Direct conversion of chitosan to 5-hydroxymethylfurfural in water using Brønsted–Lewis acidic ionic liquids as catalysts

Abstract

The dehydration of chitosan to 5-hydroxymethylfurfural (HMF) via hydrothermal conversion was investigated in the presence of Brønsted–Lewis acidic ionic liquids. The catalytic effect of different Brønsted- and Lewis-acidic sites and the molar fraction of Lewis-acidic sites were investigated in detail. It was found that Brønsted–Lewis acidic ionic liquids ([Hmim][HSO₄]–0.5FeCl₂) exhibited good catalytic performance, furnishing HMF in 44.11% yield. Furthermore, the effects of reaction temperature, time, and amount of catalyst were also investigated. The optimal reaction conditions were obtained for a 1.25 wt% aqueous solution of the catalyst under hydrothermal conditions (180 °C for 4 h). A high HMF yield of 44.11% was obtained starting from 50 mg of chitosan. The Brønsted–Lewis acidic ionic liquid catalysts could be recycled by simple separation and exhibited constant activity in four successive trials.

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1. Introduction

Increasing carbon dioxide emissions and energy demands have drawn worldwide attention, with consequent exploration and development of green renewable energy systems gradually becoming a mainstream research direction in recent years. Biomass, as a substitute for fossil fuels, is expected to be the renewable resource of the future due to the vast number of chemical products that can be potentially manufactured from it, such as 5-hydroxymethylfurfural (HMF).^1,2 HMF, an aromatic aldehyde, is a resourceful intermediate linking biomass-based carbohydrate chemistry with conventional petroleum-based industrial chemical technology.^3,4 HMF can be used for the production of various chemicals and liquid fuels such as 2,5-dihydroxy-methylfuran (DHMF),⁵ 5-formyl-2-furancarboxylic acid (FFCA),⁶ 2,5-dimethyltetrahydrofuran (DMTHF),⁷ 2,5-furandicarboxylic acid (FDCA),⁸ adipic acid,⁹ caprolactam,⁹ 1,6-hexanediol,⁹ and levulinic acid (LA).¹⁰ Of so many furan derivatives, FDCA, which is a promising new biomass-derived chemical building block with a huge market potential, was recently achieved in 100% yield through the oxidation of HMF.⁸ Based on its wide application, the industrial potential of HMF as a key platform chemical is restricted by its relatively high production cost due to the difficulty of achieving the necessary high selectivity and separation purification yield.^11,12

Currently, major efforts have been directed at developing effective pathways for converting cellulose and its hydrolytic carbohydrates (glucose, fructose, sucrose) to HMF.^13–21 In general, the degradation of cellulose or its hydrolytic carbohydrates to HMF was achieved in the past using different stoichiometric catalysts like imidazoles ionic liquid catalysts,^13,15,19 metal catalyst,^20,21 Lewis acid,^14,16,17 and ion-exchange resin.¹⁸ Beyond cellulose, chitosan is the second most abundant polysaccharide after cellulose; however, despite the structural similarity between these two polysaccharides, there are few reports on conversion of chitosan to HMF. Recently, Yan et al.^22,23 proposed the concept of the shell biorefinery, whereby crustacean shells are separated into different fractions, and each fraction is upgraded or transformed into value-added products. Recently, Wang et al. reported that HMF could be obtained from chitin biomass (chitin 9.0%, chitosan-1 K 12.8%, chitosan-5 K 12.2%, chitosan-50 K 8.0%, chitosan-100 K 8.6%, and chitosan–COOH 9.2%) using a concentrated aqueous solution of ZnCl₂.²⁴ Additionally, Omari et al. reported the hydrolysis of chitosan and chitin in water under microwave irradiation using SnCl₄·5H₂O as a catalyst, producing HMF in 10.0 wt% yield.²⁵ Subsequently, Szabolcs et al.²⁶ reported that glucosamine-based glycans (D-glucosamine, N-Ac-D-glucosamine, LMw-chitosan, MMw-chitosan, chitin) were also converted to HMF and levulinic acid (LA) by using controlled microwave irradiation. The highest yield of 37.8% LA was obtained with 2 M H₂SO₄ at 190 °C for 30 min. However, the Lewis- and Brønsted-acidic catalysts used in these studies are difficult to recycle and pollute the environment. A most recent study showed that the 1-methylimidazolium hydrosulfate ([MIM]HSO₄) ionic liquid can catalyze the formation of HMF from chitosan under hydrothermal conditions.²⁷ The abovementioned ionic liquid can be reused; however, this leads to a lower HMF yield. Therefore, efficient transformation of chitosan to HMF should be investigated. This transformation of sustainable chitosan into high-value fuel and chemical products is of huge economic and environmental interest.

