14 research outputs found
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 O<sub>2</sub>-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 O<sub>2</sub>-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