Copper-zeolites facilitate partial methane oxidation (PMO) to methanol, with various mononuclear and multinuclear active site structures proposed in the literature, and which form upon dioxygen activation in amounts that depend on both bulk and atomic-scale zeolite properties. Here, we use a high-symmetry zeolite framework (CHA) as a model support to investigate the effects of mononuclear Cu site type and proximity on the formation of binuclear Ox-bridged Cu sites upon dioxygen activation and the consumption of binuclear sites in reduction (CH4, CO, He auto-reduction) treatments. CHA was synthesized to vary the relative amounts of 6-MR paired and isolated Al sites, which respectively exchange either Cu2+ (Z2Cu) or [CuOH]+ (ZCuOH) site motifs, with the latter forming binuclear Ox-bridged Cu2+ complexes upon O2 activation (723 K). Cu-CHA samples containing solely Z2Cu sites do not undergo auto-reduction at high temperatures (He, 723 K), do not form multinuclear Ox-bridged structures upon dioxygen activation (723 K), and do not form methanol from stoichiometric PMO cycles as reported previously. In contrast, the quantity of binuclear Ox-bridged Cu sites that reduce in methane to form methanol during stoichiometric PMO cycles and that undergo auto-reduction in He are similar, and generally increases as the mean distance between ZCuOH sites decreases; ZCuOH sites are thus precursors to PMO-active sites. Increasing ZCuOH proximity leads to increasing numbers of binuclear Ox-bridged Cu structures (i.e., trans-μ-1,2-peroxo and mono-μ-oxo dicopper(II)); identified experimentally by in situ UV-Visible and Raman spectroscopies) formed upon dioxygen activation (723 K), which are quantified experimentally with CO reduction (TPR) and predicted by theory from statistical Cu siting models that account for both mononuclear and binuclear structures. In situ Cu K-edge X-ray absorption spectra show that inert (He, 723 K) and reducing (CH4, 473 K) treatments form Cu(I) in increasing fractions with decreasing mean ZCuOH distance. This investigation illustrates how integrating synthesis routes to precisely control Cu site speciation and density, in situ methods to characterize and quantify Cu sites, and theoretical Cu siting models can elucidate the mechanisms of Cu(I) oxidation and Cu(II) reduction in different gas environments (reductants, oxidants, inerts) and the atomic-scale properties that determine the number of Cu sites that facilitate partial methane oxidation.

Quantitative connections between copper site proximity and binuclear active sites for partial methane oxidation to methanol in Cu-CHA zeolites

ACS Fall 2021

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