As an environmentally friendly catalyst, Brønsted–Lewis acidic ionic liquids (ILs) attract the attention of researchers, as it combines the characters of Brønsted–Lewis solid acids and ionic liquids. It is a novel type of environmentally friendly catalysts and is easy to recycle by simple separation. The utilization of ILs as reaction media or catalysts for the dehydration of hexoses to HMF has been reported. For example, Liu et al.²⁸ found that Brønsted–Lewis acidic ionic liquids [HO₃S–(CH₂)₃–NEt₃]Cl–CrCl₂ exhibited good catalytic performance, furnishing a 93.4% yield of HMF. The catalytic activity of the above ionic liquid in the dehydration of fructose to HMF was confirmed. Recently, metal chlorides in the neutral IL l-alkyl-3-methylimidazolium chloride ([EMIm]Cl) were used to effectively catalyze the formation of HMF from hexoses.⁴ The ionic liquid 1-methylimidazolium chlorosulfate ([HMIm]SO₃Cl), possessing dual Brønsted–Lewis acidity, showed better catalytic performance in the dehydration of fructose to HMF than ionic liquids with only Brønsted or Lewis acidity.²⁹ Brønsted–Lewis acidic N-methyl-2-pyrrolidonium metal chlorides ([Hnmp]Cl/MCl_x), employed in the hydrolysis of microcrystalline cellulose (MCC) and cotton, gave the reducing sugar in 98.8% yield.³⁰ The results indicated that the combined Brønsted- and Lewis-acidic character of ILs played a positive role in the dehydration of carbohydrates to HMF.³¹

As can be seen from the above overview, imidazole ionic liquids, Lewis acids (ZnCl₂ or SnCl₄), Brønsted acids (H₂SO₄), or their mixtures may promote the degradation of chitosan and its derivatives. Brønsted–Lewis acidic ionic liquids combine the characters of Brønsted–Lewis solid acids and ionic liquids. However, the use of Brønsted–Lewis acidic ionic liquids for the degradation of chitosan to HMF has not been reported. Due to the presence of Brønsted- and Lewis-acidic sites, these ILs can selectively catalyze different reaction mechanisms to enhance the conversion and selectivity of the reaction. We herein present our attempts to synthesize a series of Brønsted–Lewis acidic ionic liquids which efficiently catalyze the degradation of chitosan to HMF and can be recycled by simple separation. Hence, we reason that a Brønsted–Lewis catalytic system can not only be highly active and efficient in achieving high yields of HMF formation from chitosan, but also be recyclable. Accordingly, this contribution presents a full account of our study towards this main objective.

2. Experimental section

2.1 Materials

Chitosans (white crystalline powders) were purchased from Shandong Aokang Biotech Co., Ltd. (Jinan, China). N-Methylimidazole and 1,4-butane sultone were purchased from Nanjing Xiezun Chemical Co., Ltd. (Nanjin, China). HMF (>97%) was purchased from Heowns Biochem Technologies LLC (Tianjin, China). Triethylamine (Et₃N, AR grade), methanol (HPLC grade), and ethyl acetate (AR grade) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Vitriolic acid and hydrochloric acid were purchased from Tianjin Chemical Reagent No. 5 plant (Tianjin, China). All other chemicals were supplied by local suppliers and used without further purification.

