Synthese, Charakterisierung und in situ Katalyse von Silika SBA-15 getragenen Molybdänoxidmodellkatalysatoren

This work describes the synthesis, the structural characterization and the catalytic testing of silica SBA-15 supported molybdenum oxide model catalysts. Additionally, a solvent induced washing effect during the SBA-15 synthesis and its scale up on the SBA-15 structure was studied. The MoxOy/SBA-15 catalysts were prepared with a Mo loading between 1.0 and 21.1 wt.% Mo (0.2 to 9.8 Mo/nm2) by a grafting/ion exchange procedure and incipient wetness impregnation. The catalyst structure was analyzed with Raman-, UV/Vis-, Photoelectron-, X-ray absorption- and IR spectroscopy. Furthermore, the catalyst structure was studied under reaction conditions in presence of propylene/oxygen and steam in an in situ Raman-MS setup, which was constructed in the course of this work. The study of washing SBA-15 with two solvents (water and ethanol) with nitrogen adsorption and XRD revealed that the surface area can be increased by 25% to 800 m2/g compared to washing with a single solvent. Moreover, it was found that huge solvent quantities for washing should be avoided, as they induce hydrolysis reactions, which alter the SBA-15 structure and lead to a widening and narrowing of certain mesopore sections. This decreases the surface area and pore volume of the support. The effect decreases with up-scaling of the synthesis. For the MoxOy/SBA-15 catalysts the molybdenum oxide was observed on the support in a dispersed state up to a loading of 12.1 wt.% Mo (3.5 Mo/nm2). Above this loading crystalline -MoO3 and as minority phase-MoO3 are formed. In this loading region the fraction of dispersed molybdenum oxide was almost constant at 12 to 13 wt.% Mo. Regarding the dispersed molybdenum oxide two structurally different states were observed. The hydrated state exists in the presence of moisture at room temperature, whereas the dehydrated state is obtained after treatment over 350°C in synthetic air. The molybdenum oxide structure in the hydrated state can be described as preferentially octahedral centres, which also exhibit connections and form di- or oligomers beside monomers. Comparing the XPS data of the dehydrated and the hydrated state showed that the molybdenum oxide was higher dispersed in the dehydrated state. UV/Vis spectroscopy supported this finding and showed that beside monomeric also di- or oligomeric species were present at Mo densities in the range of dispersed molybdenum oxide (≤3.5 Mo/nm2). Connections between neighbouring molybdenum oxide centres were detected by EXAFS and IR Spectroscopy using nitric oxide as probe molecule. Furthermore, a octahedral to tetrahedral ratio of 1 : 3 was found by a XANES fit. Therefore, the molybdenum oxide structure of the dehydrated state can be described as dispersed tetrahedral and octahedral molybdenum oxide centres, which coexist as monomers and connected di- or oligomers on the silica support surface. Moreover, the size distribution of the dispersed molybdenum oxide species in the hydrated and dehydrated state seems to be independent of the loading. MoxOy/SBA-15 was catalytically tested in the selective oxidation of propylene and propane. Whereas propylene was oxidized selectively to propionaldehyde, acetone and acrolein as major products, the oxidation of propane led unselectively to COx. In situ Raman investigations reveal that during the propylene oxidation or in the presence of steam, the structure of the catalyst largely resembles the structure of dehydrated dispersed molybdenum oxide.1. Introduction 1 1.1 Motivation 1 1.2 Molybdenum Oxides Systems in Partial Oxidation Reactions 2 1.3. Nanostructured Support Model Catalysts 3 1.3.1. Surface Structure of Amorphous Silica 5 1.3.2. Structure of Supported Molybdenum Oxide 7 1.4. Selective Oxidation of Propylene 11 2. Experimental 13 2.1. Raman Spectroscopy 13 2.2. In situ Raman-MS Setup 13 2.2.1. Introduction 13 2.2.2. Construction Details on the Setup 14 2.2.3. The in situ Raman Setup 17 2.2.4. The in situ Raman Cell 19 2.2.5. The Heating Block 19 2.2.6. The Saturator Unit 21 2.3. FTIR Spectroscopy 24 2.4. X-Ray Absorption Spectroscopy 24 2.5. NEXAFS 26 2.6. UV/Vis Spectroscopy 27 2.7. Nitrogen Adsorption 27 2.8. X-Ray Diffraction (XRD) 28 2.9. X-Ray Photoelectron Spectroscopy (XPS) 29 2.10. Catalysis Setup 29 3. Synthesis and Properties of SBA-15 31 3.1. Formation of SBA-15 31 3.2. Pore Structure of SBA-15 33 3.3. Influence of Washing and Scale Up on to the Pore Structure and Surface 36 Area of Silica SBA-15 3.3.1.Introduction 36 3.3.2. Synthesis of SBA-15 37 3.3.2.1. Synthesis 37 3.3.2.2. Details of the Washing Procedure 38 3.3.3. Template Removal by Washing with a Single Solvent 40 3.3.4. Influence of Combinational Washing with Ethanol and Water 43 3.3.5. Influence of Scale Up on the Washing Effect 47 3.3.6. Conclusions 53 4. Characterization of MoxOy/SBA-15 54 4.1. Introduction 54 4.2. Synthesis of MoxOy/SBA-15 54 4.3. Characterization of the MoxOy/SBA-15 Synthesis by Raman 55 Spectroscopy 4.4. TG-MS and DSC 57 4.5. Characterization of the Minority Phase 61 4.6. Distribution of Crystalline and Disperse Molybdenum Oxide 64 4.7. Conclusions 67 5. Catalysis of MoxOy/SBA-15 69 5.1. Propylene Oxidation 69 5.2. Conclusions 72 6. Detailed Structural Characterization of MoxOy/SBA-15 73 6.1. Introduction 73 6.2. Raman Characterization of MoxOy/SBA-15 73 6.3. UV/Vis Spectroscopy 79 6.4. Electronic Structure Characterization by XPS 83 6.5. Nitric Oxide Adsorption and Oxidation on SBA-15 Supported 87 Molybdenum Oxide Studied by Transmission IR Spectroscopy 6.5.1. Introduction 87 6.5.2. Results and Discussion 88 6.5.3. Conclusions 99 6.6. Structure of Molybdenum Oxide Supported on Silica SBA-15 103 Studied by X-Ray Absorption Spectroscopy 6.6.1. Introduction 103 6.6.2. Results and Discussion 104 6.6.3. Conclusions 113 6.7. Structure of Molybdenum Oxide Supported on Silica SBA-15 115 Studied by O K-Edge NEXAFS 6.7.1. Introduction 115 6.7.2. Results and Discussion 116 6.7.3. Conclusions 120 6.8. In situ Raman Spectroscopy during Propylene Oxidation and 121 in the Presence of Steam 6.8.1. Introduction 121 6.8.2. In situ Examination of MoxOy/SBA-15 121 6.8.3. The Influence of Steam 128 6.8.4. Conclusions 131 6.9. Summary and Discussion of Results 133 6.9.1. The Hydrated State of MoxOy/SBA-15 133 6.9.2. The Dehydrated State of MoxOy/SBA-15 136 7. Conclusions and Outlook 140 7.1. Conclusions 140 7.2. Outlook 145 8. Appendix 147 8.1. References 147 8.2. List of Figures 158 8.3. List of Tables 166 8.4. List of Schemes 167 8.5. Publications 167 8.6. Posters 168 8.7. Talks 16