For practical applications, the concentration of feedstock in the reaction mixture should be as concentrated as possible. However, 5-HMF yield and selectivity

normally decrease with increasing initial carbohydrate concentration since the formed 5-HMF can combine with monosaccharides, and cross-polymerize to form humins [25]. In aqueous systems including aqueous mixture systems, losses due to humin formation can be as high as 35 % for 18 wt% fructose solution, although this value decreases to 20 % for 4.5 wt% fructose solutions [25]. It was reported that in aqueous mixtures, the formation of humins for fructose conversions in ionic liquids can be largely inhibited [20, 45, 65]. Qi et al. [20] examined the effect of initial fructose concentration on fructose conversion and 5-HMF selectiv­ity in [BMIM][Cl]. When the fructose/[BMIM][Cl] weight ratio was increased from

0. 01 to 0.05, the 5-HMF selectivity changed slightly from 86 to 85.2 %. As the fructose/[BMIM][Cl] weight ratio was increased from 0.05 to 0.1, the 5-HMF selectivity decreased by about 5 %. Additional increases in the fructose/[BMIM] [Cl] weight ratio did not lead to significant lowering of the 5-HMF selectivity, which is similar to the trends noted by Yong et al. for the conversion of fructose in [BMIM][Cl] with NHC/CrCl2 (NHC=N-heterocyclic carbene) complexes as cata­lysts [45]. Xie et al. [65] studied the dehydration of fructose in [BMIM] [Cl] catalyzed by lignosulfonic acid, and the 5-HMF yields of 92.7, 88.0, 84.1, and 73 % were obtained when 5, 10, 25, and 50 wt% of fructose were used, respectively. Increasing amounts of humins (0.007, 0.019, and 0.046 g) was observed for the increasing fructose concentrations, supporting the hypothesis that high concentrations of sugars and 5-HMF in ILs promote degradation and polymerization of 5-HMF.

Liu et al. [38] presented a promising choline chloride/CO2 (ChCl/CO2) system that could tolerate high fructose concentration with stable 5-HMF yield. The research group showed that as high as 100 wt% of fructose could be dehydrated into 5-HMF in ChCl/CO2 system without substantial loss of yield (66 %) (Fig. 9.7).

This process provides a practical and eco-efficient synthesis method for 5-HMF. The authors ascribed the surprising tolerance of the ChCl/CO2 system to a high content of fructose to a change of the physico-chemical properties of the ChCl/ fructose mixture. Even in the presence of water, ChCl is capable of forming various deep eutectic solvents (DESs) with many hydrogen-bond donors such as polyols, urea, or carboxylic acid. After fructose is dehydrated into 5-HMF, ChCl and 5-HMF tend to form a new DES, which stabilizes the 5-HMF allowing its reactivity to be drastically reduced thus inhibiting its decomposition.