A review on the active sites for direct oxidation of methane to methanol by copper-zeolites: Coordination structure, formation and activity

Direct conversion of methane to methanol (MTM) remains a formidable challenge in heterogeneous catalysis because of the inert C-H bond in CH4. Cu ion-exchanged zeolites as the promising catalysts have received a wide concern in MTM. Knowledge of Cu coordination chemistry in confined zeolite environments and catalytic properties of various Cu active species (from multicopper-oxo clusters to monocopper sites) is key to understand the MTM reaction process under aerobic and anaerobic conditions. In this minireview, we summarized the current studies in respect of the types, coordination structures and formation of Cu active sites located in copper-exchanged zeolites as well as their reactivity and reaction mechanisms for direct MTM in both stepwise and continuous processes based on the reported experimental evidences and theory results. We aim to present what is known on both the physicochemical properties of various Cu active species with different coordination structures and their selective-oxidation functions in MTM reaction via the stepwise or continuous process, and what is the relationship between Cu coordination structures and methane conversion energy (including methane C-H activation and methanol desorption energies). Besides, it is revealed that water plays an important role as both methanol extractant and methane oxidant in MTM. Continuous conversion of MTM using water as the oxidant is a promising reaction process, but the oxidizing role of water remains a debate. Meanwhile, considering some gaps still existing in our knowledge, we propose to transfer the activity study from onefold active site to multiple active sites co-existed in zeolites and clear the reaction mechanism and the evolution paths of the coordination structures of various Cu active sites in the continuous process, which are apparently different with the stepwise process. These suggestions would be helpful for the design and optimization of reaction conditions and highly-effective Cu-zeolite materials and thus make the direct MTM conversion proceed further.