Hydrocarbon selectivity in fischer-tropsch synthesis in relation to textural properties of supported cobalt catalysts

Abstract

Cobalt catalysts for the synthesis of paraffins from carbon monoxide and hydrogen have been prepared by low temperature thermal treatment of dicobalt octacarbonyl impregnated porous supports (alumina, silica-alumina). An catalysts exhibited high selectivities for restrained hydrocarbon cuts, provided there was a low metal loading. The chain length of hydrocarbons was clearly related to the mean pore diameter of the supports and the hydrocarbon cut was restrained by the design of the supports. Adsorption-desorption kinetics have shown that pore condensation and filling can occur at a chain length related to the pore size and thus modify the residence times of hydrocarbons on the catalyst. It is proposed that this physical effect of the support is the limiting factor of the chain length because of the subsequent hydrogenolysis of the condensed hydrocarbons inside the pores when light paraffins are not retained and can easily desorb. With high amounts of metal and conventional catalysts, the metal is mainly at the external surface of the support and this effect can not be seen.