Infrared spectroscopic study of dimethyl ether carbonylation catalysed by TiO2-supported rhodium carbonyls

Abstract

Supported rhodium carbonyls were synthesized by deposition of Rh₆(CO)₁₆ on TiO₂. The samples were active for gas-phase dimethyl ether carbonylation in the presence of methyl iodide to give methyl acetate. Infrared spectra recorded under reaction conditions indicate that CO was activated on Rh sites in the form of Rh dicarbonyls, while dimethyl ether was dissociatively adsorbed on the support in the form of methoxy species bonded linearly to Ti⁴⁺ sites. The results also indicate the presence of Rh–acetyl and acetate complexes, as evidenced by the appearance of ν_CO bands at 1727 and 1668 cm⁻¹ during catalysis. These surface species could be regarded as reaction intermediates. The data demonstrate the bifunctional character of metal oxide-supported Rh catalysts and suggest that the carbonylation reactions of dimethyl ether and methanol catalysed by supported Rh occur by analogous mechanisms.