Spectroscopic Definition Of A Highly Reactive Site In Cu-Cha For Selective Methane Oxidation: Tuning A Mono-Mu-Oxo Dicopper(Ii) Active Site For Reactivity

Using UV-vis and resonance Raman spectroscopy, we identify a [Cu2O](2+) active site in O-2 and N2O activated Cu-CHA that reacts with methane to form methanol at low temperature. The Cu-O-Cu angle (120 degrees) is smaller than that for the [Cu2O](2+) core on Cu-MFI (140 degrees), 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.