Correlation between the acid–base properties of the La2O3 catalyst and its methane reactivity

Abstract

Density functional theory and coupled cluster theory calculations were carried out to study the effects of the acid–base properties of the La₂O₃ catalyst on its catalytic activity in the oxidative coupling of methane (OCM) reaction. The La³⁺–O²⁻ pair site for CH₄ activation is considered as a Lewis acid–Brönsted base pair. Using the Lewis acidity and the Brönsted basicity in the fluoride affinity and proton affinity scales as quantitative measures of the acid–base properties, the energy barrier for CH₄ activation at the pair site can be linearly correlated with these acid–base properties. The pair site consisting of a strong Lewis acid La³⁺ site and a strong Brönsted base O²⁻ site is the most reactive for CH₄ activation. In addition, the basicity of the La₂O₃ catalyst was traditionally measured by temperature-programmed desorption of CO₂, but the CO₂ chemisorption energy is better regarded as a combined measure of the acid–base properties of the pair site. A linear relationship of superior quality was found between the energy barrier for CH₄ activation and the CO₂ chemisorption energy, and the pair site favorable for CO₂ chemisorption is also more reactive for CH₄ activation, leading to the conflicting role of the “basicity” of the La₂O₃ catalyst in the OCM reaction. The necessity for very high reaction temperatures in the OCM reaction is rationalized by the requirement for the recovery of the most reactive acid–base pair site, which unfortunately also reacts most readily with the byproduct CO₂ to form the very stable CO₃²⁻ species.