Within the framework of theMINDO/3 method methane and oxygen interactions with pure and modified calcium oxide have been considered. It is shown that low-coordinated Ca2+3C-O2-4C and Ca2+4C-O2-3C pairs of acid-base centers are responsible for the initial step in the methane activation on pure calcium oxide. Atomic oxygen strongly adsorbs only on surface basic sites yielding surface O2-2 species. Molecular oxygen adsorbs both on acid and basic sites and forms either π-complexes or O3 species, respectively. However, the dissociative adsorption of molecular oxygen on adjacent pairs of basic sites is energetically more favorable. The energetics of the substitution of a Ca2+LC ion by Mg2+LC in Mg/CaO, Na+LC in Na/CaO and Zn2+LC in Zn/CaO and Na/Zn/CaO and the methane activation on these modified calcium oxides have also been considered. The results allow an understanding of the experimentally observed increase of the C2+ hydrocarbon selectivity in the oxidative coupling of methane both on mixed magnesium and calcium oxides containing 10 to 15% of CaO and on NaOH-promoted CaO catalysts containing minor amounts of Zn2+ cations.
- Calcium oxide
- Quantum-chemical study
ASJC Scopus subject areas
- Process Chemistry and Technology
- Physical and Theoretical Chemistry