Metal nature effect on catalytic reactions in furfural-H2O2-H2O-group V or VI d-metal salt systems in acid media

Homogeneous catalytic reactions of furfural oxidation by hydrogen peroxide in an aqueous medium in the presence of catalytic amounts of a vanadium, niobium, molybdenum, or chromium salt have been studied to compare them with the acid-autocatalytic reaction. The formation of peroxo complexes of the corresponding metals has been established. The interaction of these peroxo complexes with the carbonyl group of furfural affords organic metal-containing peroxides. The subsequent transformations of these peroxides in the presence of the indicated catalysts lead to different dominant final products. This is explained by different mechanisms of the oxidation of the key intermediates, namely, 2-formyloxy- and 2-hydroxyfurans. In the presence of the vanadium and molybdenum salts, the catalytic role of the resuting carboxylic acids is insignificant, as distinct from what is observed in the acid-autocatalyzed reaction.