ABSTRACT
Hybrid organic-inorganic anatase (hybrid-TiO2 ) is prepared by a facile hydrothermal synthesis method employing citric acid. The synthetic approach results in a high surface-area nanocrystalline anatase polymorph of TiO2 . The uncalcined hybrid-TiO2 is directly studied as a catalyst for the conversion of glucose into 5-hydroxymethylfurfural (HMF). In the presence of the hybrid-TiO2 , HMF yields up to 45 % at glucose conversions up to 75 % were achieved in water at 130 °C in a monophasic batch reactor. As identified by Ti K-edge XANES, hybrid-TiO2 contains a large fraction of fivefold coordinatively unsaturated TiIV sites, which act as the Lewis acid catalyst for the conversion of glucose into fructose. As citric acid is anchored in the structure of hybrid-TiO2 , carboxylate groups seem to catalyze the sequential conversion of fructose into HMF. The fate of citric acid bound to anatase and the TiIV Lewis acid sites throughout recycling experiments is also investigated. In a broader context, this contribution outlines the importance of hydrothermal synthesis for the creation of water-resistant Lewis acid sites for the conversion of sugars. Importantly, the use of the hybrid-TiO2 with no calcination step contributes to dramatically decreasing the energy consumption in the catalyst preparation.
