Effect of Rhodium Carbonyls on the Selectivity of Bifunctional Catalysts of Halogen-Free Carbonylation of Dimethyl Ether into Methyl Acetate

A new gas-phase process of halogen-free carbonylation of dimethyl ether (DME) into methyl acetate is a promising environmental preparation method of methyl acetate. Bifunctional catalysts with a high concentration of strong Bronsted acid sites, i.e. rhodium-promoted acid cesium salts of phosphotungstic heteropoly acid with the formula Rh/CsxH3-xPW12O40 (1.5 <= x <= 2), show the highest activity and selectivity in the carbonylation reaction of DME. Their use in a reducing medium, where there are significant transformations of the catalyst surface area until changing the phase composition, is a distinctive feature of these catalysts. In order to control the activity and selectivity of catalysts in the DME carbonylation reaction, the research on the formation of the active surface of the catalyst was carried out under reaction conditions. The formation of different rhodium carbonyls, such as Ph(CO)(2)(+) and Rh-6(CO)(16), was detected by FT IR spectroscopy of adsorbed CO on the surface of the in situ reduced Rh/Cs2HPW12O40 catalyst after filling with CO. The formation of metal particles was identified in case of changing activation conditions. The dependence was found between reduction conditions, the composition of the in situ catalyst surface area with selectivity for methyl acetate. The presence of rhodium metal particles causes C-O bond cleavage in the DME molecule and significantly reduces selectivity for methyl acetate, from 95 to 50 %. Catalysts, on the surface of which there are only rhodium carbonyls under reaction conditions upon a complete lack of trace rhodium metal, showed the highest activity and selectivity for methyl acetate. The acquired results may form the basis for developing a highly active and stable catalyst for an environmentally safe process of halogen-free DME carbonylation.