Spectroscopic Identification of the α-Fe / α-O Active Site in Fe-CHA Zeolite for the Low-Temperature Activation of the Methane C-H bond

The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from N2O to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV–vis–NIR and Mossbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe2O3 being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composi...