Solid-state NMR study of the kinetics and mechanism of dimethyl ether carbonylation on cesium salt of 12-tungstophosphoric acid modified with Ag, Pt, and Rh

摘要

By using ~(13)C magic angle spinning (MAS) NMR, the mechanism of dimethyl ether (DME) carbonylation with carbon monoxide has been studied on solid metal-containing acidic cesium salt of 12-tungstophosphoric acid, M/Cs_2HPW_(12)O_(40) (M/HPA, M = Ag, Pt, and Rh). The kinetics of the reaction has been monitored with ~1H MAS NMR in situ. Activation of DME occurs on acidic OH groups of M/HPA and gives rise to the surface methoxy groups at 293–473 K. Carbon monoxide forms the surface metal–carbonyl complexes on M/HPA at 293–473 K. The insertion of CO from the carbonyl into the CH_3-O bond of surface methoxide results in the acetate group attached to the Keggin anion, from which the target product, methyl acetate, is produced under the interaction with dimethyl ether. The reaction rate decreases in the following order: Rh/Cs_2HPW_(12)O_(40)?Pt/Cs_2HPW_(12)O_(40) > Ag/Cs_2HPW_(12)O_(40), which correlates well with concentration of the reaction intermediates, methoxy groups, and metal–carbonyls. The higher concentration of these intermediates detected on Rh/Cs_2HPW_(12)O_(40) is in charge of the higher carbonylation rate on this catalyst with respect to M/HPA (M = Ag, Pt) catalysts.