Abstract
We investigated the effect of pre-treatment conditions on the activity and selectivity of cobalt catalysts for Fischer–Tropsch synthesis (FTS) by varying both the reduction atmosphere and the reduction temperature. Catalysts supported on SiO2, Al2O3, and TiO2, prepared via incipient wetness impregnation, were evaluated, and activation temperatures in the range 250–350 °C were considered. Activation with syngas led to a better product selectivity (low CH4, high selectivity to liquid hydrocarbons, and low paraffin to olefin ratio (P/O)) than the catalysts reduced in H2 at lower activation temperatures. The CoxC species suppressed the hydrogenation reaction, and it is hypothesised that this resulted in the high selectivity of olefins observed for the syngas pre-treated catalysts. On the basis of the experimental results, we postulated that a synergistic effect between Co0 and CoxC promotes the production of the long chain hydrocarbons and suppresses the formation of CH4. In addition, for systems aimed at producing lower olefins, syngas activation is recommended, and for the FTS plants that focus on maximising the production of higher molecular weight products, H2 activation might be considered. These results provide insights for the future FTS catalyst design and for target product-driven operations.
Highlights
Fischer–Tropsch synthesis (FTS) is a structure-sensitive reaction that converts syngas derived from natural gas, coal, and biomass to valuable chemicals and synthetic fuels over a metal-based catalyst [1]
Borg [10] studied the dependency of the Co particle size on the Al2O3-support pore diameter and found that: (i) large Co particles were formed in the large pores and smaller ones formed in the narrow pores, (ii) the degree of reduction increased with the pore size, and (iii) the C5+ selectivity increased with the pore size
Our previous work over the SiO2 sample demonstrated the effect of Co–CoO bonding promoting the FT reaction, leading to a higher FT reaction rate at a lower reduction temperature (250 ◦C), when the CoO density is higher than the density observed at 350 ◦C [27]
Summary
Fischer–Tropsch synthesis (FTS) is a structure-sensitive reaction that converts syngas derived from natural gas, coal, and biomass to valuable chemicals and synthetic fuels over a metal-based catalyst [1]. Cobalt (Co) catalysts have attracted more attention in the recent years due to their high intrinsic hydrogenation activity, selectivity towards liquid hydrocarbons, and lower water gas shift (WGS) activity than iron and lower costs compared to noble metals [2,3]. The hydrogenation activity of the cobalt metal (Co), which is recognised as the active phase, is highly dependent on its structure. Borg [10] studied the dependency of the Co particle size on the Al2O3-support pore diameter and found that: (i) large Co particles were formed in the large pores and smaller ones formed in the narrow pores, (ii) the degree of reduction increased with the pore size, and (iii) the C5+ (the long chain hydrocarbons with carbon numbers equal or higher than 5) selectivity increased with the pore size
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