Acetic Acid (CH₃COOH)

The carbonylation of methanol is currently one of the major routes for acetic acid production. The basic liquid-phase process developed by BASF uses a cobalt catalyst at 250°C and a high pressure of about 70 atmospheres. The newer process uses a rhodium complex catalyst in presence of CH₃I, which acts as a promoter. The reaction occurs at 150°C and atmospheric pressure. A99% selectivity is claimed with this catalyst:

The mechanism of the carbonylation reaction is thought to involve a first-step oxidative addition of the methyl iodide promotor to the Rh(I) complex, followed by a carbonyl cis insersion step:

Carbonylation followed by reductive elimination produces back the Rh(I) catalyst:

The final step is the reaction between acetyl iodide and methyl alcohol, yielding acetic acid and the promotor:

Figure 1.1 is a flow diagram showing the Monsanto carbonylation process. Acetic acid is also produced by the oxidation of acetaldehyde and the oxidation of n-butane. However, acetic acid from the carbonylation route has an advantage over the other commercial processes because both methanol and carbon monoxide come from synthesis gas, and the process conditions are quite mild.

Figure 1.1. The Monsanto methanol carbonylation process for producing acetic acid.