Effect of catalyst layer macroporosity in high-thermal-conductivity monolithic Fischer-Tropsch catalysts

In-house-fabricated aluminum monoliths were washcoated with Co-Re/Al2O3 catalysts for Fischer-Tropsch synthesis using two alumina supports with different texture. On one hand, a commercial mesoporous alumina support was used as reference material. On the other hand, a meso-macroporous alumina was prepared in the laboratory. The macropores were expected to act as transport pores and facilitate diffusion through the catalytic phase volume. Despite showing different porosities, both catalysts presented similar cobalt dispersions and maintained the textural properties observed in the slurried samples once they were deposited onto the metallic monoliths. Being made of a high-thermal-conductivity substrate, the structured catalysts behaved like isothermal reactors with no significant radial thermal gradient being observed inside them during the reactions. Under isothermal conditions, the catalytic differences in Fischer-Tropsch synthesis tests carried out at the same conversion values were ascribed to only diffusional effects imparted by the influence of the macropores. Methane selectivity increased and C5+ selectivity decreased linearly with catalyst loading. Moreover, for the highest catalyst loading used, methane selectivity was higher and C5+ selectivity was lower for the catalyst with transport pores. However, when the results of catalysis studies were analyzed as a function of the average catalyst layer thickness; thus under equivalent diffusion lengths, the catalyst containing transport pores showed lower methane selectivity and higher C5+ selectivity than the reference catalyst over the entire range of layer thicknesses studied. Therefore, the effect of macropores was observed to reduce the diffusion limitations, although at high catalyst loadings the layer thickness determined the selectivity results more than the macroporosity of the layer.