Synthesis And Properties Of Nanoscale Metal Oxides For Oxidative Carbonylation Reaction:Ag-MnO₂ Supports With Different Morphologies And Its Catalytic Performance

Diphenyl Carbonate?DPC?is the main material for the synthesis of Polycarbonate?PC?.The synthetic route of DPC includes phosgene method,transesterification and oxidative carbonylation of phenol and so on.The oxidative carbonylation of phenol is a green synthesis route for producing DPC directly from phenol,CO,and O₂ with the catalyst.It has the advantages such as high atomic utilization and environmental friendliness.Many researches have been focused by both domestic and international scholars since 1980s.However,this method is very restrictive which requires high temperature and pressure,and the catalyst used for the reaction is more expensive than that of the phosgene and the transesterification method.In addition,its low conversion rate makes this method not be adapted to industrial production.Therefore,the key point of the process lies on the screening of catalysts with high activity,long life,and high selectivity to improve the DPC yield.Oxidativecarbonylationmethodsincludehomogeneousand heterogeneous catalysis.Homogenous catalysis has the disadvantages of adding halogen ions which leads corrosion of equipment,difficulty of catalyst recovery and product separation.Heterogeneous Pd-supported palladium catalysts for the oxidative carbonylation of diphenyl carbonate make the catalyst reusable,which can replace the homogeneous redox catalyst.It is of great significance and application value to develop green chemical processes withheterogeneousoxidativecarbonylationroute.Therefore,the heterogeneous catalytic method become the mainstream of DPC synthesis research all over the world.In recent years,metal catalysts supported by transition metal oxides is a research hotspot due to their good low-temperature catalytic performance.The transition metal manganese oxide has an excellent ability to reserve and release oxygen,and it can switch between different valence states to perform a rapid redox reaction,which makes it a prominent carrier material.Silver has the advantages of relatively low cost and excellent low temperature activity which makes it a good candidate of doping metal.In this paper,three kinds of Ag-MnO₂ carriers with different morphology were fabricated and their morphology,composition and structure were characterized by XRD,TEM,SEM,BET,and H₂-TPR.The effects of different silver to manganese ratios on the structure and properties of sea urchin-like and wires Ag-MnO₂ carriers were explored.And the effects on the DPC yield with Pd supported catalyst were also studied in this paper.The process conditions of oxidative carbonylation of phenol by Pd/Ag-MnO₂ catalyst were studied,and the following conclusions were drawn:1.The properties of manganese dioxide have been adjusted by the silver doping,and the redox properties of manganese dioxide has been strengthened,which improve its catalytic activity.The more uniform of the silver species on the surface of manganese dioxide,the smaller of the Ag particles,and the catalytic activity and selectivity of Pd/Ag-MnO₂ is higher.The sea urchin-like Ag-MnO₂ carrier has a larger specific surface area,better redox catalytic activity.But its catalyst life is less than that of the catalysts with carrier of other morphologies.The surface of wire-like Ag-MnO₂ is facilitative to form high dispersive and small particle size silver particles,which improves the interaction between silver and manganese and the activity of the catalyst significantly.2.The change of silver and manganese ratio does not influence the crystal structure of manganese dioxide.The change of silver and manganese ratio has less effect on the specific surface area of carries.Compared with the rod-like and wires-like Ag-MnO₂ carriers,the sea urchin-like Ag-MnO₂ carrier was relatively short in the catalyst life of the catalyst activities decreased faster than others.The more uniform the silver on the surface of manganese dioxide,the the pore size of the carrier is smaller and the catalytic activity of the carrier is better.The optimum ratio of silver and manganese as to the catalytic activities was:urchin-like Ag-MnO₂ 1:20,wires-like Ag-MnO₂ 1:30.3.The best condition of Pd/Ag-MnO₂ catalyst in phenol oxidative carbonylation reaction is as follows:reaction temperature 75?,the total pressure 5.5 MPa,the reaction time 4 h.The diphenyl carbonate yield can reach 20.4%under this condition.