Support effects in cobalt-based fischer-tropsch catalysis

Abstract

A range of cobalt catalysts supported on kieselguhr, silica, alumina, bentonite, zeolite Y, mordenite and ZSM-5 was examined for catalytic activity and product selectivity in the Fischer-Tropsch reaction. These results were correlated with catalyst reducibility and adsorptive properties, as well as support acidity, surface area and structure. It was found that high surface area supports give high cobalt dispersions and tend to produce highly active Fischer-Tropsch catalysts, as long as the reducibility of the cobalt is not hindered by metal-support interactions or ion exchange, or that pore diffusion or blocking effects are not taking place. All supports produced catalysts with similar methane, carbon dioxide and higher hydrocarbon selectivities, with carbon dioxide selectivity being low in all cases. The nature of the higher hydrocarbons depended strongly upon support acidity, with the non-zeolitic, low acidity supports producing the classic straight chained Fischer-Tropsch product. Of the zeolite supported catalysts, the most strongly acidic ZSM-5 supported catalyst produced the most highly branched product, followed by the zeolite Y supported catalyst. The straight chained character of the product from the mordenite supported catalyst was explained by the inaccessibility of the primary Fischer-Tropsch products to the mordenite acid sites, due to the mordenite channel system and ion exchange properties, thereby preventing secondary reactions such as isomerisation from occurring.