BASF Process—Mechanistic Studies

The rate of cobalt-catalyzed carbonylation is strongly dependent on both the pressure of carbon monoxide and methanol concentration. Complex 4.7, unlike 4.1, is an 18-electron nucleophile. This makes the attack on CH3I by 4.7 a comparatively slow reaction. High temperatures are required to achieve acceptable rates with the cobalt catalyst. This in turn necessitates high pressures of CO to stabilize 4.7 at high temperatures. 

Spectroscopic studies under actual operating conditions have not been reported. However, 4.7, 4.8, 4.10, and 4.11 are well-characterized complexes. Evidence for the intermediate 4.9 comes from kinetic studies. The structure of 4.11 has also been determined by electron diffraction. X-ray structural data are available for Co(C2F4H)(CO)3L, a model of 4.8. 

3 WATER-GAS SHIFT REACTION AND RHODIUM-CATALYZED CARBON YLATION 

The relevance of the water-gas shift reaction in the petrochemical industry has already been discussed (see Section 1.1). The significance of the water-gas shift reaction in homogeneous systems is twofold. First, it plays a crucial role in stabilizing the rhodium catalyst in the Monsanto process. Second, studies carried out in homogeneous systems employing metals other than rhodium have provided useful mechanistic insights into the heterogeneous water-gas shift reaction. We first discuss the catalytic cycle with 4.1 as one of the catalytic intermediates, and then mechanistic results that are available from an iron-based catalytic system. 

The proposed catalytic cycle for the water-gas shift reaction in the Monsanto process is shown in Fig. 4.5. This cycle operates at acidic pH and is responsible for C02 and H2 production. It has a useful function in stabilizing the rhodium 

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The first methanol carbonylation process, commercialized in the 1960 s by BASF, used an iodide promoted cobalt catalyst but required very high pressures ( 700 atm) as well as high temperatures ( 250°C), and gave only ca. 90% selectivity. Few mechanistic studies have been published and little is known for certain about the mechanism. 

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