Catalytic Oxidation of Dimethyl Ether to Dimethoxymethane over MnCl2-H4SiW12O40/SiO2 Catalyst

The H4SiW12O40/SiO2 heteropolyacid catalyst for the catalytic oxidation of dimethyl ether to dimethoxymethane was prepared by the impregnation method. MnCl2, SnCl4, and CuCl2 were used to modify the catalyst to improve its activity and selectivity. The catalytic oxidation reaction was carried out in a continuous flow fixed-bed reactor. H4SiW12O40/SiO2 is active for the oxidation of dimethyl ether, but the selectivity for dimethoxymethane is as low as 4.8%. Modification of H4SiW12O40/SiO2 with 5% MnCl2 significantly improves the dimethoxymethane selectivity up to 27.9% at 633 K, while SnCl4- and CuCl2-modified catalysts give dimethoxymethane selectivities of 16.4% and 0.4%, respectively, under the same conditions. The effects of the MnCl2 content (2%–20%) and the reaction temperature (573–633 K) on the reaction were investigated. A dimethyl ether conversion of 8.6% and a dimethoxymethane selectivity of 36.3% were obtained under the optimum conditions of 593 K and 5% MnCl2 content. X-ray diffraction patterns of the catalysts show that MnCl2 and H4SiW12O40 interact and are dispersed uniformly on the support. Infrared spectra demonstrate that the Keggin structure of H4SiW12O40 remains almost unchanged over the modified catalyst. Temperature-programmed desorption profiles of ammonia indicate that MnCl2 modification reduces the acidity of H4SiW12O40/SiO2 by decreasing the acid center numbers.