Promotion by Iodide Salts

One approach that enables the use of lower water concentrations for rhodium-complex-catalyzed methanol carbonylation is the addition of iodide salts, as exemplified by the Celanese Acid Optimization (AO Plus) technology [11,33]. A lithium iodide promoter allows carbonylation rates to be achieved that are comparable with those in the conventional Monsanto process—but at significantly lower water concentrations. The AO technology has been implemented to increase productivity at the Celanese facility in Clear Lake, Texas, and in a new 500 kt/a plant in Singapore. 

Lowering the water concentration would normally result in a decrease in the proportion of rhodium existing as [Rh(CO)2I2]. However, the lithium iodide-promoted process also employs a higher concentration of methyl acetate, which reacts with the other components as shown in Equations (8) and (9). Thus, raising the methyl acetate concentration results in a lower HI concentration, which tends to inhibit oxidation of [Rh(CO)2I2] to give [Rh(CO)2I4], thus suppressing the WGS reaction significantly, which is beneficial for catalytic carbonylation activity. 

It has been found that iodide salts can promote the oxidative addition of Mel to [Rh(CO)2I2], the rate-determining step in the cycle of the rhodium-complex-catalyzed methanol carbonylation reaction [20]. 

The precise mechanism of this promotion remains unclear formation of a highly nucleophilic dianion, [Rh(CO)2I3]2-, has been suggested, although there is no direct spectroscopic evidence of it. The possible participation of this dianion has been considered in a theoretical investigation [34]. An alternative nucleophilic dianion, [Rh(CO)2I2(OAc)]2-, has also been proposed [35,36] on the basis that acetate salts (either added or generated in situ via Equation (8)) can promote carbonylation. 

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]. 

---