The average molecular weight (M_w) of the chitosan used was approximately 350 K (deacetylation degree DD ¼ = 87.2%), based on viscosity measurements. Low-molecular-weight chitosan (LMWC 350 K) was chosen for studying the reaction conditions.

2.2 Synthesis of Brønsted–Lewis acidic ionic liquids

Brønsted-acidic [Hmim][HSO₄], [Hmim][Cl], and [HSO₃-b-N(C₂H₅)₃][Cl] were synthesized in our laboratory via a neutralization reaction and developed on the basis of the previous reports.^32–35 The structures of these Brønsted-acidic were illustrated in Fig. 1. Sulfuric acid (50 wt%) was added to an equimolar amount of N-methylimidazole over a period of 30 min while stirring and cooling to maintain a temperature of 0 °C. The reaction mixture was stirred for an additional period of 2 h at room temperature. Subsequently, water was removed under reduced pressure to obtain a colorless liquid, [Hmim]HSO₄. The crude product was washed with ethyl acetate to remove non-ionic residues and then vacuum-dried for 24 h at 80 °C. The obtained pure [Hmim][HSO₄] was treated with FeCl₂ under a protective atmosphere of nitrogen. The mixture was heated under vigorous stirring for 4 h at 100 °C and was then vacuum-dried for 24 h at 80 °C. By adjusting the amount of added FeCl₂, we prepared eight mixed-type Brønsted–Lewis acidic ionic liquids with variable FeCl₂ molar fractions, [Hmim][HSO₄]–xFeCl₂ (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8). [Hmim][HSO₄]–0.5M (M = ZnCl₂, Zn(CH₃COO)₂, FeCl₃, CrCl₃, NiCl₂, CuCl₂, CuCl, SnCl₂, CeCl₃), [Hmim][Cl]–0.5FeCl₂, [HSO₃-b-N(C₂H₅)₃][Cl]–0.5FeCl₂, and [HSO₃-b-N(C₂H₅)₃][Cl]–0.5ZnCl₂ were synthesized using similar procedures.

Fig. 1 The structures of Brønsted-acidic.

2.3 Typical procedure for the catalytic conversion of chitosan to HMF

In a typical experiment for the conversion of chitosan to HMF, chitosan (50 mg, 0.226 mmol) was added to deionized water (20 g) in a 50 mL stainless steel vessel with a Teflon lining, which was sealed by a screw cap. Brønsted–Lewis acidic ionic liquid catalysts of different structures and amounts were loaded into the reactor, which was then immersed into an oil bath pre-heated to a certain temperature and stirred for a given reaction time. Subsequently, the reactor was removed from the oil bath and immediately emerged in an ice-water bath to quench the reaction. The reaction mixture was filtered, and the residue was washed with deionized water to remove the insoluble humin polymer. Subsequently, 1 mL of the filtrate was diluted with methanol to 4.0 mL in a volumetric flask, and the diluted solution was injected into a glass tube after filtration through a 0.22 μm PTFE filter. The HMF yield was determined by high-performance liquid chromatography (HPLC) analysis of the aqueous solution, using a standard curve for quantification. The HMF yield was calculated as follows:

HMF yield [%] = mole amount of HMF/mole amount of starting chitosan × 100%

All experiments were replicated at least three times; the range of experimental errors for the HPLC analysis was ±1%.

2.4 Determination of HMF

Quantitative and qualitative analyses of HMF were performed by HPLC and gas chromatography/mass spectrometry (GC-MS). The concentration of HMF was determined with the help of HPLC (LC3000, Beijing Chuangxin Tongheng Science and Technology Co., Ltd.) using a reverse-phase C18 column (250 × 4.6 mm) and a UV detector at 284 nm. A methanol/water (23 : 77, v/v) mobile phase with a flow rate of 0.5 mL min⁻¹ was used. The column was maintained at 30 °C, and the volume used for each injection was 2 μL. In addition, we used GC-MS to determine the product structure. The GC-MS instrument (Bruker SCIONSQ/436-GC) was equipped with a 30 m × 0.25 mm × 0.25 μm HP-5 MS capillary column. Helium was used as a carrier gas at a linear velocity of 1.0 mL min⁻¹. The injector temperature was kept at 220 °C. The starting oven temperature of 50 °C (2 min) was increased at 15 °C min⁻¹ to 230 °C and was then held for 6 min, using an optimization of the method utilized previously.¹⁷ Mass spectrometric measurements were performed using electron impact ionization (EI) at 70 eV and a scan range of m/z = 50–500, at a rate of 1 scan per s. GC-MS results are shown in Fig. 2.

