On the selectivity to higher hydrocarbons in Co-based Fischer-Tropsch synthesis

Impact of process variables on selectivity to C5+ products during cobalt Fischer–Tropsch synthesis (FTS) are summarized and discussed, comprising temperature; pressure; synthesis gas composition; transport limitations of synthesis gas and products; conversion of CO; and effect of water. Further, effect of catalyst formulation, preparation and activation is included, specifically catalyst synthesis; cobalt crystallite size; crystal structure of cobalt; distribution of crystallites; support materials; pore structure; acidity; surface modifications; reduction and pretreatment. Other topics comprise promoters with focus on reduction promoters including rhenium and nickel; impurities like alkali and sulphur; deactivation and regeneration. Discussion is based on mechanistic understanding, specifically kinetics based on hydrogen assisted CO dissociation; a linear relationship between CH₄ and C5+ products; possible reinsertion of olefins; hydrogenation of olefins; and surface coverage based on SSITKA data. It is concluded that the majority of reported data on FTS can be rationalized in terms of a CHx pool mechanism on cobalt where x shifts toward monomers and high polymerization probability (α-value); or methane and light products formation, depending on synthesis conditions and catalyst properties.

Selectivity responses are illustrated mainly based on cobalt catalysts on transition alumina supports. How the support in itself affects C5+ selectivity is challenging. There clearly is a change in the composition of the cobalt surface pool of reaction intermediates, but how this change originates in support properties remains to be illuminated. So far there is no direct evidence for the termination process; β-hydrogen abstraction, to play a major role in determining FTS selectivity.