Heterogenized homogenous carbonylation

Scurrell (201) recently briefly reviewed the literature on heterogenized homogeneous rhodium catalysts for methanol carbonylation up to 1976. 

A slurry phase concurrent synthesis of methanol using a potassium meth-oxide/copper chromite mixed catalyst has been developed. This process operates under relatively mild conditions such as temperatures of 100-180°C and pressures of 30-65 atm. The reaction pathway involves a homogeneous carbonylation of methanol to methyl formate followed by the heterogeneous hydrogenolysis of methyl formate to two molecules of methanol, the net result being the reaction of hydrogen with carbon monoxide to give methanol via methyl formate ... 

Despite the apparent simpUcity of preparing heterogenized homogeneous complexes, in many cases the structure of complex-bound particles remains unknown [212], Properties of homogeneous catalysts change little during the course of catalysis. For instance, in periments conducted at 100 °C and at a CO pressure of about 6.9 MPa, aUyl chloride carbonylation catalyzed by palladium proceeds selectively with a high yield of the desired product (Fig. 33) ... 

Halide-promoted heterogeneous catalysts for carbonylation are analogues to homogeneous carbonylation catalyzed by metal carbonyls, that is, the Reppe reaction (11). The first step of the Reppe reaction involves the oxidative addition of alkyl halide promoter to carbonyl metal, for example, Rh(I) complex (Fig. 1). This step is followed by methyl migration, bonding of carbon monoxide to give a coor-dinatively saturated Rh(III) complex, and subsequent decomposition of this complex in the presence of methanol to yield a carbonylated product and regenerate the promoter and the catalyst. 

A topical review of the potential of heterogeneous metal carbonyl clusters has been published. The author describes the ideas behind the specific engineering of catalytic surfaces using metal carbonyl clusters. The utilisation of metal carbonyl clusters in homogeneous catalysis has been thoroughly reviewed (inc 410 references) by Suss-Fink and Meister. ... 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

A metal cluster can be considered as a polynuclear compound which contains at least one metal-metal bond. A better definition of cluster catalysis is a reaction in which at least one site of the cluster molecule is mechanistically necessary. Theoretically, homogeneous clusters should be capable of multiple-site catalysis. Many heterogeneous catalytic reactions require multiple-site catalysis and for these reasons discrete molecular metal clusters are often proposed as models of metal surfaces in the processes of chemisorption and catalysis. The use of carbonyl clusters as catalysts for hydrogenation reactions has been the subject of a number of papers, an important question actually being whether the cluster itself is the species responsible for the hydrogenation. Often the cluster is recovered from the catalytic reaction, or is the only species spectroscopically observed under catalytic conditions. These data have been taken as evidence for cluster catalysis. 

Oxidation is a widely used procedure in carbohydrate chemistry, mainly to access sugars that contain a carbonyl function to serve as valuable intermediates for a variety of derivatizations. Many procedures have been developed, employing either chemical or biochemical methodologies.14 148 While most of these methodologies rely on homogeneous catalysis, the use of heterogeneous catalysts has proved to be a feasible alternative.123c However, the utilization of catalysts based on silicon porous materials for the oxidation of carbohydrates is still a field to be further explored. 

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