Hydroformylation heterogeneous catalysis

An alternative interpretation of some features of the hydroformylation reaction (including the inverse CO dependence), in terms of heterogeneous catalysis by an (unidentified) insoluble cobalt component, has recently been advanced by Aldridge, Fasce and Jonassen (49a). The universal validity of this seems doubtful in the light of the considerable evidence favoring a homogeneous mechanism. 

This chapter has focused on heterogeneous catalysis in supercritical media, but the relationship between supercritical fluids and catalysis is much broader. There have been numerous studies of homogeneous catalysis in SCFs. Examples include hydroformylation via cobalt carbonyl complexes in supercritical CO2, oxidation via metal salts dissolved in supercritical water, and acid-catalyzed dehydration of alcohols in supercritical water. 

Two or three decades ago, no clear conceptual (mechanistic) or practical (experimental) bridge existed between homogeneous and heterogeneous catalysis. In some of his earliest work at Mobil, Werner demonstrated that hydroformylation, carbonylation, and other reactions catalyzed by transition metal complexes were accomphshed equally by heterogeneous catalysis when these complexes were anchored to solid organic resins. 

Unusual reaction orders are found in product-promoted or reactant-inhibited ("autocatalytic") reactions, the former with positive apparent order with respect to a product, the latter with negative apparent order with respect to a reactant (see Section 8.9). An example of a product-promoted reaction is acid-catalyzed ester hydrolysis. An example of a reactant-inhibited reaction has already been encountered, namely, olefin hydroformylation, whose order with respect to CO is negative (see eqn 6.12 in Section 6.3). Such behavior is also not uncommon in heterogeneous catalysis (see Section 9.3.2) and enzyme catalysis ("substrate-inhibited" reactions in biochemistry lingo, Section 8.3). A reaction having an order with respect to a silent partner—CO in a homogeneous hydrogenation—will be examined in some detail later in this chapter (see Examples 7.3 and 7.4). 

Using supported rhodium as catalyst, 2-nitrostyrene was converted into skatole in 70 % yield by CO/H2 (160 atm) in benzene at 160 °C [27]. However this reaction, conducted under hydroformylation conditions, involves formation of 2-(o-nitrophenyl)propionaldehyde by homogeneous catalysis, reduction of the nitro group by heterogeneous catalysis, and ring closure accompanied by thermal dehydration (Scheme 3) ... 

The technical application of the hydroformylation reaction has developed rapidly due to the industrial importance of its products. Many data were obtained during this development work, which allowed H. J. Nien-burg et aL [236] and A. J. M. Keulemans, A. Kwantes and Th. van Bavel [25] to lay down certain empirical rules for the oxo reaction. For quite some time these rules were the basis for the understanding of the product distribution in the hydroformylation reactions. There was no systematic investigation of the reaction mechanism of this process in the early years. Unsatisfactory analytical results were responsible for many misinterpretations. It was assumed that the hydroformylation proceeds through heterogeneous catalysis, an assumption which is supported by some authors even in the sixties [26, 27] (as to these papers see the critical discussion in the paper of V. Macho et aL [28]). 

In this section, we describe selected homogeneous catalytic processes that are of industrial importance. Many more processes are applied in industry and detailed accounts can be found in the suggested reading at the end of the chapter. Two advantages of homogeneous over heterogeneous catalysis are the relatively mild conditions under which many processes operate, and the selectivity that can be achieved. A disadvantage is the need to separate the catalyst at the end of a reaction in order to recycle it, e.g. in the hydroformylation process, volatile HCo(CO)4 can be removed by flash evaporation. The use of polymer supports or biphasic systems (Section 25.6) makes catalyst separation easier, and the development of such species is an active area of current research. 

An obvious way to combine the advantages of homogeneous and heterogeneous catalysis for optimized hydroformylation catalysis is to covalently anchor the molecular catalyst complex to a solid surface. Such an immobilized catalyst could be used in fixed bed or slurry type reactors and product separation would be as straightforward as for any heterogeneous catalyst. Indeed, this approach has been most widely studied as evidenced by numerous academic papers and patents, reviews, and books (Keim and Driessen-Hoelscher, 1999 and Reek et cd., 2006). 

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