In the production of 5-HMF from carbohydrates, increasing reaction time leads to an increase in the 5-HMF yield for cases in which 5-HMF decomposition does not occur. However, the yield of 5-HMF generally exhibits a maximum, indicating that decomposition of 5-HMF occurs [18, 20, 34]. The reaction mixture changing in color from yellow to deep brown is evidence for 5-HMF decomposition [5]. Gen­erally, there are three pathways for the decomposition of 5-HMF in acid catalyzed dehydration of carbohydrates [3, 21], as depicted in Fig. 9.6. The first pathway is the rehydration of 5-HMF into levulinic acid and formic acid; the second one is the self-

polymerization between 5-HMF molecules; and the third one is the cross­polymerization between 5-HMF and monosaccharides [21, 25, 40]. In non-aqueous ionic liquid systems, 5-HMF rehydration can be suppressed since the water present is limited to that of the dehydration of the carbohydrate. Control experiments without the carbohydrates exclude the possibility of self­polymerization of 5-HMF [20, 34]. Thus, it is thought that the decomposition of 5-HMF and the formation of humins are mainly due to the polymerization between 5-HMF and carbohydrates, which consume the initial carbohydrates and the formed 5-HMF, and hence reduce the 5-HMF selectivity [20].

The catalytic system and heating method have a large effect on the optimal reaction time for the production of 5-HMF. For example, when the dehydration of fructose was performed in [EMIM][Cl]-CrCl2 system, an optimal 5-HMF yield of 83 % was obtained at 80 °C in 3 h reaction time [41], but a comparable 5-HMF yield was achieved in [BMIM][Cl]-Amberlyst 15 resin system at 80 °C in only 10 min reaction time [20]. Microwave heating was found to be able to accelerate the transformation of monosaccharides and di-/polysaccharides into 5-HMF and to shorten the reaction time. Qi et al. [19] investigated the catalytic conversion of glucose in [BMIM][Cl]-CrCl3 system, and a 5-HMF yield of 71 % was achieved in 30 s for 96 % glucose conversion with microwave heating at 140 °C. In comparison, the reaction was performed with oil-bath heating and microwave heating at iden­tical conditions. Microwave heating generally gives higher yields than with oil-bath heating, which was also reported by Li et al. who obtained a 91 % 5-HMF yield with microwave irradiation at 400 W in a reaction time of 1 min [47].