Structure and Reactivity of Binuclear Cu Active Sites in Cu-CHA Zeolites for Stoichiometric Partial Methane Oxidation to Methanol

Aluminosilicate
zeolites exchanged with copper ions facilitate
partial methane oxidation (PMO) to methanol in stoichiometric oxidation
and reduction cycles, yet the identities of active Cu sites and details
of the reaction mechanism remain debated. Here, we use the high-symmetry
chabazite (CHA) zeolite framework as a model support to probe the
relationship between bulk composition, Cu speciation, and response
to various oxidizing and reducing treatments. Density functional theory
and first-principles thermodynamics combined with statistical models
reveal that Cu speciation and composition depend strongly on Al configuration
and external gas conditions. Cu-CHA samples were synthesized to survey
broad regions of Si/Al and Cu/Al composition space and framework Al
proximity. Characterization by in situ X-ray absorption
and UV–visible spectroscopy during exposure to different oxidation
conditions reveal that the extent of Cu oxidation is sensitive to
activation conditions and thus that both kinetic and thermodynamic
factors influence Cu oxidizability in a given material. Similar characterizations
during CO reduction reveal that CO titrates Cu2+ in amounts
suggesting the presence of both O- and O2-bridged species.
In contrast, CH4 and autoreduction (He) treatments reduce
similar but smaller numbers of Cu sites than CO, implicating O2-bridged Cu dimers as a potential common intermediate in the
former reduction pathways. A systematic increase in methanol yields
(per Cu) in stoichiometric PMO cycles increase with the fraction of
binuclear Ox-bridged Cu sites suggests
these species as active sites, as depicted in an updated PMO reaction
mechanism.