Converting a mixture of H and CO into aliphatic products is a multi-step reaction with several intermediate compounds. The growth of the hydrocarbon chain may be visualized as involving a repeated sequence in which hydrogen atoms are added to carbon and oxygen, the C–O bond is split and a new C–C bond is formed.
For one –CH– group produced by CO + 2 H → (CH) + HO, several reactions are necessary:
* Associative adsorption of CO
* Splitting of the C–O bond
* Dissociative adsorption of 2 H
* Transfer of 2 H to the oxygen to yield HO
* Desorption of HO
* Transfer of 2 H to the carbon to yield CH
The conversion of CO to alkanes involves hydrogenation of CO, the hydrogenolysis (cleavage with H) of C–O bonds, and the formation of C–C bonds. Such reactions are assumed to proceed via initial formation of surface-bound metal carbonyls. The CO ligand is speculated to undergo dissociation, possibly into oxide and carbide ligands. Other potential intermediates are various C fragments including formyl (CHO), hydroxycarbene (HCOH), hydroxymethyl (CHOH), methyl (CH), methylene (CH), methylidyne (CH), and hydroxymethylidyne (COH). Furthermore, and critical to the production of liquid fuels, are reactions that form C–C bonds, such as migratory insertion. Many related stoichiometric reactions have been simulated on discrete metal clusters, but homogeneous Fischer–Tropsch catalysts are of no commercial importance.
Addition of isotopically labelled alcohol to the feed stream results in incorporation of alcohols into product. This observation establishes the facility of C–O bond scission. Using C-labelled ethylene and propene over cobalt catalysts results in incorporation of these olefins into the growing chain. Chain growth reaction thus appears to involve both olefin insertion as well as CO-insertion.