Hydrogenation Fischer-Tropsch catalysis

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aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol.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. 

These observations regarding the stability of the methyl species are in line with conclusions about the frustration of methane formation in Fischer-Tropsch catalysis 40) and also the inference that the slow step in CO methanation is the reaction between methyl groups and hydrogen 41), as Fig. 8c illustrates a resistance of the methyl groups to hydrogenation. 

Kinetic studies have shown that the previously reported homogeneous Fischer-Tropsch catalysis by [Ir4(CO)i2] in molten AlCl3-NaCl(2 1) involves the homogeneous reduction of CO to MeCl followed by homologation and/or hydrogenation reactions leading to CH4, C2H6, and other hydrocarbon products/ ... 

Catalysis. Beitel et al. (1997) have employed RAIRS to study in situ the co-adsorption behaviour of CO and hydrogen on single-crystal cobalt (0001) catalysts at pressures up to 300mbar and temperatures between 298 and 490 K. The behaviour of these adsorbates is of considerable importance in relation to their commercial importance as catalysts for the Fischer-Tropsch reaction in the... 

H. Schulz, K. Beck, E. Erich. 1988. Fischer-Tropsch CO-hydrogenation, a non trivial surface polymerization Selectivity of branching. In Proceedings of the 9th International Congress on Catalysis, Calgary, Vol. 2, p. 829. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

Fischer-Tropsch and Crude Oil. Other similarities between crude oil and Fischer-Tropsch products should be mentioned. First, polynuclear condensed aromatics are produced in syntheses involving catalytic hydrogenation of carbon monoxide as shown in the Fischer-Tropsch work of Weitkamp (3). The catalysis group in our laboratory attempted catalytic hydrogenations at elevated temperatures and found production of considerable amounts of aromatics as large as pyrene. This evidence is cited to indicate that the aromatics in petroleum could possibly be derived from catalytic syntheses of the Fischer-Tropsch type. Another similarity has been shown by the product obtained from... 

While current applications include hydrogenation and fermentation, slurry bubble columns have recently been the subject of renewed study for use in two areas of hetergeneous catalysis, SRC coal liquefaction and the Fischer-Tropsch reaction to produce hydrocarbons from synthesis gas. 

The benefits of nonuniform activity distributions (site density) or diffusive properties (porosity, tortuosity) within pellets on the rate of catalytic reactions were first suggested theoretically by Kasaoka and Sakata (Ml). This proposal followed the pioneering experimental work of Maatman and Prater (142). Models of nonuniform catalyst pellets were later extended to more general pellet geometries and activity profiles (143), and applied to specific catalytic reactions, such as SO2 and naphthalene oxidation (144-146). Previous experimental and theoretical studies were recently discussed in an excellent review by Lee and Aris (147). Proposed applications in Fischer-Tropsch synthesis catalysis have also been recently reported (50-55,148), but the general concepts have been widely discussed and broadly applied in automotive exhaust and selective hydrogenation catalysis (142,147,149). 

Examination of the catalog shelf-list section for titles in the field of hydrogenation reveals a number of texts in this field—e.g., Hydrogenation of Organic Substances by C. Ellis, and The Fischer-Tropsch and Related Syntheses by H. A. Storch et al. Ellis book. Chapter 55, Reduction of Carbon Oxides, is specific to the problem in question. All the other texts listed in this section should, of course, be examined also, well as the Kirk-Othmer, Ullmann, and Thorpe encyclopedias, and the books on catalysis. 

Catalysis by Metal Ousters in Zeolites. There is an increasing interest in the use of metal clusters stabilized in zeolites. One objective of such work is to utilize the shape and size constraints inherent in these support materials to effect greater selectivities in typical metal-catalysed reactions. Much work has been concerned with carbon monoxide hydrogenation, and although the detailed nature of the supported metals so obtained is not well understood, there is clear evidence of chain limitation in the Fischer-Tropsch process with both RuY zeolites and with HY and NaY zeolites containing Fe3(CO)22- In the former case there is a drastic decline in chain-growth probability beyond C5- or C10-hydrocarbons depending upon the particle size of the ruthenium metal. 

Slurry reactors are also used in other situations, such as the polymerization of ethylene or propylene. Here the slurry consists of catalyst particles and a solvent, such as cyclohexane, into which the ethylene or propylene is bubbled and dissolved. Another illustration is the Fischer-Tropsch reaction between hydrogen and carbon monoxide, where these gases are dissolved in a slurry of hydrocarbon oil and catalyst (iron) particles. Catalysis by colloidal metal particles and colloidal enzyme particles are other examples, although not always is one reactant a gas. 

Since carbon dioxide is a thermodynamically stable, highly oxidized compound, its synthetic utilization requires some kind of a reduction -reaction with molecular hydrogen is a distinct possibility. Stepwise reduction of C02 with H2 may yield formic acid, formaldehyde, methanol and finally methane, together with CO or Fischer-Tropsch-type derivatives as shown on Scheme 3.42. In aqueous organometallic catalysis the most common product of such a reduction is formic acid. Formation of carbon monoxide, formaldehyde, and methane has already been reported, however, methanol and Fischer-Tropsch type products were not observed. 

The reaction of olefins with carbon monoxide and hydrogen in the presence of cobalt carbonyl catalysis affords inter alia aldehydes, ketones, and alcohols. These reactions are of considerable industrial importance. The industrial reactions were originally called the Fischer-Tropsch or Oxo syntheses but now are described under the general title of hydro-formylation reactions 3, 224). 

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