TheMonsanto process is an industrial method for the manufacture ofacetic acid by catalyticcarbonylation ofmethanol.[1] The Monsanto process has largely been supplanted by theCativa process, a similariridium-based process developed byBP Chemicals Ltd, which is more economical and environmentally friendly.
This process operates at apressure of 30–60atm and atemperature of 150–200 °C and gives a selectivity greater than 99%. It was developed in 1960 by the German chemical companyBASF and improved by theMonsanto Company in 1966, which introduced a new catalyst system.[2]
Thecatalytically active species is theanioncis-[Rh(CO)2I2]− (top of scheme).[3] The firstorganometallic step is theoxidative addition ofmethyl iodide tocis-[Rh(CO)2I2]− to form thehexacoordinate species [(CH3)Rh(CO)2I3]−. This anion rapidly transforms, via themigration of amethyl group to an adjacentcarbonylligand, affording the pentacoordinateacetyl complex [(CH3CO)Rh(CO)I3]−. This five-coordinate complex then reacts withcarbon monoxide to form the six-coordinate dicarbonyl complex, which undergoesreductive elimination to releaseacetyl iodide (CH3C(O)I). Thecatalytic cycle involves two non-organometallic steps: conversion of methanol tomethyl iodide and the hydrolysis of theacetyl iodide toacetic acid and hydrogen iodide.[4]
The reaction has been shown to befirst-order with respect to methyl iodide and [Rh(CO)2I2]−. Hence theoxidative addition of methyl iodide is proposed as therate-determining step.
Acetic anhydride is produced bycarbonylation ofmethyl acetate in a process that is similar to the Monsanto acetic acid synthesis. Methyl acetate is used in place of methanol as a source of methyl iodide.[5]
In this processlithium iodide converts methyl acetate tolithium acetate and methyl iodide, which in turn affords, through carbonylation, acetyl iodide. Acetyl iodide reacts with acetate salts or acetic acid to give the anhydride. Rhodium iodides and lithium salts are employed as catalysts. Because acetic anhydride hydrolyzes, the conversion is conducted under anhydrous conditions in contrast to the Monsanto acetic acid synthesis.
