Alkaline Earth Metal Modified La₂O₂CO₃ Catalysts And The Catalytic Performance For Oxidative Coupling Of Methane At Low Temperature

Non-renewable energy sources such as oil and coal are becoming depleted and the importance of renewable energy sources and natural gas is increasing.Our country’s energy consumption structure is constantly being adjusted,the proportion of coal is declining,and natural gas is the fastest growing fossil energy.The proven reserves of natural gas are huge.Methane is the main component of natural gas and is the simplest and most stable hydrocarbon.However,methane cannot be liquefied at room temperature,which not only brings a lot of inconvenience to storage and transportation,but also imposes great restrictions on subsequent use.Therefore,synthesizing light alkanes or olefins from methane is an effective way to utilize methane.At present,the relatively mature technical route is to convert methane into synthesis gas,then synthesize methanol,and then obtain hydrocarbon products,but this route has high investment cost and complicated process flow.The direct conversion of methane into hydrocarbons by oxidative coupling of methane（OCM）is the most direct and effective method,with few reaction steps and can effectively control the reaction energy consumption.However,it is currently difficult to obtain high product selectivity at higher methane conversions due to the chemical inertness of methane.Therefore,the oxidative coupling reaction of methane is less used in industry,and most of them are still in the laboratory research stage.Once breakthroughs are made in catalytic technology,natural gas will become the most ideal substitute for oil.Herein,in order to develop and enrich the research of La-based catalysts catalyzing OCM reactions,this project designed X（Mg,Ca,Sr,Ba）-La₂O₂CO₃ nanoparticle catalysts,and compared them respectively.The effects of the introduction of different alkaline earth metals on the catalytic performance of OCM were compared.In order to further improve the performance of the catalyst for OCM reaction at low temperature,a sea urchin-like La₂O₂CO₃ supported alkaline earth metal Sr catalyst（Sr/LOC-HL）with microscopic morphology was designed.The catalysts were characterized by SEM,XRD,Raman,BET,TEM,CO₂-TPD,H₂-TPR,XPS,and the dried gas products were analyzed by an online gas chromatography equipped with a flame ionization detector（FID）and a thermal conductivity detector（TCD）,and the reaction mechanism of methane oxidative coupling was studied in depth.The main innovations and findings of this paper are as follows:（1）A series of X（Mg,Ca,Sr,Ba）-La₂O₂CO₃ nanoparticle catalysts with a particle size of 5-20 nm were prepared by co-precipitation method.The effect of different alkaline earth metals on the catalytic performance was explored,and it was found that the Sr-La₂O₂CO₃nanoparticle catalyst had the best OCM reaction performance.The catalyst showed a conversion of 20.1%and a yield of 13%for the OCM reaction at 600℃.The introduction of alkaline earth metals can increase the number and density of surface active oxygen species to various degrees to improve the OCM activity.（2）The nano/micro-scale coordination polymer was prepared by the liquid phase method,and the sea urchin-like La₂O₂CO₃ catalyst was obtained by calcination.Sea urchin-like La₂O₂CO₃-supported alkaline earth metal Sr catalysts were synthesized by the incipient-wetness impregnation method.The sea urchin-like structure can effectively increase the surface area of the catalyst and improve the contact efficiency between the catalyst and the gas-phase reactants,thereby improving the catalytic performance of the catalyst.The performance of the catalysts loaded with Sr has been improved,mainly reflected in the improvement of the conversion of methane and the selectivity of C₂₊hydrocarbons.Among all the catalysts,the sea urchin-like La₂O₂CO₃ loaded with 10 wt%Sr exhibited the highest catalytic activity at both high and low temperature(20.3%CH₄conversion,69.1%C₂₊selectivity,and 13.2%C₂₊yield at 450℃).After 1000 min of reaction on the stream at 450℃,the catalyst maintains stable catalytic activity and its nanostructure is remained during the reaction.The synergistic effect of each functional component in the Sr/LOC-HL catalyst improves the catalytic behavior of methane oxidation coupling at low temperature.This work can not only provide a new strategy for the research and utilization of La-based nanostructured catalysts,but also provide theoretical guidance for the development of other advanced catalysts for low-temperature oxidative coupling of methane.