Constructing Composite Oxide Catalytic Materials With Fixed Compositions For Oxidative Coupling Of Methane To Produce Ethylene

Oxidative coupling of methane（OCM）can directly convert methane,the major component in the cheap and abundant natural gas,into high-value ethylene products in one step,which has attracted extensive interest in the past 30 years.However,the catalysts studied so far are difficult to break through the minimum industrial requirement of 30%C₂ yield.Furthemore,most of the studied catalysts need high temperatures to start the reaction to produce C₂ product.Therefore,it is of great neccessity to find more efficient catalysts,or catalysts that can start the reaction at low temperatures.In this thesis,a series of perovskite-type strontium stannate,A₂B₂O₇pyrochlore composite oxides with Y³⁺as the A-site,which have different crystal phase structures,were designed and prepared for OCM reaction.The relationship between the surface active oxygen species,acidity/alkalinity,oxygen vacancy concentrations and the crystal structures of the catalysts and their influence on the OCM reaction performance are systematically elucidated.The reaction mechanism of generating surface active oxygen by adsorbing gas-phase oxygen on the catalyst is clearly explained.The structure-activity relationship of the two types of composite oxide materials for OCM reaction has been deeply understood.It can provide new ideas for people to design more efficient OCM reaction catalysts.The main research contents are summarized here:Part 1:The perovskite-type strontium stannate composite oxides with different Sr/Sn stoichiometric ratios were synthesized by glycine-nitrate salt-assisted combustion method,including regular SrSnO₃perovskite and Ruddlesden-Popper phase mono-layered Sr₂SnO₄perovskite and double-layered Sr₃Sn₂O₇.The OCM reaction performance of the three catalysts follows the order of Sr₂SnO₄>Sr₃Sn₂O₇>SrSnO₃,among which Sr₂SnO₄ can obtain a C₂ yield around 19%at 800 ℃.It is doscovered that both the surface electrophilic oxygen(O₂^-/O₂^2-),and the surface lattice oxygen(O^2-)are the OCM selective oxygen species of the reaction.The basic sites of the catalyst are mainly provided by the A-site Sr²⁺,and both the moderate basic sites and the strong basic sites are conducive to form C₂ products.The moderate basic sites are closely related to the oxygen vacancy and the M⁺O^-ion pair.Since the mono-layer Sr₂SnO₄ perovskite and the double-layer Sr₃Sn₂O₇ perovskite have a Ruddlesden-Popper crystal structure,compared with the regular structure SrSnO₃perovskite,the Sr²⁺coordination number changes from 12 to 9.The Sn–O bond length becomes also larger.Because the Sn–O bond is more covalent than the Sr–O bond,it is easier to be broken and form more oxygen vacancies.Therefore,compared with SrSnO₃,Sr₂SnO₄ and Sr₃Sn₂O₇ have lower oxygen vacancy formation energies and higher oxygen ion conductivity,thus producing more abundant OCM selective oxygen species during the reaction.In addition,the Sr/Sn mole ratios of Sr₂SnO₄,Sr₃Sn₂O₇ and SrSnO₃ are 2/1,1.5/1 and 1/1,respectively,hence Sr₂SnO₄possesses more abundant surface basic sites,which can better stabilize the electrophilic O₂^-/O₂^2-species.Therefore,Sr₂SnO₄ with a mono-layer perovskite structure exhibits the best OCM reaction performance.Part 2:A series of Y₂B₂O₇ composite oxide catalysts with Y³⁺as the A-site cation,but with Ti⁴⁺,Sn⁴⁺,Zr⁴⁺or Ce⁴⁺as the B-site cations were synthesized by the glycine-nitrate salt-assisted combustion method,and used for OCM reaction.The characterization results have demonstrated that by decreasing the r _A/r _B ratios gradually,the crystal structure of the catalyst changes from ordered pyrochlore（Y₂Ti₂O₇）to disordered pyrochlore（Y₂Sn₂O₇）to disordered defect fluorite（Y₂Zr₂O₇）finally to C-type rare earth（Y₂Ce₂O₇）structure.The reaction performance follows the order of Y₂Zr₂O₇>Y₂Ce₂O₇>Y₂Ti₂O₇>Y₂Sn₂O₇.Y₂Zr₂O₇ having a disordered defect fluorite phase,has the highest oxygen vacancy concentration due to the largest degree of disorder in the structure,thus displaying the optimal reaction performance.With various characterization methods,the effect of crystal phase structure changes on the surface physical-chemical properties and the OCM reaction performance has been clarified for the catalysts.It is found that the OCM performance of the Y₂B₂O₇catalysts depends not only on the crystal phase structures and the amount of electrophilic O^-and O₂^-anions,but also on the surface acidity/alkalinity.Usually,a catalyst with more moderate basic sites and O^-/O₂^-oxygen species,but less strong acidic sites has better C₂ selectivity and yield.Because strong acidic sites are easy to adsorb methyl radicals to produce the CO_x deep oxidation products,which can reduce the C₂ selectivity.In addition,the basicity and acidity of all Y₂B₂O₇ catalysts are determined together by the crystal phase structure,and the properties of the metal cations at A-and B-site.