Effect of the Co-cation on Cu Speciation in Cu-Exchanged Mordenite and ZSM-5 Catalysts for the Oxidation of Methane to Methanol

摘要

Copper-exchanged zeolites are promising catalysts for the direct methane-to-methanol reaction, but the design of improved catalysts has been hampered by limited understanding of the active site structures. Here, we show that the identity of the co-cation (H vs Na) in Cu-MOR and Cu-ZSM-S catalysts significantly affects Cu speciation and the resulting reactivity of the catalysts in the cyclic methane-to-methanol reaction. The combination of reactivity results with spectroscopy and density functional theory (DFT) calculations suggests that the prevailing active site structure depends on the identity of the co-cation. The H-form of the catalysts contains a high concentration of mono-mu-oxo dicopper(II) species, which are selective for methanol formation, whereas the presence of Na appears to shift the Cu distribution toward species with greater oxygen content (attributed to mu-1,2-peroxo dicopper(II) species), which promote overoxidation of methane to carbon oxides. Results from DFT calculations indicate that Cu preferentially forms mono-mu-oxo dicopper(II) species in the 8MR side pockets of MOR, whereas the mu-1,2-peroxo dicopper(II) species is favored in the 12MR main channels of MOR for some Al pair configurations. Competition between Na and Cu for ion exchange sites in the 8MR side pockets results in displacement of some Cu into the 12MR main channels, thus affecting Cu speciation and catalyst selectivity. These findings suggest that the choice of co-cation can be used to control active site structure in transition-metal ion-exchanged zeolites.