Cobalt-catalyzed carbonylations with iodide promoters

The carbonylation of methanol is an important route to acetic acid and is the basis for two industrial processes (cobalt- and rhodium-catalyzed). The cobalt-catalyzed route uses [Co2(CO)s] as catalyst with an iodide promoter, and lies outside the scope of this work. The companion volumes Comprehensive Organometallic Chemistry should be consulted. 

In the mid-1960s, Paulik and Roth at Monsanto Co discovered that rhodium and an iodide promoter were more efficient than cobalt, with selectivities of 99% and 85%, with regard to methanol and CO, respectively. Moreover, the reaction is operated under significantly milder conditions such as 40-50 bar pressure and around 190 °C [8]. Even though rhodium was 1000 times more costly than cobalt at this time, Monsanto decided to develop the rhodium-based catalyst system mainly for the selectivity concerns, and thus for the reduction of the process cost induced by the acetic acid purification, even if it was necessary to maintain a 14% w/w level of water in the reactor to keep the stability of the rhodium catalyst. In addition, Paulik et al. [9] demonstrated that iridium can also catalyze the carbonylation of methanol although at a lower rate. However, it is noteworthy that the catalytic system is more stable, especially in the low partial pressure zones of the industrial unit. 

The mechanism of the cobalt- (BASF), rhodium- (Monsanto), and iridium- (Cativa) catalyzed reaction is similar but the rate-determining steps differ and different intermediate catalyst complexes are involved. In all three processes two catalytic cycles occur. One cycle involves the metal carbonyl catalyst (II) and the other the iodide promoter (i). For a better overview only the catalytic cycle of the rhodium-catalyzed Monsanto process is presented in detail (Figure 6.15.4). Initially the rhodium iodide complex is activated with carbon monoxide by forming the catalytic active [Rhi2(CO)2] complex 4. Further the four-coordinated 16-electron complex 4 reacts in the rate-determining step with methyl iodide by oxidative addition to form the six-coordinated 18-electron transition methyl rhodium (I II)... 

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