Inulin Dehydration to 5-HMF in Deep Eutectic Solvents Catalyzed by Acidic Ionic Liquids Under Mild Conditions

Valorization of carbohydrate-rich biomass by conversion into industrially relevant products is at the forefront of research in sustainable chemistry. In this work, we studied the inulin conversion into 5-hydroxymethylfurfural, in deep eutectic solvents, in the presence of acidic task-specific ionic liquids as catalysts. We employed aliphatic and aromatic ionic liquids as catalysts, and choline chloride-based deep eutectic solvents bearing glycols or carboxylic acids, as solvents. The reactions were performed in a biphasic system, with acetone as a benign extracting solvent, enabling continuous extraction of 5-HMF. We aimed to find the best experimental conditions for this transformation, in terms of catalyst loading, solvent, reaction time and temperature to achieve an economical and energy efficient process. We also analyzed the results in terms of solvent viscosity and structural organization as well as catalysts acidity, to elucidate which structural features mostly favour the reaction. Under optimized conditions, we obtained a yield in 5-HMF of 71 %, at 80 °C in 3 h. Our system can be scaled up and recycled three times with no loss in yield. Finally, comparison with the literature shows that our system achieves a higher yield under milder conditions than most protocols so far reported for the same transformation.