Direct Oxidation of Methane to C1 Oxygenates on Cu/ZSM-5: Reaction Property and Configuration of Cu Species

Direct oxidation of methane (DOM) under mild conditions is arguably one of the holy grails in chemistry. Cu-based zeolite catalysts have attracted much attention for their high yield and selectivity of C1 oxygenates. However, the structure-activity relationships of DOM remain controversial due to the complex configurations of Cu on zeolites. Herein, we prepared Cu/ZSM-5 catalysts with various Cu loading and investigated the effects of Cu configurations on the DOM reaction. By combining the characterization techniques and evaluating the catalytic performance, we found that the configurations of Cu species could significantly affect the yield and distribution of C1 oxygenates, which was strongly dependent on Cu loading. The highly dispersed mononuclear Cu^II in the channels of ZSM-5 is proved to be highly active. While the multinuclear Cu species inhibit the formation of HCOOH, simultaneously suppressing the overall reaction yield. The agglomerated CuO species on the surface of ZSM-5 show a relatively low correlation with DOM reaction. Thus, the relationship between the yield of C1 oxygenates and Cu loading exhibits a typical volcanic curve and 2.2 wt.% Cu/ZSM-5 possess the best performance using H₂O₂ as oxidant (productivity of 17,000 μmol g_cat⁻¹ h⁻¹ with 98.6 % selectivity at 50 °C).