Ir Catalysed Carbonylation of MeOH to AcOH

Compared with Rh systems, where the two principal species are well resolved, it can be seen that as well as more Ir species the bands also overlap, making quantification more difficult. Qualitatively some conclusions can be drawn from the spectra. Forster identified for example that in the presence of I , a potent catalyst poison, much of the Ir could still be present as [IrMe(CO)2l3] . Similarly, as [H2O] is increased the carbonylation rate falls. This is consistent with increased [T] since equilibrium [HI] increases with [H2O] as described above, inhibiting the migratory insertion reaction of [IrMe(CO)2l3] . 

When Ru carbonyl iodides are used as promoters for Ir catalysed carbonylation of MeOH to AcOH, the accurate quantification of Ir species becomes more difficult because the bands of the Ru species overlap those of the Ir species and they have larger extinction coefficients, so dominating the spectra. In batch reactions followed by HP IR, the Ru bands present initially before Ir is added and carbonylation commences have been assigned to neutral Ru carbonyl iodides and [Ru(CO)3l3]  

which has a similar promotional effect to Ru carbonyl iodides for Ir catalysed MeOH carbonylation to AcOH, of course has no carbonyl bands to obscure the Ir species. Both [Ir(CO)2l4] and [IrMe(CO)2l3] are observed under carbonylation conditions by HP IR, as in the absence of Inl3 [18]. 

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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. 

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 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. 

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]. 

A set of spectra from the carbonylation of MeOH catalysed by Ir is shown in Figure 5.4. It will be seen that background to the catalyst species is changing throughout the reaction as MeOAc and H2O are consumed to produce AcOH in the liquid phase. To obtain spectra of the catalyst, which can be compared with each other, and where the species can be reliably quantified, may require that further background spectra of the reaction composition be available. The spectra of a mid-point and an end-point composition are usually sufficient for this. 

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