Effects of Mg, Ca, Sr, and Ba Dopants on the Performance of La 2 O 3 Catalysts for the Oxidative Coupling of Methane.

Oxidative coupling of methane (OCM) is a reaction to directly convert methane into high value-added hydrocarbons (C 2+ ) such as ethylene and ethane using molecular oxygen and a catalyst. This work investigated lanthanum oxide catalysts for OCM, which were promoted with alkaline-earth metal oxides (Mg, Ca, Sr, and Ba) and prepared by the solution-mixing method. The synthesized catalysts were characterized using X-ray powder diffraction, CO 2 -programmed desorption, and X-ray photoelectron spectroscopy. The comparative performance of each promoter showed that promising lanthanum-loaded alkaline-earth metal oxide catalysts were La-Sr and La-Ba. In contrast, the combination of La with Ca or Mg did not lead to a clear improvement of C 2+ yield. The most promising LaSr50 catalyst exhibited the highest C 2+ yield of 17.2%, with a 56.0% C 2+ selectivity and a 30.9% CH 4 conversion. Catalyst characterization indicated that their activity was strongly associated with moderate basic sites and surface-adsorbed oxygen species of O 2 - . Moreover, the catalyst was stable over 25 h at a reactor temperature of 700 °C.