Mechanistic insight into the effect of active site motif structures on direct oxidation of methane to methanol over Cu-ZSM-5

Abstract

Direct oxidation of methane to methanol (DMTM), a highly challenging reaction in C₁ chemistry, has attracted lots of attention. Herein, we investigate the continuous H₂O-mediated N₂O-DMTM over a series of Cu-ZSM-5-n zeolites prepared by a solid-state ion-exchange method. Excellent CH₃OH productivity (194.8 μmol g_cat⁻¹ h⁻¹) and selectivity (67.1%) can be achieved over Cu-ZSM-5-0.3%, which surpasses most recently reported zeolite catalysts. The effect of the active site motif structure on the reaction was systematically investigated by the combined experimental and theoretical studies. It has been revealed that both the monomeric [Cu]⁺ and binuclear [Cu]⁺–[Cu]⁺ sites function to produce CH₃OH, following the radical rebound mechanism, wherein the latter one plays a dominant role due to the synergistic effect of neighboring [Cu]⁺ that can efficiently reduce the N₂O dissociation barrier to generate active oxygen for CH₄ oxidation. Microkinetic modeling results further show that the dicopper site possesses a much higher net reaction rate (1.23 × 10⁵ s⁻¹) than the monomeric Cu site (0.962 s⁻¹); moreover, H₂O can shift the rate determining step from the CH₃OH desorption step to the N₂O dissociation step over the dicopper site, thereby efficiently favoring CH₃OH production and resisting carbon deposition. Generally, the study in the present work would substantially favor other highly efficient catalyst designs.