Conversion of methane to methanol is an industrially important process, as it provides a sustainable route from an abundant and clean component of natural gas to one of the main precursors for chemicals synthesis. An efficient stepwise process catalyzed by copper-exchanged zeolites has been suggested; however, a detailed understanding of the mechanism is still missing. One of the ongoing debates is the exact configuration of the available catalytic sites. Originally, dicopper sites comprising two copper atoms connected with oxygen bridges, designed along the lines of the similar enzyme centers, have been proposed. Recently, the activity of monocopper active sites for partial methane oxidation has been theoretically investigated. At the same time, ab initio thermodynamics calculations suggested the tricopper oxide to be more stable than either mono- or dicopper sites in zeolites under treatment conditions. The formation of larger clusters, however, was not considered. Inspired by our recent experimental work suggesting the possibility of formation of small copper oxide clusters as active centers of zeolites, and not necessarily di- or tricopper sites, we use global geometry optimization to systematically assess the possibility of the formation of larger copper oxide clusters in the pores of zeolite, and their reactivity towards methane. We identify a general trend of relative stability increasing with size for copper oxide clusters of CunOn2+ and CunOn–12+ stoichiometries embedded in the 8-ring channel of mordenite. Moreover, we show that aluminium content and localization in the zeolite pore influences the cluster stability and its geometrical motif, which offers a perspective of tuning the properties of copper-exchanged zeolites by creating copper oxide clusters of given structure and size. Furthermore, the cluster stability influences the reactivity towards methane, with the adsorbed CH4 stronger stabilized on larger clusters. Possible mechanisms of methane oxidation over such copper oxide clusters are considered and critically assessed.

Copper oxide clusters as active catalytic sites of CuMOR zeolite for methane to methanol conversion: Size matters?

253rd National Meeting (2017)

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