Study On The Synthesis Of Dimethoxymethane By Methanol Oxidation With Modified Material Of G-C₃N₄

The VO_x/g-C₃N₄ composite material with g-C₃N₄ supported VO_x was prepared by impregnation method.The prepared materials were characterized by FT-IR,XRD,ICP-MS,SEM,TEM and XPS.The results show that the interaction VO_x of the V or O atom with the characteristic group-NH₂,-NH or N on the surface of g-C₃N₄ is dispersed in the molecular state on the surface of g-C₃N₄.The catalytic performance of VO_x/g-C₃N₄ materials for selective oxidation of methanol was evaluated using a micro fixed bed reactor.The catalyst exhibits excellent catalytic activity and high selectivity to dimethoxymethane?DMM?.The preparation method and reaction conditions of the catalyst were discussed,such as:calcination temperature,V loading,reaction temperature,reaction time,space velocity,the influence of the flow ratio of methanol,oxygen and nitrogen on the oxidation of methanol,showing the activity on the surface of the catalyst.The interaction between the group VO_x and the carrier g-C₃N₄ causes the V=O double bond weakening to be the reason of its high catalytic activity;the optimum process conditions for the catalyst preparation and reaction are determined as follows:the calcination temperature is 500°C,the V loading is 7.14%.The reaction temperature was 200°C and the space velocity was 2406 mL/?g·h?,wherein the methanol flux was0.05 mL/min,the oxygen flux was 5 mL/min,and the nitrogen gas flow was 15 mL/min.Under the optimum process conditions,the conversion of methanol was 38.5%and the selectivity of DMM was 92.0%.In order to further understand the industrial application prospects of this catalyst,the service life of the catalyst was studied.The results show that when the reaction continues for more than 9 h,the conversion rate of the reaction decreases rapidly,indicating that the catalyst begins to deactivate.By studying the surface properties of the deactivated catalyst by FT-IR,XRD,SEM and TEM techniques,it was determined that the deactivation of the catalyst was caused by the formation of crystalline V₂O₅ and NH₄V₃O₈ on the surface.At the same time,the adsorption behavior of methanol on VO_x/g-C₃N₄ surface and the effect of O₂ on methanol adsorption were investigated by using in situ DRIFTS spectra.The conversion of methanol to the carbonyl-containing compound on the surface of the catalyst was observed,and the introduction of O₂promoted the surface conversion reaction.On the basis of this result,the reaction mechanism of methanol on the surface of VO_x/g-C₃N₄ involving surface adsorption,surface conversion to formaldehyde,and further condensation to form DMM was discussed.