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

Bahar Ipek†⊥◆ , Matthew J. Wulfers†⊥, Hacksung Kim‡#, Florian Göltl∇ , Ive Hermans∇○, Joseph P. Smith§ , Karl S. Booksh§, Craig M. Brown∥⊥, and Raul F. Lobo*†⊥

†Center for Catalytic Science and Technology, ⊥Department of Chemical and Biomolecular Engineering, and §Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States

‡ Department of Chemistry, Center for Catalysis and Surface Science, Northwestern University, Evanston, Illinois 60208, United States

# Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

∇Department of Chemistry and ○Department of Chemical and Biological Engineering, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States

∥ Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States

ACS Catal., 2017, 7, pp 4291–4303

DOI: 10.1021/acscatal.6b03005

Abstract

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 O2or 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 [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.

Keywords: active site; Cu-SSZ-13; Cu-SSZ-39; Cu-zeolite; dicopper core; methane activation; Raman spectroscopy