Formation of Active Cu-oxo Clusters for Methane Oxidation in Cu-Exchanged Mordenite

Cu-exchanged zeolites are known to be active in the selective oxidation of methane to methanol at moderate temperatures. Among them, Cu-exchanged mordenite (MOR) is the system that has so far shown the highest methanol yield per Cu atom. This high efficiency is attributed to the ability of MOR to selectively stabilize an active tricopper cluster with a [Cu₃(μ-O)₃]²⁺ structure when activated in the presence of O₂ at high temperatures. In this study, we investigate the elementary steps in the formation of [Cu₃(μ-O)₃]²⁺ by in situ X-ray absorption spectroscopy and ultraviolet-visible spectroscopy. We demonstrate that the Cu cations undergo a series of thermally driven steps during activation that precede the formation of the active oxidizing species. We hypothesize that the thermal formation of highly mobile Cu⁺ species by autoreduction of Cu²⁺ in an inert gas is essential to enable the reorganization of Cu ions in MOR, which is necessary for the formation of a reduced precursor of [Cu₃(μ-O)₃]²⁺. Such a precursor can be oxidized in the presence of strong oxidants - such as O₂ and N₂O - to form active [Cu₃(μ-O)₃]²⁺ at temperatures as low as 50 °C.