Compared with organic and aqueous solvents, the application of ILs for the dehydration of carbohydrates can significantly reduce the reaction temperature [20, 32]. Traditional synthetic methods for 5-HMF production from carbohydrates are normally carried out at temperatures ranging from 150 to 300 °C [4, 8, 15, 18], but the reaction temperatures required for the process in ionic liquids could be reduced to 80-150 °C [19, 41,45, 47, 85], or even room temperature in some cases [34]. Normally the conversion of fructose needs a relative low temperature ranging from 80-120 °C since it is readily converted into 5-HMF through elimination of three molecules of water. Glucose is more difficult than fructose to be transformed into 5-HMF since it tends to form a stable six-membered pyranoside structure that has a low enolization rate, and its dehydration is generally carried out at 100-140 °C [19, 41, 45, 47]. The conversion of cellulose is more difficult for efficient conversion to 5-HMF than monosaccharides and requires high tempera­tures of about 150 °C in the presence of catalysts [19, 47, 53]. In general, lower temperatures lead to low 5-HMF yield (ca. 10-20 %), whereas higher temperatures promote formation of side-products and affects the 5-HMF yield so that an opti­mum temperature exists.

The reaction temperature is an important parameter for carbohydrate conver­sions since lower temperatures allow one to reduce the energy requirements. Chan et al. [84] was able to lower the dehydration reaction temperature to below 50 °C by using a system containing [BMIM][Cl] and metal salts. Chloride salts of zirconium (IV), titanium (IV), ruthenium (III), and tungsten (IV) or tungsten (VI), the most efficient chloride salt was that of tungsten (VI) that gave a 5-HMF yield of 63 % at 50 °C.

Reactions that can be promoted at ambient conditions are considered as one of the key goals among the 12 principles of green chemistry [92, 93]. Remarkably, it was found that efficient dehydration of fructose to 5-HMF could be carried out at room temperature in ionic liquids provided that the ionic liquid was brought about its melting point and the substrate was pre-dissolved in the ionic liquid before cooling to room temperature. Qi et al. [34] developed a green catalytic system for the production of 5-HMF from fructose catalyzed by a strong cation exchange resin, by the addition of different cosolvents such as DMSO, acetone, methanol, ethanol, ethyl acetate, and supercritical carbon dioxide to the ionic liquid 1-butyl-3- methylimidazolium chloride ([BMIM][Cl]). In a typical reaction, fructose was first dissolved in [BMIM][Cl] in a water bath at 80 °C for 20 min. After the mixture was cooled down to room temperature, the solution appeared gel-like with a very high viscosity. Subsequently, the catalyst (Amberlyst-15 sulfonic ion-exchange resin) and some amount of co-solvent were added for viscosity reduction and the reaction proceeded smoothly at 25 °C. Through addition of a co-solvent, the viscosities could be greatly reduced from an estimated value of 6,800 mPa s to values of around 2,000 mPa s, with the best results being obtained for acetone (1,850 mPa s) and ethyl acetate (1,930 mPa s). Interestingly, a gaseous co-solvent, such as CO2 or supercritical CO2 (>31 °C) was tried and found to provide comparable results to the organic solvents. Thus, use of CO2 can possibly provide viscosity reduction and make it simple to regenerate the solvent system. Reductions in viscosity allowed the transformations to be carried out at close to room temper­ature. For this case [34], 5-HMF was obtained at yields of 78-82 %. The time for reaction was longer than in the previous work (6 h vs. 10 min) [20], but this method has the advantage of being performed at room temperature.