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 RhCH3 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.

Methyl iodide provides acceleration of dimethyl ether carbonylation on Rh/Cs2HPW12O40 catalysts by opening a new reaction pathway with the reactive species arranged at a mononuclear rhodium center. 13C solid-state NMR revealed the stages of formation and interconversions of the reaction intermediates—surface Rh methyl, Rh carbonyls, surface acetates, and methoxy groups.Figure optionsDownload high-quality image (101 K)Download as PowerPoint slideHighlights
► Methyl iodide facilitates dimethyl ether carbonylation on Rh/Cs2HPW12O40.
► Methyl iodide provides the formation of surface Rh methyl groups.
► Reactive species, methyl, and carbonyl groups are arranged at mononuclear Rh center.
► Dimethyl ether is activated at Brønsted acid sites of Cs2HPW12O40.
► Dimethyl ether is involved into the process via the interaction with hydrogen iodide.