Fig. 2 Experimental and internal standard mass spectra of HMF.

2.5 Recycling of catalysts

The conversion of chitosan to HMF was chosen as a model reaction, and experiments were carried out at 180 °C for 4 h in [Hmim][HSO₄]–0.5FeCl₂ aqueous solution. After the reaction, most of the HMF was removed by repeated extraction with ethyl acetate. The water in the aqueous phase was completely removed by evaporation in vacuum, and the residue was subsequently oven-dried at 80 °C for 12 h in a draught drying cabinet. The thus obtained [Hmim][HSO₄]–0.5FeCl₂ was directly used in the next run by adding a fresh chitosan sample and deionized water under the same reaction conditions.

3. Results and discussion

3.1 FT-IR characterization of Brønsted–Lewis acidic ionic liquids

So far, research on Brønsted–Lewis acidic ionic liquids is still in its infancy. Due to the acidity being an important property of Brønsted–Lewis acidic ionic liquids, the corresponding characterization methods have attracted attention and include potentiometric titration, the Hammett indicator method, and FT-IR spectroscopy.³⁶ Among the above methods, only FT-IR spectroscopy can estimate the presence of the Brønsted acid and the strength of the Lewis acid.³⁷ Consequently, we preferred to characterize the acidity of Brønsted–Lewis acidic ionic liquids by FT-IR spectroscopy. Pyridine is widely used as an FT-IR probe molecule for the determination of Lewis and Brønsted acidities, since it can react with Brønsted and Lewis acids to produce the [PyH]⁺ cation or the Py–Lewis acid complex, respectively.^38,39 In the FT-IR spectra, the absorption peak of [PyH]⁺ is close to 1540 cm⁻¹, while the Py–Lewis acid complex exhibits an absorption peak close to 1450 cm⁻¹. By observing these two peaks, the sample acidity type can be inferred. This method has been used to identify whether the Brønsted–Lewis acidic ionic liquid exhibits both Brønsted and Lewis acidity types, and we used this method to characterize [Hmim][HSO₄]–FeCl₂. As shown in Fig. S6 (ESI†), compared with the spectra of pure pyridine and [Hmim][HSO₄]–FeCl₂, two new absorption peaks at 1551 and 1443 cm⁻¹ appeared in the FT-IR spectra of pyridine/[Hmim][HSO₄]–0.5FeCl₂ and pyridine/[Hmim][HSO₄]–0.7FeCl₂, which indicated that these ionic liquids were both Brønsted- and Lewis-acidic.

Analogously, the strength of Lewis acids can usually be characterized using acetonitrile as an FT-IR spectroscopy probe.^39–41 The nitrile group reacts with the Lewis acid to produce a CN–Lewis acid complex, which shows a new absorption peak at 2200–2400 cm⁻¹. With the increase of Lewis acid strength, this absorption peak moves to higher wavenumbers. Herein, we employed this method to determine the Lewis acid strength of [Hmim][HSO₄]–FeCl₂. As shown in Fig. S7 (ESI†), pure acetonitrile showed two absorption peaks at 2250 and 2289 cm⁻¹, attributed to the stretching vibrations of the nitrile group. It is clear that in the spectra of acetonitrile/[Hmim][HSO₄]–0.5FeCl₂ and acetonitrile/[Hmim][HSO₄]–0.7FeCl₂ new absorption peaks at 2324 and 2330 cm⁻¹ appeared in addition to the two peaks mentioned above, respectively. The new absorption peak was attributed to the CN–Lewis acid complex, which signifies that both [Hmim][HSO₄]–0.5FeCl₂ and [Hmim][HSO₄]–0.7FeCl₂ exhibit Lewis acidity. In addition, the peak of [Hmim][HSO₄]–0.7FeCl₂ was located at higher wavenumbers than that of [Hmim][HSO₄]–0.5FeCl₂, implying that the Lewis acidity of the former exceeded that of the latter.

