Monsanto process — catalytic carbonylation of methanol to acetic acid

Catalyst and reaction features

The classic Monsanto system uses a rhodium complex together with iodide promoters and operates with very high selectivity (commonly reported >99%). Reaction conditions are moderately severe: elevated pressure (roughly 30–60 atm) and elevated temperature (typically about 150–200 °C) are used to achieve practical rates and yields.

Mechanism (simplified)

• Formation of methyl iodide from methanol and an iodide source.
• Oxidative addition of methyl iodide to the rhodium center.
• Insertion of carbon monoxide (carbonylation) to form an acetyl‑rhodium intermediate.
• Reductive elimination gives acetyl iodide, which is hydrolysed to acetic acid, regenerating iodide.

History and development

The Monsanto carbonylation route was developed and commercialized in the 1960s by industrial research teams; related carbonylation chemistry was studied by several firms and laboratories, including work at BASF. The process brought a major improvement in selectivity and efficiency compared with earlier heterogeneous routes.

Industrial importance and successors

For decades the Monsanto process supplied a substantial fraction of global acetic acid demand owing to its efficiency and selectivity. In later years it was largely supplanted by the Cativa process, an iridium-based catalytic system that offers improved economics, lower water requirements and reduced corrosion issues. Both approaches illustrate how catalyst choice governs plant design and environmental footprint.

Notable practical points

Operators must manage iodide corrosion, catalyst recovery and CO handling when running the process at scale. Although the original Monsanto technology is less common in new plants today, its mechanism and development remain a key case study in homogeneous catalysis and industrial chemistry.