Selective oxidation of methane by molecular oxygen catalyzed by a bridged binuclear ruthenium complex at moderate pressures and ambient temperature

The oxidation of methane by molecular oxygen was found to be efficiently catalyzed by a binuclear bridged ruthenium complex, bis-(μ-acetato)(μ-oxo) bis-salen ruthenium (III), [L2 Ru2 (μ-O)(μ-CH3COO)2] 1 (L = Hsalen), in which methanol was observed to be formed selectively with small formation of formaldehyde at moderate total pressure 10–15 atm and at 30 °C in a 1:1 (v/v) mixture of acetone–water solvent. In typical experiments conducted in a pressure reactor of 100 ml capacity at 30 °C and 15 atm pressure, 27 × 10−3 M methanol and 2.4 × 10−3 M formaldehyde were found to be formed with 5 × 10−4 M catalyst. The employed partial pressures of methane and molecular oxygen in these catalytic experiments were at 10 and 5 atm, respectively. Complex 1 was synthesized, characterized and evaluated for catalytic oxidation of methane in detail as a function of total pressure, CH4:O2 pressure ratio, concentration of the catalyst and the pressures of methane and molecular oxygen in which oxidation proved to be favorably effected by these parameters. The rates of the oxidation of methane to methanol were linearly increased on increasing the concentrations of the catalyst, methane and molecular oxygen under employed reaction conditions showing first order dependence kinetics in each concentration parameter. Based on the kinetics and experimental results, a non-radical, ionic mechanism is suggested for the oxidation of methane to methanol.

Graphical abstractA binuclear bridged ruthenium complex was found to be an efficient catalyst for methane oxidation by molecular oxygen yielding selectively methanol with small amount of formaldehyde at moderate pressure and ambient temperature. Methanol formation was found to be favorable on increasing the total pressure, CH4:O2 ratio, catalyst concentration and the partial pressures of CH4 and O2.Figure optionsDownload full-size imageDownload as PowerPoint slide