3.2 Thermal stability of Brønsted–Lewis acidic ionic liquids

TGA was performed on the prepared Brønsted–Lewis acidic ionic liquids to characterize their thermal stability. The thermogravimetric analysis is illustrated in Fig. S8 (ESI†). There is a mass loss on the TG curve near 100 °C which results from the evaporation of water. One can note that the weight loss rate of [Hmim][HSO₄]–0.5FeCl₂ and [Hmim][HSO₄]–0.7FeCl₂ were much larger than that of [Hmim][HSO₄], further indicating that the Lewis-acidic sites FeCl₂ were successfully combined with the Brønsted-acidic sites [Hmim][HSO₄]. Furthermore, the decomposition temperatures of Brønsted–Lewis acidic ionic liquids [Hmim][HSO₄]–0.5FeCl₂ and [Hmim][HSO₄]–0.7FeCl₂ are 337.00 °C and 338.04 °C, respectively. Therefore, Brønsted–Lewis acidic ionic liquids [Hmim][HSO₄]–xFeCl₂ exhibit high thermal stability within the scope of the reaction temperature.

3.3 Catalyst screening

The catalytic conversion of chitosan to HMF by various Brønsted–Lewis acidic ionic liquids was carried out under hydrothermal conditions. We varied the ionic liquid structure (e.g., [Hmim][HSO₄], [Hmim][Cl], and [HSO₃-b-N(C₂H₅)₃][Cl]) and the Lewis acid component (e.g., ZnCl₂, Zn(CH₃COO)₂, FeCl₂, FeCl₃, CrCl₃, NiCl₂, CuCl₂, CuCl, SnCl₂, and CeCl₃), with the corresponding effects on catalytic performance summarized in Table 1. The molar fraction of Lewis acids in the Brønsted–Lewis acidic ionic liquids was 0.5 (x = 0.5), and all reactions utilized deionized water as a clean, non-corrosive, non-flammable, renewable, readily available, cheap, and environmentally friendly solvent. As shown in Table 1, different types of Brønsted–Lewis acidic ionic liquids exhibited diverse catalytic effects on the conversion of chitosan. A yield of 44.11% HMF was obtained (Table 1, entry 5) with [Hmim][HSO₄]–0.5FeCl₂ as a catalyst under hydrothermal conditions. However, little catalytic performance (only 1.61% HMF yield) was detected for reactions using the conventional method (stir-reflux) with the same [Hmim][HSO₄]–0.5FeCl₂ loadings and volumes of deionized water under reflux in a 50 mL round-bottom flask at 180 °C in an oil bath (Table 1, entry 6). These results suggest that pressure played a critical role in these reactions, which possibly require subcritical or superheated water conditions. Three kinds of Brønsted–Lewis acidic ionic liquids (Table 1, entries 2, 5, and 8) were selected as catalysts, and it was found that [Hmim][HSO₄]–0.5FeCl₂ showed the best performance. In contrast, blank experiments were carried out in absence of any Brønsted ionic liquids (Table 1, entries 1, 13, and 15) or the corresponding Lewis acids (Table 1, entries 18–20). Much to our disappointment, reduced HMF yields were observed. Our results are consistent with previous ones,^25,42 where Brønsted-acidic and Lewis-acidic ionic liquids exhibited a significant effect on the dehydration of chitosan to HMF due to their unique advantages.

