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. My contributions to the understanding of the process are described below.

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.

References

D. Forster, Adv. Organomet. Chem., 1979, 17, p. 255.

A. G. Kent, B. E. Mann, and C. P. Manuel, "The solution structure of [Rh(COMe)(CO)₂I₃]^- and the absence of methyl migration in [Rh₂(COMe)₂(CO)₂I₆]^2- ", J. Chem. Soc., Chem. Commun., 1985, pp. 728-729.

A. G. Kent, B. E. Mann, and C. P. Manuel, "The solution behaviour of [₂(COMe)₂(CO)₂I₆]^2-, its reactions with CO, pyridine, and methanol, and the X-ray structure of [Ph₄As][Rh(COMe)(CO)(NC₅H₅)I₃]", J. Chem. Soc., Dalton Trans., 1987, pp. 489-496.

A. Haynes, B. E. Mann, D. J. Gulliver, G. E. Morris, and P. M. Maitlis, "The direct observation of [MeRh(CO)₂I₃]^-, the key intermediate in rhodium-catalysed methanol carbonylation", J. Am. Chem. Soc., 1991, 112, pp. 8567-8569.

A. Haynes, B. E. Mann, G. E. Morris, and P. M. Maitlis, "Mechanistic studies on rhodium-catalysed carbonylation reactions – Spectroscopic detection and reactivity of a key intermediate, [MeRh(CO)₂I₃]^- ", J. Am. Chem. Soc., 1993, 115, pp. 4093-4100.