Hybrid Organic–Inorganic Anatase as a Bifunctional Catalyst for Enhanced Production of 5‐Hydroxymethylfurfural from Glucose in Water

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 Ti^IV 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 Ti^IV 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.