Fischer-Tropsch synthesis

Key point: Hydrogen and carbon monoxide can be converted to hydrocarbons and water by reaction over iron or cobalt catalysts.

The conversion of syngas, a mixture of H₂ and CO, to hydrocarbons over metal catalysts was first discovered by Franz Fischer and Hans Tropsch at the Kaiser Wilhelm Institute for Coal Research in Mullheim in 1923. In the Fischer—Tropsch reaction, CO reacts with H₂ to produce hydrocarbons, which can be written symbolically as the formation of the chain extender ( —CH₂— ), and water:

CO + 2H₂ --> -CH₂- + H₂0

The process is exothermic, with ∆_rH = —165 kJ mol^-1. The product range consists of aliphatic straight-chain hydrocarbons that include methane (CH₄) and ethane, LPG (C₃ to C₄), gasoline (C₅ to C₁₂), diesel (C₁₃ to C₂₂), and light and heavy waxes (C₂₃ to C₃₂ and >C₃₃, respectively). Side reactions include the formation of alcohols and other oxygenated products. The distribution of the products depends on the catalyst and the temperature, pressure, and residence time. Typical conditions for the Fischer—Tropsch synthesis are a temperature range of 200-350^oC and pressures of 15-40 atm.

There is general agreement that the first stages of the hydrocarbon synthesis involve the adsorption of CO on the metal, followed by its cleavage to give a surface carbide (and water), and the successive hydrogenation of such species to surface methyne (CH), methylene (CH₂), and methyl (CH₃) species, but there is still debate on what happens next and how chain growth occurs. One proposal suggests a polymerization of bridging surface —CH₂— groups initiated by a surface —CH₃ group. However, the fact that many such species have been isolated and are stable as metal complexes suggests that the mechanism is probably not so simple. Other mechanistic investigations of these processes have suggested an alternative possibility for chain growth in the hydrocarbon synthesis, namely that it proceeds by combination of surface bridging —CH₂— groups and alkenyl chains (M—CH=CHR), rather than the combination of alkyl chains (M—CH₂CH₂R) with methylene groups in the surface. Several catalysts have been used for the Fischer—Tropsch synthesis; the most important are based on Fe and Co. Cobalt catalysts have the advantage of a higher conversion to and a longer life (of over 5 years). The cobalt catalysts are in general more reactive for hydrogenation and produce fewer unsaturated hydrocarbons and alcohols than iron catalysts. Iron catalysts have a higher tolerance for sulfur, are cheaper, and produce more alkene products and alcohols. The lifetime of the iron catalysts is short, however, and in commercial installations generally limited to about 8 weeks.

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