Production of 5-hydroxymethylfurfural from Japanese cedar (Cryptomeria japonica) in an ionic liquid, 1-methylimidazolium hydrogen sulfate

Production of 5-hydroxymethylfurfural (5-HMF) from Japanese cedar (Cryptomeria japonica) using an ionic liquid, 1-methylimidazolium hydrogen sulfate ([MIM]HSO4), was investigated. 5-HMF can be produced from C. japonica at temperatures above 120 °C. The maximum yield of 5-HMF was about 9 wt% after 15 min of treatment at 160 °C. However, 5-HMF produced in this process tended to decompose as the treatment continued. To avoid decomposition and to provide a means of recovering 5-HMF from [MIM]HSO4, three reaction systems based on [MIM]HSO4 were investigated: biphasic [MIM]HSO4/organic solvent system, [MIM]HSO4 with vacuum distillation, and [MIM]HSO4 with vacuum steam distillation. The [MIM]HSO4 reaction system combined with vacuum steam distillation was most effective. The maximum yield of 5-HMF was 17.5 wt% after treatment for 45 min at 160 °C. The combination of [MIM]HSO4 treatment with vacuum steam distillation is suitable for 5-HMF production because it is a one-pot process without the need for catalysts or pretreatment.

www.nature.com/scientificreports/ Cl with CrCl 2 have encouraged studies of lignocellulosic materials with ionic liquids [22][23][24][25][26][27][28] . Pine wood was converted to 5-HMF in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) with CrCl 3 ·6H 2 O as catalyst in 52 mol% yield by reaction at 200 °C 29 . These studies suggest that 5-HMF can be produced from lignocellulosics without pretreatments such as delignification and hydrolysis. However, typical studies of the formation of 5-HMF from lignocellulosics in ionic liquid have involved the use of catalysts 30 . In addition, extraction of 5-HMF from ionic liquid is also important. 5-HMF is not stable in various reaction media and degrades to levulinic acid (LA) and formic acid (FA) 31,32 . Once generated, LA and FA can polymerize with 5-HMF to form humin, which is known as a by-product of saccharide-based biorefinery processes 8,12 . Our previous studies also revealed that 5-HMF produced in ionic liquids, [EMIM]Cl 33 or 1-methylimidazolium hydrogen sulfate ([MIM]HSO 4 ) 34 as reaction medium is not stable under heating. Thus, 5-HMF should be continuously removed from the reaction medium to prevent the generation of by-products. As a fundamental research, biphasic reaction/separation coupling system was studied. In this system, conversion of glucose or fructose to 5-HMF in ionic liquids and extraction of 5-HMF produced from ionic liquids with supercritical carbon dioxide was applied 35,36 . In addition, there have been reports on a method of extracting 5-HMF from dissolved in ionic liquids using organic solvents 37 , or a method of extraction after derivatizing of 5-HMF in ionic liquids to another furan compounds 37,38 . However, extraction systems to remove 5-HMF produced from lignocellulosics in ionic liquids have received little attention. Recently, there is a study in which the reaction that produces humin from 5-HMF is not regarded as a side reaction, but is positively used to produce humin. The humin produced was revealed to show the metal ion adsorption capacity 39 . In this study, the non-catalytic production of 5-HMF from Japanese cedar (Cryptomeria japonica) was studied in [MIM]HSO 4 , which is a protic ionic liquid. Protic ionic liquids lead to proton donor and acceptor sites. Furthermore, the H + provided by protic ionic liquids give various applications to these ionic liquids. The production cost of protic ionic liquid is lower than that of other ionic liquids 19,[40][41][42] . The influence of water on the production of 5-HMF was studied because wood is normally humidified at ambient atmosphere. Furthermore, extraction systems to separate 5-HMF from [MIM]HSO 4 were also investigated.  Fig. 1. 5-HMF was produced from wood in [MIM]HSO 4 at 120 and 140 °C, and the yield of 5-HMF reached a maximum of 6.3 wt% after treatment for 5 h at 120 °C. Given that it is known that 5-HMF degrades under acidic conditions and [MIM]HSO 4 is an acidic ionic liquid, any 5-HMF that is produced in [MIM]HSO 4 is likely to degrade. As the treatment temperature increased, the treatment time required to reach maximum yield became shorter and the decomposition became faster. However, lowering the treatment temperature to 100 °C showed very low production of 5-HMF.

