Selective Oxidation of Methane into Formic Acid Over Zif-8-Encapsulated Mononuclear Fe Species Under Mild Conditions

Abstract

The direct catalytic oxidation of methane into C1 oxygenates is a promising approach for methane valorization. However, the difficulties in activation of CH4 and inhibition of over-oxidation hinder the development of efficient catalysts for this transformation. Herein, highly dispersed Fe species confined by ZIF-8 (Fe-ZIF-8) are developed for the direct transformation of methane into value-added C1 oxygenates with H2O2 as oxidant at 323 K. The as-obtained Fe-ZIF-8 can afford an optimal turnover rate (TOR) of ~ 45 molC1·molFe-1·h-1. Note that the selectivity to formic acid is ~97%, reflecting one of highest values for Fe-based heterogeneous catalysts under similar conditions to date. Combined analysis of X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), ultraviolet-visible diffuse reflectance (UV-vis DR) spectroscopy and Mössbauer spectroscopy suggests that the catalytically active sites in Fe-ZIF-8 are mononuclear six-coordinated Fe3+ species, which could not only activate the primary C-H bonds, but also efficiently catalyze successive oxidation to formic acid. 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.

Keywords: Methane oxidation, Monomeric Fe species, Formic acid, Metal-organic frameworks, Encapsulation