Oxygen Precursor to the Reactive Intermediate in Methanol Synthesis by Cu-ZSM-5

The reactive oxidizing species in the selective oxidation of methane to methanol in oxygen activated Cu-ZSM-5 was recently defined to be a bent mono(μ-oxo)dicopper(II) species, [Cu2O]2+. In this communication we report the formation of an O2-precursor of this reactive site with an associated absorption band at 29 000 cm−1. Laser excitation into this absorption feature yields a resonance Raman (rR) spectrum characterized by 18O2 isotope sensitive and insensitive vibrations, νO−O and νCu−Cu, at 736 (Δ18O2 = 41 cm−1) and 269 cm−1, respectively. These define the precursor to be a μ-(η2:η2) peroxo dicopper(II) species, [Cu2(O2)]2+. rR experiments in combination with UV−vis absorption data show that this [Cu2(O2)]2+ species transforms directly into the [Cu2O]2+ reactive site. Spectator Cu+ sites in the zeolite ion-exchange sites provide the two electrons required to break the peroxo bond in the precursor. O2-TPD experiments with 18O2 show the incorporation of the second 18O atom into the zeolite lattice in the transformation of [Cu2(O2)]2+ into [Cu2O]2+. This study defines the mechanism of oxo-active site formation in Cu-ZSM-5.