Studies on the reaction mechanism of the Fischer–Tropsch synthesis on iron and cobalt

Abstract

A new mechanism of the Fischer–Tropsch synthesis is proposed based on the hypothesis that two incompatible mechanisms are involved resting exclusively on methylene and on carbon monoxide insertion, respectively. This hypothesis is reflected by the well known superposition of two Anderson–Schulz–Flory distributions. Experiments with co-feeding of ethene, 1-alkenes and diazomethane as a source of surface methylene and also the carbon number distribution of branched hydrocarbons strongly support the hypothesis of two independent mechanisms and the methylene insertion mechanism of one of them. Co-feeding of alcohols, the dependence of the ratio of the two mechanisms on the pressure of hydrogen and carbon monoxide and the promoter effect of alkali on iron catalysts also prove the hypothesis of the two mechanisms and point to the carbon monoxide insertion mechanism as the second mechanism that is characterized by the higher growth probability of the resulting Anderson–Schulz–Flory distribution. Furthermore new interpretations of the crucial steps of C–C linkage and chain termination are given. The insertion of methylene is interpreted by coupling of an alkylidene and a methylene surface species towards a coordinated olefin with the chance of chain growth termination by 1-alkene desorption. For the carbon monoxide insertion mechanism the termination of chain growth is assumed to occur by the formation of 1-alkenes and of alcohols via an alcoholate intermediate. The new mechanism gives without any exception a sound interpretation of a great variety of experiments and contributes also to the interpretation of the promoter effect of alkali and of the different performance of cobalt and iron catalysts.