Selective Conversion of Cellobiose and Cellulose into Gluconic Acid in Water in the Presence of Oxygen, Catalyzed by Polyoxometalate-Supported Gold Nanoparticles

Gold nanoparticles loaded onto Keggin-type insoluble polyoxometalates (CsxH3-xPW12O40) showed superior catalytic performances for the direct conversion of cellobiose into gluconic acid in water in the presence of O-2. The selectivity of Au/CsxH3-xPW12O40 for gluconic acid was significantly higher than those of Au catalysts loaded onto typical metal oxides (e.g., SiO2, Al2O3, and TiO2), carbon nanotubes, and zeolites (H-ZSM-5 and HY). The acidity of polyoxometalates and the mean-size of the Au nanoparticles were the key factors in the catalytic conversion of cellobiose into gluconic acid. The stronger acidity of polyoxometalates not only favored the conversion of cellobiose but also resulted in higher selectivity of gluconic acid by facilitating desorption and inhibiting its further degradation. On the other hand, the smaller Au nanoparticles accelerated the oxidation of glucose (an intermediate) into gluconic acid, thereby leading to increases both in the conversion of cellobiose and in the selectivity of gluconic acid. The Au/CsxH3-xPW12O40 system also catalyzed the conversion of cellulose into gluconic acid with good efficiency, but it could not be used repeatedly owing to the leaching of a H+-rich hydrophilic moiety over long-term hydrothermal reactions. We have demonstrated that the combination of H3PW12O40 and Au/Cs3.0PW12O40 afforded excellent yields of gluconic acid (about 85%, 418 K, 11 h), and the deactivation of the recovered H3PW12O40Au/Cs3.0PW12O40 catalyst was not serious during repeated use.