Fischer-Tropsch reaction formation

As was demonstrated in the preceding sections, structure-sensitivity phenomena are mostly confined to particle size regimes smaller than 3-4 nm. A process of industrial relevance was investigated by de Jong et al. [127] in their study on cobalt particle size effects in the Fischer-Tropsch reaction. Earlier works noted distinct drop in activity for Co particles smaller than lOnm and ascribed this phenomenon to either a partial oxide or carbide formation which should be enhanced for particles in this size regime [128-139]. In order to avoid similar effects, de Jong used... 

Xiong, J., Ding, Y., Wang, T., Yan, L., Chen, W., Zhu, H., and Lu, Y. 2005. The formation of Co2C species in activated carbon supported cobalt-based catalysts and its impact on Fischer-Tropsch reaction. Catal. Lett. 102 265-69. 

In reality, not only the main reaction (the Fischer-Tropsch reaction) leading to the formation of higher hydrocarbons (Equation 12.1), but also methane formation (Equation 12.2) and the water-gas shift reaction (Equation 12.3) have to be considered. The rate equations for these three reactions on a commercial Fe-catalyst were determined by Popp8 and Raak2 and summarized by Jess et al.9 However, to simplify matters, just the Fischer-Tropsch reaction forms the basis of the approach presented here ... 

As a check to confirm that no extraneous non-polymer-attached catalytic species were present, the following experiment was performed. Polystyrene without attached cyclopentadiene was exposed to Co2(C0)e, extracted using a Soxhlet extractor and dried in vacuo in exactly the same manner as was used to synthesize 5. When used under the above Fischer-Tropsch reaction conditions, these treated, white polystyrene beads did not discolor, release any detectable species into solution, cause a CO/H2 pressure drop, or result in the formation of any detectable amounts of methane. These observations argue against the presence of small amounts of occluded Co2(C0)e or C04 (CO) 12 which could conceivably have been active or precursors to active species. It should be noted that the above clusters were reported to be essentially inactive under Fischer-Tropsch conditions (140°C, toluene, 1.5 atm., 3/1 H2/CO, three days) leading to mere traces of methane (11). The lack of products under our conditions also indicates that, at least in the absence of resin-bound CpCo(C0)2 or its derivatives, the polystyrene support did not degrade. 

The extremely low turnover rate (0.011 mmol CO/nmol Co/h, production of 0.003 mmol CH /mmol Co/h average), stimulated attempts to increase the efficiency of the reaction. Indeed, pretreatment involving removal of CO via vacuum thermolysis of CpCo(C0)2 5 to yield " CpCo", followed by its use in a Fischer-Tropsch reaction, led to improved activity (turnover of 0.130 mmol CO/irmol Co/h, production of 0.053 mmol CH /mmol Co/h average). It appears that decarbonylation is necessary for formation of the catalytically active species. 

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... 

Metal molybdates421 and cobalt-thoria-kieselguhr422 also catalyze the formation of hydrocarbons. It is believed, however, that methanol is simply a source of synthesis gas via dissociation and the actual reaction leading to hydrocarbon formation is a Fischer-Tropsch reaction. Alumina is a selective dehydration catalyst, yielding dimethyl ether at 300-350°C, but small quantities of methane and C2 hydrocarbons423 424 are formed above 350°C. Heteropoly acids and salts exhibit high activity in the conversion of methanol and dimethyl ether.425-428 Acidity was found to determine activity,427 130 while hydrocarbon product distribution was affected by several experimental variables.428-432... 

Dr. Rosscup. If the formation of hydrocarbons is kinetically rather than thermodynamically controlled, does this suggest that the crude oil was formed at a temperature near that of the Fischer-Tropsch reaction ... 

The intrinsic nature of tungsten carbide catalyst in CO-H2 reactions is to form hydrocarbons. This property can be modified by oxidic promoters as for the case of noble metals like Pt or Rh or by the presence of carbon vacancies at the surface. To increase the production of alcohols in the Fischer-Tropsch reaction, the catalyst should be bifunctional, with oxidic and carbidic components as in the case of WC on Ti02. Overcarburization of WC on supports like Si02 or Zr02 where the W-O-metal interaction is weak leads to C/W ratios close to unity and does not result in alcohol formation. 

In this chapter we also discuss two other catalytic reactions that involve CO as one of the reactants. These are the water-gas shift (see Section 1.2) and Fischer-Tropsch reactions. Although for these reactions homogeneous catalysts are not used industrially, for explaining the formation of by-products in the... 

Although it would be valuable to positively identify the second phase particles formed by hydrothermal treatment so that a mechanism for their formation and growth could be understood, it is not essential to its use as a catalyst. Fischer-Tropsch reactions occur in a reducing atmosphere and the actual iron catalyst is probably a reduced iron compound that could only exist under the reaction conditions. 

The hydroformylation (or 0x0 ) reaction was discovered in 1938 by Roelen who was working on the formation of oxygenates as by-products of the Fischer-Tropsch reaction over cobalt catalysts. It soon became clear that the aldehydes and alcohols found were the products of secondary reactions undergone by the 1-alkenes (which are the primary products of the Fischer-Tropsch reaction, Section 4.7.2) with syngas. Further work showed that Roelen had discovered a new reaction, in which the elements of H and CHO were added to an olefin (hence hydroformylation), and which was catalyzed by cobalt. It was later found that the true precatalyst was not cobalt metal but derivatives of dicobalt octacarbonyl, such as the hydride, CoH(CO)4. 

