Towards a chemical understanding of the Fischer-Tropsch reaction: alkene formation

A new understanding of the Fischer-Tropsch reaction to alkenes on a chemical basis is developed. This uses the ideas of organometallic chemistry and homogeneous catalysis derived from model systems, combined with the results of experiments using (CHCH)-C-13-C-13-X (X = H, Br, etc.) compounds as probes to investigate the heterogeneously catalysed hydrogenation of CO over metal catalysts. The labelled probe experiments on the hydrogenation of carbon monoxide, to alkenes and alkanes over heterogeneous catalysts under Fischer-Tropsch (FT) conditions, have confirmed that the key species in the reactions are surface methylenes (formed from adsorbed CO) which are polymerised. However, new evidence favours coupling of surface methylenes with alkenyl intermediates rather than with the alkyl intermediates which were previously proposed. Mass-spectroscopic investigations of the hydrocarbon products show the incorporation of two C-13 atoms when C-13(2)-vinyl probes are added over rhodium, ruthenium or cobalt catalysts; for Ru and Co the hydrocarbons also contain significant amounts of C-13(1) arising from cleavage of the vinylic probe. iron is a poorer catalyst under these conditions but resembles rhodium. The involvement of vinyl/alkenyl intermediates has led to the elaboration of a new alkenyl cycle for the FT reaction, based on the principles of organo-transition metal reactions developed during studies of the coupling of organic ligands on model complexes. The quantitative data from the probe reactions can conveniently be mathematically modeled using a simple steady state approach; good agreement between experiment and theory is found. Oxygenates such as ethanol are also formed in the Fischer-Tropsch reactions, especially over Rh, but their origin is not yet clear. (C) 1999 Elsevier Science B.V. All rights reserved.