Oxygen precursor to the reactive intermediate in methanol synthesis by Cu-ZSM-5

Abstract

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, [Cu_2O]^(2+). In this communication we report the formation of an O_2-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 ^(18)O_2 isotope sensitive and insensitive vibrations, νO-O and νCu-Cu, at 736 (Δ^(18)O_2 = 41 cm^(-1)) and 269 cm^(-1), respectively. These define the precursor to be a μ-(η^2:η^2) peroxo dicopper(II) species, [Cu_2(O_2)]^(2+). rR experiments in combination with UV-vis absorption data show that this [Cu_2(O_2)]^(2+) species transforms directly into the [Cu_2O]^(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. O_2-TPD experiments with ^(18)O_2 show the incorporation of the second ^(18)O atom into the zeolite lattice in the transformation of [Cu_2(O_2)]^(2+) into [Cu_2O]^(2+). This study defines the mechanism of oxo-active site formation in Cu-ZSM-5.