The effect of Zn doping on active Cu species and its location of Cu-exchanged mordenite for the stepwise oxidation of methane to methanol

The effect of Zn doping (0.5-3 wt%) on Cu-exchanged mordenite (Cu-MOR) was investigated during the stepwise oxidation of methane to methanol. The strong interaction between Cu and ZnOx stabilized the highly-dispersed state of Cu2+ but reduced the Cu2+ bounded to extra-framework oxygen (active site), as demonstrated by the H-2-TPR and XPS results. The Cu/Al and Zn/Al ratios suggested that Zn preferably bonded to the sites in the 8-MR channel, which led to highly dispersed Cu2+ anchored onto the highly accessible sites (12-MR and 8-MR side pocket). The reactivity indicated that highly dispersed Cu2+ can be gradually transformed into active Cu2+ species during contact with methane. Bimetallic Cu-ZnOx was also able to activate methane, resulting in a product complex. Although Zn-doped Cu-MOR catalysts gave a lower methanol yield at 2 h, a higher methanol yield could be achieved at saturation methane loading time. Interestingly, 3 wt% Zn doping on Cu-MOR showed superior activity due to the increase of methanol yield up to 20% at 5 h of methane loading time. This work paves the way for the design of highly dispersed Cu2+ in the 12-MR channel of mordenite zeolite via the control of strong Cu-ZnOx interaction.