Kinetics and mechanism of the fischer tropsch hydrocarbon synthesis on a cobalt on alumina catalyst

The kinetics of the Fischer Tropsch reaction have been studied on a cobalt on alumina catalyst. The kinetics can be explained satisfactorily by a model in which the initiation proceeds via carbon monoxide dissociation and formation of a CH₂-surface intermediate. The formation of this intermediate is the rate determining step. The chain growth is thought to proceed via the addition of the same CH₂ groups to the growing molecule. The model also explains the influence of the hydrogen and carbon monoxide partial pressures on the olefin/paraffin ratio as determined experimentally for the C₃ fraction, and on the methane selectivity. For high ratios of hydrogen to carbon monoxide, the methane selectivity is higher than predicted, while at the same time a corresponding deficit in the ethene yield is noticed. These two facts are tentatively ascribed to an ethene hydrocracking reaction. The experimental data show that the Schulz Flory (S.F.) "constant" α has a tendency to increase from C₃ to C₇. An analysis on the basis of our model shows that the S.F. constant is a complex function of the partial pressures of hydrogen and carbon monoxide.