The Effect Of Support Modification On The Gas-phase Epoxidation Of Propylene In MoO₃-Bi₂SiO₅/SiO₂

Propylene epoxidation is a important chemical intermediate.Compared with the traditional chlorohydrin method and hydrogen method,gas-phase epoxidation of propylene is a sipmle,safe,low cost and green process when oxygen is used as oxidant.The key of process is to develop suitable catalysts.Molybdenum-based catalysts,by contrast,have the adventage of low cost and possess the better catalytic activity and stability,but there are still some problem restricting its industrial application:strong acidity on the surface of the support,poor dispersion of molybdenum components,low PO selectivity,more reaction coupling products,and furher improvement in the stability of the catalyst.In the paper,the support were modified to regulate the surface physicochemical properties,inhibit the formation of Bi₂Mo₃O₁₂,improving the dispersion and stability of molybdenum components,promote the synergistic effect,enhence the gas-phase propylene epoxidation performance of catalyst.Firstly,Bi₂SiO₅/SiO₂support of MoO₃-Bi₂SiO₅/SiO₂catalysts doped with the different transition metals（copper,nickel,cobalt and iron）were screened.The influence of doping concentration of modifier on the gas-phase propylene epoxidation performance of catalyst was investigated.Research results show that the doping modification of the copper,nickel,cobalt and iron can effectively control the proformance of catalyst for propylene epoxidation.The MoO₃-Bi₂SiO₅/SiO₂catalysts modified by iron exihibited the higher catalytic activity with 71.6%PO selectivity and26.0%C₃H₆conversion and the selectivity of acetaldehyde（AA）was reduced to less than 7%.The propylene epoxidation activity got great improvement by the modification of iron.The test results of different reaction temperatures showed that the appropriate reaction temperature could reduce the secondary conversion of PO.Secondly,on the basis of the research results of modifier screening,XRD,BET,UV-vis,XPS,NH₃-TPD,Raman,HRTEM was used conduct an in-depth study on the structure of catalyst after the modification of iron,revealing the influence mechanism of iron modification on the catalytic performance of catalyst.The results show the support modification of iron can effectively inhibit the formation of Bi₂Mo₃O₁₂,besides,the iron on the surface of support can effctively capture the molybdenum components to improve the dispersion of molybdenum components and inhibit the migration of molybdenum components by forming Fe-Moriveted interface structure.The modification of iron can also promote the synergistic effect of different active components and improve the catalytic activity and stability.Finally,the catalytic reaction kinetics of catalyst modified by iron was studied and the technological parameters（reaction pressure,reactant concentration,space velocity）of catalytic reaction were systematically investigated,besides,the apparent activation energy was calculated.The results showed the best epoxidation perfomance was obtained at 400^oC,0.15 MPa,15000 m L·g^-1_cat·h^-1,C₃H₆:O₂:N₂=1:4:20 m L·min^-1with 71.6%PO selectivity and 26.0%C₃H₆conversion.The appropriate partial pressure of propylene and reacition temperature can inhibit the side reaction and improve the selectivity of PO.