Carbonylation of dimethyl ether on solid Rh-promoted Cs-salt of Keggin 12-H3PW12O40: A solid-state NMR study of the reaction mechanism

The carbonylation of dimethyl ether (DME) with carbon monoxide on Rh-promoted cesium salt of 12-tungstophosphoric acid, Rh/Cs2HPW12O40 (HPA), has been studied with 13C solid-state NMR. The bi-functional character of Rh/Cs2HPW12O40 catalyst in halide-free carbonylation of DME has been directly demonstrated. The activation of the C–O bond of DME proceeds on Brønsted acid sites of HPA with the formation of the methyl group attached to the surface of HPA (methoxy species), whereas the role of rhodium consists in trapping carbon monoxide from gaseous phase and a transfer of CO to the center of DME activation, acidic OH-group of the catalyst, in the form of rhodium carbonyls. The lattice of Cs2HPW12O40 makes it possible to locate these two different active centers in close proximity to each other, e.g., on two adjacent oxygen atoms, terminal and bridging, of one Keggin anion, thus facilitating the insertion of carbon monoxide from rhodium carbonyl into the C–O bond of methoxy-group to produce the acetate group bound to the Keggin anion. The latter offers finally methyl acetate under the interaction with DME, the intermediate surface methoxy-groups being restored.

The mechanism of the halide-free carbonylation of dimethyl ether with carbon monoxide on Rh/Cs2HPW12O40 has been established. The formation and inter-conversions of the reaction intermediates – surface methoxy-groups, Rh-carbonyls and surface acetates, has been monitored with 13C solid-state NMR. The bi-functional character of the catalyst has been directly demonstrated.Figure optionsDownload high-quality image (102 K)Download as PowerPoint slideResearch highlights
► Activation of DME on Rh/Cs2HPW12O40 occurs on Brønsted acid sites.
► The activation of DME results to the formation of surface methoxy species.
► The role of rhodium in Rh/Cs2HPW12O40 consists in trapping carbon monoxide from gaseous phase.
► The formed rhodium carbonyl transfers CO to the center of DME activation.
► Location of rhodium carbonyl in vicinity of methoxy group facilitates formation of acetate group.