Conversion of Glucose to 5-Hydroxymethylfurfural in Deep Eutectic Solvent of Choline Chloride–Chromium Chloride

5-Hydroxymethylfurfural (HMF) is a crucial biobased compound for synthesizing chemicals and biofuels, but the direct conversion of glucose to HMF suffers low yield and purification problems. In this paper, a cheap and efficient deep eutectic solvent (DES) of choline chloride–chromium chloride (ChCl–CrCl3·6H2O) was developed to convert glucose and other carbohydrates to HMF. High HMF yields of 70.2, 78.6, 72.6, and 71.7% were achieved from glucose, fructose, high fructose corn syrup, and sucrose, respectively. The DES ChCl–CrCl3·6H2O could be reused nine times without a significant loss of catalytic activity. In addition, a method was developed to regenerate humins-polluted DES. Combined with ultraviolet and visible spectrum data and density functional theory calculation, [Cr(H2O)2Cl3Ch]+ was identified as the active species in DES ChCl–CrCl3·6H2O. This paper proposed a new effective system to convert carbohydrates to HMF. The discovery of active species provides a base for future studies on dehydration mechanisms.