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

Graphical Methane oxidation : Highly dispersed mononuclear Fe species encapsulated by ZIF-8 (Fe-ZIF-8) afford an optimal turnover rate of ∼45 mol C1 mol Fe −1 h −1 and a formic acid selectivity of ∼97 % in the direct methane oxidation at 323 K. The direct catalytic oxidation of methane into C1 oxygenates is a promising approach for methane valorization. However, the difficulties in CH 4 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 H 2 O 2 as oxidant at 323 K. The as-obtained Fe-ZIF-8 can afford an optimal turnover rate (TOR) of ∼45 mol C1 mol Fe −1 h −1 . Note that the selectivity to HCOOH is ∼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 57 Fe Mössbauer spectroscopy suggests that the catalytically active sites in Fe-ZIF-8 are mononuclear six-coordinated Fe 3+ 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.