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

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• 作者：Takashi Yumura ; Yuuki Hirose ; Takashi Wakasugi ; Yasushige Kuroda ; Hisayoshi Kobayashi
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：April 1, 2016
• 年：2016
• 卷：6
• 期：4
• 页码：2487-2495
• 全文大小：631K
• ISSN：2155-5435

文摘

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.