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

Direct oxidation of CH4 to CH3OH using O-2 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-O-2-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, O-18(2), 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 O-2 and OH group activation rate on Cu-CHA.

Automated summary

Cu-CHA zeolites show high catalytic activity in the direct oxidation of CH4 to CH3OH due to their redox properties, particularly the high reducibility of Cu2+ involved in CH4 activation.