Constructing A₂B₂O₇ Composite Oxidescatalytic Materials For Oxidativoncoupling Of Methane Into Ethylene

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 Ca_0.5La₂Ce₂O₇ and Ca_0.5La_1.5Ce₂O₇as dispersed CaCO₃,which covers the active sites and is harmful to the OCM reaction.However,for La₂Ce_1.5Ca_0.5O₇,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.La₂Ce_1.5Ca_0.5O₇ 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?.