Effect of phase transformation of La₂Zr₂O₇ catalysts on catalytic performance for the oxidative coupling of methane

La₂Zr₂O₇ catalyst was prepared using co-precipitation method and then calcined at different temperatures to obtain a series of catalysts with different phase structures. Their catalytic performances for oxidative coupling of methane were evaluated in a fixed bed micro-reactor. Meanwhile, the changes of phase structure, surface base sites and surface oxygen species upon these samples were characterized with XRD, Raman, CO₂-TPD, and XPS. With the increase of the calcination temperature from 700 to 1200℃, the crystallinity of La₂Zr₂O₇ catalysts increased continuously and the crystal phase changed obviously. The structure of the catalysts gradually changed from amorphous to disordered defective fluorite structure, and ultimately to phrochlore structure. Both the number of medium basic sites and the electrophilic oxygen species, such as O₂^2- and O₂^-, on the catalysts decreased with the phase transition caused by the temperature raising, resulting in the decrease of CH₄ conversion and C₂₊ selectivity. The amorphous LZO-CP-700 catalyst showed the best performance in methane oxidation coupling reaction.