production method: The Monsanto process

This process was first developed by BASF in 1960 using the reaction of methanol and carbon monoxide with co-catalysts of cobalt and iodide at conditions of 300 ºC and 700 atmospheres.

chemical equation: Monsanto Process

This was taken further by Monsanto in 1966, who used the same reaction, but a new catalyst system of Rhodium/iodide ion co-catalysts, which operated at milder conditions of 150 - 200 ºC and 30 - 60 atmospheres. The methanol used is produced from synthesis gas (a mixture of CO, CO2/ H2). The reaction can be viewed as inserting carbon monoxide into the C-O bond of the methanol and is known as methanol carbonylation and also commercially, as the Monsanto process.

This process was first commercialised in 1970 and was a great advance on the previous processes in several respects:

  • It has an atom economy of 100%, with all atoms in the reactants going into the product (see equation above). In contrast, the older direct oxidation of butane and naphtha had a very poor atom economy with respect to ethanoic acid (about 35%). Consequently, there is a huge reduction in waste, and purification of the product is much easier
  • The whole process uses less energy, particularly for product separation and purification.
  • Has a high yield, approximately 98% based on methanol (90% based on carbon monoxide).
  • Uses methanol, a cheaper feedstock than the previous naphtha/ butane.
  • Although methanol is usually manufactured from synthesis gas, produced from oil, it can also be produced from biomass (wood), municipal wastes and sewage. This may eventually lead to the process being no longer dependent on oil.
  • The reaction is extremely fast, and the catalyst has a long life

The Monsanto process has been the dominant process used in the production of ethanoic acid from 1970 to the late 1990s.

Reaction mechanism

diagram: Rhodium catalyst

The mechanism of the reaction has been studied in great detail, in particular the role of the catalyst. For more detailed information on the mechanism visit the catalysis site.

diagram: commercial Monsanto plan

Monsanto Process Weaknesses

The catalyst system does have some drawbacks:

  • Rhodium metal is very expensive - more expensive than gold (current prices can be found at the Johnson Matthey web site
  • Rhodium and iodide form insoluble salts such as RhI3, so water content in the reaction vessel has to be kept relatively high to prevent this. A final distillation step is required to remove water, adding to the costs and energy demand. Any precipitation occurring removes catalyst, which must be recovered and returned to the main reactor.
  • Rhodium also catalyses side reactions such as: -

          chemical equation: CO + H20 = CO2 + H2
This reduces the partial pressure of carbon monoxide, so the mixture must be removed from the reaction vessel and replaced with more carbon monoxide.

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