Investigation On Lanthanum-cerium Composite Oxides As Effective Catalyst For Oxidative Coupling Of Methane

Oxidative coupling of methane（OCM）is an ideal process to produce highly value added C₂ products.While CH₄ molecule is thermaldynamically stable,C-H bond energy is 439k J/mol,and therefore it has the difficulty to be activated,especially being selectively activated.Since OCM was firstly proposed by UCC of America,lots of catalysts have been explored,including one type of alkali/alkaline metal oxide catalysts such as Li/Mg O,and Na WO₄ type of catalysts like Mn/Na WO₄/SiO₂.While the Li type of catalysts have the problems of Li loss during the reaction,and significant activity and selectivity could be achieved generally above800℃over the Na WO₄ type catalysts.Therefore,catalyst design is still the key for OCM reaction,which deserves much effort.A₂B₂O₇ is a kind of pyrochlore,which has the characteristics of high thermal stability and A/B element modulations,showing prospective applications in OCM reactions.Herein,La₂Ce₂O₇ was synthesized by sol-gel method,which was further modified by the following strategies:（i）being doped with alkaline metal,through optimizing the type of alkaline metals and methods for doping,the selectivity of C₂ was improved;（ii）through morphology control,non-stoichiometric La-Ce mixed oxide synthesis,and Sr doping,the catalysts’activity and selectivity were tuned.By using XRD,N₂-physical absorption-desorption,Raman spectra,H₂-TPR,XPS and CO₂-TPD characterizations,the crystal structures,surface oxygen distributions,mobility of lattice oxygen and surface basicity of the La-Ce based catalysts were investigated.By correlation with the OCM performance,the structural and property correlations were established.The results were summarized as followed:（Ⅰ）La₂Ce₂O₇ catalyst was synthesized by sol-gel method,and doped with alkaline metals of Mg,Ca,Sr,Ba to further modulate the catalysts’activity and selectivity.Ca.57%C₂selectivity and 14%C₂ yield(CH₄:O₂:N₂=4:1:5,WHSV=72,000 m L?g^-1?h^-1)could be obtained at 800℃over the La_1.5Sr_0.5Ce₂O₇ catalyst,and the catalysts showed stable performance during the tested 30 hours,showing the prospective applications of the pyrochlore based materials in OCM reactions.Characterization results indicate that even after Sr doping,the defective cubic fluorite phase was kept for the La_1.5Sr_0.5Ce₂O₇ sample.Meanwhile the ratio of reducible lattice oxygen（>500℃）become pronounced,being consistent with the enhanced relative lattice oxygen contents up Sr doping,especially by sol-gel method.By correlating with OCM activity results,it comes out that the mobility of lattice oxygen,relative content of lattice oxygen as well as strong surface basicity show positive relations with C₂ selectivity and yield.Doping Sr into La₂Ce₂O₇ enables partial Sr incorporate into the lattice of pyrochlore,the mobility of lattice oxygen was enhanced,and therefore more lattice oxygen could participate into the OCM reaction.Meanwhile,some strong basic centers were generated,and thereby the C₂ selectivity and yield were enhanced.（Ⅱ）Through morphology control and non-stoichiometric La-Ce mixed oxide synthesis,the catalysts’activity and selectivity were further tuned.When La₂Ce₂O₇ nanobelts were synthesized by hydrothermal（HT）method,the low temperature performance was improved,as compared with La₂Ce₂O₇ nanoparticles.Doping Sr into it increased the high temperature C₂selectivity and yield,but depressed the low temperature performance.The C₂ selectivity can reach 63%at 800℃and C₂ yield was 16%(CH₄:O₂:N₂=4:1:5,WHSV=72,000 m L?g^-1?h^-1)over the La_1.5Sr_0.5Ce₂O₇-HT（1）catalyst.Non-stoichiometric La₃Ce O_x was synthesized by sol-gel method,and its properties was found to be better than stoichiometric La₂Ce₂O₇.When Sr was introduced into La₃Ce O_x,the C₂ selectivity can reach 57%at 800℃and C₂ yield was13%(CH₄:O₂:N₂=4:1:5,WHSV=72,000 m L?g^-1?h^-1).Characterization results indicate that La₂Ce₂O₇ nanobelts could activate oxygen at low temperatures and further Sr doping enhanced the mobility of lattice oxygen.As for La₃Ce O_x,La³⁺can partially replace Ce⁴⁺,and therefore enlarging the lattice distance and enhancing the mobility of lattice oxygen.Further Sr doping made more active lattice oxygen participate in the OCM reaction and generate much stronger basicity site,and thereby further enhanced the C₂ selectivity and yield.