SYNTHESIS AND CATALYTIC PROPERTIES OF EGGSHELL COBALT CATALYSTS FOR THE FISCHER-TROPSCH SYNTHESIS

CO diffusional restrictions decrease the rate and C-5(+) selectivity in large (1-3 mm) catalyst pellets required for Fischer-Tropsch synthesis in packed bed reactors. Eggshell catalysts, in which the active Co component is preferentially located near the outer pellet surface, decrease these transport restrictions and increase Fischer-Tropsch synthesis rates and C-5(+) selectivity. Maximum C-5(+) selectivities occur on catalysts with intermediate shell thickness, because these catalysts avoid intrapellet CO concentration gradients but still restrict the diffusive removal of reactive olefin products, which can readsorb and continue to grow to higher molecular weight hydrocarbons. Eggshell catalysts were prepared by a novel impregnation technique using molten cobalt nitrate. The eggshell thickness is controlled by the melt viscosity and by the contact time between the melt and the support pellet. These impregnation procedures and the slow reduction of the impregnated nitrate salts lead to relatively high cobalt dispersions (0.05-0.07) even at the high Co concentrations (40-50 wt%) present within the shell region. (C) 1995 Academic Press, Inc.