28: Transition Metal Catalysed Methanol Carbonylation

The production of acetic acid, via the transition metal catalysed carbonylation of methanol, is well established as a major commercial application of homogeneous catalysis. Since the 1960s, when a cobalt-based catalyst was first used industrially by BASF, methanol carbonylation processes have continuously evolved and improved. Notably, Monsanto developed a rhodium/iodide catalyst system, variants of which have been operated by a number of companies. More recently, the journey down group 9 of the periodic table was completed by the commercialisation of an iridium/iodide based catalyst system by BP Chemicals. This chapter deals with the fundamental chemistry underlying these processes. The catalytic mechanisms are discussed in the context of the activity, selectivity and stability of rhodium and iridium catalysts, together with examples from the recent literature of attempts to improve catalyst behaviour. Approaches such as catalyst immobilisation, ligand modification and promoter effects are described. The importance of mechanistic understanding, accomplished through a range of experimental and theoretical studies, is emphasised.