Fast Cyclohexane Oxidation Under Mild Reaction Conditions Through a Controlled Creation of Redox-Active Fe(II/III) Sites in a Metal-Organic Framework

MIL-100(Fe), a metal-organic framework containing coordinatively unsaturated iron sites, was prepared by a solvothermal method and utilized in cyclohexane oxidation. Interestingly, the composition and fraction of redox-active Fe(II/III) sites in MIL-100(Fe) could be tuned by using different pretreatment temperatures. The obtained materials were characterized by means of field-emission scanning electron microscopy (FE-SEM), powder X-ray diffraction (PXRD), N-2 adsorption-desorption isotherms, and X-ray photoelectron spectroscopy (XPS). Due to the presence of redox-active coordinatively unsaturated sites (CUS) at the iron atoms, MIL-100(Fe) exhibited good performance for cyclohexane oxidation under mild reaction conditions. Oxidation rates were significantly enhanced when reduced Fe-II sites were generated by high-temperature pretreatments. In either high-temperature or low-temperature pretreatment, the isolated Fe(II/III) sites in MIL-100 were far more reactive in cyclohexane oxidation than were bulk iron oxide materials. Finally, possible reaction pathways were proposed based on radical, Haber-Weiss routes.