Fischer carbide-methylene mechanism

A new mechanism to interpret alkene formation in Fischer-Tropsch synthesis has been presented 499-501 There is a general agreement that hydrocarbon formation proceeds according to the modified carbene mechanism. Specifically, CO decomposes to form surface carbide and then undergoes hydrogenation to form surface methine (=CH), methylene (=CH2), methyl and, finally, methane. Linear hydrocarbons are formed in a stepwise polymerization of methylene species. When chain growth is terminated by p-hydride elimination [Eq. (3.61)], 1-alkenes may be formed,502 which is also called the alkyl mechanism ... 

The carbide mechanism was originally proposed by Fischer and Tropsch as early as 1926 [20] and later by Craxford and Rideal [156]. The reaction was considered as a polymerization of methylene groups formed by surface carbide species. Considering the work of Pettit 1121. 125] and Sachtler (109-111 ]. the following scheme can be delineated ... 

In a recent study, R. Pettit et at. examined the validity of tire Fischer-Tropsch carbide mechanism, the Anderson-Emmett hydroxy carbene mechanism and the Pichlcr-Schulz mediaiiism [174. In a first experiment, the Schulz Flory distribution obtained by CO/H conversion over a cobalt catalyst in the absence and in the presence of CH N] was studied. It was found that addition of CHjN resulted in a signillcant increase of the propagation rate which is in favour of the assumption of methylene as a building block, as predicted by the carbide mechanism. Furthermore, the reaction was carried out using labeled CO (90% CO and 10% CO), H2. and CHjNj in variable ratios. The number of atoms in the propenc fraction was calculated according to the three... 

Pettit [44] has stressed the importance of bridge melhylene ligands un transition metal surfaces- Similarities in product formation with CO/H] and surest a common mechanism for both processes, with methylene possibly being formed by carbide hydrogenation in Fischer-Tropsch reactions. 

F. Fischer s hypothesis concerning the mechanism of the S3mthesis postulated the intermediary formation of cobalt or iron carbide and reduction of the carbide to methylene (CHj) groups. The latter would subsequently polymerize to yield unsaturated paraffin hydrocarbons. Although direct hydrogenation of preformed cobalt or iron carbide yields largely... 

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