Everything about The Monsanto Process totally explained
The
Monsanto process is an important method for the manufacture of
acetic acid. This process operates at a
pressure of 30-60
atm and a
temperature of 150-200 °C and gives a selectivity greater than 99%. The Monsanto process has largely been supplanted by the
Cativa process, a similar
iridium-based process developed by
BP Chemicals Ltd which is more economical and environmentally friendly.
Catalytic cycle
The
catalytically active species is the
anion cis-
Rh(CO)2I2- (1). The
catalytic cycle has been shown to involve six steps, two of which don't involve rhodium: conversion of methanol to methyl iodide and the hydrolysis of the acetyl iodide to acetic acid. The first
organometallic step is the
oxidative addition of
methyl iodide to
cis-
Rh(CO)2I2- to form the
hexacoordinate species
(CH3)Rh(CO)2I3- (2). This anion rapidly transforms, via the migration of a
methyl group to the
carbonyl ligand, affording the pentacoordinate
acetyl complex
(CH3CO)Rh(CO)I3- (3). This five-coordinate complex then reacts with
carbon monoxide to form the six coordinate dicarbonyl complex
(4) which decomposes by
reductive elimination to form
acetyl iodide (CH
3COI) and regenerate the active form of the
catalyst. Acetic acid iodine is then hydrolyzed to
acetic acid.
The reaction has been shown to be
first order with respect to methyl iodide and the catalyst
(1). Hence the
rate-determining step of the catalytic cycle has been proposed to be the
oxidative addition of methyl iodide to the catalyst
(1). It is believed that this occurs via the
nucleophilic attack by the rhodium centre on the carbon of methyl iodide.
Tennessee Eastman Acetic Anhydride Process
Acetic anhydride is produced by
carbonylation of
methyl acetate in a process that was inspired by the Monsanto acetic acid synthesis.
» CH
3CO
2CH
3 + CO → (CH
3CO)
2O
In this process methyl acetate converts to methyl iodide, which in turn affords acetyl iodide, which reacts with acetate salts or acetic acid to give the product. Rhodium iodides and lithium salts are employed as catalysts. Because acetic anhydride isn't stable in water, the conversion is conducted under anhydrous conditions in contrast to the Monsanto acetic acid synthesis.
Further Information
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