Heterogeneous catalysis with homogeneous carbonylation reaction

Solutions of ruthenium carbonyl complexes in acetic acid solvent under 340 atm of 1 1 H2/CO are stable at temperatures up to about 265°C (166). Reactions at higher temperatures can lead to the precipitation of ruthenium metal and the formation of hydrocarbon products. Bradley has found that soluble ruthenium carbonyl complexes are unstable toward metallization at 271°C under 272 atm of 3 2 H2/CO [109 atm CO partial pressure (165)]. Solutions under these conditions form both methanol and alkanes, products of homogeneous and heterogeneous catalysis, respectively. Reactions followed with time exhibited an increasing rate of alkane formation corresponding to the decreasing concentration of soluble ruthenium and methanol formation rate. Nevertheless, solutions at temperatures as high as 290°C appear to be stable under 1300 atm of 3 2 H2/CO. 

The great majority of intramolecular cyclopropanation reactions of unsaturated a-diazo-carbonyl compounds have been achieved with copper catalysts, especially with copper powder or copper bronze, copper(II) sulfate, and bis(acetylacetonato)copper(II). Homogeneous catalysis by bis(salicylaldimato)copper(II) or copper(I) halide/trialkyl phosphite complexes has repeatedly been reported to be superior to heterogeneous catalysis by other copper(I) and cop-per(II) salts, e.g. formation of and 2. ... 

An important point, difficult in many cases to be clarified, is related to the nature o-f the catalytic system, that is which is the real catalytical 1y active species and moreover in which cases we can clearly distinguish between homogeneous and heterogeneous catalysis. For these reasons, the subdivision between heterogeneous(4,2.5.1., 4.2.6.1.) and homogeneous (4.2.5.2., 4.2,5.3., 4.2.6.2.) catalysts is in some cases somewhat arbitrary and uncertain. This unsatisfactory situation is primarily due to the fact that with few exceptions the catalytic carbonylation reactions of nitro compounds proceed at high pressures and temperatures, and these are reaction conditions which interfere with studies on the mechanism of these reactions. 

As shown by reaction 1.6.4.3, depending on the requirement for hydrogen, carbon monoxide could be further reacted with water to yield more hydrogen. This reaction is called water gas shift reaction and is of relevance in homogeneous catalyst-based carbonylation reactions (see Section 4.7.1). The heterogeneous industrial catalyst for water gas shift reaction contains copper and zinc oxide on alumina. By the standards of heterogeneous catalysis, it operates at a relatively low temperature ( 230°C). 

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