We interrogate Cu-zeolites with well-defined Cu speciation and proximity, prepared using judiciously chosen synthetic and treatment procedures, to facilitate identifying partial methane oxidation (PMO) active sites. The distribution of framework Al in chabazite (CHA) zeolites influences the speciation of mononuclear Cu
2+ (Z
2Cu) and [Cu(II)OH]
+ (ZCuOH) [1] and binuclear Cu
2+ di-oxo complexes [2]. Cu-CHA were synthesized to contain solely Z
2Cu or ZCuOH sites of varying density, or a mixture of Z
2Cu and ZCuOH sites [1], and studied for stoichiometric PMO (oxidative treatment: 20 kPa O
2, 723 K, 6 h, methane exposure: 21 kPa CH
4, 473 K, 0.5 h, methanol extraction: 2.5 kPa H
2O, 473 K, 1 h). Methanol yields (per Cu) do not depend systematically on total Cu or Z
2Cu content, but increase systematically with ZCuOH density. ZCuOH sites are precursors to polynuclear Cu-oxo structures that form after O
2 treatments [2] and behave as PMO active sites [3].
In-situ X-ray absorption spectroscopy showed that inert (He, 723 K) and reducing (CH
4, 473 K) environments led to increasing fractions of Cu(I) with ZCuOH density.
In-situ UV-Visible spectroscopy identified mononuclear Cu
2+ species from d-d transitions (~8,000-16,000 cm
-1) and binuclear Cu
2+ di-oxo from LMCT (24,000-30,000 cm
-1), and spectral changes were monitored while reducing Cu di-oxo species by CO (523 K) [2] and CH
4 (473 K). O
2 activation of Cu-CHA forms bis(μ-oxo) dicopper(II),
trans-1,2-peroxo dicopper(II), and μ-(η
2:η
2)peroxo dicopper(II) intermediates identified by Raman spectra, whose assignments were confirmed with computational modeling. Multivariate curve resolution-alternative least squares was used to extract time-resolved contributions from Raman spectra to investigate O
2 activation kinetics, which reveal two Cu site types with distinct kinetic behavior.
[1] Paolucci, C. et al., J. Am. Chem. Soc., 2016, 138, 6028.
[2] Li, H., et al., Chem. Sci., 2019, 10, 2373.
[3] Pappas, D. K. et al., J. Am. Chem. Soc., 2017, 139, 14961.
