The Monsanto process is an industrial method for the manufacture of

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

by catalytic carbonylation of

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

. 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. This process operates at a

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

of 30–60 atm and a

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

of 150–200 °C and gives a selectivity greater than 99%. It was developed in 1960 by the German chemical company

BASF BASF SE (), an initialism of its original name , is a European Multinational corporation, multinational company and the List of largest chemical producers, largest chemical producer in the world. Its headquarters are located in Ludwigshafen, Ge ...

and improved by the Monsanto Company in 1966, which introduced a new catalyst system.

Catalytic cycle

The catalytically active species is the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

''cis''- h(CO)₂I₂sup>− (top of scheme). The first

organometallic Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

step is the oxidative addition of methyl iodide to ''cis''- h(CO)₂I₂sup>− to form the hexacoordinate species CH₃)Rh(CO)₂I₃sup>−. This anion rapidly transforms, via the migration of a methyl group to an adjacent carbonyl

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

, affording the pentacoordinate

acetyl In organic chemistry, an acetyl group is a functional group denoted by the chemical formula and the structure . It is sometimes represented by the symbol Ac (not to be confused with the element actinium). In IUPAC nomenclature, an acetyl grou ...

complex CH₃CO)Rh(CO)I₃sup>−. This five-coordinate complex then reacts with

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

to form the six-coordinate dicarbonyl complex, which undergoes reductive elimination to release acetyl iodide (CH₃C(O)I). The catalytic cycle involves two non-organometallic steps: conversion of methanol to methyl iodide and the hydrolysis of the acetyl iodide to

and hydrogen iodide. The reaction has been shown to be first-order with respect to methyl iodide and h(CO)₂I₂sup>−. Hence the oxidative addition of methyl iodide is proposed as the rate-determining step.

Tennessee Eastman acetic anhydride process

Acetic anhydride is produced by carbonylation of methyl 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. : CH₃CO₂CH₃ + CO → (CH₃CO)₂O In this process lithium iodide converts methyl acetate to lithium 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.

References

External links

* https://web.archive.org/web/20050412040850/http://www.uyseg.org/catalysis/ethacid/ethacid2.htm {{Organometallics Organometallic chemistry Rhodium Catalysis Chemical processes Monsanto