Selective Oxidation Of Methanol To Dimethoxymethane Over Bifunctional Vanadium Catalyst

Dimethoxymethane （DMM） is an important intermediate, and has been widelyused as diesel fuel additive and organic synthesis reagent. It has been also utilized asan excellent solvent in pharmaceutical and perfume industries due to low toxicity.Bifunctional vanadium catalysts for selective oxidation of methanol to DMM werestudied.VO_x/TS-1catalysts were prepared by impregnation and modified via dopingSO42-and PO₄^3-. The VO_x/TS-1catalys exhibited high catalytic activity and DMMselectivity in selective oxidation of methanol to DMM.423K was the optimumreaction temperature under atmospheric pressure. The structural and physicochemicalproperties of the catalyst were characterized by XRD, N₂adsorption-desorption, FTIR,Py-FTIR, NH₃-TPD, XPS and other technical analysis. When vanadium loading wasless than20wt%, vanadium was highly dispersed on the surface of the support.Addition of SO42-and PO₄^3-could generate strong and moderate acidic sites over thecatalysts. The total amount of acidic sites and the reducibility of catalysts were bothenhanced. This greatly improved the yield of DMM as well as the conversion ofmethanol, due to the enhanced condensation reaction of formaldehyde with methanol.VO_x/TS-1-PO₄^3-catalyst showed a good methanol conversion of56.7%and a DMMselectivity of73%.Mesoporous Al-P-V-O catalyst for the DMM synthesis reaction had an excellentcatalytic activity with methanol conversion up to55%and DMM selectivity above80%.Al-P-V-O catalysts with the formation of amorphous material showed the mesoporousstructural features such as high specific surface area and larger pore volume. Thesurface properties of the catalyst were characterized by NH₃-TPD, O₂-TPD, H2-TPRand XPS. The ratio of Al/V/P influenced the porosity, redox property and surfaceacidity of the catalysts. The activity of the reaction depended on the redox propertiesof catalyst while the DMM selectivity was significantly influenced by strong acidicsites. The synergistic effect of vanadium and phosphorus enhanced the redox propertyof the catalyst. Higher DMM yield was obtained owing to the better coupledreducibility with the acidic sites, which can be controlled by the ratio of Al/V/P of thecatalysts.In situ infrared characterization and temperature-programmed surface reactionwere carried out to investigate the reaction mechanism of selective oxidation of methanol to DMM over VO_x/TS-1catalyst. Methanol was first adsorbed on thecatalyst surface to generate methoxy, and dehydrogenated to form formaldehyde atredox sites of VO_x. Then it condensed to DMM, dimethyl ether （DME） and methylformate （MF） on acidic sites. Active lattice oxygen of vanadium oxide was providedto the reactants during the oxidation reaction process. After reaction, vanadium oxidewas left with vacancy, resulting in the lost of activity. Oxygen from the air couldadsorb on the vacancy of the vanadium oxide and restored vanadium oxide to theinitial state, which ensured the continuity of the methanol oxidation reaction.