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−CrCl 3 • 6H 2 O) 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−CrCl 3 •6H 2 O 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(H 2 O) 2 Cl 3 Ch] + was identified as the active species in DES ChCl−CrCl 3 •6H 2 O. 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.
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.
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