Conclusions Spectroscopy and Understanding Carbonylation Mechanisms

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. 

As well as characterizing complexes involved in the main catalyst cycles, spectroscopy has contributed to the measurement of the kinetics of these cycles and to byproduct reactions. The major catalyst species present under working conditions of the catalyst systems have been identified for all the systems. Individual reaction steps involving interconversion of catalyst complexes have been isolated and studied in model reactions. IR has been very important in these studies with metal carbonyl species, including the identification of Ru promoter species in MeOH carbonylation. 

At the same time NMR has proved particularly useful in determining factors influencing the important reaction of HI or Acl with MeOAc and understanding how different classes of promoters can affect the standing concentrations of these intermediates under process conditions. 

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. 

Finally, in the Rh catalysed carbonylation of MeOAc to AcjO it is the control of [Acl] rather than [HI] which determines the overall carbonylation rate because Acl is involved in the rate controlling reaction with MeOAc. 

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