Carbide mechanism

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 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 ... 

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). 

Recombination of CH. fragments is an essential step to initiate the chain-growth reaction according to the Sachtler-Biloen carbide mechanism. In a series of elegant papers, Cheng et al. (31-33) reported on the structure dependence as well as on the metal dependence of this class of reactions. Activation energies for CH. —CH recombination on flat and stepped surfaces of cobalt are listed in Table 4. 

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 ... 

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... 

Davis26 pointed out in his review on FT synthesis mechanism that the reaction pathway is also dependent on catalyst compositions and operation conditions. Their experimental data on FT synthesis with iron catalysts at low temperatures suggested that instead of carbide mechanism, an oxygenate intermediate, similar as the formate species responsible of the WGS reaction, existed on the surface and initiated carbon chain propagation. The final chain termination step was accompanied by elimination of the oxygen atom. 

Kummer et al. also considered whether synthesis could occur on a few active sites of the type proposed by Taylor. They performed two types of experiments. In one group of runs, care was taken to synthesize an amount of hydrocarbon corresponding to only a very small fraction of the surface. If active points covering a significant percentage of the surface were involved, one would expect that when the total amount of hydrocarbon synthesized was made to correspond to a smaller fraction of the surface carbon, the apparent percent of the reaction going via carbide mechanism would rise. In contrast to this, even for the smallest (0.62 percent of a monolayer) hydrocarbon synthesis the reaction going via a carbide was only 10 percent. Thus, if active points were responsible for most of the synthesis, they must correspond to much less than 0.5 percent of the surface of the catalyst. 

Percy and Walter converted a mixture of doubly labeled ethene ( CH2 CH2 2 mol percent), CO (24.5 mol percent) and H2 (73.5 mol percent). The propene in the effluent from runs with low (3 to 4 percent) conversions was trapped and analyzed with NMR to determine the distribution of C at each carbon position in the propene. It was possible to quantify the amount of each of the eight isotopomers that can result from various combinations of C and C that were present. Singly labeled propenes were formed from the doubly labeled ethene this required that some ethene dissociates to form C fragments. No evidence was obtained that would indicate that two Cj units, one from CO and one from a C2 unit, recombine to give C2H4 in the product. Thus, ethene is incorporated in part as C2 units and in part by dissociation to Cj units. Incorporation of single Cj units from ethene is equally probable at the three carbon positions of the propene that is eventually formed. The authors state that they see no contradiction between any of their data and the widely accepted carbide mechanism for FTS. 

The gaseous carbide mechanism of oxide reduction by carbon, noted in the table but left out of consideration in this book, is in many ways close to the CDV mechanism. It will suffice to mention that both mechanisms involve gasification of reactants. About 30 articles by L vov and colleagues published during the period 1981-2000 are devoted to studies of this mechanism (see [17, 18] for a list of these papers). Nevertheless, this mechanism was excluded from consideration owing to the author s wish to focus on the main subject of this monograph. 

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. 

Particulate microcomposite Silica, carbon black, calcium carbonate, glass bead, glass balloons, silicon carbide Mechanical mixing and casting, compression moulding, matched-die moulding... 

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. 

FTS Mechanism and Molecular Reaction 2.2.1 Surface Carbide Mechanism... 

Figure 2 Diagram of FTS surface carbide mechanism (Sarup and Wojciechowski, 1989).

Due to the limitation of surface carbide mechanism, Anderson and his coworkers proposed (Storch et al., 1951) a more detailed mechanism to explain FTS product distribution. The theory regards that the chemical adsorption of H2 and CO would generate surface enol complex (HCOH), then it would generate methylene after dehydration and then the hydrocarbon would be formed. In surface enol mechanism, CO has been hydrogenated without dissociation in advance, as shown in Fig. 3, which is well explained on the energy level. 

Dry (1976) and Huff and Satterfield (1984) derived the same equation from the combined enol/carbide mechanism, assuming strong adsorption of CO and water relative to H2 and CO2 as shown in Eq. (2). 

In 1993, Lox and Froment (1993) proposed detailed dynamic model mechanism which based on FTS carbide mechanism, the model clarify the association between operating conditions change and FTS product distribution under classical ASF frame. This was the first attempt to develop detailed dynamic model. Recently, Ma and coworkers (Ma et ah, 1999a, 1999b) proposed a FTS detailed dynamic-olefin readsorption mechanism model, which indicates eigenactivity of olefin readsorption, the formation of which is shown in Eqs. (39)—(45), where R is the reaction rate of different hydrocarbons, R, is the reaction rate constant of different reactions, a is the chain-growth possibility corrected by is the... 

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.

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