Temperature-Responsive Deep Eutectic Solvents for Highly Selective Separation of 5-Hydroxymethylfurfural from Reaction Mixture

5-Hydroxymethylfurfural (HMF) is a key biobased platform compound for the production of high-value chemicals and fuels. However, separation and purification of HMF is currently the bottleneck for HMF production. In this work, a novel class of temperature-responsive deep eutectic solvent (DES)–water systems have been explored to selectively separate HMF from reaction mixtures. The fructose dehydration reaction was conducted in homogeneous DES–H2O systems, and then, the produced HMF was migrated to the DES-rich phase while the unreacted fructose was retained in the H2O-rich phase upon cooling. Thus, selective separation of HMF was realized. It was found that water–water and DES–DES interactions predominate over DES–water interactions during cooling, thus resulting in upper critical solution temperature phase separation. Under optimal conditions, the partition coefficient and separation factor of HMF could reach up to 78 and 609, respectively, which is about 5 and 13 times those reported in ionic liquid-based aqueous biphasic systems. Furthermore, HMF could be removed from the DES-rich phase, giving a high recovery efficiency of 69%, which is much higher than the best value reported in the literature. At the same time, DES was regenerated and reused in the next process, and no obvious decrease in partition coefficient of HMF was observed after six cycles.