Production of 2-hydroxyacetylfuran from lignocellulosics treated with ionic liquid–water mixtures

Abstract

Japanese cedar (Cryptomeria japonica) was treated with 12 ionic liquid (IL)–water mixtures at 120 °C for 1 h. Production of 5-hydroxymethylfurfural, furfural and 2-hydroxyacetylfuran (2-HAF) was observed by HPLC and GC-MS. This is the first report to identify 2-HAF from lignocellulosics using ILs. The optimal IL–water mixture was found to be a 90% pyridinium chloride and 10% water w/w solution, although any IL–water mixture that contained pyridinium or imidazolium salts produced all three compounds in varying yields.

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1. Introduction

Lignocelluloses are the most abundant renewable resource on the planet and are mainly comprised of three biopolymers: cellulose, hemicellulose and lignin. Recently, they have attracted attention as viable alternative renewable carbon resources to fossil fuels. Cellulose, which has a β-1,4-glucan chemical structure, and hemicellulose, which is mainly composed of hexoses and pentoses, can be converted to biofuels¹ and various raw materials (Fig. 1)² via chemical and biological reactions. Lignin is an aromatic biopolymer biosynthesized from phenylpropane via oxidative radical coupling, which can be transformed to significant aromatic building blocks of chemicals.³

Fig. 1 Chemical structures of furan compounds derived from polysaccharides in lignocellulosics.

After the United States Department of Energy established 12 chemical building blocks in 2004,⁴ research into the production of useful compounds and materials from carbohydrates in lignocellulosics has increased significantly. 5-Hydroxymethylfurfural (5-HMF), which can be synthesized from fructose and glucose, is a key intermediate and can be transformed into any one of the twelve chemical building blocks. 2,5-Furandicarboxylic acid, which is a candidate fuel source and a raw material in the production of plastics, can be prepared by oxidation of 5-HMF. Levulinic acid (Fig. 1), which is also one of the 12 building blocks, can be obtained from 5-HMF by hydrolysis reactions under acidic conditions.⁵

Swatloski et al. have reported that ionic liquids (ILs), which are organic salts with melting points below around 100 °C, can dissolve cellulose.⁶ Subsequently, studies on the treatment of lignocellulosics with ILs has been increasing.^7,8 The reaction behaviors of cellulose and wood with imidazolium or pyridinium-based ILs have been investigated⁹ and a morphological study of wood with ILs has been published.¹⁰

Many groups have attempted to convert lignocellulosics, or lignocellulosics-related compounds, into useful compounds using different ILs. Saccharides (e.g., glucose, fructose and cellulose) with different ILs have been used mainly as starting materials in the production of 5-HMF.^11–16 In some examples, 5-HMF could be produced from the saccharides with the use of a catalyst.^11–16 There have been few studies, however, examining the production of lignocellulosics from wood, and all of these have used ILs with lower volumes of water and dried samples. Application of IL–water systems for the treatment of lignocellulosics provides several advantages over other methods.

First, the drying of lignocellulosics is often required because they contain water, but if an IL–water method is used, then this drying step can be removed from the production process. Second, the solid-state ILs at ambient temperature and dissolved in water can behave as solvents for the biorefinery of lignocellulosics. Thereby, many different IL–water solvent systems can be used for the treatment of lignocellulosics. Third, the addition of water to IL reduces the financial cost and the potential environmental impact because the total volume of ILs that is needed can be reduced. Recently, Abe et al. reported tetra-n-butylammonium hydroxide and tetra-n-phosphonium hydroxide that contained water successfully dissolved wood at room temperature,¹⁷ and solutions of ILs and acids were employed as pretreatments in the production of bioethanol via saccharification.¹⁸

Although IL–water mixtures are attractive solvent systems, and have great potential for the conversion of lignocellulosics to valuable compounds, studies on the treatment for lignocellulosics with IL–water mixtures have not been undertaken in detail. In this paper, we report the analysis of the soluble fractions obtained from Cryptomeria japonica treated with IL–water mixtures, without catalysts, and describe the identification of 2-HAF from the sample.

