Control of Metal Dispersion and Structure by Changes in the Solid-State Chemistry of Supported Cobalt Fischer–Tropsch Catalysts

Controlling preparation variables in supported cobalt Fischer–Tropsch catalysts has a dramatic effect on the dispersion and distribution of cobalt, and determines how active and selective the resulting catalyst will be. We detail specific examples of catalyst synthesis strategies for modifying interactions between the support and the cobalt precursor, promoting reduction, stabilizing catalysts to high-temperature treatments, minimizing deleterious support metal interactions, and controlling the distribution of cobalt on large support particles. It is important to optimize the support and precursor interaction strength, so that it is strong enough to obtain good dispersion but not too strong to prevent low temperature reduction. We show examples in which formation of surface complexes and epitaxial matching of precursor and support structures improves dispersion dramatically. Reduction promoters can help in those cases where support–precursor interactions are too strong. We show how substitutions of silicon into a titania lattice stabilizes surface area and retards formation at high oxidation temperatures of cobalt ternary oxides that reduce only at very high temperatures—an important consideration if oxidative coke removal is necessary. In addition, surface treatment of TiO₂ with an irreducible oxide like ZrO₂ can inhibit deleterious support interactions that can block surface cobalt sites. Selectivity can also be dramatically altered by catalyst synthesis. We illustrate a case of large (2 mm) SiO₂ particles onto which cobalt can be added either uniformly or in discrete eggshells, with the eggshell catalysts having substantially higher C₅₊ selectivity. These approaches can lead to optimal Fischer–Tropsch catalysts with high activity and C₅₊ selectivity, good physical integrity, and a long life.