Catalytic oxidative coupling of methane over mixed-anion rare-earth oxyfluorides: composition/structure–activity relationships

Abstract

The incorporation of fluoride anions into oxide catalysts can modify their structural and surface properties, consequently influencing their catalytic performance. In this work, a series of rare-earth oxyfluorides (REOF; RE = La, Sm, Eu, Dy, Y, and Yb) was investigated for their catalytic activity in the oxidative coupling of methane (OCM). All REOFs crystallize in a trigonal structure, except YbOF, which adopts either monoclinic (YbOF-m) or tetragonal (YbOF-t) polymorphs. The CH₄ conversion and C₂ selectivity on the REOF catalysts at 600 and 800 °C exhibit systematic increasing or decreasing trends that correlate with the ionic radii of the RE³⁺ cations. Among the trigonal REOFs, YOF shows the highest CH₄ conversion and C₂ selectivity at 800 °C, with values of 24.8% and 44.9%, respectively. CO₂- and O₂-temperature-programmed desorption (TPD) analyses reveal that OCM activity, lattice size, basicity, and surface oxygen species follow similar trends, indicating correlations among these factors. Larger lattice parameters and longer RE-(O,F) bond lengths are associated with a higher density of moderate basic sites and moderately bound surface oxygen species. Finally, YbOF-m exhibits superior CH₄ conversion and C₂ selectivity compared to its tetragonal polymorph, YbOF-t. At 700 °C, the C₂ selectivities are 45.2% and 14.5% for the monoclinic and tetragonal phases, respectively.