The promoter effect of alkali in Fischer-Tropsch iron and cobalt catalysts

The promoter effect of alkali on Fischer-Tropsch iron catalysts causes an increased 1-alkene selectivity, a slightly increased reaction rate, an increased growth probability of hydrocarbon chains and also an increased resistance against oxidation of iron by the reaction product water. Experiments are presented which show that for cobalt catalysts alkali addition also leads to increased 1-alkene selectivity. However, the reaction rate is markedly reduced. The effect on the 1-alkene selectivity is without doubt due to increased adsorption strength of carbon monoxide causing an enhanced displacement of 1-alkenes while the propensity towards hydrogenation is hardly reduced. For iron catalysts the 1-alkene selectivity increases in the turn of Li, Na, K, Cs. With respect to the bimodal Anderson-Schulz-Flory (ASF) distribution the strong effect on the growth probability α₂ is independent of the nature of the alkali cation while the fraction f₂ of the distribution that is characterized by α₂ increases in the turn Li, Na, K, and Cs. These strong promoter effects are interpreted on the basis of a novel mechanism of Fischer-Tropsch synthesis whereby the alkali cation takes part in the catalytic circle. Finally an analogy of the promoter effect of alkali on iron for the ammonia synthesis and the Fischer-Tropsch synthesis is suggested

Further support for the two-mechanisms hypothesis of Fischer–Tropsch synthesis

The recently proposed hypothesis of two incompatible mechanisms of chain growth based on CH₂- and CO-insertion is furthermore supported by a detailed interpretation of the ¹³CH₂N₂ co-feeding experiments of Maitlis and co-workers and of the selectivity of branched hydrocarbons. The carbon number distribution is discussed considering in particular the role of readsorption and incorporation of 1-alkenes and its subsequent chain growth

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

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