Hybrid-organic-inorganic anatase as a bifunctional catalyst for enhanced production of HMF from glucose in water

Herein, we report a synthetic route for the preparation of hybrid-organic-inorganic anatase (hybrid-TiO2) via 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 here as the catalyst for the conversion of glucose into 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 oC in a monophasic batch reactor. As identified by Ti K-edge XANES, hybrid-TiO2 contains a large fraction of five-fold 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 bounded to anatase and the Ti(IV) Lewis acid sites throughout recycling experiments is also investigated. In a broader context, the contribution outlines the importance of hydrothermal synthesis for the creation of water-resistant Lewis acid sites for the conversion of sugars. Most importantly, the utilization of the hybrid-TiO2 with no calcination step contributes to dramatically decreasing the energy consumption in the catalyst preparation