Abstract
The topic of this thesis are zeolite based bifunctional catalysts, used for the conversion of hydrocarbons in the presence of hydrogen. The bifunctionality stems from the fact that metal nanoparticles on the surface of the catalyst provide the metal function for (de)hydrogenation, while the zeolite provides acid sites for isomerization
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and cracking. These catalysts are used for the production of liquid fuels and chemicals in large-scale industrial processes such as hydrocracking and hydroisomerization. The feedstock for these processes is mostly derived from fossil resources such as crude oil or Fischer-Tropsch products from natural gas. Bifunctional catalysts can also be applied for the conversion of more sustainable hydrocarbon feedstocks, such as biomass or waste streams. In some cases the feedstocks can be converted directly, or they need to undergo a sequence of processes involving gasification towards synthesis gas, a Fischer-Tropsch process and finally an hydrocracking process.
Bifunctional catalysts (based on zeolite Y), have already been studied for several decades and many fundamental studies on these catalysts have appeared too. Several studies have focused on the effect of the ratio between metal and acid sites, or the maximal distance between sites required for efficient transport of reaction intermediates between sites for optimal catalytic performance. The intimacy criterion for bifunctional catalysts, proposed by Paul B. Weisz in 1962, can be used to predict the maximal distance (often referred to as intimacy) between metal and acid sites when the reaction conditions, rate of reaction and intermediate diffusivity are known. Usually quantification of the criterium leads to distances within the micrometer length scale (1-1000 µm) and is often interpreted as ‘the closer the better’. However, a number of recent studies on zeolite-based bifunctional catalysts suggest that locating metal nanoparticles within a zeolite crystal, providing a very close intimacy between metal and acid sites, is detrimental for the catalytic performance.
The work in this thesis explores the effects of the proximity of metal and acid sites at the nanoscale for zeolite-based composite catalysts. It was proven that by using either of two different Pt-precursors for catalyst synthesis, Pt could be selectively deposited on the zeolite or on the γ Al2O3 component of a composite support, with limited heterogeneities within samples. A positive effect on the isomer selectivity for Pt nanoparticles located on the γ Al2O3 binder could be confirmed, and was extended to other zeolites and physical mixtures. For short-chain alkanes having relatively high diffusivities, the proximity between metal and acid sites was less important than for long-chain alkanes, but still relevant. Moreover, possible side-effects of residual chlorine from the Pt-precursor on the catalytic performance could be excluded. Overall, the results point to enhanced cracking for composite catalysts based on large pore zeolites and Pt nanoparticles located in zeolite crystals. The insights from the work in this thesis provide guides to further optimize current industrial catalysts, whereas also strategies and clues for future studies are provided.
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