Unveiling the Role of Copper Valence States in Enhancing the Catalytic Performance of Copper-Modified ZSM-5 for Direct Methane Conversion

The conversion of methane (CH4) to methanol (CH3OH) under mild conditions remains a significant challenge in catalysis. In this study, we introduce a method to adjust the surface valence states of copper species in Cu-ZSM-5 catalysts by annealing under different atmospheres (N2, air, and H2). Among these, the 10% Cu-ZSM-5 catalyst calcined in H2 showed outstanding performance, achieving a methanol productivity of 8.08 mmol/(gcat·h) and 91% selectivity at 70 °C and 3 MPa using H2O2 as the oxidant. Comprehensive characterization revealed that H2 annealing optimized the Cu surface to a lower valence state (predominantly Cu+), enhancing CH4 adsorption and promoting H2O2 activation to generate ·OH and ·CH3 radicals, which drive selective CH3OH formation. In situ DRIFTS and radical trapping experiments further confirmed the critical role of Cu+ in facilitating C-H bond cleavage and suppressing overoxidation.