Developing Novel Metal Oxide Compounds for Efficient Oxidative Coupling of Methane(OCM)

【作者】 王翔； Xu Junwei； Xi Rong； Zhang Yan； Liu Yameng； Xu Xianglan； Fang Xiuzhong；

【Author】 Wang Xiang;Xu Junwei;Xi Rong;Zhang Yan;Liu Yameng;Xu Xianglan;Fang Xiuzhong;Key Lab of Jiangxi Province for Environment and Energy Catalysis, College of Chemistry,Nanchang University;

【机构】 Key Lab of Jiangxi Province for Environment and Energy Catalysis, College of Chemistry,Nanchang University；

【摘要】 OCM reaction is one of the important routes for value-added transformation of methane, which has aroused much interest over past decades. At present, the large scale industrialization of this important reaction is still restricted by lacking catalysts having enough one-way C₂ product yield. Therefore, with open thoughts to design novel kinds of efficient catalysts that can be operated at relatively lower temperature region, or can meet the minimum industrialization requirement is of great necessity.Over recent years, we discovered that some A₂B₂O₇ pyrochlore compounds,which owns bulk 8 a oxygen vacancies, intrinsic alkalinity and high thermal stability,can match most of the active site requirements for a good OCM catalyst^[1]. When studying La₂B₂O₇（B=Ti, Zr and Ce） compounds for OCM, we found that by decreasing the rA/rB radius ratio from La₂Ti₂O₇> La₂Zr₂O₇>La₂Ce₂O₇, a monoclinic layered perovskite, ordered cubic pyrochlore or defective cubic fluorite phase can be formed in sequence, which induces the change of the surface alkalinity and the amount of active oxygen sites. As a consequence, the reaction performance of the catalysts follows the order of La₂Ce₂O₇>La₂Zr₂O₇>La₂Ti₂O₇, testifying that the change of the crystalline structures impacts the reactivity of the catalysts eventually.To further confirm this, two La₂Zr₂O₇ compounds with ordered cubic pyrochlore or defective cubic fluorite phase have also been intentionally designed and used for OCM. The catalyst with a cubic fluorite structure displays much better reaction performance than the one with a pyrochlore structure. More recently, we have constructed strontium stannate catalysts with different crystalline phases for OCM reaction. A Sr₂SnO₄ catalyst with a perovskite-like stack of mono-layered structure demonstrates the best reaction performance among all the samples.It is concluded that that the abundance of surface alkaline sites and active electrophilic oxygen species is the major factor accounting for the OCM performance.In comparison with the State-of-the Art Mn/NaWO₄/SiO₂, La₂Ce₂O₇ and La₂Zr₂O₇ display much improved reaction performance below 750 ℃. In addition, these catalysts possess potent resistance to sulfur and lead poisoning. By further tuning the chemical compositions, optimizing the preparation methods and adding certain additives, catalysts with low temperature OCM performance could be achieved. The structure-reactivity relationship of these metal oxide compounds have been systematically elucidated.更多还原

【Abstract】 OCM reaction is one of the important routes for value-added transformation of methane, which has aroused much interest over past decades. At present, the large scale industrialization of this important reaction is still restricted by lacking catalysts having enough one-way C₂ product yield. Therefore, with open thoughts to design novel kinds of efficient catalysts that can be operated at relatively lower temperature region, or can meet the minimum industrialization requirement is of great necessity.Over recent years, we discovered that some A₂B₂O₇ pyrochlore compounds,which owns bulk 8 a oxygen vacancies, intrinsic alkalinity and high thermal stability,can match most of the active site requirements for a good OCM catalyst^[1]. When studying La₂B₂O₇（B=Ti, Zr and Ce） compounds for OCM, we found that by decreasing the rA/rB radius ratio from La₂Ti₂O₇> La₂Zr₂O₇>La₂Ce₂O₇, a monoclinic layered perovskite, ordered cubic pyrochlore or defective cubic fluorite phase can be formed in sequence, which induces the change of the surface alkalinity and the amount of active oxygen sites. As a consequence, the reaction performance of the catalysts follows the order of La₂Ce₂O₇>La₂Zr₂O₇>La₂Ti₂O₇, testifying that the change of the crystalline structures impacts the reactivity of the catalysts eventually.To further confirm this, two La₂Zr₂O₇ compounds with ordered cubic pyrochlore or defective cubic fluorite phase have also been intentionally designed and used for OCM. The catalyst with a cubic fluorite structure displays much better reaction performance than the one with a pyrochlore structure. More recently, we have constructed strontium stannate catalysts with different crystalline phases for OCM reaction. A Sr₂SnO₄ catalyst with a perovskite-like stack of mono-layered structure demonstrates the best reaction performance among all the samples.It is concluded that that the abundance of surface alkaline sites and active electrophilic oxygen species is the major factor accounting for the OCM performance.In comparison with the State-of-the Art Mn/NaWO₄/SiO₂, La₂Ce₂O₇ and La₂Zr₂O₇ display much improved reaction performance below 750 ℃. In addition, these catalysts possess potent resistance to sulfur and lead poisoning. By further tuning the chemical compositions, optimizing the preparation methods and adding certain additives, catalysts with low temperature OCM performance could be achieved. The structure-reactivity relationship of these metal oxide compounds have been systematically elucidated.更多还原