Abstract
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 Ca 2+ 3C-O 2− 4C and Ca 2+ 4C-O 2− 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 O 2− 2 species. Molecular oxygen adsorbs both on acid and basic sites and forms either π-complexes or O 3 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 Ca 2+ LC ion by Mg 2+ LC in Mg/CaO, Na + LC in Na/CaO and Zn 2+ 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 C 2+ 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 Zn 2+ cations.
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