Electrical energy from carbohydrate oxidation during viologen-catalyzed O2-oxidation: Mechanistic insights

Catalytic oxidation of carbohydrates by oxygen using various viologens under mild conditions was recently shown to occur at high efficiency with formate and carbonate formed as products. However, high efficiency was only achieved at viologen/carbohydrate ratios >10. The use of such high catalyst ratios is undesirable and a mechanistic study was initiated to better understand how catalytic efficiency is influenced by catalyst ratio. The results suggest that the dipositive viologen reacts with the enediol form of the carbohydrate, initiating carbohydrate oxidation with subsequent reduction of the viologen to the radical cation form. The latter is then oxidized by air or by current collecting electrodes when the reaction is conducted in a fuel cell. If the viologen/carbohydrate ratio is low, electron transfer from the carbohydrate to the oxidized form of the viologen becomes limiting and the carbohydrate undergoing oxidation rearranges into unreactive intermediates such as carboxylic acids, and alcohols. However, at high ratios, excess viologen more efficiently oxidizes the carbohydrate and minimizes formation of unreactive intermediates. Viologen polymers were more efficient than an equivalent concentration of monomers, suggesting that the higher localized concentration in polymeric viologen acts to efficiently oxidize carbohydrates and simulates high viologen/carbohydrate ratios. The use of viologen films on electrodes or conducting viologen polymers should facilitate use of carbohydrates for electricity production in fuel cells.