New Composite Catalysts For One-step Synthesis Of Dimethyl Carbonate From Methanol:Preparation And Catalytic Mechanism

As a green organic solvent,dimethyl carbonate?DMC?can be used as fuel additive,and has a wide range of application prospects.The gas-phase oxidative carbonylation of methanol is an economic and environmental friendly route for dimethyl carbonate synthesis,the key of the process is the development of efficient catalysts.On the basis of PdCl₂-CuCl₂-KOAc/AC catalyst,the composite catalyst?PdCl₂-CuCl₂-KOAc/SAC@Al₂O₃?and the supported ionic liquid membrane catalyst?PdCl₂-CuCl₂-KOAc/SAC?were prepared to solve the problems of poor stability and deactivation of the catalyst due to chloride ion loss.The PdCl₂-CuCl₂-KOAc/SAC@Al₂O₃ catalyst was prepared by alumina ultrasonic dispersion rotary evaporation method.The effects of dispersing solvent and alumina content over the structure and catalytic activity of PdCl₂-CuCl₂-KOAc/SAC@Al₂O₃ catalyst were investigated.Al₂O₃ package turned to be uniform as 1.0 g of alumina dispersed in hydrochloric acid solution?0.1 mol/L?.The results showed that compared with PdCl₂-CuCl₂-KOAc/SAC catalyst,the PdCl₂-CuCl₂-KOAc/SAC@Al₂O₃ catalyst had an induction period,but its activity stability was improved.After the induction period,the maximum space-time yield of DMC reached to917 g·L-cat^-1·h^-1,and its active stability could be maintained for 28 hours.The catalyst exhibited a significant inactivation state when the runtime past 35 hours.The results also showed that the specific surface area and pore size of the catalysts were significantly lower than those of the fresh catalysts.The decreasing of surface area of PdCl₂-CuCl₂-KOAc/SAC@Al₂O₃ catalyst was obviously smaller than that of PdCl₂-CuCl₂-KOAc/SAC catalyst.It might be an important reason of the catalyst deactivation that the decrease in the specific surface of the catalyst was resulted from the formation of KCl crystals during the reaction.The results of XPS characterization showed that the reduction of low-valence copper content on the PdCl₂-CuCl₂-KOAc/SAC@Al₂O₃ catalyst surface caused by the encapsulation treatment of alumina should be the main reason for the decrease of the initial activity and the cause of the induction period.PdCl₂-CuCl₂-KOAc-IL/SAC catalyst was prepared by using three kinds of chlorine-containing liquid[Bmim]Cl,[BMImCl][AlCl₃]and[HMMIm]Cl as liquid film.It was found that the catalyst performance was better when[Bmim]Cl was used as the liquid film phase.The highest space-time yield of DMC was 878 g·L-cat^-1·h^-11 as the catalyst stabilized.The experiments of stability evaluation showed that compared with that of the catalyst with no liquid film,the stability of the catalyst with the supported liquid film was improved to some extent,yet the effect was not rather obvious.