Acetic acid is typically produced by methanol carbonylation. In this process, methanol and carbon monoxide react to produce acetic acid according to the equation:
CH3OH + CO = CH3COOH
Two related processes 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, often in the same production plants. 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 on the same plant using the rhodium catalysts.