Construction Of SnO₂ And Ln₂Zr₂O₇ Pyrochlores Catalyzed Materials For Methane Oxidation Coupling Into Ethylene

The main component of natural gas is 83%⁹9%methane.Therefore,it is of great importance to utilize methane industrially.The oxidative coupling of methane?OCM?into ethylene is one of the important routes to transform it into added value chemicals,which can be finished in one step.Therefore,it has both academic and economic values.However,at present,high reaction temperature,large energy consumption and low one way C₂ product yield are still the problems that need to be solved.Therefore,seeking highly efficient catalysts that can be operated at low temperature is a hot topic which is concerned by many scientists.Based on the deep understanding of the OCM reaction mechanism,this thesis has designed and prepared SnO₂-based and Ln₂Zr₂O₇pyrochlore catalysts for OCM reaction.The main results are summarized here:Part one:A series of catalysts modified by different alkali metal oxides been prepared.In comparison with the un-modified SnO₂,the reaction performance of all the modified catalyst can be significantly improved,among which lithium oxide shows the best promotional effects,and the optimal catalyst was achieved with a Sn/Li molar ratio of 5/5.Over this catalyst,the highest C₂ product yield of 16%has been achieved at 750?.XPS and CO₂-TPD results have revealed that for those catalysts with evident enhanced OCM reaction performance,the co-existence of suitable amount of surface alkaline and electrophilic oxygen sites is indispensable.Furthermore,over the catalysts modified by different amount of lithium oxide,the C₂yield at different temperatures is nearly proportional to the amounts of both surface intermediate alkaline sites and electrophilic oxygen species.Therefore,it is concluded that the abundance and the concerted interaction of these two types of surface active sites are the major factors determining the reaction performance of the SnO₂-based catalysts.Last but not the least,the optimized catalyst,which has a Sn/Li molar ratio of 5/5 and also contains equal amounts of Sn O₂ and Li₂SnO₃ crystalline phases,exhibits much better reaction performance than Mn/Na₂WO₄/SiO₂,the most promising catalyst at present,at low temperature region?below 750??.Part two:A series of Ln₂Zr₂O₇?Ln=La,Pr,Sm and Y?pyrochlore compounds with different A sites have been prepared as catalysts for OCM reaction.XRD results show that the pure Ln₂Zr₂O₇ pyrochlore compounds have been successfully prepared by co-precipitation method.In comparison with pure Zr O₂,all the Ln₂Zr₂O₇ catalysts also exhibit higher activity and selectivity to C₂ products.La₂Zr₂O₇ owns the most abundant electrophilic oxygen and surface alkaline sites,it thus exhibits the best performance among all the catalysts,on which the highest C₂ product yield of 15.1%can be achieved at 750°C.In comparison with Mn/Na₂WO₄/SiO₂,the most promising catalyst at present,La₂Zr₂O₇ exhibits a superior reaction performance at low temperature region?<750°C?.