5-Hydroxymethylfurfural (5-HMF) Production from Hexoses: Limits of Heterogeneous Catalysis in Hydrothermal Conditions and Potential of Concentrated Aqueous Organic Acids as Reactive Solvent System

5-Hydroxymethylfurfural (5-HMF) is an important bio-sourced intermediate, 17 formed from carbohydrates such as glucose or fructose. The treatment at 150–250 °C of 18 glucose or fructose in pure water and batch conditions, with catalytic amounts of most of 19 the usual acid-basic solid catalysts, gave limited yields in 5-HMF, due mainly to the fast 20 formation of soluble oligomers. Niobic acid, which possesses both Lewis and Brønsted 21 acid sites, gave the highest 5-HMF yield, 28%, when high catalyst/glucose ratio is used. By 22 contrast, we disclose in this work that the reaction of fructose in concentrated aqueous 23 solutions of carboxylic acids, formic, acetic or lactic acids, used as reactive solvent media, 24 leads to the selective dehydration of fructose in 5-HMF with yields up to 64% after 2 hours 25 at 150 °C. This shows the potential of such solvent systems for the clean and easy 26 production of 5-HMF from carbohydrates. The influence of adding solid catalysts to the 27 carboxylic acid media was also reported, starting from glucose. 28