In-situ generation of highly-reactive FeIV=O and its contribution during CH4 conversion to CH3OH

Abstract

Photocatalytic CH4 conversion to CH3OH under mild conditions is undoubtedly an efficient strategy to realize CH4 upgrading. In this work, in-situ formed highly reactive FeIV=O complex with a bound Cl axial ligand was detected at ambient temperature and its contribution to CH4 photoxidation to CH3OH was deeply investigated. Under optimized experimental condition, a high CH3OH yield of 3026.1 mmol molFe−1 (6.76 mmol gcat−1 h−1) with selectivity of 73.6 % was obtained during 8 h. Isotope labeling mass spectroscopy experiments manifested that O2 was the only source of oxygen in products. The formation of Cl-(H2O)4FeIVO complex was confirmed using low-temperature electron spin resonance spectroscopy and cryogenic Raman spectroscopy (ν(FeO) = 828 cm−1). Through density functional theory (DFT) calculation, we showed that the Cl axial ligand acts as an “electron reservoir” during CH4 conversion, i.e., electron acceptor during the process of CH4 oxidation to CH3OH, while electron donor in CH3OH desorption. This not only lowered C-H cleavage energy, but also facilitated CH3OH desoption and inhibited overoxidation. As far as we know, Cl-(H2O)4FeIVO is the first detected FeIVO complex generated directly from O2 and Fe3+ at ambient temperature.