Fischer-Tropsch synthesis: Cobalt catalysts on alumina having partially pre-filled pores exhibit higher C5+ and lower light gas selectivities

• Co/alumina catalysts were prepared with pores pre-filled with alumina.
• With increased pore filling, methane and C2C4 selectivities decreased and C5+ increased.
• Comparisons were made at two different conversion levels using a CSTR: ∼25% and ∼50%.
• Co loading was adjusted so that Co size and extent of reduction were similar.

Sequential impregnation and calcination were used to fill the pores of a commercial alumina support with alumina. In this manner, two different pore-modified alumina supports were synthesized. To account for the diminished pore size, cobalt loadings were lowered as appropriate to ensure a similar cobalt size range (e.g., clusters 14–18 nm in diameter) and comparable extents of reduction. This was confirmed by measurements from TPR, hydrogen chemisorption/pulse reoxidation, and EXAFS/XANES spectroscopies. Catalysts were tested using a slurry phase CSTR reactor at commercially relevant FTS conditions. Selectivities were compared at both high and low levels of conversion for the catalyst series. Decreasing the pore length, as measured by PSD data, appears to lessen the adverse effect that the higher relative diffusional rate of hydrogen versus carbon monoxide has on the H2/CO surface fugacity ratio on the catalyst surface. That is, moving the cobalt particles closer to the pore mouth may prevent the H2/CO ratio from being augmented at the surface of cobalt particles due to diffusion or other factors, and inhibits excessive chain termination that would lead to higher light product selectivities. Pore filling thus results in a catalyst that is similar to an egg-shell catalyst. With increased pore filling by alumina, C1C4 light gas selectivities decreased, in a systematic way, by as much as 33% (relative basis), and the C5+ selectivity improved by as much as 10.3% (relative basis) in comparison with the 22.4%Co/Al2O3 (IWI) reference catalyst. This greater effectiveness offers an opportunity for more environmentally benign chemical processing.

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