The Monsanto Process

In the Monsanto process, methanol is combined catalytically with carbon monoxide to give acetic (ethanoic) acid. This reaction is done in the presence of a catalyst. Much of the original work was done by Forster. This process is now run by British Petroleum.

Forster established the mechanism of the process. For a number of years a team in Sheffield lead by Professor P.M. Maitlis has been working on the details of the mechanism. Dr. A. Haynes has carried out detailed kinetic measurements and I have been involved using NMR spectroscopy to study some details of the process. In this web page, my contributions to the understanding of the process are described.

The basic catalytic pathway is

The feedstock is methanol which reacts with HI to give methyl iodide.

The methyl iodide reacts with [Rh(CO)2I2]- to give [Rh(CH3)(CO)2I3]-. This compound had not been previously identified. It is in equilibrium with [Rh(CO)2I2]-. It rearranges to give [Rh{C(O)CH3}(CO)I3]-. Dr. A. Haynes had shown that a significant concentration of [Rh(CH3)(CO)2I3]- can be obtained in neat methyl iodide. [Rh(CH3)(CO)2I3]- was identified in solution using 1H and 13C NMR spectroscopy.

Forster had isolated [Rh{C(O)CH3}(CO)I3]2-2 from the reaction mixture and identified it by crystallography. We showed that in solution it is almost certainly present as [Rh{C(O)CH3}(CO)I3]2-2.

Reaction of [Rh{C(O)CH3}(CO)I3]2-2 with carbon monoxide gives [Rh{C(O)CH3}(CO)2I3]-2. At low temperature the rotation about the Rh-acetyl bond slows and the two carbonyl groups become inequivalent. By use of 13CO, J(13CO,13CO) showed that the two carbonyls are mutually trans and established the stereochemistry of this intermediate.

The reaction cycle is completed by the loss of CH3COI to regenerate [Rh(CO)2I2]-. The CH3COI reacts with water to generate ethanoic (acetic) acid and HI.


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[Brian Mann's home page] [Department of Chemistry] [University of Sheffield]

Date:- 6 January, 1997