Fischer carbide mechanism

The mechanisms proposed over the last 50 years for the Fischer-Tropsch synthesis, principally on the basis of studies using heterogeneous catalyst systems, may be divided into three main classes (a) metal-carbide mechanisms (b) hydroxyl carbene, =CH(OH), condensation mechanisms and (c) CO insertion mechanisms. 

The question of the mechanism of Fischer-Tropsch reaction is of considerable controversy. Three principal routes for product formation have been proposed the carbide mechanism, the hydroxymethylene mechanism, and the CO insertion mechanism. Numerous modifications were also introduced in attempts to account for some details in the complex chemistry of the process.205 207 208 211 229-233... 

An important subsequent observation seemed to indicate that carbides are not reactive under Fischer-Tropsch conditions.235 When carbon was deposited on a surface by the decomposition of l4CO, labeled carbon was not incorporated into the products. This and other evidence accumulated against the carbide mechanism by the 1950s led to the formulation of other mechanisms. The hydroxymethylene or enolic mechanism191 assumes the formation via the hydrogenation of carbon monoxide [Eq. (3.13)] of a surface-bound hydroxymethylene species (2) ... 

The revival of interest in Fischer-Tropsch chemistry in the 1970s resulted in new observations that eventually led to the formulation of a modified carbide mechanism, the most widely accepted mechanism at present.202-204,206,214 Most experimental evidence indicates that carbon-carbon bonds are formed through the interaction of oxygen-free, hydrogen-deficient carbon species.206 Ample evidence shows that carbon monoxide undergoes dissociative adsorption on certain metals to form carbon and adsorbed oxygen ... 

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... 

The first one is the carbide mechanism (Fig. 10), which was initially advanced by Fischer and Tropsch in 1925 and later further developed by Sachtler, Biloen and others. Therefore it is also known as Sachtler-Biloen mechanism. The main steps of this mechanism involve CO cleavage to a surface carbide (CHj as the monomer for chain propagation, hydrogenation (eventually to methane), and the formation and coupling of surface hydrocarbyl (C Hj species, from which 1-alkenes are produced by p-elimi nation. 

The mechanism of the Fischer-Tropsch reactions has been the object of much study (note Eqs. XVI11-55-XV111-57) and the subject of much controversy. Fischer and Tropsch proposed one whose essential feature was that of a metal carbide—patents have been issued on this basis. It is currently believed that a particular form of active adsorbed carbon atoms is involved, which is then methanated through a series of steps such as... 

Many attempts have been made to elucidate the mechanism(s) of this reaction. The original proposal by Fischer and Tropsch is based on carbide intermediates [2]. 

Subsequent studies have failed to support the carbide theory, and it is now generally accepted that carbides of the type proposed by Craxford play little or no part in the Fischer-Tropsch synthesis (86, 87). It has, however, recently been suggested, by analogy with the mechanism proposed for the Haber synthesis of ammonia, that carbides formed by dissociative absorption of carbon monoxide would be expected to be readily hydrogenated and could therefore be of importance in Fischer-Tropsch synthesis over heterogeneous catalyst (88). 

With the recent development of zeolite catalysts that can efficiently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol. Two possible mechanisms for methanol formation are suggested by literature discussions of Fischer-Tropsch catalysis (9-10). These are shown in Schemes 1 and 2. 

Catalyst preparation is crucial in successful Fischer-Tropsch synthesis. Appropriate catalyst composition and delicate pretreatment and operating conditions are all necessary preconditions to achieve the desired results. Catalyst disintegration brought about by oxidation and carbide formation is a serious problem that can be prevented only by using catalysts with adequate chemical and mechanical stability under appropriate operating conditions. 

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 ... 

Good evidence has been obtained that heterogeneous iron, ruthenium, cobalt, and nickel catalysts which convert synthesis gas to methane or higher alkanes (Fischer-Tropsch process) effect the initial dissociation of CO to a catalyst-bound carbide (8-13). The carbide is subsequently reduced by H2to a catalyst-bound methylidene, which under reaction conditions is either polymerized or further hydrogenated 13). This is essentially identical to the hydrocarbon synthesis mechanism advanced by Fischer and Tropsch in 1926 14). For these reactions, formyl intermediates seem all but excluded. 

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. 

Here Z is a Ni surface site. The equation they derive is complex but can be simplified (see Table 4) for full-scale application. These workers point out that the same equation can be derived from a mechanism involving surface carbon as an intermediate similar to carbide theories for Fischer-Tropsch synthesis. In that case steps (b) and (c) in the above equation would be replaced by (b ) and (c ). 

Fischer and Tropsch assumed an intermediate formation of carbides (carbide theory) as mechanism of the reaction of carbon monoxide and hydrogen to higher hydrocarbons. Methane was assumed to be formed via an intermediate formation of hydrides. The competition of carbon monoxide and hydrogen in connection with the formation of carbides and hydrides was considered to be responsible for the tendency of different catalysts to form preferentially either higher hydrocarbons or methane. With this theory, Fischer and Tropsch explained why iron presented an... 

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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