Oxidations by the reagent 'O-2-H2O2 vanadate anion pyrazine-2-carboxylic acid'. Part 10 - Oxygenation of methane in acetonitrile and water

The oxidation of methane by a combination of air and hydrogen peroxide is effectively catalyzed in solution by a system composed of vanadate and pyrazine-2-carboxylic acid (PCA). In acetonitrile solution, containing the vanadate anion as tetrabutylammonium salt, the reaction gives, over a temperature range of 25 to 50 degrees C, methanol, carbon monoxide, formaldehyde, formic acid and carbon dioxide, the latter three compounds, however, being partially due to the oxidation of the acetonitrile used as the solvent, especially at higher temperatures. In aqueous solution, containing the vanadate anion in the form of the sodium salt, the reaction affords, over a temperature range of 40 to 70 degrees C, selectively methyl hydroperoxide within 4 h. The yield of CH3OOH attains 24%, based on H2O2, after 24 h at 50 degrees C, the catalytic turnover number being 480. The process seems to involve hydroxyl radicals, generated by the catalyst from H2O2 even at low temperatures. At 120 degrees C, methane is oxidized by O-2 and H2O2 to give formaldehyde and formic acid, even in the absence of the catalyst, presumably due to the formation of HO radicals from H2O2 in the presence of very low concentrations of metal ions from the autoclave under high temperature conditions. (C) 1998 Elsevier Science B.V.