Iridium-complex catalyzed carbonylation iodide concentration

Iridium complexes catalyze carbonylation of CH3OH to acetic acid, also with an iodide promoter. The reaction rate relative to Rh is much slower. The steps in the reaction sequence are similar to those for Rh, but the kinetics are more complex . Complex interactions involve HjO, the form of the iodide promoter and CO pressure . For example, at high concentration of 1 ion, the rate increases with increasing pressure. At low 1 levels and low HjO concentration, the reaction rate is inversely dependent on CO pressure. Catalyst species under these different reaction conditions include IrfCOljI, IrH(CO)2l2(OH2), [Ir(CH3)(CO)2l3] and [IrH(CO)2l3] . In acetophenone solvent at 175°C and 3 MPa, the reaction is first-order in CH3OH and independent of CH3I at concentrations in which the I Ir ratio is >20 . Under some conditions, the water gas shift reaction becomes important careful control is necessary for high efficiencies to acetic acid. 

A process for the coproduction of acetic anhydride and acetic acid, which has been operated by BP Chemicals since 1988, uses a quaternary ammonium iodide salt in a role similar to that of Lil [8]. Beneficial effects on rhodium-complex-catalyzed methanol carbonylation have also been found for other additives. For example, phosphine oxides such as Ph3PO enable high catalyst rates at low water concentrations without compromising catalyst stability [40—42]. Similarly, iodocarbonyl complexes of ruthenium and osmium (as used to promote iridium systems, Section 3) are found to enhance the activity of a rhodium catalyst at low water concentrations [43,44]. Other compounds reported to have beneficial effects include phosphate salts [45], transition metal halide salts [46], and oxoacids and heteropolyacids and their salts [47]. 

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