Carbonylation of dimethyl ether on Rh/Cs2HPW12O40: Solid-state NMR study of the mechanism of reaction in the presence of a methyl iodide promoter

Using 13C solid-state nuclear magnetic resonance, the carbonylation of dimethyl ether (DME) with carbon monoxide has been studied on solid Rh/Cs2HPW12O40 in the presence of a methyl iodide promoter. The observed decrease in the reaction temperature in comparison with halide-free systems is caused by a change of the reaction mechanism. At first, the activation of the promoter and CO on Rh/Cs2HPW12O40 produces methyl rhodium carbonyl species. Carbon monoxide is then embedded into the Rhsingle bondCH3 bond to afford Rh acetyl. Rapid migration of the acetyl group from the Rh center to a Brønsted acid site of Rh/Cs2HPW12O40 gives rise to an acetate group attached to the Keggin unit (Keggin acetate). Brønsted acid sites provide dimethyl ether activation with the formation of a surface methoxy group. The latter, through reaction with hydrogen iodide, which is produced at the stage of Rh acetyl-to-Keggin acetate transformation, restores methyl iodide for subsequent carbonylation stages. Methyl acetate formation from a Keggin acetate and DME closes a catalytic cycle.