For illustration purposes, we briefly discuss oxygen removal on cobalt. The Fischer-Tropsch reaction on this catalyst is known to be only weakly suppressed by the product water (41). The available computational results indicate that the activation energy for the reaction of adsorbed hydrogen, Hads/ with Oads to adsorbed OH species, OHads/ on cobalt is about 166 kj/mol for the flat Co(OOOl) surface and 70 kj/mol for sfepped cobalf surfaces (42). For comparison, fhe activation energy for fhis reaction on rhodium is 90 kJ/mol (43) Subsequent water formation occurs by recombination of OHads with Hads this reaction has a barrier of befween 5 and 10 kj/mol. 

It is now widely accepted that the activation of CO is highly structure sensitive (II). The activation of CO on most of the transition metals has been investigated. The computational results for cobalt (6) and ruthenium (5) are of particular relevance to us because these elements in the metallic state are active for the Fischer-Tropsch reaction. These results can be compared with those obtained for rhodium (40), which selectively catalyzes the formation of alcohols from CO and H2, and for nickel (30), which is a methanation catalyst. 

Assuming BEP-type relationships to be valid, we can make a prediction of the selectivity of fhe Fischer-Tropsch reaction as a function of the M—C bond energy. In Figure 10, a schematic representation is given of the relative rates of production of particular groups of Fischer-Tropsch products as a function of fhe M—C interaction energy. Four types of reaction are compared coke or carbide formation, hydrocarbon chain growth, CH4 formation, and CO dissociation. 

Surface reconstruction is driven by stabilization of the adsorbate after adsorption of carbon atoms on more reactive surface atoms. Ciobica et al. (74) demonstrated that an overlayer of Cads leads to the Co(lll) to Co(lOO) reconstruction on fee cobalt (the stable phase of small cobalt particles). Because of the change in metal atom density in the surface layer, the reconstruction may be associated with faceting and hence creation of step-edge sites, which are highly active in the Fischer-Tropsch reaction (5). Hence, surface reconstruction and formation of a stable carbide overlayer may actually be the processes occurring during the initial activation of the catalyst. This phenomenon has been described by Schulz (101) as self-organization. 

It is often proposed that "Ci" formation is also the rate-limiting step of the Fischer-Tropsch reaction. This scenario can only become true if chain growth proceeds through CO insertion, which we suggest to be unlikely. 

Equation (2) is the actual Fischer-Tropsch reaction of hydrocarbon formation which is best catalyzed by cobalt catalysts, and Equation (3) represents the water-gas shift reaction, which has to be anticipated with Fe catalysts. 

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. 

Fig. 1. If equilibrium is maintained on cooling, CO will be converted largely to CH4 (solid lines) before metastable formation of more complex hydrocarbons by the Fischer-Tropsch reaction becomes possible (dashed lines). However, the reaction is very slow in the absence of catalysts, and may not have begun until about 400 K, when catalysts such as serpentine and magnetite became available through the hydration of olivine. Thus CO may have persisted metastably between 600 and 400 K...

It turns out, however, that the Fischer-Tropsch reaction gives an isotopic fractionation of just the right sign and magnitude, owing to a kinetic isotopic effect (Lancet and Anders, 1970 Lancet, 1972). The temperature dependence of the fractionation between 375 and 500 K suggests that the observed fractionations in Cl and C2 chondrites correspond to about 360 to 400 K (Fig. 11). These values agree rather well with the formation temperatures of carbonates and silicates, based on ratios, 360 K for Cl s and 380 K for C2 s (Onuma et al., 1972,... 

Shift Conversion. Improved LT shift catalysts can operate at lower temperatures to achieve a very low residual CO content and low byproduct formation. A new generation of HT shift catalysts largely avoids hydrocarbon formation by Fischer-Tropsch reaction at low vapor partial pressure, thus allowing lower steam to carbon ratio in the reforming section (see Section 4.2.1.1.1). 

Reactions. The Fischer-Tropsch reaction converts synthesis gas into a mixture of alkanes and alkenes over a solid catalyst usually containing iron, The basic reaction for paraffin formation is as follows... 

As a matter of fact, olefin-consuming reactions (by H2) may be a serious problem in some technical reactions. Palladium complexes and Co2(CO)g (commercial products) are typical catalysts. Problems may also arise in the Fischer-Tropsch reaction [19, 20] where iron oxides of a certain basicity (alkaline-metal doping) are being used to catalyze the formation of hydrocarbons according to (the simplified) eq. (15). More details are provided in Section 3.1.8. Since water is inevitably formed, carbon dioxide can also occur. On the other hand, it is doubtful whether the CO/H2O system will be used for directed reductions of organic compounds, since hydrogen is an extremely abundant industrial chemical. The water-gas shift reaction is thus to be avoided in the vast majority of cases. 

Reduction of Ta(silox)3Cl2 with Na/Hg leads to a three-coordinate alkoxide complex Ta(silox)3. The coordinatively unsaturated tantalum complex is capable of cleaving H2 and O2 bonds resulting in the hydride and 0x0 complexes as illustrated in Scheme 7.14. Carbon monoxide is also split upon carbonylation of Ta(silox)3 generating the 0x0 and p-dicarbide complexes. This reaction models the C—O bond cleavage and C—C bond formation believed to occur in the Fischer-Tropsch reaction, and the ketenylidene complex Ta(silox)3(=C=C=0) was postulated as the key intermediate. On the other hand, when Ta(silox)3 was treated with pyridine and benzene, remarkable T -coordinated complexes were formed. 

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