MnCl2 modified H4SiW12O40/SiO2 catalysts for catalytic oxidation of dimethy ether to dimethoxymethane

The catalytic oxidation of dimethy ether (DME) to dimethoxymethane (DMM) was carried out over MnCl2 modified H4SiW12O40/SiO2 catalysts prepared by impregnation method in a continuous flow type fixed-bed reactor with ratio of n(DME)/n(O2) = 1 : 1 at 593 K and 0.1 MPa. The effect of calcination temperatures, different impregnation sequences, reaction time and different feeding modes over MnCl2(5 wt%)-H4SiW12O40/SiO2 catalyst were all investigated. The results show that the catalyst, which was prepared by first H4SiW12O40 then MnCl2 impregnated in SiO2 and calcined at 673 K demonstrates the optimum activity with 8.6% of DME conversion and 39.1% of DMM selectivity. The pure DME reactants form higher selectivity of DMM than pure CH3OH reactants. The BET surface area, total pore volume, X-ray diffraction (XRD), NH3-TPD, TEM, FT-IR, XPS and ICP-AES were used to characterize the structure and performance of the catalysts deeply. XPS shows that Mn4+ species of the fresh catalyst play important role and may be reduced to Mn2+ or Mn3+ species in the synthesis of DMM from DME oxidation reaction. Besides, the amount of superficial Mn declines by 15.5% after the reaction. ICP-AES indicates that the total amount of Mn after the reaction decreases by only 4.9% than that of the fresh sample. XRD patterns of the catalysts calcined at different temperatures show that the Keggin structure of the catalysts are destroyed when the temperature exceeds 673 K, Besides, NH3-TPD profiles also indicate that the acidity of the catalyst becomes weaker with the calcination temperature increased. Compared to the fresh catalyst, the acidity of the catalyst after the reaction is also weakened. It is found that Mn modified H4SiW12O40/SiO2 improves the catalyst oxidative activity and moderates the acidity of the catalyst. The mechanism of this reaction may be that DMM can be synthesized via methoxy groups formed by DME dissociation under the cooperation of the acid sites and redox sites of Mn modified H4SiW12O40/SiO2 catalysts. (c) 2006 Elsevier B.V. All rights reserved.