The correlation between catalyst surface basicity and hydrocarbon selectivity in the Fischer-Tropsch synthesis

Abstract

Carbon dioxide chemisorption studies at 0 °C were used to determine the relative basicities of alkali-promoted reduced magnetite catalysts. Catalysts promoted with oxides of Li, Na, K, Ca, and Ba were studied. For the equivalent amounts of alkali present the basicity of the reduced catalysts increased in the order Ba, Li, Ca, Na, and K. Data are given which show that a higher surface basicity correlates with a lower methane selectivity in the Fischer-Tropsch synthesis. Impregnating the catalyst with alkali after fusion rather than addition of the alkali at the fusion stage of preparation, results in catalysts with higher surface basicities and consequently lower methane selectivities. Silica depresses surface basicity and results in increased methane production. CO 2 appears to chemisorb on the oxide promoters present on the surface of the reduced catalyst and possibly also on the metallic iron surface. The BET surface areas of the catalysts decrease with increasing basicity of the alkali promoters.