5-Hydroxymethylfurfural (HMF) synthesis in a deep eutectic solvent-based biphasic system: closing the loop of solvent reuse, product isolation and green metrics

Abstract

The scale up and recycling of all process streams in the H₄SiW₁₂O₄₀ catalyzed dehydration of D-fructose (Fru) to 5-hydroxymethylfurfural (HMF) were investigated. For this, a biphasic system based on a self-consuming deep eutectic solvent (DES) consisting of choline chloride (ChCl) and Fru in a molar ratio of 5 : 1 as the reaction phase with in situ extraction of HMF employing acetonitrile was employed. In addition to ChCl : Fru being a cost-effective DES of renewable origin, it provides a way to suppress side reactions to levulinic and formic acid, particularly. The scale-up of the reaction system to a total volume of 180 mL resulted in a reaction time of 12.5 minutes to achieve quantitative conversion reaching high yields of 76% and selectivities as high as 83% whilst operating temperature was only at 80 °C, while proceeding twice as fast compared to the smaller scale reaction of previous work. The system shows easy separation of the upper extraction phase from the reaction phase due to the solidification of ChCl and the catalyst H₄SiW₁₂O₄₀ upon cooling to room temperature showing partition coefficients of about 4 to 5. HMF could be isolated from the extraction phase, recovering HMF crystals of >99% purity. The system has the potential for numerous recycling runs up to a water content of 7.5 wt%, beyond which the DES phase undergoes a loss of activity due to the system transitioning to an aqueous solution. The extraction phase shows full recyclability and can be reused after simple distillation to separate HMF, showing promise for further implementation. Finally, considering the mass balance of the system, the basic green metrics of the system are calculated to show its potential compared to other similar concepts in the literature.