Cobalt/iridium-catalyzed processes

For the cobalt- and iridium-catalyzed processes the intermediate catalyst complexes are shown in Figures 6.15.5 and 6.15.6, respectively. 

Reaction (78) regenerates Mel from methanol and HI. Using a high-pressure IR cell at 0.6 MPa, complex (95) was found to be the main species present under catalytic conditions, and the oxidative addition of Mel was therefore assumed to be the rate determining step. The water-gas shift reaction (equation 70) also occurs during the process, causing a limited loss of carbon monoxide. A review of the cobalt-, rhodium- and iridium-catalyzed carbonylation of methanol to acetic acid is available.415... 

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

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)... 

It is clear that for all the three carbonylation processes, the organic part of the catal5hic cycles does not depend on the metal and remains the same. The difference between the cobalt-catalyzed cycle on the one hand and the rhodium and iridium cycles on the other is that in the former there are no oxidative addition or reductive elimination steps. Between the rhodium and iridium cycles, the difference is that in the former all the... 

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