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TheCativa process is a method for the production ofacetic acid by thecarbonylation ofmethanol. The technology, which is similar to theMonsanto process, was developed by BP Chemicals and is under license by BP Plc.[1]: 293–307 The process is based on aniridium-containingcatalyst, such as theanioniccomplex diiododicarbonyliridate(i) [Ir(CO)2I2]− (1).
The Cativa and Monsanto processes are sufficiently similar that they can use the same chemical plant. Initial studies by Monsanto had shown iridium to be less active thanrhodium for the carbonylation of methanol.[2] Subsequent research, however, showed that the iridium catalyst could be promoted by ruthenium, and this combination leads to a catalyst that is superior to the rhodium-based systems. The switch fromrhodium toiridium also allows the use of less water in the reaction mixture. This change reduces the number of drying columns necessary, decreases formation of by-products, such aspropionic acid, and suppresses thewater gas shift reaction.
Thecatalytic cycle for the Cativa process, shown above, begins with the reaction ofmethyl iodide with the square planar active catalyst species (1) to form the octahedral iridium(III) species (2), thefac-isomer of [Ir(CO)2(CH3)I3]−. This oxidative addition reaction involves the formalinsertion of the iridium(I) centre into the carbon-iodine bond of methyl iodide. After ligand exchange (iodide forcarbon monoxide), the migratory insertion of carbon monoxide into the iridium-carbon bond, step (3) to (4), results in the formation of asquare pyramidal species with a boundacetyl ligand. The active catalyst species (1) is regenerated by thereductive elimination ofacetyl iodide from (4), a de-insertion reaction.[1]: 94–105 The acetyl iodide ishydrolysed to produce the acetic acid product, in the process generatinghydroiodic acid which is in turn used to convert the starting material (methanol) to the methyl iodide used in the first step.