| Propylene oxide?PO?as an important organic intermediate,is mainly used to produce polyether polyols,propylene glycol and various surfactants.The main production processes of propylene oxide are chlorohydrin method,Halcon method and hydrogen peroxide oxidation processes.However,these three processes have their own shortcomings,such as high requirements for equipment,long process flow and high energy consumption,which do not meet the requirements of green development.In contrast,direct oxidation of propylene with oxygen is considered to be the most promising production process because of its high atomic utilization and environmental friendliness.At present,Ag-based catalysts have shown good catalytic activity in the direct oxidation of propylene with oxygen.Compared with other catalytic systems,Ag-based catalysts have low cost and good stability,and have important industrial application value.Therefore,it is of great significance to develop an efficient and stable Ag-based catalyst for this green process.Metal carbonate?BaCO3,La2?CO3?3?is a simple and cheap catalyst support,which has good catalytic performance and is widely used in the field of catalysis.BaCO3 support with controllable morphology were synthesized by liquid phase direct precipitation,homogeneous precipitation and low temperature solid phase method,respectively.Ag/BaCO3 catalysts were prepared by chemical reduction,sedimentation and impregnation methods.The physicochemical properties of the catalyst,such as phase composition,specific surface area and morphology,were studied by corresponding characterization methods.The catalyst was applied to the oxidation of propylene to PO by oxygen.The effects of preparation method,crystal form control agent and reaction conditions on the performance of the catalyst were investigated.Characterization and experimental results show that 4 wt%Ag/BaCO3 catalyst has the best epoxidation activity for propylene by NaBH4 reduction?SBR?,which prepared by direct precipitation of liquid phase with 20%EDTA as crystal control agent.The optimal performance with propylene conversion of 2.32%,PO selectivity of 53.87%and PO formation rate of 46.45?molPO·g-1cat·h-1 were obtained at 280oC.In addition,La2?CO3?3 was used as support to prepared Ag/La2?CO3?3 catalyst for propylene epoxidation.The effects of crystal control agent,Ag loading,temperature and other factors on propylene epoxidation were investigated,and the physicochemical properties of Ag/La2?CO3?3 catalyst were investigated by corresponding characterization methods.The preparation method is as follows:La2?CO3?3 support was synthesized by direct precipitation method with 5%citric acid?CA?as crystal control agent,and Ag/La2?CO3?3 catalyst was prepared by SBR method.The experimental results show that 9.0 wt%Ag/La2?CO3?3 catalyst has the best catalytic activity.At 230oC,PO formation rate is 18.14?molPO·g-1cat·h-1,propylene conversion and PO selectivity are 1.56%and 23.29%,respectively.Although the activity of the catalyst is slightly lower than that of Ag/BaCO3 catalyst,the reaction conditions are milder and the catalyst has good industrial application prospects and research value.In this paper,the preparation of Ag supported on metal carbonate catalyst and its catalytic performance for propylene gas phase epoxidation were studied in detail,and the possible reaction mechanism was proposed.It provides a new idea for the study of Ag-based catalysts for propylene gas phase epoxidation. |
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