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Designing novel amorphous catalysts for the propane dehydrogenation reaction

Bere, Takudzwa 2022. Designing novel amorphous catalysts for the propane dehydrogenation reaction. PhD Thesis, Cardiff University.
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Global demand for propene, a major platform chemical with a myriad of uses in the manufacturing and chemical industry, is anticipated to continue to grow annually. The expected growth in propene demand cannot be met by existing processes, therefore direct or on-purpose processes are being developed to fill the so-called ‘propene gap’. Many of the technologies employed for the commercial dehydrogenation of propane operate using various Pt-based catalysts. This work addresses the rationale catalyst design of the support material and the supported metal catalyst with the aim of unlocking new catalyst design strategies. Hence, the investigations into catalyst design based on amorphous/disordered materials with an anticipated higher density of active sites for acid-catalysed catalytic reactions, are of great importance. This work addresses the systematic design of a new supercritical antisolvent-mediated (SAS) route to amorphous silica-alumina materials exploring the effect of solvent composition, process temperature, process pressure, calcination conditions, and choice of metal precursors. A route to a series of optimized and systematically varied amorphous silica-alumina was realised and the experimental approach was backed up by detailed advanced characterization. The synthesis strategy has not been previously reported in literature and preliminary investigations revealed bulk (microstructural) and local (nanoscale) structural similarities to analogous state-of-the-art, flame-spray pyrolysis (FSP) synthesized materials. Subsequent work focussed on finding and applying a reproducible method to deposit platinum nanoparticles with small particle size and size distribution onto the support. Catalytic evaluation was carried out on two acid-catalysed reactions, the propane dehydrogenation reaction and catalytic dehydration of methanol-to-DME (MTD). A combination of XRD, TGA, NH3-TPD, Pyridine-DRIFTS, (heteronuclear 1D MAS and 2D 27Al MQMAS) Solid-state NMR, XPS, SEM/EDX, FTIR/ATR, HRTEM, and SAED helped establish structure-performance relations. Through careful catalyst design, Pt/SAS-4 and Pt/FSP-4 catalysts with moderate surface acidity displayed the highest catalyst activity, productivity, and stability. The results found the catalyst performance to be comparable to and/or superior to analogous Pt-based catalysts supported on crystalline supports and reported in literature. Similar activity correlations were realised in the methanol-to-dimethyl ether reactions, and the key active component of the catalyst was surface acidity namely the nature, concentration, density, and balance of acid sites. A combination of several factors including aluminium speciation, morphology and surface acidity of the support explained the variations in catalytic activity. In the supported metal catalyst morphology and surface acidity played an active role in the redox properties of the support and its interaction with supported metal particles. The high propene yield, propene productivity and stability of Pt supported on supercritical antisolvent precipitation and flame spray pyrolysis synthesized SiO2-Al2O3 was attributed to a high proportion of coexistent AlIV - and AlV -based Brønsted acid sites within the support. The former was responsible for propane activation and the latter for anchoring and stabilising deposited nanoparticles; a key observation over a 16-hour non-oxidative propane dehydrogenation reaction. Therefore, the presence of a high proportion of AlV polyhedral, increased elemental homogeneity and high density of homotopic acid sites was used to rationalise a lot of the fundamental findings and relationships observed during this work. From the combined experimental, characterization and catalytic study, moderate aluminium content (Si/Al) and thus surface acidity was pertinent to enhanced catalyst activity, selectivity and stability in acid-catalysed reactions. The implications of this work in improving the understanding of novel, robust catalyst design and subsequent catalytic applications in the field of acid-catalyzed reactions has been explored.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 31 May 2022
Last Modified: 01 Jun 2022 09:13

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