Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites

Two [Cu₂O]²⁺ cores have been identified as the active sites of low temperature methane hydroxylation in the zeolite Cu-MOR. These cores have similar geometric and electronic structures, yet different reactivity with CH₄: one reacts with a much lower activation enthalpy. In the present study, we couple experimental reactivity and spectroscopy studies to DFT calculations to arrive at structural models of the Cu-MOR active sites. We find that the more reactive core is located in a constricted region of the zeolite lattice. This leads to close van der Waals contact between the substrate and the zeolite lattice in the vicinity of the active site. The resulting enthalpy of substrate adsorption drives the subsequent H atom abstraction step - a manifestation of the "nest" effect seen in hydrocarbon cracking on acid zeolites. This defines a mechanism to tune the reactivity of metal active sites in microporous materials.