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, [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.