Ln2Zr2O7 compounds (Ln=La,Pr, Sm,Y) with varied rare earth A sites for low temperature oxidative coupling of methane

With the objective to develop catalysts having application potential for oxidative coupling of methane (OCM) at relatively lower temperature. A series of Ln₂Zr₂O₇ compounds with varied rare earth A sites have been prepared by a co-precipitation method. XRD and Raman have proved that pure Ln₂Zr₂O₇ compounds have been successfully prepared for all the catalysts. By decreasing the r_A/r_B ratio, their crystalline structure transform from an ordered pyrochlore (La₂Zr₂O₇) to a less ordered pyrochlore (Pr₂Zr₂O₇ and Sm₂Zr₂O₇) and eventually to a defective cubic fluorite phase (Y₂Zr₂O₇). H₂-TPR, O₂-TPD and XPS have testified that the amount of surface active O₂^- species follows the order of La₂Zr₂O₇ > Pr₂Zr₂O₇ > Sm₂Zr₂O₇ > Y₂Zr₂O₇, which is well consistent with the reaction performance, indicating that the abundance of surface O₂^- sites is a critical factor influencing the reaction performance. CO₂-TPD has demonstrated that a better catalyst generally possesses a larger amount of surface moderate alkaline sites, which is another factor to affect the reaction performance. It is concluded that the concerted interaction between the two types of surface active sites controls the reaction performance of the Ln₂Zr₂O₇ catalysts. In comparison with the state-of-the art Mn/Na₂WO₄/SiO₂, La₂Zr₂O₇, the best catalyst, exhibits much improved reaction performance below 750℃.