2. Experimental

2.1 Materials

All solvents were of analytical grade and purchased from Wako Pure Chemical Industries Ltd (Osaka, Japan) or Sigma-Aldrich (St. Louis, MO). Ionic liquids and authentic samples of furan compounds were purchased from Tokyo Chemical Industry Co., Ltd (Tokyo, Japan), Wako Pure Chemical Industries Ltd or Sigma-Aldrich. Chloroform-d was purchased from Cambridge Isotope Laboratories Inc. (Andover, MA). Milli-Q water was used for IL treatments and HPLC analysis. C. japonica (Japanese cedar) was ground in a Wiley-mill to a particle size between 90 and 180 μm and then successively extracted with ethanol/benzene (1/2, v/v) for 6 h in a Soxhlet extractor to remove several extractives in C. japonica. The extracted wood powders were dried in an oven at 105 °C for 24 h prior to further processing (see Section 2.4).

2.2 Instruments

¹H NMR spectra were recorded on a Bruker AVANCE400 operating at 400 MHz. Chemical shifts (δ) are given in ppm relative to tetramethylsilane (δ 0.00 ppm) or CHCl₃ (δ 7.26 ppm) as internal standards. Mass spectra were obtained on a Shimadzu GC-MS (Shimadzu Co., Ltd, Kyoto, Japan) equipped with a DB-5MS column (30 m × 0.25 mm i.d., 0.25 μm film thickness, Agilent Technologies, USA). Analytical thin-layer chromatography (TLC) was performed on Merck pre-coated silica gel plates (Kiesel gel 60F₂₅₄ plate, 0.25 mm thick; Merck, USA) and silica gel column chromatography was carried out on a Kanto Chemical silica gel 60N (spherical, neutral, 63–210 μm). HPLC separation was conducted on a Shimadzu Prominence (Shimadzu Co., Ltd, Kyoto, Japan) equipped with an LC-20AD pump, a CTO-20A column oven, and an SPD-M20A photodiode array. The HPLC conditions were: Aminex HPX-87H column (Bio-Rad Laboratories, Inc., Hercules, CA), a flow rate of 0.6 mL min⁻¹, 5 mM H₂SO₄ as the mobile phase using a column temperature of 45 °C, and sample detection via UV absorbance at 280 nm.

2.3 Preparation of authentic 2-hydroxyacetylfuran (2-HAF)

2-HAF was prepared as previously conducted in the paper.¹⁹ Sucrose (30 g) and oxalic acid (0.3 g) were dissolved in 100 mL of water. The solution was heated at 140 °C in an autoclave for 3 h. After cooling to room temperature, the reaction mixture was neutralized with saturated NaHCO₃ aq. and filtered with 0.45 μm membrane filter. The filtrate was evaporated and then extracted with ethyl acetate four times before the combined organic layers were washed with water and dried over Na₂SO₄. Finally, the crude sample was obtained after drying in vacuo at room temperature then purified by flash chromatography on a silica gel (ethyl acetate : hexane = 1 : 2, v/v) to give 2-HAF (30 mg) in 0.1% yield. The identity of 2-HAF was established by ¹H NMR and GC-MS. Spectra data of 2-HAF: ¹H NMR (400 MHz, CDCl₃) δ 3.26 (1H, t, J = 4.4 Hz), 4.75 (2H, d, J = 4.4 Hz), 6.60 (1H, dd, J = 1.6, 3.6 Hz), 7.31(1H, dd, J = 0.8, 3.6 Hz), 7.64 (1H, dd, J = 0.8, 1.6 Hz), GC-MS m/z 126 [M]⁺, 95, 67, 51, calculated mass of C₆H₆O₃ is 126.03.

