Bei der Deutschen Forschungsgemeinschaft (DFG) bedanke ich mich
für die finanzielle Unterstützung.
Ich danke meiner Frau Hanna und meiner kleinen Tochter Nell für die familäre Ablenkung
nebem dem Forschungsalltag und die uneingeschränkte Unterstützung und
Rücksichtnahme während der Anfertigung dieser Arbeit.