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Methanol carbonylation

Most acetic acid is produced by methanol carbonylation. In this process, methanol and carbon monoxide react to produce acetic acid according to the equation:

The process involves iodomethane as an intermediate, and occurs in three steps. A metal carbonyl catalyst is needed for the carbonylation (step 2).

Two related processes exist for the carbonylation of methanol: the rhodium-catalyzed Monsanto process, and the iridium-catalyzed Cativa process. The latter process is greener and more efficient and has largely supplanted the former process. Catalytic amounts of water are used in both processes, but the Cativa process requires less, so the water-gas shift reaction is suppressed, and fewer by-products are formed.

By altering the process conditions, acetic anhydride may also be produced in plants using rhodium catalysis.