2.4 Treatment of C. japonica with ionic liquid–water mixtures

Typical experiments using ionic liquids are shown in Table 1. Air-dried wood powders from section 2.1 (0.18 g), ionic liquids (3.0 g), and Milli-Q water (0.33 g) were added to a 15 mL ace glass pressure tube (Ace glass, Inc., Vuneland, NJ). For entry 3, air-dried cedar powder (0.12 g), 1-ethylpyridinium chloride (2.0 g), and Milli-Q water (0.22 g) were added to a glass tube. For entry 8, air-dried cedar powder (0.18 g), and pyridinium chloride ([Py][Cl], 3.0 g) were added to a 15 mL glass tube and the mixture was stirred for 20 min at 160 °C in an oil bath. For entry 13, air-dried cedar powder (0.18 g) was added to 2.78 mL of a 1.5% v/v hydrochloric acid solution in a 15 mL glass tube. The mixtures were stirred for 1 h at 120 °C in an oil bath. After each reaction, 50 μL of each solution was taken and diluted with 450 μL of water. The solutions were shaken and filtered through a 0.45 μm syringe filter. The filtrate (10 μL) was then analyzed by HPLC. The reaction solutions using imidazolium chloride (entry 4), hydrazinium chloride (entry 7) and pyridinium chloride (entry 8) were diluted with water in order to take them easily.

Table 1 Furan yields from C. japonica after treatment with different IL–water mixtures

Entry	Ionic liquids	Water content (wt%)	5-HMF^a (wt%)	2-HAF^a (wt%)	Furfural^a (wt%)
a per dry wood.b not detected.
1	Pyridinium chloride	10	2.96	3.40	1.45
2	Pyridinium p-toluenesulfonate	10	0.69	Trace	0.86
3	1-Ethylpyridinium chloride	10	2.79	1.28	0.59
4	Imidazolium chloride	10	0.48	0.16	1.14
5	1-Methylimidazolium chloride	10	0.69	0.29	1.11
6	1-Methylimidazolium hydrogen sulfate	10	1.56	0.40	1.87
7	Hydrazinium chloride	10	n.d.^b	n.d.^b	n.d.^b
8	Pyridinium chloride	0	0.59	0.48	0.18
9	Pyridinium chloride	20	1.50	0.88	0.94
10	Guanidinium chloride	20	n.d.^b	n.d.^b	n.d.^b
11	Pyridinium bromide	30	0.72	0.19	0.57
12	Piperidinium chloride	30	n.d.^b	n.d.^b	n.d.^b
13	1.5% v/v hydrochloric acid	—	0.29	n.d.^b	0.48

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Air-dried wood powder (0.36 g), [Py][Cl] (6.0 g) and Milli-Q water (0.66 g) were added to a 35 mL ace glass pressure tube (Ace glass, Inc., Vuneland, NJ). The residual reaction solution was filtered through a glass filter with 1 μm of pore size and the filtrate was evaporated. Next, the reduced filtrate was dried in vacuo at room temperature and the dark colored residue obtained was extracted with acetone. Finally, the acetone extract was filtered through a 0.45 μm syringe filter before it was analyzed by GC-MS.

3. Results and discussion

We first chose [Py][Cl] for our IL–water mixtures because of its success in previous studies.^20–23 The HPLC chromatogram of the soluble fraction obtained from C. japonica treated with a [Py][Cl]–water mixture at 120 °C for 1 h is shown in Fig. 2. By comparison with authentic samples, the compounds corresponding to the peaks at 31 min (peak 1) and 47 min (peak 3) were identified as 5-HMF and furfural, respectively; however, the peaks at 21, 23, 34 and 44 min in the chromatogram could not be immediately identified. Structure elucidation of the unknown compound for the distinctive peak at 34 min was investigated. Using UV absorbance and HPLC retention time, we hypothesized that the compound had a furan skeleton.

Fig. 2 HPLC chromatogram of the soluble fraction obtained from C. japonica treated with a 90%/10% w/w solution of [Py][Cl] and water. Peaks 1, 2 and 3 were identified as 5-HMF, 2-HAF and furfural, respectively.

Moreau et al. have proposed conversion routes of monosaccharides to furan compounds such as furfural, 5-HMF and 2-HAF (Fig. 1).^24,25 Their results show that the three different furan compounds can be formed via a 1,2-enediol intermediate from monosaccharides by the acyclic route. They demonstrated that furfural and 5-HMF were produced from fructose by dehydration, but extraction of 2-HAF in the mixture was not observed.²⁴ However, volatile compounds, including 2-HAF, have been observed in foods^26–30 and other materials^31–36 using GC-MS. A trace amount of 2-HAF was detected in the acid catalyzed dehydration of fructose,³⁷ and conversion of cellulose to 2-HAF was demonstrated in a concentrated zinc chloride solution under microwave irradiation at 135 °C.^38,39 Glucose and mannose, which can be converted to fructose by isomerization, are in high concentration in lignocellulosics, even though production of 2-HAF from lignocellulosics with IL has not been reported as far as we know.

