Highly Efficient and Selective Preparation of 5-Hydroxymethylfurfural from Concentrated Carbohydrates Using Deep Eutectic Solvents

Converting carbohydrates to 5-hydroxymethylfurfural (HMF) is an important goal for biorefinery. Although various deep eutectic solvents (DESs) have been reported for HMF production, the field lacks systematic research on the effect of cations and anions and an effective catalytic system for converting concentrated carbohydrates. Here, 12 kinds of DESs containing four cations and six anions were systematically evaluated and tetraethylammonium bromide (TEAB) was confirmed as the most effective DES, which provided HMF yields of 90 and 76% from fructose and glucose, respectively. The stable bromide-substituted intermediate may explain the positive effect of bromide based on the results of density functional theory calculation. A high concentration substrate has an adverse impact on the selectivity of HMF due to the HMF concentration caused by glucose, CrCl3·6H2O, and concentrated HMF. To inhibit the condensation, a biphasic system of TEAB/THF is developed to convert a highly concentrated glucose of 62.5 wt % (83.3 wt % with respect to TEAB), providing an excellent HMF yield of 72%. This study has revealed the impact of cations and anions of DESs on HMF production, which will serve as a base for designing task-specific DESs. Moreover, an efficient catalytic system to produce HMF from concentrated glucose was also provided.