On the support in cobalt Fischer–Tropsch synthesis—Emphasis on alumina and aluminates

Abstract

Impact of the support in Fischer–Tropsch synthesis is discussed with emphasis on alumina and aluminates; in particular γ-alumina, α-alumina, Mg-aluminate and Ni-aluminate. Topics covered are the support in industrial type catalysts, pore characteristics, attrition of particles, pore diffusion, loading and heat treatment of aluminates, turn-over-frequency and selectivity to C5+ products. Comparison is made between the supports including γ-alumina with narrow, medium, large and ultra-large pore size. An attempt is made to decouple the effects of cobalt crystallite size, pore size and type of support; all of which are important for the degree of polymerization. Selectivity to C5+ is favored by supports containing large pores, i.e., pores made up of large crystallites of the support material that also are more inert in nature. Cobalt surface coverage of CHx reaction intermediates is coupled directly to product selectivity as well as to catalytic activity. A model is suggested to link changes in composition of the surface pool of intermediates with properties of the support. Strain in cobalt crystallites as they extend over several crystallites of the support is expected to play a decisive role. Finally, the effect of conversion and added water on Fischer–Tropsch synthesis is analyzed. It is shown that all supports exhibit a positive selectivity response to water vapor pressure, but more so for the large pore supports titania, Ni-aluminate and γ-alumina with larger pores. Adsorbed water on cobalt may facilitate CO activation and suppress hydrogen coverage. Activity response of water is similar, although an anomalous reduction in activity upon added water for narrow pore alumina is observed. The reduction in activity is ascribed to oxidation of cobalt, possibly due to condensation of water vapor.