Acid-Promoted Selective Oxidation of Methane to Formic Acid over Dispersed Rhodium Catalysts under Mild Conditions

Direct oxidation of methane into value-added liquid chemicals is a promising route for wide applications. However, the efficient conversion of methane with high selectivity under mild conditions is still challenging. Herein, the catalysts of rhodium dispersed on ZSM-5 were synthesized for the methane conversion utilizing molecular O2 as the oxidant together with CO and H2O under a total pressure of 40 bar at <100 °C. Moreover, the conversion of methane and the selectivity of formic acid could be improved by increasing acidity over the Rh/ZSM-5 catalysts. Promoted by H+, the activities of methane oxidation can reach 3.56 molHCOOH/molRh/h and 23.96 molHCOOH/molRh/h at 50 and 90 °C over the 5.0Rh/H-ZSM-5 catalyst, and the selectivity of HCOOH of 98.48% to all organic oxygenates can be acquired at 90 °C. Dramatically, the activation of the C–H bond can even happen at room temperature. Electron paramagnetic resonance results showed that acidity can promote the generation of ·OH with CO, H2O, and O2 as the reactants, resulting in high catalytic activity under mild conditions.