Rh Catalysed Carbonylation of MeOH to AcOH

The Rh catalysed carbonylation of MeOH to AcOH was studied at Monsanto by HP IR under working reaction conditions using a short path length transmission cell coupled to a stirred reactor [12]. The presence of [Rh(CO)2l2] as the principal Rh species was generally noted. Consistent with the model studies and the kinetics of the carbonylation reaction, which tended to first order in total Rh and Mel, the rate controlling step was of course the reaction of [Rh(CO)2l2r with Mel. 

Later Eisenberg reported that the water gas shift reaction catalysed by Rh and HI involved [Rh(CO)2l2] and [Rh(CO)2l4] , (Eq. (48)) and (Eq. (49)), though the original publication was not substantially based on spectroscopic studies [48, 49]. 

This increased [HI] leads to the formation of [Rh(CO)2l4] , a decrease in [Rh(CO)2l2] and thus a decrease in carbonylation rate. Carrying out the carbonylation of MeOH to AcOH at lower [H2O] leads to decreased activity. At the same [MeOAc] and lower [H20[ more [Rh(CO)2l4] is present. 

Just as intermediates such as [RhMe(CO)2l3] are not generally observed under the usual working conditions of MeOH carbonylation to AcOH, no intermediate species are observed directly for Rh catalysed water gas shift. However, the first step in oxidation of [Rh(CO)2l2] to [Rh(CO)2l4] , and thus a key step in the Rh/HI catalysed water gas shift reaction, the formation of [RhH(CO)2l3] , has been reported by Bunel, who followed the reaction by NMR (Eq. (50)) [50]. 

The overall decrease in carbonylation activity in Rh catalysed MeOH carbonylation at lower [H2O] can be explained both by an increase in the rate of formation of [Rh(CO)2l4] due to the increased acidity of HI at lower [H2O] and a decrease in the rate of reduction of [Rh(CO)2l4] by H2O and CO at lower [H2O]. 

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Many other modifications, particularly of the Rh and Mel catalysed carbonylation of MeOH, have been proposed and some of these have been operated commercially or may have been tested at significant pilot plant scale. These include, for example, the use of phosphine oxide species such as PPh30 [20] as promoters and systems involving immobilizing the Rh on ion exchange resins [21]. Numerous examples of ligand modified catalysts have been described, particularly for Rh, though relatively few complexes have been shown to have any extended lifetime at typical process conditions and none are reported in commercial use [22, 23]. The carbonyl iodides of Ru and Os mentioned above in the context of the Cativa process are also promoters for Rh catalysed carbonylation of MeOH to AcOH [24]. 

The result of these studies has been to show how the differences between these apparently very similar processes arise. In the Rh catalysed carbonylation of MeOH to AcOH, it is the control of [HI] which determines how much of the catalyst is present in the active form as well as the relative rate of the competing water gas shift cycle and it is the property of HI as an acid, which is important. In the Ir catalysed carbonylation of MeOH to AcOH, it is again the control of [HI] which is important, not so much because of the shift between active and inactive forms of the catalyst as with Rh but because of the inhibition of the carbonylation cycle by F and thus because of the property of HI as an iodide rather than as an acid. 

The present understanding of the mechanism of Rh or Ir catalysed carbonylation of MeOH to AcOH and Rh catalysed carbonylation of MeOAc to AC2O is due in large part to the application of spectroscopy, particularly IR and NMR. 

In 1986, BP Chemicals became the owners of the Monsanto technology. They subsequently also developed their own Cativa process, aimounced in 1996, carbonylation of MeOH to AcOH catalysed by Ir and Mel and promoted with specific metal iodides [8]. As with the improvements in the original Monsanto Rh process, Cativa had benefits such as improved catalyst stability and more favorable operating conditions [9]. 

The reaction of HI with MeOH to regenerate Mel, as well as the esterification of MeOH and AcOH were recognized as interrelated equilibria by the team at Monsanto who first investigated the Rh catalysed carbonylation of MeOH, (Eq. (3)), (Eq. (10)), (Eq. (31))... 

Since then MeOH carbonylation to AcOH has been developed to commercial scale so successfully that it has displaced other technologies for the manufacture of AcOH. The first of these processes, operated from the 1960s, was that of BASF using Co promoted by iodide [2]. However, Monsanto introduced their Rh and Mel catalysed process in 1968, which operated at lower temperatures and pressures and with much improved selectivity, and this immediately became the technology of choice, being licensed to a number of other companies [3],... 

At about the same time, BP Chemicals, ivho ivere also licensors of the Monsanto process, developed their oivn process for carbonylation of a mixed MeOH/MeOAc/ H2O feed to AcOH and AC2O using a Rh and Mel catalysed process promoted with a quaternary ammonium iodide salt ([QAS]I), [7]. 

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