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

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-5 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-μ-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 μ-1,2-peroxo
dicopper­(II) species), which promote overoxidation of methane to carbon
oxides. Results from DFT calculations indicate that Cu preferentially
forms mono-μ-oxo dicopper­(II) species in the 8MR side pockets
of MOR, whereas the μ-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.