A significant problem with supported cobalt catalysts for Fischer-Tropsch synthesis is the upward deviation of methane from the ASF trend. One possibility is that the faster diffusion rate of H2 versus CO can influence the relative surface fugacities on the surface of Co metal particles. This, in turn, may lead to a higher termination rate. Therefore, catalysts are often designed with an eggshell configuration [E. Iglesia, Appl. Catal. A: General 161 (1997) 59-78]. In order to investigate this effect experimentally, gamma alumina was pre-filled with alumina by sequential impregnation of Al(NO3)3 and calcination. To maintain a relatively uniform size of cobalt particles, lower loadings were utilized with increased pore filling of the base gamma alumina support. Catalysts were tested using a CSTR slurry reactor at similar conversion levels - using both a high (50%) and a low (25%) conversion level - in order to make fair comparisons of product selectivity. Dispersions were comparable (5 - 6%) and % reduction ranged from 40 to 51%. With increased pore filling the light gas selectivities decreased (with methane decreasing by 2% and C2 - C4 selectivities decreasing between 3 and 5%, on an absolute basis). C5+ selectivities were found to increase by between 5 and 8% with pore modification. The results suggest that decreasing the pore length attenuates the effect of the higher relative diffusional rate of hydrogen versus CO. This helps to prevent the relative surface fugacities of hydrogen to carbon monoxide from being augmented on the cobalt active surface due to diffusion. Thus, the increased termination rate of having the higher hydrogen to CO ratio can be avoided. The modification by pore filling effectively makes the catalyst behave more like an egg-shell type catalyst.

Fischer-Tropsch synthesis: Improved C5+ selectivity with pore-modified alumina

251st National Meeting (2016)

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