Organometallic chemistry and surface science: mechanistic models for the Fischer-Tropsch synthesis

The Fischer-Tropsch synthesis is an industrially important process for the conversion of synthesis gas (CO/H-2) into hydrocarbons and oxygenates. Synthesis gas can be obtained from coal or natural gas. Organometallic model complexes and surface science techniques have been widely used to obtain mechanistic information about this heterogeneous process. A review of the mechanisms for the Fischer-Tropsch synthesis and the evidence for these mechanisms is presented. It is generally accepted that the Fischer-Tropsch reaction may be viewed as a polymerisation of surface methylene (=CH2) species, which are formed by the dissociation and hydrogenation of CO on the catalyst surface. The alkyl mechanism proposes that the reaction is initiated by the formation of a surface methyl species, and that chain growth takes place by successive insertions of methylene into the metal-alkyl bond. The alkenyl mechanism proposes that the formation of a surface vinyl species (-CH=CH2) initiates chain formation, and that chain growth is facilitated by methylene insertion into the metal-vinyl bond to form an allyl species (-CH,-CH=CH2). The allyl species isomerises to form a surface alkenyl (-CH=CH-CH3) which may propagate further. These mechanisms are discussed. (C) 2000 Elsevier Science S.A. All rights reserved.