Propylene Epoxidation By Oxygen On Na₂CO₃ Modified Cu/SiO₂ And Zn-Cu/SiO₂

Propylene oxide is one of important chemical raw material.At present,two major methods for preparing propylene oxide still have some drawbacks at home and abroad.For example,the chlorohydrin method pollutes the environment and does not conform to the concept of green sustainable development advocated by the world;the Halcon co-oxidation method produces a large amount of co-products while producing propylene oxide,and the the propylene yield is not high.Therefore,using molecular oxygen as an oxidant to produce propylene oxide is the most potential method for the preparation of propylene oxide.This method has many advantages,such as a wide range of molecular oxygen sources,low production costs,environmental friendliness,and high atom utilization.In this paper,Cu/SiO2 and Zn-Cu/Si02 catalysts were synthesized,and Na2CO3 was used as a modifier to study the epoxidation of propylene.Firstly,Cu/SiO2 catalyst was prepared by urea deposition precipitation method.Then,six kinds of modifiers such as NaCl,Na2SO4,NaNO2,Na2CO3,NaOH and Na2C2O4 were loaded on the surface by impregnation method.It was found that all six Na salt modifiers can be used to accelerate the formation of propylene oxide,respectively.The epoxidation activity of the Na2CO3 modified Cu/SiO2 catalyst was the highest,indicating that the modification effect of Na+ was significantly higher than that of Cl-.When the reaction temperature was 220?,the conversion of propylene and the selectivity of propylene oxide were 1%and 18%,respectively.XRD results showed that the addition of Na salt modifier could promote the reduction of CuO to Cu2O species.The results of TEM showed that the addition of Na2CO3 had little effect on the particle size of Cu species.XPS results showed that the content of Cu+ species increased from 13%to 76%after Cu/SiO2 catalyst was modified by Na2CO3.Combined with the reaction and characterization results,the valence state of Cu species is the key to affect the epoxidation activity of the catalyst propylene,and the Cu+ species is more favorable for the formation of propylene oxide.In addition,Zn-Cu/SiO2 composite catalysts with different Zn contents were prepared by urea combustion method.When Zn accounts for 8%of the mass content of Zn-Cu bimetal,Zn-Cu/Si02 has the highest epoxidation activity of propylene,and the addition of an appropriate amount of Na2CO3 modifier can further promote the formation of propylene oxide.On the Zn-Cu/SiO2-Na2CO3 catalyst,when the reaction temperature was 220?,the conversion of propylene was 1%,and the selectivity of propylene oxide was maintained at 26%.XRD and TEM results indicate that ethanol is more likely to cause partial agglomeration of Cu species than solvent water.The addition of Zn species and Na2CO3 has little effect on the particle size of Cu species,and the particle size of Cu species remains at around 9 rm.The XPS results showed that the addition of Zn and Na2CO3 could promote the reduction of Cu2+ species on the surface of the catalyst to Cu+ species,and the content of Cu species on the surface of Zn-Cu/SiO2-Na2CO3 increased from 9%before modification to 38%.The results of XRD,TEM,XPS and other experimental results show that the Cu+ species in the catalyst is the active center of propylene epoxidation and has the highest epoxidation activity of propylene.