Roles of Water Molecules in Modulating the Reactivity of Dioxygen-Bound Cu-ZSM-5 toward Methane: A Theoretical Prediction

We propose theoretically that the reactivity of O₂-bound Cu-ZSM-5 toward methane is enhanced by the presence of one water molecule near a dinuclear copper site inside a 10-membered ring of the zeolite cavity. The current study employed density functional theory (DFT) calculations with the B3LYP functional to elucidate reaction intermediates during dioxygen activation by Cu-ZSM-5 in the presence of one water molecule attached to a dicopper site. The initial event is the formation of a hydroperoxo species bridged by the dicopper site via an H atom transfer from an attached water to the bound dioxygen. After the formation of the intermediate, the hydroperoxo O-O bond is completely cleaved to form radical oxygen containing intermediates, such as a Cu-O-Cu species bound by two OH groups (HO-Cu-O-Cu-OH), as well as a copper oxyl group containing intermediate (HO-Cu-OH-CuO). The radical oxygen containing intermediates can cleave a methane C-H bond in a homolytic fashion. Examining the barrier for the C-H bond activation obtained from DFT calculations, we found that the two types of intermediates have the power to more effectively cleave methane C-H bonds than the Cu-O-Cu intermediate that has been proposed to be formed in the absence of a water molecule. The current DFT findings propose that O₂-bound Cu-ZSM-5 in the presence of one water molecule is a potential candidate for catalysts desired for methane to methanol conversion under mild conditions. Recently, techniques for controlling the number of water molecules near the active site of a ZSM-5 zeolite have been developed, and therefore the DFT findings should stimulate experimental efforts for constructing catalysts for direct methane hydroxylation. (Chemical Equation Presented).