Isotopic Oxygen Exchange between Dioxygen and MgO Catalysts for Oxidative Coupling of Methane

Abstract

The isotopic oxygen exchange between dioxygen and MgO based catalysts (Li/MgO, Zr/MgO, and MgO) was investigated at 773 to 1173 K in order to clarify the mechanism behind the generation of the active sites. The rate analysis was successful when two exchange steps having different rates (fast and slow) and different amounts of exchangeable (surface and lattice) oxygen atoms were assumed. The fast step was proposed as the exchange between dioxygen and the surface oxygen anion, whereas the oxide anion flow from the bulk to the surface was proposed for the slow step. However, both the MgO and Zr/MgO catalysts exhibited similar results regarding the two different rates and exchangeable oxygen atoms as well as their temperature dependencies. Zr/MgO had slightly higher rates due to the Zr4+added, in contrast to the results for Li/MgO; a faster exchange per surface area and a larger amount of exchangeable oxygen (8.8 times as large as surface oxygen atoms at 1073 K). Li/MgO was proposed as having both O−(MgO) and Li+O−as the active sites. The isotopic oxygen exchange (fast and slow) becomes measurable above ca 873 K, where hydrogen evolution starts during the TPD run (generation an O−defect) while the oxidative coupling of methane (OCM) reaction becomes appreciable. This suggests that both the exchange and the OCM reactions occurred through the active sites of O−produced at above 973 K. The exact rate of oxygen transfer in the exchange was much lower than the rate of the OCM reaction. From these results, we have concluded that the measurable “fast” exchange was the rate of exchange between the surface oxygen and the active center O−, while the exchange between dioxygen and O−(also a step of OCM) may be much faster.