Reaction Conditions of Methane-to-Methanol Conversion Affect the Structure of Active Copper Sites

Determining the structure of the active Cu sites, which are associated with the methane conversion intermediate during stepwise, low-temperature, methane-to-methanol conversion, represents an important step for the upgrade of this reaction route to a viable process. Quick X-ray absorption spectroscopy allowed us to follow the electronic and structural changes to the active Cu sites during reaction with methane and during desorption of the activated intermediate. A large fraction (41%) of the oxygen-activated Cu-II reacted with methane and underwent reduction to Cu-I. When the intermediate was released as the product MeOH into the gas phase after reaction with water, the fraction of Cu-I was simultaneously converted back to Cu-II. Therefore, the activation of methane involves a change in the copper oxidation state. Density functional theory calculations identified [Cu-I-OCH3-Cu-II] and [Cu-I-OH-Cu-II] as energetically plausible structures of the adsorbed intermediates. The structure of the active Cu sites is also a function of conditions. In a dry pretreatment environment, the Cu sites took the form of dehydrated Cu-II oxide species, well characterized in the literature as mono(mu-oxo) and bis(mu-oxo)dicopper species. Under wet conditions, the dicopper species was destabilized to a hydrated Cu-II species, but a small amount of water-stable Cu-II oxide remained that was also active for conversion of methane to methanol.