La₂Ce₂O₇ Compound Catalysts For Oxidative Coupling Of Methane To Produce Ethylene

Natural gas,whose main component is methane,is a cheap resource and has been widely used.Through oxidative coupling of methane(OCM)reaction,it can be effectively converted into ethylene,an important platform molecule for chemical industry.Over past thirty years,people have been performing a lot of researches on this reaction,but it is not yet industrialized.Up to now,several issues are still remained un-solved.For example,on all the catalysts,the one-way C2 product yield is still lower than the 30%minimum requirement for industrialization.In addition,the reaction temperature is too high and the stability of the catalysts is too low.Therefore,people are still seeking better catalysts to industrialize this process.La2Ce2O7 has a disordered defective flurotite structure,which possesses abundant surface oxygen defects,electrophilic oxygen and alkaline sites,and displays good performance for OCM reaction.Hence,in this study,we have tried to optimize the low temperature reaction performance of La2Ce2O7,and investigate its bulk and surface properties systematically.The main results are listed here:Part 1:With the objective to develop OCM catalysts having high reaction performance at low temperature,a La2Ce2O7 compound has been optimized to tailor its surface reactive sites by using co-precipitation(CP),hydrothermal(HT)and glycine nitrate combustion(GNC)methods.All the catalysts show considerable low temperature reaction performance,following the sequence of La2Ce2O7-GNC>La2Ce2O7-HT>La2Ce2O7-CP.On La2Ce2O7-GNC,the best catalyst in this study,even at 450?,2.5%C2 product yield have already been achieved,indicating its superior low temperature reaction performance.All the La2Ce2O7 catalysts consist of a disordered defective fluorite crystalline phase.It is revealed that superoxide O2-anions with a bond order of 1.5 have been formed on the surfaces of all the catalysts.However,peroxide O22-anions with a bond order of 1.0,which are more reactive than O2-anions,is uniquely formed on La2Ce2O7-GNC,which explains its superior low temperature reaction performance.A La2Ce2O7 catalyst with higher reaction performance usually possesses a larger amount of surface electrophilic oxygen and alkaline sites.Therefore,it concluded that the interaction between these two kinds of surface active sites in fact controls the OCM reaction performance of the La2Ce2O7 catalysts.Part 2:Based on the work in part ?,to probe deeper the active sites of La2Ce2O7 disordered cubic defective flurotite for OCM reaction,CeO2 La2O3 La2Ce2O7 samples have been prepared by a sol-gel method.To compare the structure effect,a La2O3:CeO2=1:2 catalyst were prepared by a solid phase physical mixing method.XRD and Raman results have shown that both CeO2 and La2O3 samples consit of pure oxide phase,and La2O3:CeO2=1:2 catalyst is composed of separated CeO2 and La2O3 phases,indicating that they are presented as a simple mixture in the catalyst.In contrarys,a disordered defective fluorite structure has been successfully formed in La2Ce2O7 catalyst.Compared with individual CeO2,La2O3 and La2O3:CeO2=1:2 catalyst,La2Ce2O7 c,atalyst displays the best reaction performance.Moreover,La2Ce2O7 catalyst shows also better stability and low temperature reaction performance than La2O3 catalyst.In situ Raman?XPS and CO2-TPD results have demonstrated on the surface of La2Ce2O7 catalyst,the largest amount of both O2-species and moderate alkaline sites have been formed thus exhibiting the optimum reaction performance among all the catalyst.