Synthesis Of Dimethyl Carbonate Via Catalytic Reactive Distillation Over Heterogeneous Base Catalyst

Dimethyl carbonate（DMC）is a kind of green organic chemicals due to its excellent chemical properties and non-toxic properties,which plays an increasingly important role in the chemical industry in recent years.The transesterification of propylene carbonate（PC）with methanol to produce DMC is an environmentally friendly synthesis method,which has the advantages of mild reaction conditions,high yield,high economy and safe operation.Because the reaction is reversible and the reaction equilibrium constant is low,the traditional production process needs a lot of material recycling to improve the reaction conversion,which leads to high equipment investment and operation cost.The reaction products can be separated from the reaction system in time by reactive distillation,which is helpful to improve the conversion rate of reactants.At present,the catalyst commonly used in this reaction is homogeneous catalyst sodium methoxide.Although its catalytic performance is good,there are still some problems in the separation and purification of follow-up products,catalyst recycling,and by-product post-treatment as its high energy consumption and environmental unfriendly.Therefore,the study of solid catalysts for this reaction have received extensive attention.In this paper,4.6%CaO/NaY catalysts were prepared by equal volume impregnation.Their structures and surface properties were studied by XRD,BET,CO₂-TPD and NH₃-TPD,and their catalytic properties for the synthesis of dimethyl carbonate by transesterification were further studied.It was clear that the acid-base sites on the surface of the catalysts were the important factors restricting the transesterification performance of PC and methanol.In order to further improve the reaction performance of DMC synthesis,ZrO₂-Al₂O₃ composite oxide was prepared by coprecipitation.The results show that ZrO₂-Al₂O₃is a kind of catalyst with good thermal and chemical stability.It has high hardness and good acid-base properties,which makes it superior catalytic to 4.6%CaO/NaY in the process of transesterification.The controllable preparation of ZrO₂-Al₂O₃ catalyst with different structural characteristics and surface acid-base center properties was realized by adjusting the content of Al ion in the catalyst,the type of precipitant and calcination temperature.After the introduction of Al element into ZrO₂,the solid solution structure is formed,the number of weak acid sites is increased,and the strong base sites are emerged.When Zr/Al ratio is 1,the weak acid sites and strong base sites reach the maximum.Data analysis shows that the activity in the transesterification of PC and methanol to DMC increases linearly with the number of weak acid sites and strong base sites on the catalyst surface.More strong base sites contributed to the dissociation of methanol,and weak acid sites with specific strength contributed to the stabilization of reaction intermediates and the acceleration of reaction rate.The effects of feed alcohol ester ratio,reflux ratio,PC LHSV,catalyst loading on the catalyst stability were investigated in the reactive distillation colume with ZrO₂-Al₂O₃catalyst.The optimum process conditions were determined:n（MeOH）/n（PC）was 4:1,reflux ratio was 5:1,catalyst loading height was 0.25 m,PC LHSV was 0.26 h^-1.The conversion of PC and selectivity to DMC can reach 98.14%and 99.96%,respectively.