Influence of the La/Ce ratio on La-Ce oxides promoted by Sr in the methane oxidative coupling reaction

Abstract

The high methane availability and its significant role as a greenhouse gas have led the scientific community to pursue methods for its use. This paper proposes the usage of the oxidative coupling of methane (OCM) as a promising path to transform methane directly into value-added hydrocarbons, mostly ethylene, which serves as a crucial compound in the petrochemical sector. An efficient OCM catalyst should contain substantial basicity and oxygen vacancies for providing surface electrophilic oxygen species, such as superoxides and peroxides, instrumental for boosting selectivity towards C₂ products. Mixed lanthanum-cerium oxide (La-Ce) catalysts have emerged as strong candidates in OCM due to their high thermal stability, oxygen mobility, and high alkalinity. In this study, they were prepared through a surfactant-assisted hydrothermal method with different La/Ce ratios for fine-tuning their basic properties and promoting oxygen mobility on the surface of the catalysts. Samples followed a volcano-shaped relationship between C₂ yield and La content, with optimal performance at a La/Ce molar ratio of 2.1, attributed to the interplay between the high amount of basic sites and oxygen vacancies, increasing the presence of superoxide species over lattice oxygen. Moreover, the Sr-promoted catalyst showed high density of strong basic sites while preserving the reactive oxygen species, achieving 20 % CH₄ conversion, with 57 % C₂ compounds selectivity at 750 °C and GHSV of 18.000 mL.g_cat⁻¹.h⁻¹.