Monsanto process
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The '''Monsanto process''' is an important method for the manufacture of [[acetic acid]]. This process operates at a [[pressure]] of 30-60 [[atmosphere (unit)|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|BP Chemicals Ltd]] which is more economical and environmentally friendly.
==Catalytic cycle==
[[Image:monsantocycle.jpg|center|The Mosanto process]]
The [[catalysis|catalytically]] active species is the [[anion]] cis-[[Rh(CO)<sub>2</sub>I<sub>2</sub>]]<sup>-</sup> '''(1)'''. The [[catalytic cycle]] has been shown to involve six steps, two of which do not involve rhodium: conversion of methanol to methyl iodide and the hydrolysis of the acetyl iodide to acetic acid. The first [[organometallic chemistry|organometallic]] step is the [[oxidative addition]] of [[methyl iodide]] to ''cis''-[[Rh(CO)<sub>2</sub>I<sub>2</sub>]]<sup>-</sup> to form the [[octahedral molecular geometry|hexacoordinate]] species [[(CH<sub>3</sub>)Rh(CO)<sub>2</sub>I<sub>3</sub>]]<sup>-</sup> '''(2)'''. This anion rapidly transforms, via the migration of a [[methyl]] group to the [[carbonyl]] [[ligand]], affording the pentacoordinate [[acetyl]] complex [[(CH<sub>3</sub>CO)Rh(CO)I<sub>3</sub>]]<sup>-</sup> '''(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<sub>3</sub>COI) 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 reaction|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.<ref>Zoeller, J. R.; Agreda, V. H.; Cook, S. L.; Lafferty, N. L.; Polichnowski, S. W.; Pond, D. M. "Eastman Chemical Company Acetic Anhydride Process" Catalysis Today (1992), volume 13, pp.73-91. doi:10.1016/0920-5861(92)80188-S</ref>
: CH<sub>3</sub>CO<sub>2</sub>CH<sub>3</sub> + CO → (CH<sub>3</sub>CO)<sub>2</sub>O
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 is not stable in water, the conversion is conducted under anhydrous conditions in contrast to the Monsanto acetic acid synthesis.
==References==
<references/>
* http://www.uyseg.org/catalysis/ethacid/ethacid2.htm
* http://www.uyseg.org/greener_industry/pages/ethanoicAcid/6ethanoicAcidPM2.htm
{{Organometallics}}
[[Category:organometallic chemistry]]
[[Category:Rhodium]]
[[Category:Industrial processes]]
[[id:Proses Monsanto]]
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[[pt:Processo Monsanto]]