Water Molecules Facilitate Hydrogen Release in Anaerobic Oxidation of Methane to Methanol over Cu/Mordenite

Development of a suitable mild-condition process for direct conversion of methane to methanol faces multiple challenges, the principal ones being the higher reactivity of the primary oxidation products and the need for temperature swings in the typically employed chemical looping procedures. To circumvent these problems, the use of water as a mild oxidant has been recently suggested, leading to the concurrent formation of molecular hydrogen. By means of ab initio calculations, we address the experimentally observed features of the reaction to identify possible reaction pathways of such hydrogen release. We propose that, along with a strong stabilizing effect of water, short-lived [Cu-H] intermediate species play a crucial role in the mechanism of the reaction. Proton transfer from the Bronsted acid site of the zeolite framework via an adsorbed water molecule to the Cu-I species generates a [Cu-H] intermediate, which then facilitates the release of molecular hydrogen. This allows the reaction to proceed over a relatively low-energy transition state configuration. At the same time, excess of water leads to increased complexity of the concerted transition state, which results in hindering of the hydrogen transfer and increase of the corresponding energy barrier.