Selective Oxidation of Methane into Formic Acid over ZIF-8-Encapsulated Mononuclear Fe Species under Mild Conditions

The direct catalytic oxidation of methane into C1 oxygenates is a promising approach for methane valorization. However, the difficulties in CH4 activation and inhibition of over-oxidation hinder the development of efficient catalysts for this transformation. Herein, highly dispersed Fe species confined by ZIF-8 are developed for the direct transformation of methane into value-added C1 oxygenates with H2O2 as oxidant at 323K. The as-obtained Fe-ZIF-8 can afford an optimal turnover rate (TOR) of similar to 45mol(C1)mol(Fe)(-1)h(-1). Note that the selectivity to HCOOH is similar to 97%, outstanding among the reported Fe-based heterogeneous catalysts. Combined analysis of X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy and Fe-57 Mossbauer spectroscopy suggests that the catalytically active sites in Fe-ZIF-8 are mononuclear six-coordinated Fe3+ species. The confinement of mononuclear Fe species within a porous metal-organic framework material can provide an exquisite, enzyme-like performance for highly selective oxidation of methane towards formic acid under mild conditions.