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 O2 activation
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-resolved in situ Raman spectroscopy
reveals the kinetics of O2 adsorption and activation within
these well-defined Cu-CHA materials and the concomitant structural
evolution of copper–oxygen (CuxOy) complexes, which are interpreted
alongside Cu­(I) oxidation kinetics extracted from in situ X-ray absorption spectroscopy (XAS). Raman spectra of several plausible
CuxOy species
simulated using density functional theory suggest that experimental
spectra (λex = 532 nm) capture the formation of mono­(μ-oxo)­dicopper
species (ZCuOCuZ). Transient experiments show that the timescales
required to form CuxOy structures 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 CuxOy complexes continue to evolve structurally following
rapid Cu­(I) oxidation. The dependence of ZCuOCuZ formation rates on
O2 pressure, H2O pressure, and temperature is
consistent with a mechanism in which ZCuOH reduces to form ZCu+ sites that bind molecular oxygen and form ZCu-O2 intermediates. 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 O2 to form active
CuxOy complexes,
which underscore the insight afforded by judicious combinations of
experimental and theoretical techniques.