Inulin, which is also called fructan, is a carbohydrate consisting of fructose units that vary in the degree of polymerization (DP) from 2 to 60, or higher. The fructosyl units in inulin are linked by P(2 ! 1) linkages with the polymer chains terminating with a glucose unit [62]. Inulin is indigestible. The production of 5-HMF from inulin in ionic liquids has not been studied as extensively as cellulose, since inulin is not so abundant in nature. However, the carbohydrate provides a different feedstock that might give insight into mechanism of its transformation into 5-HMF. The hydrolysis of inulin to fructose followed by the dehydration of fructose to produce 5-HMF is a possible two-step reaction pathway. Because both reaction steps are catalyzed by acid catalysts, it is interesting to consider the production of 5-HMF from inulin as a one pot reaction, since this would avoid the separation of the fructose in the intermediate step.

Hu et al. [63] developed a process for the direct conversion of inulin to 5-HMF in choline chloride (ChoCl)/oxalic acid and ChoCl/citric acid deep eutectic solvents (DES), for which a 5-HMF yield of 56 % was obtained at relatively low temper­atures (80 °C). When a biphasic system with IL and ethyl acetate was used for the in-situ extraction of 5-HMF, the 5-HMF yield was enhanced to 64 %, since the product 5-HMF is soluble in ethyl acetate, while the reactant inulin and fructose are insoluble in the DES, and ethyl acetate is only slightly soluble in the DES, which is favorable for the reaction in the biphasic system [63]. Although SnCl^EMIM] [BF4] system is efficient for glucose dehydration, there is no advantage in using the system for inulin conversion to 5-HMF since only moderate 5-HMF yields of 40 % are obtained [50].

Qi et al. [64] used the characteristics of two kinds of ionic liquids to develop an efficient process for the direct conversion of inulin to 5-HMF in one pot with two reaction steps under mild conditions. In the first step, the ionic liquid 1-butyl-3- methyl imidazolium hydrogen sulfate ([BMIM][HSO4]) was employed as both solvent and catalyst for the rapid hydrolysis of inulin into fructose with 84 % yield in 5 min reaction time at 80 °C. In the second step, 1-butyl-3-methyl imidazolium chloride ([BMIM][Cl]) and a strong acidic cation exchange resin were added to the mixture to selectively convert the generated fructose into 5-HMF, achieving a 5-HMF yield of 82 % in 65 min, which is the highest 5-HMF yield reported to date for an inulin feedstock. These authors proposed a conceptual process that the ionic liquid, resin, catalyst, ethyl acetate, and the supplied chemicals are internally recycled and represent an efficient method for producing 5-HMF from inulin (Fig. 9.4).

Xie et al. [65] studied the catalytic conversion of inulin into 5-HMF in ionic liquids by lignosulfonic acid catalyst, and found that that consecutive hydrolysis of inulin was very fast, with a maximum yield of 47.0 % achieved in only 5 min, and the yield could be maintained even with prolonging the reaction time to 90 min. The effect of water addition was examined and it was demonstrated that the addition of

H2O does not affect the reaction, with comparative yields obtained since the dehydration of one molecule of fructose produces three molecules of H2O, which is sufficient for the previous hydrolysis of inulin into fructose, therefore, additional water does not seem to be necessary for this reaction system. Bennoit et al. [66] tried to partially substitute ionic liquids with glycerol or glycerol carbonate as cheap, safe and renewable sourced co-solvents for the acid-dehydration of inulin to 5-HMF. They found that a system that used [BMIM][Cl]/glycerol carbonate in a 10:90 ratio as solvent and a wet Amberlyst 70 acidic resin as catalyst resulted in a 5-HMF yield of 60 % at 110 °C. A major part of the ionic liquid was substituted with glycerol carbonate which allows the cost and environmental impact of the process to be potentially be lower than methods that petroleum derived co-solvents.

Although ionic liquids have been investigated as solvent for the synthesis of 5-HMF, they still have some shortcomings such as price and toxicity that hamper their industrial applications. Liu et al. [38] developed an interesting choline chlo­ride (ChCl)/CO2 system, where the addition of CO2 could decrease the pH of the system through formation of carbonic acid and played as an efficient catalyst for the hydrolysis of inulin and the dehydration of fructose into 5-HMF. The inulin conversion with or without water addition in ChCl/CO2 system was studied. The yield of 5-HMF was rather disappointing (12 %) in the absence of water (120 °C, 4 mPa CO2, 90 min). The authors attributed the low 5-HMF yield to the low content of water at the initial stage of the reaction that does not favor the hydrolysis of inulin to fructose. Hence, water was initially added (16 wt%), and the yield of 5-HMF improved to 38 % when the reaction time was prolonged to 6 h. The 5-HMF was recovered with 41 % yield after 15 h of reaction, demonstrating the stability of 5-HMF in ChCl as compared to other solvents. This process has many advantages: (1) it is not necessary to remove the trace amounts of catalyst contained in 5-HMF after extraction; (2) the use of cheap and safe CO2 as a source of acid catalyst;

(3) the low ecological risk of the ChCl/CO2 system that is renewable; and (4) the system is still efficient at high loadings of fructose (up to 100 wt%).