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Structure and Functional Properties of Heteropolyoxomolybdates Supported on Silica SBA-15

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Structure and Functional Properties of Heteropolyoxomolybdates Supported on Silica SBA-15 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. 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 VII 3.6 Summary.................................................................................................................. 41 4 Characterization of P(V,W)xMo12-x-SBA-15 (x
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