Monsanto mechanism

A particular type of rheometer developed by Monsanto, now known as Alpha Technologies. The mechanical portion of the rheometer consists of a biconical disc embedded in an elastomeric specimen contained in a specially designed... 

It was discovered by Monsanto that methanol carbonylation could be promoted by an iridium/iodide catalyst [1]. However, Monsanto chose to commercialise the rhodium-based process due to its higher activity under the conditions used. Nevertheless, considerable mechanistic studies were conducted into the iridium-catalysed process, revealing a catalytic mechanism with similar key features but some important differences to the rhodium system [60]. 

Monsanto, 34 208 acetic acid process, 34 85 proposed mechanism for, 34 89 Monte Carlo simulations... 

In 1970, the first rhodium-based acetic acid production unit went on stream in Texas City, with an annual capacity of 150 000 tons. Since that time, the Monsanto process has formed the basis for most new capacities such that, in 1991, it was responsible for about 55% of the total acetic acid capacity worldwide. In 1986, B.P. Chemicals acquired the exclusive licensing rights to the Monsanto process, and 10 years later announced its own carbonylation iridium/ruthenium/iodide system [7, 8] (Cativa ). Details of this process, from the viewpoint of its reactivity and mechanism, are provided later in this chapter. A comparison will also be made between the iridium- and rhodium-based processes. Notably, as the iridium system is more stable than its rhodium counterpart, a lower water content can be adopted which, in turn, leads to higher reaction rates, a reduced formation of byproducts, and a better yield on CO. 

As with Rh catalysed carbonylation of MeOH, much enduring work on the mechanism of Ir catalysed carbonylation was carried out and reported by Forster at Monsanto [11], Combining data from catalytic reactions, synthesis and reactions of intermediates and IR studies of reaction solutions, three regimes or cycles, designated I, II and III, were identified, compared with the one for Rh. A key reactor composition variable determining which regime is operating is [H2O]. 

In Rh catalysed systems, where the metal acyl species also clearly contained iodide, a further possibility was introduced, compared with the mechanism postulated by BASF for their Co systems, that elimination of Acl could occur. The earliest publications from Monsanto which described the proposed mechanism noted that they could not distinguish between a final step involving (i) reductive elimination of Acl followed by hydrolysis of Acl (Eq. (32)), (Eq. (33)) and (ii) hydrolysis at the metal center followed by some other initially unspecified mechanism of recycling HI to Mel (Eq. (34)) [3]. 

As mentioned in the previous section, the carbonylation of methanol to acetic acid is an important industrial process. Whereas the [Co2(CO)s]-catalyzed, iodide-promoted reaction developed by BASF requires pressures of the order of 50 MPa, the Monsanto rhodium-catalyzed synthesis, which is also iodide promoted and which was discovered by Roth and co-workers, can be operated even at normal pressure, though somewhat higher pressures are used in the production units.4,1-413 The rhodium-catalyzed process gives a methanol conversion to acetic acid of 99%, against 90% for the cobalt reaction. The mechanism of the Monsanto process has been studied by Forster.414 The anionic complex m-[RhI2(CO)2]- (95) initiates the catalytic cycle, which is shown in Scheme 26. 

Assume that the basic mechanism of the Monsanto process is valid for the cobalt-based process. What would be the crucial hypothetical catalytic... 

Scheme 1. Proposed mechanism for the Monsanto acetic acid synthesis.

Monsanto also discovered significant catalytic activity for iridium/iodide catalysts however, they chose to commercialize the rhodium-based process due to its higher activity under conventional high water conditions. Despite this, detailed mechanistic studies by Forster and his colleagues were undertaken at Monsanto and revealed a catalytic mechanism for iridium which is similar to the rhodium system in many respects, but with additional complexity due to participation of both anionic and neutral complexes (see below). 

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