Direct conversion of methane to oxygenates catalyzed by iron( III ) chloride in water at near ambient temperature

The conversion of natural gas (mainly methane) to more valuable compounds is an attractive topic in energy field. While heterogeneously catalytic conversion of methane with hydrogen peroxide (H2O2) has been extensively studied, in-depth study on similar homogeneous systems is lacking. In this work, FeCl3 was demonstrated as an excellent homogeneous catalyst for direct oxidation of methane in the presence of H2O2 at near ambient temperature (50°C) in water, with significant yields of methanol (1972.2 μmol/gcat) and formic acid (33 273.5 μmol/gcat) and a high turnover frequency (TOF) of 5.7 hours−1. The superiorities of both the cation (Fe3+) and the anion (Cl−) were revealed by studying the cation and anion effects in comparison with CoCl2 and NiCl2 (TOF of 0.035 and 0 hour−1, respectively) as well as Fe(NO3)3 and FeBr3 (TOF of 3.3 and 0.73 hour−1, respectively). Furthermore, it was found that the hydrated ferryl ion [(H2O)5FeIVO]2+ could be the major active species for the activation of CH4 and the formation of CH3OH, whereas the hydroxyl radicals (·OH) were responsible for the formation of HCOOH, CO and CO2. Moreover, theoretical study based on density function theory (DFT) suggested that the cation effect could be related to the Gibbs free energies for CH4 activation on their corresponding active species. Meanwhile, the anions affect the performance mainly by the structural features.