Formation of [Cu₂O₂]²⁺ and [Cu₂O]²⁺ toward C-H Bond Activation in Cu-SSZ-13 and Cu-SSZ-39

Cu-exchanged small-pore zeolites (CHA and AEI) form methanol from methane (>95% selectivity) using a 3-step cyclic procedure (Wulfers et al. Chem. Commun. 2015, 51, 4447-4450) with methanol amounts higher than Cu-ZSM-5 and Cu-mordenite on a per gram and per Cu basis. Here, the Cu_xO_y species formed on Cu-SSZ-13 and Cu-SSZ-39 following O₂ or He activation at 450°C are identified as trans-μ-1,2-peroxo dicopper(II) ([Cu₂O₂]²⁺) and mono-(μ-oxo) dicopper(II) ([Cu₂O]²⁺) using synchrotron X-ray diffraction, in situ UV-vis, and Raman spectroscopy and theory. [Cu₂O₂]²⁺ and [Cu₂O]²⁺ formed on Cu-SSZ-13 showed ligand-to-metal charge transfer (LMCT) energies between 22,200 and 35,000 cm^-1, Cu-O vibrations at 360, 510, 580, and 617 cm^-1 and an O-O vibration at 837 cm^-1. The vibrations at 360, 510, 580, and 837 cm^-1 are assigned to the trans-μ-1,2-peroxo dicopper(II) species, whereas the Cu-O vibration at 617 cm^-1 (Δ¹⁸O = 24 cm^-1) is assigned to a stretching vibration of a thermodynamically favored mono-(μ-oxo) dicopper(II) with a Cu-O-Cu angle of 95°. On the basis of the intensity loss of the broad LMCT band between 22,200 and 35,000 cm^-1 and Raman intensity loss at 571 cm^-1 upon reaction, both the trans-μ-1,2-peroxo dicopper(II) and mono-(μ-oxo) dicopper(II) species are suggested to take part in methane activation at 200°C with the trans-μ-1,2-peroxo dicopper(II) core playing a dominant role. A relationship between the [Cu₂O_y]²⁺ concentration and Cu(II) at the eight-membered ring is observed and related to the concentration of [CuOH]⁺ suggested as an intermediate in [Cu₂O_y]²⁺ formation. (Chemical Equation Presented).