Results and discussion
The change in yield of 5-HMF over time for treatment of dried wood in [MIM]HSO 4 at 160 °C is shown in Fig. 2. The yield of 5-HMF reached a maximum of 7.1 wt% after 15 min of treatment. The rate of formation of 5-HMF at 160 °C was higher than those at 100, 120 and, 140 °C as shown in Fig. 1. In addition, the maximum yield of 5-HMF at 160 °C was higher than those observed at 100, 120, and 140 °C. A similar tendency for changes in 5-HMF yield was observed in the treatment of rice straw 34 . A higher reaction temperature reduced the duration of treatment required to achieve a maximum yield. The maximum value of 5-HMF was 6.8 wt% at 160 °C around 30 min treatment.
It is reported that the water content in the reaction system affect the production of 5-HMF from fructose, which is different from this study in the raw material and an ionic liquid used 43 . However, to investigate the influence of water content in wood, moisture-conditioned wood was treated in [MIM]HSO 4 . Table 1   www.nature.com/scientificreports/ HSO 4 at 140 °C, the yield of 5-HMF reached a maximum of about 6 wt% after 2 h (Fig. 3a). A similar trend was observed for changes in yield from the treatment of dried wood (Fig. 1). For treatment at 160 °C, the yields of 5-HMF from moisture-conditioned woods (Fig. 3b) were higher than those from dried woods (Fig. 2). The yield of 5-HMF reached a maximum of about 9 wt% after about 15 min when moisture-conditioned woods were treated in [MIM]HSO 4 at 160 °C. Moisture-conditioned wood can be converted efficiently to 5-HMF in [MIM] HSO 4 . Given that 5-HMF is produced from glucose obtained by hydrolysis of cellulose, water is required to hydrolyze cellulose to glucose. However, 5-HMF is also produced from dried wood, so it is considered that some wood decomposition occurs, and water necessary for hydrolysis is generated from the wood during decomposition. It is speculated that water in the moisture-conditioned wood was not effectively used for the hydrolysis reaction at 140 °C, whereas the water was used for hydrolysis at 160 °C. Therefore, the temperature used to produce 5-HMF was increased to 160 °C, and moisture-conditioned wood (moisture content 4.8%) was used in subsequent experiments.  4 were also extracted (70% and 55%, respectively) as shown in Fig. 4a. However, other organic solvents showed poor ability to extract 5-HMF from the reaction media. To improve the selectivity of 5-HMF extraction from the reaction medium, the reaction medium was modified by adding water or ethanol. The extraction yields of 5-HMF from the mixture of [MIM]HSO 4 and water by organic solvents are shown in Fig. 4b. After the addition of water     4 . In addition, THF has the advantages of low reactivity and low boiling point, which allow it to be easily separated from 5-HMF without denaturation.

Production of 5-HMF by biphasic reaction system ([MIM]HSO 4 /organic solvent).
To continuously extract 5-HMF during the process of its formation from wood in [MIM]HSO 4 , we used a biphasic reaction system based on [MIM]HSO 4 and an organic solvent. Table 2 shows the yields of 5-HMF extracted in the organic solvents of the biphasic reaction systems. Using benzaldehyde as the cosolvent, 5-HMF was extracted from the biphasic system in low yield at 140 and 160 °C, although 5-HMF was also present in the [MIM]HSO 4 phase in low yield. The yield of 5-HMF extracted by benzaldehyde reached a maximum of 3.4 wt% after treatment at 160 °C for 1 h, while 5-HMF was present in the [MIM]HSO 4 phase at 3.2 wt%. 5-HMF was considered to be unstable in benzaldehyde at 140 and 160 °C because the yield of extracted 5-HMF in benzaldehyde decreased  In another effort to improve the recovery of 5-HMF, vacuum steam distillation of the reaction system was studied. This process can effectively allow distillation of chemicals at lower temperatures. In addition, it was found that the inclusion of water in the reaction system increased the yield of 5-HMF in [MIM]HSO 4 (see Fig. 3). Table 4 shows the yields of recovered 5-HMF from wood by using vacuum steam distillation. The yield of 5-HMF reached a maximum of 17.5 wt% after treatment at 160 °C for 45 min. This system increased the yield of 5-HMF from wood in [MIM]HSO 4 compared with the other reaction systems used in this study. It is suggested that almost all wood was converted to 5-HMF by the treatment at 160 °C for 30 min or by the treatment at 180 °C for 20 min. The yields of 5-HMF at 200 °C were lower than those at 160 and 180 °C. However, the residues of 5-HMF at 200 °C were higher than those at 160 and 180 °C. This is because the 5-HMF produced was rapidly decomposed at 200 °C before being evaporated and converted into other compounds, which was recovered as residue. These results indicate that vacuum steam distillation at 160 and 180 °C is suitable for recovery of 5-  Preparation of moisture-conditioned wood. To prepare moisture-conditioned wood, wood flours were placed in desiccators at 20 °C for 2 weeks in which the relative humidity (RH) was controlled by an aqueous saturated salt solution. The salt solutions used to control RH in the desiccators were LiCl (11.3% RH), MgCl 2 (32.8% RH), and KCl (85.0% RH) 45 . To prepare water-saturated wood, dried wood flour was impregnated with distilled water under conditions of reduced pressure. Moisture content of moisture-conditioned wood was determined according to the equation: where W 0 is the oven-dried mass of wood after drying at 105 °C for 24 h, and W 1 is the mass of wood after moisture conditioning for 2 weeks.

