Spectroscopic Definition of a Highly Reactive Site in Cu-CHA for Selective Methane Oxidation: Tuning a Mono-μ-Oxo Dicopper(II) Active Site for Reactivity
Hannah M. Rhoda, Dieter Plessers, Alexander J. Heyer, Max L. Bols, Robert A. Schoonheydt, Bert F. Sels, Edward I. Solomon
Abstract
Using UV–vis and resonance Raman spectroscopy, we identify a [Cu2O]2+ active site in O2 and N2O activated Cu-CHA that reacts with methane to form methanol at low temperature. The Cu–O–Cu angle (120°) is smaller than that for the [Cu2O]2+ core on Cu-MFI (140°), and its coordination geometry to the zeolite lattice is different. Site-selective kinetics obtained by operando UV–vis show that the [Cu2O]2+ core on Cu-CHA is more reactive than the [Cu2O]2+ site in Cu-MFI. From DFT calculations, we find that the increased reactivity of Cu-CHA is a direct reflection of the strong [Cu2OH]2+ bond formed along the H atom abstraction reaction coordinate. A systematic evaluation of these [Cu2O]2+ cores reveals that the higher O–H bond strength in Cu-CHA is due to the relative orientation of the two planes of the coordinating bidentate O–Al–O T-sites that connect the [Cu2O]2+ core to the zeolite lattice. This work along with our earlier study ( J. Am. Chem. Soc, 2018, 140, 9236−9243) elucidates how zeolite lattice constraints can influence active site reactivity.