The Carbide 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  

Tlie first step comprizes the dissociative adsorption of CO on the catalyst surface, as is shown in Equation (23). 

The feasibility of this step has been shown for numerous transition metals and 

For this step model reactions are known, such as Utc hydrogenation of the carbide cluster Fe CfCO) (1) IU8-120). The fac de hydrogenation of surface carbon has been pointed out in Section 4.5. Methane can be formed in this step by further hydrogenation. 

Chain propagation is initiated by the reaction of surface methyl with surface methylene. Further chain propagation ensues via a continued insertion of methylene groups, as shown in Equation (25). 

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The carbide mechanism, however, does not explain the formation of oxygenates in FTS products. 

On the basis of the nature of CO adsorption and of the nature of chain initiator intermediates, popular mechanistic proposals include the carbide mechanism,1-2 wherein CO adsorbs dissociatively and the carbide (C ) is the chain initiator intermediate, and the enolic mechanism,3 involving molecular adsorption of CO and the formation of an oxygen intermediate, the enol (HC OH). 

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) ... 

We then focus on the Pichler-Schulz CO insertion mechanism (39). This reaction has been much less investigated than the carbide mechanism. We recognize that in homogeneous catalysis, alkene hydroformyla-tion has been investigated extensively it appears that hydroformylation is much more difficult on metallic surfaces than in the presence of mononuclear cationic metal complexes (40). 

The carbide mechanism fails, however, to explain the formation of oxygenated products such as alcohols, aldehydes and acids which are common sidc-products in FT synthesis and are especially formed in the first stage of the rcaclion [159]. Assumption of CO insertion as the terminating step to produce oxygenated products would be a viable explanation. Such a scheme has been proposed by Ichikawa to account for the formation of oxygenated Cj products over rhodium (Equation (29)) [149]. 

A combination of the insertion and the carbide mechanism has been provided by Ponec [ 173]. It is assumed that CO is dissociated from surface carbon, which is partially hydrogenated to a (CH ) species. Chain propf tion is envisioned by repeated CO insertion and partial hydrogenolysis. 

All the mechanisms described rely more or less on experimental results. Quite contradictory assumptions have been made and it seems difficult to find definite evidence for proving or disproving one of these mechanistic proposals. Considering the recent results concerning catalyst surfaces and active species which were briefly covered in Section 4.5, a mechanistic picture, excluding dissociative adsorption of CO, seems improbable. On the other hand, the carbide mechanism cannot explain oxygenated products wheie. at least in the terminating step, CO insertion is necessary. 

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... 

Todic et al. [14] developed a comprehensive micro-kinetic model based on the carbide mechanism that predicts FT product distribution up to carbon number 15. This model explains the non-ASF product distribution using a carbon number dependent olefin formation rate (e term). The rate equations for the olefins and paraffins used in the model are shown in Figure 2. The derivation of the rate equations and physical meaning of the kinetic parameters, as well as their fitted values, can be found in Todic et al. [14]. In the current study, a MATLAB code which uses the Genetic Algorithm Toolbox has been developed, following the method of Todic et al. [14], to estimate the kinetic model parameters. In order to validate our code, model output from Todic et al. [14] was used as the input data to our code, and the kinetic parameter values were back-calculated and compared to the values fi om [14], as shown in Table 1. The model has 19 kinetic parameters that are to be estimated. The objective function to be minimized was defined as... 

We have developed a MATLAB code that uses Genetic Algorithm technique to estimate the parameters of the kinetic model based on the carbide mechanism, and demonstrated that this code works using data from the literature [14]. A high pressure bench-scale reactor unit has been commissioned to conduct FT experiments in the gas phase and supercritical hydrocarbon solvent phase. Here, we have reported the preliminary results from a set of gas phase FT experiments conducted using our rig. 

General consensus is reached on the carbide mechanism [4,5], which is used here to describe the formation of the monomer and the paraffinic and olefinic products. 

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. 

Fig. 10 Schematic diagram of the carbide mechanism. Reprinted from ref. 16 with permission from Elsevier.

Chain initiation. As illustrated by both the carbide mechanism and the CO insertion mechanism, the FTS reaction is initiated through CO activation. The divergence for two mechanisms is that the carbide mechanism requires CO to firstly dissociate, while the CO insertion mechanism proposes CO to be hydrogenated at the initial step. Some recent density functional studies based on periodic catalyst models have evaluated the possibility of these different activation pathways by calculating the activation energy barriers of the involved steps under different reaction conditions (temperatures, pressures and coverages). Here, we will discuss these results in both direct and H-assisted routes. 

In a subsequent step, the CH species can recombine to form a CjHj, species. Higher hydrocarbons will be formed by subsequent insertion steps of the higher oligomers with the CH species initially generated by CO dissociation. This is called the carbide mechanism. Many different intermediates have been suggested as actual intermediates in this process [10]. 

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