Table 1 Effect of different Brønsted–Lewis acidic ionic liquids on the conversion of the chitosan into HMF under hydrothermal conditions^a

Entry	Catalyst	HMF yield (%)
		160 °C	180 °C	200 °C
a Experiment conditions: chitosan, 50 mg; deionized water, 20 g; concentration of Brønsted–Lewis acidic ionic liquid aqueous solution, 1.25 wt%; treatment time, 4 h; temperature, 160 °C, 180 °C, and 200 °C.b The experiment were refluxed in 50 mL round-bottom flask at 180 °C in the oil bath.c The Brønsted-acidic and Lewis-acidic as two separate catalysts in reaction, respectively.
1	[Hmim][HSO4]	Trace	20.41	13.80
2	[Hmim][HSO4]–0.5 ZnCl2	14.51	35.72	16.17
3	[Hmim][HSO4]–0.5 Zn(CH3COO)2	4.94	12.93	3.01
4	[Hmim][HSO4]–0.5 SnCl2	Trace	13.46	9.53
5	[Hmim][HSO4]–0.5 FeCl2	10.18	44.11	29.16
6	[Hmim][HSO4]–0.5 FeCl2^b	0.52	1.61	Trace
7	[Hmim][HSO4]–0.5 CrCl3	Trace	13.62	2.51
8	[Hmim][HSO4]–0.5 NiCl2	5.24	33.47	12.90
9	[Hmim][HSO4]–0.5 CuCl2	Trace	4.36	Trace
10	[Hmim][HSO4]–0.5 CuCl	Trace	4.88	Trace
11	[Hmim][HSO4]–0.5 FeCl3	2.96	15.60	3.55
12	[Hmim][HSO4]–0.5 CeCl3	5.27	12.08	8.11
13	[Hmim]Cl	Trace	3.74	Trace
14	[Hmim]Cl–0.5 FeCl2	Trace	2.69	Trace
15	[HSO3-b-N-(C2H5)3]Cl	11.25	15.95	14.21
16	[HSO3-b-N-(C2H5)3]Cl–0.5 ZnCl2	15.79	24.62	19.78
17	[HSO3-b-N-(C2H5)3]Cl–0.5 FeCl2	8.94	14.39	10.02
18	FeCl2	9.83	12.12	6.53
19	ZnCl2	Trace	1.48	Trace
20	NiCl2	Trace	3.57	Trace
21	[Hmim][HSO4], FeCl2^c	6.34	30.5	26.57
22	[Hmim]Cl, FeCl2^c	Trace	1.03	Trace
23	[HSO3-b-N-(C2H5)3]Cl, ZnCl2^c	8.34	14.46	2.69

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To examine the influence of the molar fraction of FeCl₂, the best-performing catalyst [Hmim][HSO₄]–0.5FeCl₂ was selected for the conversion of chitosan to HMF under the same conditions. As shown in Fig. 3, increasing the amount of FeCl₂ in [Hmim][HSO₄]–FeCl₂ gradually increased the yield of HMF and subsequently decreased it after reaching a maximum. This could be caused by the stronger acidity of the catalyst at increased FeCl₂ molar fractions. The intramolecular hydrogen bonds of chitosan break more easily when more acidic catalysts are used, further contributing to the accelerated fracture of glucosidic bonds and to the simultaneous improvement of the chitosan hydrolysis speed.²⁷ However, too strong acidity causes further decomposition of HMF to levulinic acid. The most appropriate Lewis acid molar fraction in the Brønsted–Lewis acidic ionic liquids for the conversion of chitosan to HMF was equal to 0.5.

Fig. 3 Effect of the molar fraction (x) on HMF production under hydrothermal conditions. Reaction conditions: chitosan, 50 mg; deionized water, 20 g; concentration of Brønsted–Lewis acidic ionic liquid aqueous solution, 1.25 wt%; reaction temperature, 180 °C, reaction time, 4 h.

