Studies And Application Of The La-based Catalysts Modified By Different Anions For Oxidative Coupling Of Methane

The oxidative coupling of methane(oOCM)is an important approach to the utilization of natural gas.Development of efficient and high selective catalyst is an important research direction of OCM reaction.La2O3 is widely used as the main component of OCM catalysts owing to its high oxygen storage and oxygen release capacity.In this work,a series of anion modified La-based catalysts was prepared with critic acid method to improve the catalytic activity of La2O3.In order to explore the relationship between catalytic performance for OCM and physical-chemical properties of La-based catalysts.The main results obtained are summarized as follows,(1)When La-based catalysts were modified by different anions(PO43-,SiO32-,SO42-),they showed different promoting effect to their OCM performance,especially for PO43-;It was found that the addition of anions could inhibit the formation of carbonate in the reaction,resulting in increasing the amount of active sites of the catalyst and then improving catalytic activity of OCM perfomance.Meanwhile,the addition of PO43' enhanced the ability of activation of oxygen molecules and promoted the activation of methane;and the SiO32-promoted the formation of moderate alkaline site which is benefit to the activation of C-H bond.(2)When La-based catalysts were modified by different amount of PO43-,the crystalline phases of the catalysts were changed from La2O3,La3PO7 to LaPO4,and decreasing crystalline size were showed with the addition of phosphate.When the catalyst was composed by La2O3 with optimal amount of La3PO7,it showed the better performance in OCM.The promotion of the OCM performance was related to the higher quantities of surface defect sites and the larger ratio of electrophilic oxygen species/lattice oxygen species on the sample.(3)On the 0.13P/La catalysts prepared by different citric acid content,the OCM performance of catalysts show trends of volcano shape with the increasing amount of critic acid.It was found that the P/La atomic ratio on surface was greatly influenced by the amount of critic acid.The.catalyst with CA/(P+La)= 2 showed best OCM performance.Higher concentration of the surface of P species promoted dispersion of La2O3,which is the main factor affecting the reaction activity of OCM,and lead to better OCM performance.