Dioxygen Activation Kinetics over Distinct Cu Site Types in Cu-Chabazite Zeolites

Cu-exchanged zeolites activate dioxygen to form active sites for partial methane oxidation (PMO), nitrogen oxide decomposition, and carbon monoxide oxidation. Apparent rates of O₂activation depend both on the intrinsic kinetics of distinct Cu site types and the distributions of such sites within a given zeolite, which depend on the density and arrangement of the framework Al atoms. Here, we use hydrothermal synthesis methods to control the arrangement of framework Al sites in chabazite (CHA) zeolites and, in turn, the distinct Cu site types formed. Time-resolvedin situRaman spectroscopy reveals the kinetics of O₂adsorption and activation within these well-defined Cu-CHA materials and the concomitant structural evolution of copper-oxygen (Cu_xO_y) complexes, which are interpreted alongside Cu(I) oxidation kinetics extracted fromin situX-ray absorption spectroscopy (XAS). Raman spectra of several plausible Cu_xO_yspecies simulated using density functional theory suggest that experimental spectra (λ_ex= 532 nm) capture the formation of mono(μ-oxo)dicopper species (ZCuOCuZ). Transient experiments show that the timescales required to form Cu_xO_ystructures that no longer change in Raman spectra correspond to the durations of oxidative treatments that maximize CH₃OH yields in stoichiometric PMO cycles (approximately 2 h). Yet, these periods extend well beyond the timescales for the complete conversion of the initial Cu(I) intermediates to their Cu(II) states (<0.3 h, reflected in X-ray absorption near edge spectroscopy spectra), which demonstrates that Cu_xO_ycomplexes continue to evolve structurally following rapid Cu(I) oxidation. The dependence of ZCuOCuZ formation rates on O₂pressure, H₂O pressure, and temperature is consistent with a mechanism in which ZCuOH reduces to form ZCu⁺sites that bind molecular oxygen and form ZCu-O₂intermediates. Subsequent reaction with proximate ZCu⁺forms bridging peroxo dicopper complexes that cleave O-O bonds to form ZCuOCuZ in steps facilitated by water. These data and interpretations provide evidence for the chemical processes that link rapid and kinetically irrelevant Cu oxidation steps (frequently probed by XAS and UV-vis spectroscopy) to the relatively slow genesis of reactive Cu complexes that form CH₃OH during PMO. In doing so, we reveal previously unrecognized complexities in the processes by which Cu ions in zeolites activate O₂to form active Cu_xO_ycomplexes, which underscore the insight afforded by judicious combinations of experimental and theoretical techniques.