3.4 Exploring optimum reaction conditions

In the above analyses, [Hmim][HSO₄]–FeCl₂ showed excellent catalytic performance in the conversion of chitosan to HMF. We next investigated other factors to further optimize experimental conditions, including the molar fraction of FeCl₂ in [Hmim][HSO₄]–FeCl₂ (x), temperature, time, and the concentration of the [Hmim][HSO₄]–FeCl₂ aqueous solution. The results are presented in Fig. 4, 5, and 6, respectively. By varying the temperature between 140 °C and 220 °C, we established that the reaction carried out at 180 °C gave the best HMF yield (Fig. 4), with the quantity of deionized water kept constant at 20 g, the concentration of [Hmim][HSO₄]–FeCl₂ aqueous solution maintained at 1.25 wt%, and the reaction time kept at 4 h. The results were consistent with those previously reported for the effect of reaction temperature on the synthesis of HMF,⁴³ and Khaled et al.²⁵ surmised that this decrease was likely due to more rehydration reactions occurring at higher temperatures to yield levulinic acid from the initially generated HMF. The effect of reaction time was also investigated, with the results presented in Fig. 5. Obviously the reaction time was also a critical factor determining the conversion of chitosan and the yield of HMF. At long reaction time, the yield of HMF sharply increased up to 4 h, slightly decreasing at longer reaction time. These results indicated that long reaction time induced further decomposition of HMF to levulinic acid,⁴² simultaneously resulting in greater conversion of HMF to humin.⁴⁴ Thus, it was shown that 4 h was an appropriate reaction time for the conversion of chitosan to HMF in our catalytic system.

Fig. 4 Effect of the temperature on HMF production under hydrothermal conditions. Reaction conditions: chitosan, 50 mg; deionized water, 20 g; concentration of Brønsted–Lewis acidic ionic liquid aqueous solution, 1.25 wt%; reaction temperature, 180 °C, reaction time, 4 h.

Fig. 5 Effect of reaction time on HMF production under hydrothermal conditions. Reaction conditions: chitosan, 50 mg; deionized water, 20 g; concentration of Brønsted–Lewis acidic ionic liquid aqueous solution, 1.25 wt%; reaction temperature, 180 °C.

Fig. 6 Effect of the different concentration of [Hmim][HSO₄]–0.5FeCl₂ aqueous solution on HMF production under hydrothermal conditions. Reaction conditions: chitosan, 50 mg; deionized water, 20 g; reaction temperature, 180 °C, reaction time, 4 h.

As shown in Fig. 6, the yield of HMF varied with the concentration of the [Hmim][HSO₄]–FeCl₂ aqueous solution, illustrating the great influence of the amount of catalyst. The yield gradually increased when the concentration of [Hmim][HSO₄]–0.5FeCl₂ increased from 0.25 to 1.25 wt%, but decreased when more than 1.5 wt% were added. The maximum (44.11%) HMF yield was obtained in the presence of 1.25 wt% [Hmim][HSO₄]–0.5FeCl₂ aqueous solution at 180 °C for 4 h. Low concentrations of catalyst are probably not sufficient to fully degrade chitosan, while high catalyst loadings not only accelerate the conversion of chitosan to HMF, but also promote other side-reactions, such as rehydration of HMF to levulinic acid and cross-polymerization reactions,⁴⁵ resulting in a low HMF yield. Furthermore, due to the high cost of [Hmim][HSO₄]–0.5FeCl₂, 1.25 wt% [Hmim][HSO₄]–0.5FeCl₂ in water was deemed an appropriate and economical concentration.

3.5 Reaction pathways for chitosan dehydration in the binary system

Based on all the above results, a possible mechanism for the dehydration of chitosan to HMF using aqueous [Hmim][HSO₄]–FeCl₂ as a catalyst was proposed, as shown in Scheme 1. This mechanism has much in common with the previously studied glucosamine²⁵ and starch⁴⁶ dehydration processes. Ionic liquids containing halide anions tend to destroy the semicrystalline structure of chitosan due to its strong ability to form hydrogen bonds with hydroxyl groups.^46,47 The ILs disrupt the original inter- and intramolecular hydrogen bonds between the hydroxyl groups of chitosan molecules, thereby favoring their solvation. In the system used herein for the conversion of chitosan to HMF, hydrolysis of the dissolved chitosan occurred in water, leading to the formation of partially depolymerized chitosan, glucosamine oligomers, and eventually glucosamine⁴⁸ via cleavage of glycosidic bonds in acidic aqueous solution. Furthermore, by performing the reactions in water, both hydronium and hydroxide ions were made readily available for reactions at this site. However, the above interaction may be insufficient for further degradation of chitosan to HMF. When metal halides were present in ionic liquids, the glycosidic linkage weakened, facilitating the subsequent ring-opening reaction to obtain an open-chain aldose. In the following sections, we present the calculated mechanistic details of pathways I and II to understand the synergetic effects of Brønsted–Lewis acidity on the conversion of chitosan to HMF.

