Catalytic oxidation of methane to methanol over Cu-CHA with molecular oxygen

Abstract

Direct oxidation of CH₄ to CH₃OH using O₂ is challenging because of the high stability of CH₄ and the relatively high reactivity of CH₃OH. Here, Cu-CHA zeolites are tested for the direct oxidation of CH₄. Catalytic production of CH₃OH in a CH₄–O₂–H₂O flow reaction is improved using CHA type zeolites compared to other zeolites including MOR, BEA, MFI, and FAU zeolites reported previously. In situ X-ray absorption fine structure (XAFS) spectroscopy reveals that the high catalytic activity of Cu-CHA is derived from its redox properties, particularly, the high reducibility of Cu²⁺ involved in CH₄ activation. The reaction gas concentrations are varied to find the optimized reaction conditions on Cu-CHA. In addition, the reaction mechanism of the direct CH₄ oxidation on Cu-CHA is investigated based on not only the effects of gas concentrations but also the isotope gas effects using CD₄, ¹⁸O₂, and D₂O. It is suggested that the rate-determining step is the C–H activation of CH₄, and the selectivity of CH₃OH is determined by the oxidation rate of CH₃OH, which is affected by the O₂ and OH group activation rate on Cu-CHA.