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

2017-07-01
İpek Torun, Bahar
Wulfers, Matthew J.
Kim, Hacksung
Goltl, Florian
Hermans, Ive
Smith, Joseph P.
Booksh, Karl S.
Brown, Craig M.
Lobo, Raul F.
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 CuxOy species formed on Cu-SSZ-13 and Cu-SSZ-39 following O-2 or He activation at w450 degrees C are identified as trans-mu-1,2-peroxo dicopper(II) ([Cu2O2](2+)) and mono-(mu-oxo) dicopper(II) ([Cu2O](2+)) using synchrotron X-ray diffraction, in situ UV vis, and Raman spectroscopy and theory. [Cu2O](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 0-0 vibration at 837 cm(-1). The vibrations at 360, 510, 580, and 837 cm(-1) are assigned to the trans-mu-1,2-peroxo dicopper(II) species, whereas the Cu-O vibration at 617 cm(-1) (Delta O-18 = 24 cm(-1)) is assigned to a stretching vibration of a thermodynamically favored mono-(mu-oxo) dicopper(II) with a Cu-O-Cu angle of 95 degrees. 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-mu-1,2-peroxo dicopper(II) and mono-(mu-oxo) dicopper(II) species are suggested to take part in methane activation at 200 degrees C with the trans-mu-1,2-peroxo dicopper(II) core playing a dominant role. A relationship between the [Cu2Oy](2+) concentration and Cu(II) at the eight-membered ring is observed and related to the concentration of [CuOH]+ suggested as an intermediate in [Cu2Oy](2+) formation.