Given this, we assumed that 2-HAF could be obtained from lignocellulosics and further analysis of the soluble fraction from C. japonica was attempted. Unfortunately, an authentic sample of 2-HAF is not available commercially so we prepared an authentic sample of 2-HAF by the degradation of sucrose.¹⁹ The authentic 2-HAF was characterized by ¹H NMR and GC-MS, and gave an HPLC retention time of 34 min. The results therefore indicate that the unidentified peak 2 in Fig. 2 was 2-HAF. Additionally, the acetone extract from the soluble fraction of C. japonica treated with [Py][Cl]–water at 120 °C for 1 h was subjected to GC-MS. A peak at 9 minutes showed a mass spectrum with a molecular ion at m/z 126 and fragment ions at m/z 95, 67 and 51. The fragmentation pattern of the peak was the same as authentic 2-HAF. Therefore, we conclude that the compound corresponding to the peak at 34 min was 2-HAF.

Samples of C. japonica were also treated with other IL–water mixtures and quantitative analysis of the furan compounds in each soluble fraction was undertaken (Table 1). For entry 8, C. japonica was treated with [Py][Cl] at 160 °C without the addition of water because its melting point is around 144–148 °C. 5-HMF and furfural were successfully formed in the soluble fractions obtained from C. japonica treated with different IL–water mixtures, except for the ILs that contained hydrazinium chloride, guanidinium chloride and piperidinium chloride–water mixtures (entries 7, 10 and 12).

Production of 2-HAF from lignocellulosics depends on the ILs as 1.5% v/v hydrochloric acid was not a suitable solvent for producing 2-HAF (entry 13). Overall the results indicate that it is likely that pyridinium and imidazolium play an important role in the production of 2-HAF from saccharides in lignocellulosics. Consequently, it was found that a 90% [Py][Cl] and 10% w/w water solution is the most effective for the production of 2-HAF from C. japonica (entry 1).

There are two stages in the reaction to produce furan compounds from polysaccharides in lignocellulosics. First stage is hydrolysis of polysaccharides to produce various monosaccharides. Second stage is the conversion of the obtained monosaccharides into furan compounds by dehydrations as described in the previous paper.^24,25,38,39 10% of water in [Py][Cl] seems to be appropriate ratio for the production of monosaccharides. The role of water at the second stage, however, has not been clarified as far as we know.

Because acidity of pyridinium chloride is stronger than that of imidazolium chloride in water, pyridinium chloride is thought to promote the hydrolysis of polysaccharides in the first stage. Thus, the yields of 5-HMF and 2-HAF in the reaction with pyridinium chloride were superior to those with imidazolium chloride since monosaccharides, the precursors of furan compounds, are rich in the reaction media with pyridinium chloride.

We are presently investigating the reaction behavior of lignocellulosics with [Py][Cl]–water mixtures and the formation mechanism of 2-HAF using model compounds related to lignocellulosics.

4. Conclusions

We have demonstrated that the treatment of C. japonica with different IL–water mixtures produces furfural, 5-HMF and 2-HAF with yields dependent on which organic compounds are used in the IL–water mixtures. This paper is the first report to identify 2-HAF in the reaction mixtures obtained from woody biomass using ILs indicating that 2-HAF has potential as a new platform^40,41 in lignocellulosic biorefinery.

Acknowledgements

The authors wish to thank Ms Chihiro Kohsaka of the Kyoto Municipal Institute of Industrial Technology and Culture for her assistance with GC-MS and Mr Ryoya Ito for great help with the preparation of 2-HAF. This work was supported by a Science and Technology Research Promotion Program for Agriculture, Forestry, Fisheries and Food Industry grant (No. 26052A) from the Ministry of Agriculture, Forestry and Fisheries of Japan.

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