Constructing A2B2O7 Composite Oxidescatalytic Materials for Oxidativoncoupling of Methane into Ethylene

Author:Xu Jun Wei

Supervisor:wang xiang


Degree Year:2019





Natural gas is widely utilized as a feedstock for the production of energy and chemicals,which comprises up to 90%of methane.Oxidative coupling of methane(OCM)into ethylene is one of the important routes for the value-added transformation of natural gas.Seeking highly efficient catalysts that can be operated at low temperature and improving C2 selectivity at relatively high methane conversion,thus achieving one-way C2 yield above 30%,is still the core issue to be solved for the successful industrialization of this process.To achieve this goal,it is of great necessity to design and prepare catalysts with high performance.A2B2O7 compounds possess high thermal stability,intrinsic 8a structure oxygen vacancies and certain surface alkalinity,which fits to most of the active site requirements for an OCM catalyst.However,people still lack systematic investigation and understanding on the structure and reactivity relationship for the application of this category of materials for OCM reaction.Based on the deep understanding of A2B2O7 compounds for OCM,the alteration trend of the phase compositions,the properties of the active sites,intrinsic oxygen vacancies,surface electrophilic oxygen and the relationship between structure and reactivity for OCM reaction on A2B2O7 compounds have been elucidated and unraveled in-depth.On the base of this,by adjusting the A and B site compositions and dopping low valence ions,novel type of catalysts with appropriate active sites highly matching low temperature OCM reaction,which have also the industrial application potential will be constructed.The main results are summarized here:Part 1:Aiming to elucidate the relationship between the phase structures of A2B2O7 compounds and its reactivity for OCM reaction,three model La2B2O7 catalysts possessing a fixed La3+A-site but with varied Ti4+,Zr4+or Ce4+B sites have been purposely designed and prepared successfully.By using various characterization methods such as XRD,Raman,Mapping,H2-TPR,O2-TPD,CO2-TPD,EPR,insitu FT-IR and XPS,it has been proved that the crystalline phase of La2B2O7 varies from monoclinic layered perovskite(La2T2O7)to ordered cubic pyrochlore(La2Zr2O7)and defective cubic fluorite(La2Ce2O7)with decreasing rA/r B.It can be concluded that the surface electrophilic O2-species determines the OCM reactivity of the La2B2O7compounds,the moderate alkaline sites are intimately related to the surface O2-sites and the formation of active surface O2-species on La2B2O7 catalysts could proceed via two pathways.Part 2:Two different crystalline phase of La2Zr2O7 composite oxides have been prepared as catalysts for OCM reaction.XRD and Raman results show that the pure pyrochlore and defective cubic fluorite phase of La2Zr2O7 composite oxides have been successfully prepared by co-precipitation method.By using various characterization methods such as Mapping,H2-TPR,O2-TPD,CO2-TPD,EPR,XPS,UV-vis spectra and PL spectroscopy,it has been proved that La2Zr2O7 possessing defective cubic fluorite phase can generate more amount of oxygen vacancies thus obtaining catalysts with much improved reaction performance due to the more amount of O2-species generated by oxygen vacancies.Part 3:A series of Ln2Ce2O7 composite oxides with a fixed Ce B site cation but with altered rare earth La3+,Pr3+,Sm3+and Y3+A sites have been prepared with a sol-gel method.By using various characterization methods such as XRD,Raman,H2-TPR,CO2-TPD,EPR and XPS,it has been proved that the crystalline phase of Ln2Ce2O7varies from defective cubic fluorite(La2Ce2O7)to rare-earth cubic-type(C-type)(Sm2Ce2O7)and(Y2Ce2O7)with decreasing rA/r B.It is believed that the concerted interaction between surface intermediate basic sites and selective electrophilic oxygen species O2-is the predominant reason controlling the reaction performance of the catalysts.La2Ce2O7 exhibits the best performance among all the catalysts,on which the highest C2 yield of 16.6%is achieved at 750℃.In comparison with Mn/Na2WO4/SiO2,the most promising catalyst at present,La2Ce2O7 display much improved reaction performance at low temperature region(<750℃).Part 4:To optimize the reaction performance of La2Ce2O7 for OCM,catalysts with varied La/Ce molar ratios and doped by Ca additive have been synthesized and characterized by different techniques.XRD and Raman results are discovered that a La2Ce2O7 compound having a disordered cubic defective fluorite phase is predominantly formed in all the samples.By using various characterization methods such as Mapping,EPR,XPS,TGA and CO2-TPD,it has testified that varying the La/Ce ratio influences the fine crystalline structure of the catalysts.As a result,the abundance of the surface facile oxygen and alkaline sites increase by increasing the La/Ce ratio.Moreover,the Ca additive exists on the surface of Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7as dispersed CaCO3,which covers the active sites and is harmful to the OCM reaction.However,for La2Ce1.5Ca0.5O7,the major part of the Ca additive has entered into the lattice of La2Ce2O7 phase as Ca2+cations,thus creating more oxygen vacancies and improving the surface alkalinity of the catalyst,which is favorable to the OCM reaction.By increasing La/Ce ratio and doping Ca2+into La2Ce2O7 lattice,the quantities of both kinds of active sites can be significantly improved,thus obtaining catalysts with much improved reaction performance.La2Ce1.5Ca0.5O7 owns the largest amount of O2-,hence depicting the best reaction performance,over which the highest C2 yield of 22.5%can be achieved even at 750℃.