Homogeneous catalysis Monsanto process

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

As in the original Monsanto process involving homogeneous catalysis and catalyst recycle, the product is removed as a liquid, because the gas phase of a stripping reactor would contain a low concentration of the high-boiling acetic acid. In the Acetica process, no catalyst recycle is needed, as the solid catalyst stays in the reactor. 

An important modern example of homogeneous catalysis is provided by the Monsanto process in which the rhodium compound 1.4 catalyses a reaction, resulting in the addition of carbon monoxide to methanol to form ethanoic acid (acetic acid). Another well-known process is hydro-formylation, in which the reaction of carbon monoxide and hydrogen with an alkene, RCH=CH2, forms an aldehyde, RCH2CH2CHO. Certain cobalt or rhodium compounds are effective catalysts for this reaction. In addition to catalytic applications, non-catalytic stoichiometric reactions of transition elements now play a major role in the production of fine organic chemicals and pharmaceuticals. 

The Monsanto process, one of the most successful industrial homogeneous catalytic processes, uses a Rh complex and catalytic HI to carbonylate MeOH to MeC02H. A Rh precatalyst (almost any Rh complex will do) is converted into Rh(CO)2l2, the active catalyst, under the reaction conditions. The mechanism of the reaction involves three steps. In the first step, MeOH and HI are converted to Mel and H20 by an Sn2 mechanism. In the second step, Mel and CO are converted to MeCOI under Rh catalysis. In the third step, H2O (generated in the first step) hydrolyzes MeCOI to afford MeC02H and to regenerate HI. 

Carbonylation of methanol catalyzed by soluble Group IX transition metal complexes remains the dominant method for the commercial production of acetic acid. The Monsanto process stands as one of the major success stories of homogeneous catalysis, and for three decades it was the preferred technology because of the excellent activity and selectivity of the catalyst. It has been demonstrated by workers at Celanese, however, that addition of iodide salts can significantly benefit the process by improving the catalytic reaction rate and catalyst stability at low water concentrations. Many attempts have been made to enhance the activity of... 

G. G. Stanley (1994) Carbonylation processes by homogeneous catalysis in Encyclopedia of Inorganic Chemistry, ed. R.B. King, Wiley, Chichester, vol. 2, p. 575 - A well-referenced overview which includes hydroformylation, Monsanto and Tennessee-Eastman processes. 

The Monsanto process for acetic acid production from methanol has been a very successful application of homogeneous catalysis with rhodium. ... 

From a historical perspective, the Monsanto process for the preparation of (l.)-DOPA in 1974 laid the foundation stone for industrial enantioselective catalysis. Since then it has been joined by a number of other asymmetric methods, such as enantioselective Sharpless epoxidation (glycidol (ARCO) and disparlure (Baker)), and cyclopropanation (cilastatin (Merck, Sumitomo) and pyre-throids (Sumitomo)). Nevertheless, besides the enantioselective hydrogenation of an imine for the production of (S)-metolachlor(a herbicide from Syngenta), the Takasago process for the production of (-)-menthol remains since 1984 as the largest worldwide industrial application of homogeneous asymmetric catalysis. [124]... 

Although rhodium and iridium are high-priced precious metals, their complexes are indispensable for homogeneous catalysis. Prominent examples for their use on the industrial scale are, among others, asymmetric hydrogenation, hydroformyla-tion, and the Monsanto acetic acid or the Cativa process [1]. 

The industrial manufacture of acetic acid by methanol carbonylation (Equation (1)) has utilized catalysts based upon all three of the group 9 metals, since the initial development by BASF of a cobalt/iodide-based system. " The BASF process required harsh conditions of temperature and pressure, and suffered from relatively low selectivity. It was soon superceded by highly selective, low-pressure rhodium/iodide-based catalysts developed by Monsanto. The Monsanto process (and related variants operated by other manufacturers) quickly became dominant and remains one of the most successful examples of the commercial application of homogeneous catalysis.Rhodium catalysts for methanol carbonylation are discussed in Chapter 7.03. 

The Monsanto, E-H, and Cativa processes represent triumphs in the application of organotransition metal chemistry to catalysis, using homogeneous transition metal compounds to promote production of valuable materials cheaply, efficiently, and selectively. Without question, fundamental work accomplished previously on the understanding of basic organometallic reaction types helped... 

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