Scheme 1 Proposed mechanism of Brønsted–Lewis acidic ionic liquid promoted conversion of chitosan into HMF.

In pathway I, the subsequent ketone–enol tautomerism and dehydration afford the five-membered furan ring, with the NH₂ group removed from glucosamine, similar to the mechanism proposed in an earlier study.⁴⁹ In pathway II, alternatively, we assume that coordination of the amine group to either a proton or a metal center facilitates this bond-breaking process by weakening the proximal C₁–O bond and making it more amenable to hydrolysis.²⁵ Therefore, the open-chain aldose intermediate is dehydrated to an enamine, followed by electrolytic rearrangement, deaminization, and dehydration to form a five-membered furan ring. Finally, HMF can be generated by further dehydration of the five-membered furan ring. However, to date, attempts to detect ammonia or reducing sugars in the aqueous phase after extracting the products from the reaction mixtures have been unsuccessful. Therefore, we cannot provide unequivocal evidence for all steps of the proposed mechanism.

3.6 Recycling experiments

Traditionally, some organic solvents, including tetrahydrofuran, toluene and ether, ethyl acetate, have been employed to extract HMF from reaction systems.^50–52 Recently, the efficient extraction of 5-hydroxymethylfurfural (HMF) from the conversion products of glucose in an ionic liquid has been achieved via charging compressed CO₂ into the mixtures.⁵³ In our experiments, we found ethyl acetate was the best solvent for extraction HMF in this reaction. Most of the HMF was separated by repeated extraction with ethyl acetate. The present method can easily remove the product and achieve the subsequent recycling of the IL.

Catalyst recycling is one of the key elements of green and sustainable chemistry. Hence, the reusability of the [Hmim][HSO₄]–0.5FeCl₂ catalyst was studied in this work. As shown in Fig. 7, the processes were evaluated over four cycles, the yields of HMF being 44.11, 37.31, 31.32, and 29.51%, respectively. The catalytic activity was slightly reduced with time, possibly due to the mass loss during the recycling procedure and the determination of HMF yield. Therefore, it can be concluded that the catalyst was stable in this system, having potential applications in large-scale industrial synthesis.

Fig. 7 Recycling of [Hmim][HSO₄]–0.5FeCl₂ for the synthesis of HMF from chitosan under hydrothermal conditions. Reaction conditions: chitosan, 50 mg; [Hmim][HSO₄]–0.5FeCl₂, 250 mg; deionized water, 20 g; reaction temperature, 180 °C; reaction time, 4 h.

4. Conclusion

In conclusion, an environmentally friendly catalyst has been prepared and used for the direct conversion of chitosan to HMF. Investigating the effect of different reaction conditions on the formation of HMF from chitosan showed that the 44.11% yield was obtained in the presence of 1.25 wt% [Hmim][HSO₄]–0.5FeCl₂ aqueous solution at 180 °C for 4 h. This conversion reaction is carried out in water, and the ionic liquids catalysts can be readily recycled, showing no noticeable loss in their catalytic activity throughout four consecutive recycles. Thus, these desired features possessed by the current catalyst system have enabled the development of the greener and more environmentally benign process for chitosan conversion into HMF. We expect that the present work will boost the rapid development of chitin biomass towards their novel applications.

Acknowledgements

The authors would like to acknowledge the financial support from National Natural Science Foundation of China (No. 21406166, 51678411) and the National Key Technology Support Program (2015BAE01B03).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21289a

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