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ispartof: Progress In Energy And Combustion Science vol:96 status: published

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 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.

The iron and steel industry is a carbon-intensive industry and one of the largest industrial sources of CO2 emissions. In this work, we show how the steel mill gases can be conditioned using three metal oxides to produce a CO/CO2 stream that can be used for the production of chemicals, thereby preventing the emission of carbon to the atmosphere as CO2. Abundant oxides of iron and manganese, characterised by their readiness to capture and release gaseous O2, and calcium oxide, characterised by its capacity to capture and release gaseous CO2 can be deployed in this process. Process analysis indicates that by fully utilising the chemical energy of the carbon-rich blast furnace gas (BFG) of the steel mill, 37% of the associated CO2 emissions can be eliminated. A techno-economic evaluation shows that further reduction of CO2 emissions is viable. Preliminary estimations indicate that the cost for processing BFG through the proposed process is 46 EUR2020 per tonneBFG. The sources of revenue are the product CO/CO2 stream (0.2 tonneproduct per tonneBFG) and electricity constituting 85% and 14% of the total revenue with the remaining 1% obtained by the sale of spent metal oxides used in the process. The technical feasibility of the process was experimentally proven in a fixed bed reactor to produce a CO/CO2 stream and an H2O-rich stream while the metal oxides were periodically regenerated in alternating redox conditions. Thirty executed cycles indicated stable performance of the process. The proposed process concept can be applied to any gas stream containing CO2 and fuel. Catalytic fast pyrolysis of polyolefinic waste streams was investigated to recover valuable base chemicals at high selectivity. HZSM-5 zeolite with different properties, affected by Si/Al, mesoporosity, phosphorus stabilization, and steaming, were tested and thoroughly characterized. Different feeds, catalyst/feed ratios and reaction temperatures were evaluated in a micropyrolysis reactor coupled to two-dimensional gas chromatography. While unmodified HZSM-5 rapidly deactivated, phosphorus-modified and steamtreated HZSM-5 showed almost no deactivation due to its lower coking propensity during 130 runs with stable conversion towards C5+ aliphatics and high C-2-C-4 olefins selectivity (-75%) using post-consumer mixed polyolefins. The performance of this direct olefins production route with unprecedented high olefin selectivity was further evaluated in a plantwide context. It was found that it requires-37% lower energy input than the plastics pyrolysis followed by pyrolytic oil steam cracking, while it results to at least a one order of magnitude lower environmental burden as compared to waste incineration.

In previous studies H2PtCl6⋅6H2O or Pt(NH3)4(NO3)2 were used to control the location of Pt nanoparticles in Zeolite/γ-Al2O3 composites for the synthesis of metal-acid bifunctional catalysts. Use of H2PtCl6⋅6H2O led to Pt nanoparticles located on the γ-Al2O3 binder, in a ‘nanoscale’ intimacy with zeolite acid sites being beneficial for performance in the conversion of hydrocarbons, compared to catalysts with Pt nanoparticles located in zeolite Y (‘closest’ metal-acid intimacy) obtained by use of Pt(NH3)4(NO3)2. A side effect of using H2PtCl6∙6H2O was a higher NH3 uptake in NH3-TPD experiments, that suggested a higher acidity possibly affecting catalysis. In this study the effect of residual chlorine on Pt/Zeolite Y/γ-Al2O3 catalysts (0.1-0.6 wt.% Cl) was investigated by (pyridine) FTIR, NH3-TPD and n-alkane hydro-isomerization. The coordination of chlorine to γ-Al2O3 was observed and led to a higher NH3 uptake, but no effect of chlorine on the catalytic performance of the catalysts was detected.