A mechanistic study of the Fischer-Tropsch synthesis using transient isotopic tracing. Part 2: Model quantification

A quantitative mechanistic model for the low-pressure Fischer - Tropsch synthesis reaction on a Co/Ru/TiO2 catalyst is presented. Although the Fischer - Tropsch synthesis is operated at dry conditions, the presence of a physisorbed state is essential in the mechanism. The monolayer coverage of the C-3 to C-20 hydrocarbons in the physisorbed state is low at 0.3%. The most abundant chemisorbed surface species are COads and two single-C species, C(alpha,ad)s and C(beta,ad)s. The fractional surface coverage of growing hydrocarbon chains is low at 1.4%. With increasing H-2/CO feed ratio, the surface concentrations of H-ads and free sites increase. The rate coefficient for chain initiation is one order of magnitude lower than that for chain growth. The rate coefficient of ethene readsorption is one order of magnitude higher than that for the readsorption of higher 1-olefins. Chain branching and bond shift are important secondary reactions at atmospheric pressure, transforming reactive 1-olefins into unreactive internal and isoolefins and thus decreasing the asymptotic chain growth probability. As is to be expected, the termination to paraffin is represented by a hydrogenation reaction. The termination to olefin, however, appears to be a desorption reaction rather than a hydrogenation or a dehydrogenation reaction. The growing hydrocarbon chain is therefore represented by a CiH2i species. The quantitative mechanistic model as presented in this paper in combination with additional assumptions is used to successfully predict the characteristics of the product distribution of the high-pressure Fischer - Tropsch reaction.