The ability of Cu species to bind O2, cleave the O-O bond, and form distinct CuxOy structures depends strongly upon the spatial and crystallographic distribution of the Al atoms within the zeolite and the fraction of this distribution associated with Cu ions. The Al distribution in a given zeolite depends on its bulk Si/Al ratio and framework topology, and the conditions used for its synthesis, which can collectively lead to significant Cu site heterogeneity among experimental studies. Divalent copper complexes are particularly sensitive to non-uniformities in the arrangement of AlO4 - sites, because they either require charge compensation from two AlO4 -centers or coordination to anionic ligands if only one AlO4 -center is present. As stated by Borfecchia et al., the differences between the many proposals for active CuxOy sites for stoichiometric PMO cycles likely reflect complexities both among the zeolite topologies studied (e.g., ZSM-5, MOR, CHA) and among the varied reaction protocols used (e.g. O2 activation conditions, CH4 reaction conditions). The high symmetry chabazite (CHA) framework provides a model material to study the nature of O2 activation, because CHA contains a single crystallographically unique tetrahedral site. Moreover, CHA zeolites can be synthesized intentionally with precise control over the relative proximity of framework Al atoms. For example, Dusselier and coworkers used this capability to demonstrate on Fe-CHA zeolites that paired Al atoms in the 6-membered ring (MR) (i.e., in second-or third-nearest neighbor configurations) stabilize the PMO active site (α-Fe) and correlate to CH3OH yields (per Fe) from PMO cycles. Well-defined Cu-CHA materials with controlled speciation of Cu ions can be synthesized wherein 6-MR paired Al sites (2 Al per 6-MR) exchange Cu 2+ ions (Z2Cu), while 6-MR isolated framework Al atoms (1 Al per 6-MR) nominally exchange [CuOH] + species (ZCuOH). Upon O2 activation of Cu-CHA, a fraction of ZCuOH species form binuclear O2-bridged structures observed within in situ UV-Vis spectra, while Z2Cu sites do not. These findings agree with reports Z2Cu sites do not participate in PMO. The elementary steps by which O2 activates over ZCuOH sites to form PMO-active species, however, remain elusive.