Extraction of 5-HMF in [MIM]HSO 4 by organic solvents.
[MIM]HSO 4 (3 g) was heated at 160 °C in an oil bath with temperature controller (EO-200R, AS ONE Corporation, Osaka, Japan; Precision: ± 1 °C). Moisture-conditioned wood (0.09 g; moisture content 4.8%) was added to the [MIM]HSO 4 and the reaction mixture was gently stirred for 30 min. After the reaction mixture was cooled to room temperature, 50 μL of the mixture was added into 500 μL of various organic solvents and the biphasic mixtures were agitated. A 100 μL aliquot of the organic solvent phase was sampled and evaporated to dryness. The residue was then diluted with 200 μL of distilled water.
As an alternative procedure, the reaction mixture prepared as described above was mixed with water or ethanol (1/1, v/v). The mixture obtained was also extracted with organic solvents following the procedures described above.

Production of 5-HMF in biphasic [MIM]HSO 4 /organic solvent reaction system. [MIM]HSO 4
(3 g) was heated at 140 or 160 °C in an oil bath with temperature controller (EO-200R, AS ONE Corporation, Osaka, Japan; Precision: ± 1 °C). Moisture-conditioned wood (0.09 g; moisture content 4.8%) and 6 mL of organic solvent were added to the [MIM]HSO 4 and the reaction mixture was gently stirred. A condenser was also connected to the reaction vessel. After treatment, 200 μL of organic phase was withdrawn and evaporated to dryness. The obtained residues were diluted with 5 mL of ethanol or 15 mL of distilled water.

Production of 5-HMF by vacuum distillation.
[MIM]HSO 4 (3 g) was heated at 140, 160, 180, or 200 °C in an oil bath with temperature controller (EO-200R, AS ONE Corporation, Osaka, Japan; Precision: ± 1 °C). Moisture-conditioned wood (0.09 g; moisture content 4.8%) was added to the [MIM]HSO 4 and the reaction mixture was gently stirred. The pressure in reaction system was reduced to vacuum (2.5 kPa approximately) by connection to a vacuum pump (TST-100, Sato Vac, Tokyo, Japan). The products ware recovered by cold trap cooled with ice-water. Distilled water was added to the recovered products in the cold trap to give a final volume of 20 mL. Steam was supplied to the reaction medium, which was generated from distilled water heated at same temperature as the reaction mixture. The reduced pressure in the reaction system (2.5 kPa) was achieved by connection to a vacuum pump (TST-100, Sato Vac). The steam was recovered by a cold trap cooled with ice-water. Aqueous solution was recovered in the cold trap at 0.9 mL/min. Evaluation methods. 5 4 were analyzed by HPLC. The samples were prepared by dilution with distilled water and were filtered through a 0.45-μm filter. The filtrates were analyzed under following conditions: column, Aminex HPX-87H (Bio-Rad); flow rate, 0.6 mL/min; eluent, 5 mM H 2 SO 4 ; column temperature, 45 °C; detector, RID and UV detector set at 280 nm.
The yield of product was calculated using the following equation: The molar yield of product was calculated using the following equation: The number of moles of hexose units in Japanese cedar was determined as follows. The Japanese cedar was added to distilled water and sodium chlorite was added to the mixture to a final concentration of 0.09 M, along with a small quantity of acetic acid. The obtained reaction mixture was heated at 80 °C, and sodium chlorite with acetic acid was added to the reaction mixture every hour for 4 h. The reaction mixture was filtered and the residue, which was regarded as holocellulose, was oven-dried at 105 °C for 24 h and then weighed. The number of moles of hexose units in Japanese cedar was calculated on the basis of holocellulose and the reported composition of hexose and pentose in Japanese cedar 46 .
The extraction yield was calculated using the following equation: In vacuum steam distillation, 10 mL of distilled water was poured into the reaction mixture to stop the reaction at a predetermined time. After stirring for 24 h at room temperature, the solution was filtered and washed with distilled water. The obtained residue was dried in an oven at 105 °C for 24 h and weighed to calculate the yield.

Conclusions
Treatment of moisture-conditioned wood in [MIM]HSO 4 is suitable for the production of 5-HMF without catalysts above 120 °C. However, 5-HMF is unstable in [MIM]HSO 4 and decomposes at temperatures above 120 °C.
In the biphasic reaction system with organic solvent, 5-HMF was extracted by benzaldehyde, but the yield was low and the ability to separate the solvent from 5-HMF was strongly compromised by the high boiling point of benzaldehyde. The use of vacuum distillation to separate 5-HMF from [MIM]HSO 4 was ineffective; however, when steam was supplied to the system at 160 or 180 °C, 5-HMF was efficiently recovered as it formed in [MIM] HSO 4 . The [MIM]HSO 4 reaction system combined with vacuum steam distillation is effective for production of 5-HMF from wood among various reaction systems studied in this paper because it is a one-pot process that requires no catalyst or pretreatment.