Oxidative coupling of methane and oxidative dehydrogenation of ethane reactions over La-based monoclinic layered perovskite and double perovskite

Abstract

Herein, we partially doped the A-site or B-site cations of the monoclinic layered perovskite La₂Ti₂O₇ with alkali metal ions (Na⁺ and K⁺) and alkaline earth metal ions Mg²⁺ to form double perovskites (NaLaTi₂O₆, KLaTi₂O₆, and LaTiMgO₆) for the oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODHE), demonstrating that the compatibility between the radii of the heterovalent doping ions and the radii of the host perovskite A-site and B-site cations is an important factor affecting their reaction performance. Doping with heterovalent ions changes the crystal symmetry and structure of the host perovskite La₂Ti₂O₇, and when the radius of the doping ion is closer to that of the host A- or B-site ion, more oxygen vacancies can be generated and more chemisorbed oxygen species can form. The oxygen vacancies are closely related to the moderate basic sites, and the reactive oxygen species in OCM and ODHE are O₂^δ- and O₂⁻. Doping with B-site cations is more beneficial for improving the reaction performance than doping with A-site cations. This is attributed to the covalent property of the B-O bond in perovskite, as doping with B-site cations can weaken the covalent bond, thereby facilitating the formation of oxygen vacancies. Moreover, the additional chemisorbed oxygen generated by oxygen vacancies resulting from high-temperature crystal phase transitions/lattice distortions in perovskite can further enhance reaction performance.