Heterogenized homogenous carbonylation catalysts

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

Recently, a more stable Rh catalyst for methanol carbonylation based on the crosslinked polyvinylpyridine system has been disclosed in which the degree of crosslinking of the resin support is as high as 60 % [115c-e]. This catalyst improvement is the basis for the potential development of a commercial methanol carbonylation acetic acid process named Acetica . This process is being offered for license by Chiyoda and UOR Even with this announcement, there are still considerable doubts whether heterogenized carbonylation catalyst systems can compete with the low-water homogeneous Rh- and Ir-catalyzed processes (cf. Sections 2.1.1 and 3.1.1.3). 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

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 ... 

One of the earliest reactions involving the insertion of CO into a Cn-olefin molecule to produce an aldehyde with one greater Cn+i carbon number is the so-called hydroformylation or "0x0 " reaction. The 0x0 reaction is carried out over homogeneous catalysts, rhodium or cobalt carbonyls, and is an important industrial process. Recently the production of acetic acid, acetaldehyde, and glycol from CO and H2 over heterogeneous and homogeneous rhodium catalysts have been reported. Straight-chain saturated hydrocarbons are not the only molecules that may be produced in the Fischer-Tropsch reaction. There have been... 

Fig. 33. Carbonylation of allyl chloride in the presence of heterogeneous (1) and homogeneous (2) catalysts homogeneous catalyst - [Pd(NH3)4] [ -CH2CgH4S03]2, heterogeneous catalyst - [Pd(NH3)4] supported on polystyrenesulfoacid [212]...

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) ... 

The activities of heterogeneous catalysts of carbonylation are usually lower as compared to those in analogous homogeneous systems (eg. Table 1) (10) however, the advantages of heterogeneous catalysts remain attractive for industrial purpose. Therefore, many attempts have been made to develop carbonylation catalysts with activities approaching those of homogeneous systems. 

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. 

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). 

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