Reaction kinetic studies are not only important for uncovering the mechanism of 5-HMF formation on a molecular level, but also useful for process development studies to optimize process conditions and reactor configurations to obtain high 5-HMF yields. A number of kinetic studies have been carried out on the transfor­mation of various mono — or poly-saccharides into 5-HMF in different reaction medias, but herein we will mainly overview the kinetics studies reported in the ionic liquids system.

Moreau et al. [29] investigated the reaction kinetic of the conversion of fructose into 5-HMF in ionic liquid, 3-methylimidazolium chloride acting as both solvent and catalyst. The reactions were carried out at the temperature range of 90-120 °C with the initial fructose concentration of 0.01-2.5 M. A reaction order of one was applied for the kinetic analysis of the dehydration of fructose, obtaining an activa­tion energy of 143 kJ/mol for the reaction of fructose to 5-HMF. Wei et al. [96] reported the conversion of fructose to 5-HMF in 1-butyl-3-methyl imidazolium chloride using IrCl3 as the catalyst. The reactions were performed at temperatures ranging from 80 to 120 °C and a fructose concentration of 10 wt%. A kinetic network involving fructose conversion to 5-HMF and byproducts was proposed to model the experimental data using first order reaction kinetics. The activation energies for fructose conversion were estimated to be 165 and 124 kJ/mol for the formation of 5-HMF and formation of byproducts, respectively. Based on a reaction order of one, Qi et al. [20] reported an activation energy of 65 kJ/mol for the formation of 5-HMF from fructose, which was performed in 1-butyl-3-methyl imidazolium chloride in the presence of a strong cation exchange resin Amberlyst 15A. The authors contributed the lower activation energy to higher acidity of the strong sulfonic ion-exchange resin over that of the other catalysts.

Compared with fructose, fewer studies have been carried out for the kinetic study on the dehydration of glucose to 5-HMF, and these studies reported so far have been performed in aqueous systems, except for one work [19], which was performed in the ionic liquid 1-butyl-3-methyl imidazolium chloride using CrCl3 as catalyst under microwave irradiation. In that reaction system, an activation energy of 115 kJ/mol was reported using first-order reaction approach to model the experimental data [19]. The value of the activation energy of this work was comparable with the values (108-137 kJ/mol) reported for the dehydration of glucose in water with sulfuric acid as catalyst. However, the pre-exponential factor determined in that work was 3.5 x 1014 min-1, which was 3-8 orders of magnitude larger than those of previous works. The difference in the pre-exponential factors should be a result of the enhanced effective collision among reactant molecules in the homogeneous phase [19, 97]. It has been found that the activation energy for the side reactions that glucose decomposed to undesired humins is higher than for the desired reaction of glucose to 5-HMF, indicating that lower temperatures favor 5-HMF formation [98]. Therefore, the reaction temperature should be optimized for 5-HMF production.

In many kinetic studies of carbohydrates, 5-HMF is involved as an intermediate or decomposition product in reaction pathway to obtain sugars or target compounds and so the kinetics of 5-HMF is not well known. The decomposition of cellulose in aqueous systems is often modeled with first-order approaches that give activation energies for the decomposition of cellulose in water to be between 140 and 190 kJ/ mol [2]. Vanoye et al. [99] conducted a kinetic study on the acid-catalyzed hydrolysis of cellobiose in [EMIM][Cl] with 3.5 mM methanesulfonic acid catalyst in the presence of small amounts of water (3.5 mM) as the cosolvent. Activation energies of 111 and 102 kJ/mol were obtained for cellobiose hydrolysis and glucose degradation, respectively. Dee and Bell [100] performed kinetic studies on the cellulose hydrolysis in a batch set up in ionic liquids ([EMIM][Cl] and [BMIM] [Cl]) with mineral acid catalysts, and glucose, 5-HMF and cellobiose were the primary reaction products. The reaction kinetics for glucose formation were fit with first-order reaction rate equation and an activation energy of 96 kJ/mol was determined.

For 5-HMF production, the undesired decomposition of 5-HMF to levulinic acid, formic acid, and humins should be suppressed as much as possible. To gain insight in the reactivity of 5-HMF, kinetic studies using 5-HMF as the starting material should be investigated. However, kinetic studies on the decomposition of 5-HMF in ionic liquids system thus far are not available. Instead, there are many works that focus on the reaction kinetic of 5-HMF decomposition with 5-HMF as starting material in aqueous systems and organic-aqueous mixtures. For the rehy­dration of 5-HMF to levulinic acid, activation energies vary from 47 to 210 kJ/mol, and the activation energies range between 100 and 125 kJ/mol for the decomposi­tion of 5-HMF to humins. Asghari et al. [21] reported a higher activation energy for the formation of humins from 5-HMF (122 kJ/mol) than that for the decomposition of 5-HMF to levulinic acid (94 kJ/mol) catalyzed by HCl in subcritical water. On the contrary, Girisuta et al. [101] found similar activation energies of about 111 ± 2 kJ/mol for both the rehydration of 5-HMF to levulinic acid and the side reaction of decomposition of 5-HMF to humins.