9.2.1 Starting Material

9.2.1.1 Fructose

Fructose is the most common studied substrate for the preparation of 5-HMF in aqueous solutions, organic solvents, and water-organic solvent mixtures. In an early work that applied molten salts to the synthesis of 5-HMF from carbohydrates, fructose was converted to 5-HMF with a high yield of 70 % in the presence of pyridinium chloride in 1983 [27]. However, this pioneering work did not stimulate wide investigations on the 5-HMF production from biomass in melt salt solutions, until the beginning of the twenty-first century. Biomass conversions in ionic liquids became a hot topic when Lansalot-Matras and Moreau reported the acid-catalyzed dehydration of fructose in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM] [PF6]) with dimethyl sulfoxide (DMSO) as co-solvent in the presence of Amberlyst® 15 sulfonic ion-exchange resin as catalyst, to obtain a 5-HMF yield as high as 87 % at 80 °C for 32 h reaction time [28]. It was demonstrated that the addition of ionic liquids had a positive effect on the fructose dehydration to 5-HMF. They also studied acid-catalyzed dehydration of fructose in ionic liquid 1-H-3- methylimidazolium chloride ([HMIM][Cl]), which acted both as solvent and cata­lyst, and demonstrated a 5-HMF yield as high as 92 % [29]. According to the activation parameters calculated from the Arrhenius plots for formation and decom­position of 5-HMF, activation energies are found to be similar to those obtained in reactions catalyzed by zeolite solid catalyst. The authors attribute the high yields observed for the formation of 5-HMF in ionic liquids as solvent to be due to the differences in the preexponential factors [28]. Thereafter, many papers on the selective dehydration of fructose in ionic liquids with various catalysts, have appeared in the literature. Efficient dehydration of fructose to 5-HMF in ionic liquids was reported at much lower temperatures than in aqueous, organic solvents, and water-organic mixture systems [20, 30—34].

Qi et al. reported that 1-butyl-3-methylimidazolium chloride, [BMIM][Cl], used with a sulfonic ion-exchange resin catalyst could efficiently dehydrate fructose into 5-HMF to give a fructose conversion of 98.6 % and a 5-HMF yield of 83.3 % in 10 min reaction time at 80 °C. The reaction time could be reduced to 1 min when the temperature was increased to 120 °C which resulted in a 5-HMF yield of 82.2 % and nearly 100 % fructose conversion [20]. Comparison of the [BMIM][Cl] and [BMIM][BF4] systems that has the same sulfonic ion-exchange resin (Amberlyst®

15) as the catalyst, indicates that the higher efficiency and selectivity observed in the [BMIM][Cl] system can be attributed to a higher tendency towards concerted catalysis due to the greater hydrogen-bonding character, nucleophilicity or basicity of the chloride ion.

Shi et al. [35] used trifluoromethanesulfonic acid (TfOH), which is an interesting catalyst, to promote the conversion of fructose into 5-HMF in imidazolium ionic liquids. They studied the reaction system for different alkyl chain length ionic liquids and used various kinds of anions. In that work, yields of 5-HMF were strongly affected by aggregation of cations and the hydrogen bonds between fructose and anions of ionic liquids. Imidazolium cationic ILs with alkyl chain lengths of the cations being shorter than four carbons were found to be suitable for 5-HMF formation. They found that the anion of an IL forms strong hydrogen bonds with fructose molecules, and thus, the acid radical leads to high reaction activity. These results not only provide evidence to explain the interaction of the structure at the molecular level in 5-HMF preparation in ionic liquids, but also provide some hints on choosing suitable ILs for 5-HMF preparation [35].

Imidazolium-based ILs provide efficient dehydration of fructose into 5-HMF, however, eutectic mixtures of choline chloride with acids have been identified as a promising catalytic system for the process. Hu et al. [36] investigated the conver­sion of fructose to 5-HMF in choline chloride/citric acid at 80 °C, and obtained a 5-HMF yield of 77.8 % without in situ extraction and a yield of 91.4 % when continuous extraction with ethyl acetate was used for a 1 h reaction time. The main advantage of this process was not only high 5-HMF yields obtained but also the chemical system components (fructose, choline chloride and citric acid) used that all originate from renewable sources. Ilgen et al. studied choline chloride for the purpose of using highly concentrated mixtures of fructose (up to 50 wt%). The resulting solutions had a melting region of 79-82 °C, which is much lower than the melting point of pure choline chloride (300 °C), thus processing the solutions at low temperatures was possible. The highest 5-HMF yield obtained from the choline chloride-fructose system was 67 % for which the reaction conditions were p-TsOH as catalyst, reaction temperature of 100 °C and reaction time of 0.5 h [37]. Further­more, they made a screening study to compare the environmental impact of the choline chloride system with different conventional solvents for the conversion of carbohydrates into 5-HMF, and indicated advantages of the choline chloride sys­tems in terms of low toxicity and reduced mobility [37]. Liu et al. [38] developed a cheap and sustainable choline chloride/CO2 system for the dehydration of highly concentrated fructose solutions into 5-HMF with a yield of up to 72 %. In addition to the environmental benefits of this strategy, they found that in the presence of ChCl, 5-HMF is stabilized, probably due to hydrogen bonding that allows forma­tion of a eutectic mixture between choline chloride and 5-HMF. This aspect allows fructose can be converted with a high content (up to 100 wt%) as compared to traditional procedures where 5-HMF is obtained in yields higher than 60 % only from a fructose concentration lower than 20 wt% [20].