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aside from the reports of Bradley (6), Bathke and Feder (]), and the work of Pruett (8) at Union Carbide (largely unpublished), very little is known about the homogeneous catalytic hydrogenation of CO to methanol.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

Much of the justification for the extensive study of transition metal cluster chemistry is embedded in the assumption that reactions of metal clusters are realistic structural models for reactions at metal surfaces in such processes as heterogeneous catalysis (9,10,11). For example, the metal carbonyl clusters, Ir4(CO)i2 and Os3(CO)i2, were demonstrated to be effective homogeneous catalysts for methanation (12). Additionally, Demitras and Muetterties (13) have found Ir4(CO)i2 to be a homogeneous catalyst in the Fischer-Tropsch synthesis of aliphatic hydrocarbons. Homogeneous catalysis of the water gas shift reaction by metal carbonyl clusters (e.g., Ru3(CO)i2) in alkaline solution has been reported by Laine, Rinker, and Ford (14), and more recently by Pettit s group (15). Nevertheless, mononuclear metal carbonyls (e.g., Fe(CO)s and the group VIb metal hexacarbonyls) have been demonstrated to have considerable activity above 120°C as soluble catalysts for Reaction 2 (16),... 

The first catalyst used in the oxo-reaction was the solid Fischer-Tropsch catalyst consisting of 66% of silica, 30% of cobalt, 2% of thorium oxide and 2% of magnesium oxide. It took quite some time until the homogeneous nature of the catalysis in the oxo reaction was discovered [1, 2, 6] and proved [100]. After this discovery a large number of metals and metal salts were investigated for their use as oxo catalysts. 

In spite of such catalytic effects of the metal cluster, the rate of this reaction remains very low. Nevertheless, this reaction is an example of a very interesting type of homogeneous catalysis. Here the activation of carbon monoxide appears to be achieved by interaction of both the carbon and oxygen atoms with the metal cluster atoms in a similar way to the CO-chemisorption on metals in heterogeneous Fischer-Tropsch processes. 

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/ ... 

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