Eric van Steen

Polymerisation kinetics of the Fischer-Tropsch CO hydrogenation using iron and cobalt based catalysts

Abstract Many equations describing the intrinsic rate of Fischer–Tropsch CO hydrogenation have been proposed in literature. Most of them are empirical, but a number of them have been based on a mechanism with a postulated rate determining step. For the derivation of these rate equations it is usually assumed that the rate of formation of the monomer is rate limiting, i.e. the slowest step in the product formation in the Fischer–Tropsch synthesis. This approach is fundamentally flawed since then methane would be the most abundant product. Furthermore, it does not take into account the polymerisation character of the Fischer–Tropsch synthesis and it cannot account for the observed product distributions. It will be shown that a different approach, which accounts for the polymerisation character of the Fischer–Tropsch synthesis and the irreversibility of this reaction, leads to a rate expression which is similar to the ones derived previously in many aspects. The theoretical implications of the rate determining step, however, no longer apply. The new rate equation proposed is based on the assumption that the rate of the reaction in the Fischer–Tropsch synthesis is governed by the rate of hydrogenation of surface carbon. The proposed rate equation will be used successfully to describe the kinetics of the consumption of carbon monoxide for the formation of organic compounds in the Fischer–Tropsch synthesis for both iron and cobalt based catalysts.

Regularities of selectivity as a key for discriminating FT-surface reactions and formation of the dynamic system

Primary product compositions from FT-CO-hydrogenation have been obtained. A kinetic model has been used to calculate rate constants, rates and probabilities of the elemental surface reactions as a function of carbon number from product composition data. Formation of the catalytic system in the initial stage of an experiment is characterized by selectivity changes as related to elemental surface reactions.