Developing Efficient Rare Earth Compound Catalysts With A Cubic Flourite Structure For Oxidative Coupling Of Methane To Produce Ethylene

Oxidative coupling of methane,which can convert methane into an important industrial platform molecule ethylene in one step,has received increasing attention over recent years.However,most of the catalysts show lower reaction performance at lower reaction temperature,and the single-pass C₂ yield still cannot meet the minimum requirements for industrialization.People are trying to find new catalysts with high performance at low temperature or with sufficient single-pass C₂ yield to realize OCM industrialization.In this thesis,three different methods were used to prepare Y₂Zr₂O₇composite oxides with disordered defect fluorite structure,and A₂Ce O₃ composite oxides with three alkali metal ions（Li⁺,Na⁺,and K⁺）at A-site for OCM reaction.The relationship between the surface active oxygen species,basic sites,oxygen vacancy concentration,and crystal phase structure of the composite oxides,and their effects on OCM reaction performance of the two types of catalysts has been investigated.The main contents are summarized here:Part 1:In order to study and develop OCM catalysts with better low-temperature reaction performance,we prepared three kinds of disordered defects by coprecipitation method（CP）,citric acid sol-gel method（SOL）,and glycine nitrate combustion（GNC）method for OCM reaction.All the Y₂Zr₂O₇ composite oxides form a disordered defect fluorite structure.The reaction performance test results show that Y₂Zr₂O₇-GNC can obtain a C₂ product yield about 3.5%even at 500 ^oC.The OCM performance order of the Y₂Zr₂O₇ composite oxides prepared by the three methods is Y₂Zr₂O₇-GNC>Y₂Zr₂O₇-SOL>Y₂Zr₂O₇-CP.XRD and Raman results show that all the three Y₂Zr₂O₇composite oxide catalysts formed a disordered defect fluorite structure.The larger the lattice disorder of the composite oxide,the better its OCM reaction performance.CO₂-TPD results show that the moderate alkaline sites can stabilize the active oxygen species on the catalyst surface,which is beneficial to C₂ selectivity.The higher the amount of surface moderate alkaline sites,the better the C₂ selectivity.XPS O 1s results showed that O^-and O₂^-electrophilic oxygen species were the OCM active oxygen sites for this series of catalysts,and the higher their content,the higher the C₂ products yield.The ion conductivity test results show that the higher the oxygen ion conductivity,the greater the oxygen vacancies concentration on the catalyst surface.Y₂Zr₂O₇-GNC has the largest specific surface area,the largest amount of moderate alkaline sites,electrophilic oxygen species and the highest oxygen ion conductance.Therefore,it has the best OCM reaction performance.Part 2:In order to research and develop OCM catalysts with excellent reaction performance,a series of A₂Ce O₃ composite oxides with Li⁺,Na⁺and K⁺alkali metal ions as A-site ions and Ce⁴⁺as B-site ions were prepared by citric acid sol-gel method,and used for the OCM reaction.As a comparison,Ce O₂ was synthesized by the same method.The reaction performance test results show that Na₂Ce O₃ can obtain a C₂product yield about 16.3%at 800 ^oC.The OCM reaction performance sequence of the four catalysts is Na₂Ce O₃>K₂Ce O₃>Li₂Ce O₃>Ce O₂.XRD and Raman results show that all the four catalysts formed a disordered defect fluorite structure.CO₂-TPD results show that the moderate and strong alkaline sites can stabilize the active oxygen species on the catalyst surface,which is beneficial to C₂ selectivity.The results of Raman and XPS O 1s indicated that O₂^2-and O₂^-electrophilic oxygen species were the OCM active oxygen sites of this series of catalysts.The higher the content,the higher the C₂ product yield.XPS Ce 3d test results show that the higher the Ce³⁺amount,the higher the oxygen vacancies concentration.The Na₂Ce O₃ catalyst possesses the largest amount of moderate and strong alkaline sites,electrophilic oxygen species and the largest oxygen vacancy concentration,so it has the best OCM reaction performance.