A potassium-promoted Mo carbide catalyst system for hydrocarbon synthesis

Abstract

Potassium-promoted Mo carbide catalysts prepared by temperature-programmed carburization with H₂–C₃H₈ have been evaluated for Fischer–Tropsch synthesis (FTS). Temperature-programmed carburization of MoO₃–Al₂O₃ with a mixture of H₂–C₃H₈ was a two-stage conversion involving the transformation of Mo oxide to the final carbide phase via an intermediate oxycarbide phase. Compensation effect and isokinetic relationship were observed for both oxycarbide and carbide phase formation suggesting that a similar topotactic mechanism was involved in the solid phase transformation. CO seemed to adsorb more strongly than H₂ as implicated by its higher heat of desorption and site concentration. Both acid and basic centres were detected on the Mo carbide surface. The K-promoter increased site concentration for both weak and strong basic sites. The relationship between FT activity and K loading paralleled the trend in CO₂ heat of desorption with K addition. FT activity attained a maximum at about 3%K loading. CO hydrogenation over the Mo carbide catalyst was most suitably described by an enolic intermediate mechanism. A generalized kinetic model was derived for predicting the reaction rate for individual hydrocarbons as a function of chain growth propagation and olefin-to-paraffin ratio.