Application And Investigation Of Hierarchically Structured Porous Lanthanum-Based Catalysts For Oxidative Coupling Of Methane

Oxidative coupling of methane(OCM)is a promising technology for converting methane into C2 hydrocarbons(ethylene/ethane).Whereas the harsh reaction temperature(>700?)restricts the selectivity of C2 species due to the formation of by-products(CO and CO2).Thus,it is necessary to develop an effective catalyst for low-temperature OCM.Hierarchically structured porous materials are often used in catalytic oxidation reactions to reduce the reaction temperature due to their open pores,surfaces accessible to reactant molecules,large specific surface area and pore volume,and surface oxygen vacancies.In this work,we investigated the relationship between OCM performance and physicalchemical of hierarchically structured porous La2O3-based catalysts with the help of characterization.The main results are as follows1.The flower-like hierarchical porous La2O3(La2O3-F)catalysts exhibit better catalytic performance for OCM reaction than that on dense microsphere La2O3 catalysts(La2O3-M).In particular,La2O3-F catalysts exhibit higher methane conversion(34.9%)and C2 selectivity(40.5%)at low reaction temperature(430?).The main reason is that the surface defects structure is formed in hierarchical pores(including mesopores and macropores)of La2O3-F.These surface defects structure enhances the ability of La2O3-F to activate oxygen and promotes the formation of chemisorbed oxygen species(electrophilic oxygen species),which is beneficial to the selective oxidation of methane to C2 at low temperature2.The C2 yield of Sr-doped flower-like hierarchical porous La2O3(Sr0.1La-F,n(Sr)/n(La)=0.1)is increased by 4.7%relative to La2O3-F(19.1%vs.14.4%),while the C2 yield of Sr-doped La2O3-M(Sr0.1La-M)is only increased from 9.8%for La2O3-M to 11.8%.The results indicate that this is related to the interaction between Sr and La2O3 The high-dispersed Sr can be obtained over hierarchical porous Sr0.1La-F,which prevents the agglomeration of La2O3 particles and isolats the active sites.The surface defects and the highly dispersed Sr enhances the interaction between Sr and La2O3,which promotes the insertion of Sr into the lattice of La2O3 and causes changes in the electronic properties of La2O3,thus promoting the formation of electrophilic oxygen species.These optimized physicochemical properties over Sr0.1La-F are beneficial to improve C2 selectivity and methane conversion.3.The catalytic performance of Sr0.1La-F would be lost due to collapse of the hierarchical porous under the action of high-temperature steam.Adding trace metals with higher valence than La3+,for example Zr4+,Nb5+,and Mo6+,can maintain the porous structure of catalysts and thus improve the stability of the Sr0.1La-F.However,the valence and the amount of high valence added metals affect catalytic performance since the oxygen vacancies decrease as the valence of the added metal increases or the amount of addition increases,which leads to a decrease in methane conversion and C2 selectivity.