Direct conversion of chitosan to 5-hydroxymethylfurfural in water using Br?nsted-Lewis acidic ionic liquids as catalysts

Article abstract of DOI:10.1039/c6ra21289a

The dehydration of chitosan to 5-hydroxymethylfurfural (HMF) via hydrothermal conversion was investigated in the presence of Br?nsted-Lewis acidic ionic liquids. The catalytic effect of different Br?nsted- and Lewis-acidic sites and the molar fraction of Lewis-acidic sites were investigated in detail. It was found that Br?nsted-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 Br?nsted-Lewis acidic ionic liquid catalysts could be recycled by simple separation and exhibited constant activity in four successive trials.

Full text of DOI:10.1039/c6ra21289a

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Direct conversion of chitosan to 5-hydroxymethylfurfural in
water using Brønsted-Lewis acidic ionic liquids as catalysts
Yi Jiang, Hongjun Zang,∗ Sheng Han, Bing Yan, Songbai Yu, and Bowen Cheng
State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Department
of Environmental and Chemistry Engineering, Tianjin Polytechnic University, 300387
Tianjin, China.
ABSTRACT
The dehydration of chitosan to 5ꢀhydroxymethylfurfural (HMF) via hydrothermal
conversion was investigated in the presence of BrønstedꢀLewis acidic ionic liquids. The
catalytic effect of different Brønstedꢀ and Lewisꢀacidic sites and the molar fraction of
Lewisꢀacidic sites were investigated in detail. It was found that BrønstedꢀLewis 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 BrønstedꢀLewis acidic ionic liquid catalysts could be recycled by simple
separation and exhibited constant activity in four successive trials.
∗a Eꢀmail addresses: zanghongjun@tjpu.edu.cn or chemhong@126.com ( H. J. Zang)
Corresponding author. Tel.: +86 22 2401 9762; fax: +86 22 2452 8055.

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Key words: chitosan dehydration, HMF, BrønstedꢀLewis acidic ionic liquid,
hydrothermal synthesis
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 of 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 5ꢀhydroxymethylfurfural (HMF).1,2 HMF, an aromatic aldehyde, is a resourceful
intermediate linking biomassꢀbased carbohydrate chemistry with conventional
petroleumꢀbased industrial chemical technology3,4. HMF can be used for the production
of various chemicals and liquid fuels such as 2,5ꢀdihydroxyꢀmethylfuran (DHMF),5
5ꢀformylꢀ2ꢀfurancarboxylic acid (FFCA),6 2,5ꢀdimethyltetrahydrofuran (DMTHF),7
2,5ꢀfurandicarboxylic acid (FDCA),8 adipic acid,9 caprolactam,9 1,6ꢀhexanediol,9 and
levulinic acid (LA).10 Of so many furan derivatives, FDCA, which is a promising new
biomassꢀderived 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 ne1cessary 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

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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 ionꢀexchange
resin.18 Beyond cellulose, chitosan is the second most abundant polysaccharide after
cellulose; however, despite the structural similarity between these two polysaccharides,
22,23
there are few reports on conversion of chitosan to HMF. Recently, Yan et al
proposed the concept of the shell biorefinery, whereby crustacean shells are separated
into different fractions, and each fraction is upgraded or transformed into valueꢀadded
products. Recently, Wang et al. reported that HMF could be obtained from chitin
biomass (chitin 9.0%, chitosanꢀ1K 12.8%, chitosanꢀ5K 12.2%, chitosanꢀ50K 8.0%,
chitosanꢀ100K 8.6%, and chitosanꢀCOOH 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 glucosamineꢀbased
glycans (Dꢀglucosamine, NꢀAcꢀDꢀglucosamine, LMwꢀchitosan, MMwꢀchitosan, 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 Lewisꢀ and Brønstedꢀacidic catalysts used in these studies are
difficult to recycle and pollute the environment. A most recent study showed that the
1ꢀmethylimidazolium 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,

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efficient transformation of chitosan to HMF should be investigated.
This
transformation of sustainable chitosan into highꢀvalue fuel and chemical products is of
huge economic and environmental interest.
As an environmentally friendly catalyst, BrønstedꢀLewis acidic ionic liquids (ILs)
attract the attention of researchers, as it combines the characters of BrønstedꢀLewis
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 BrønstedꢀLewis 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 lꢀalkylꢀ3ꢀmethylimidazolium chloride
([EMIm]Cl) were used to effectively catalyze the formation of HMF from hexoses.4 The
ionic liquid 1ꢀmethylimidazolium chlorosulfate ([HMIm]SO3Cl), possessing dual
BrønstedꢀLewis acidity, showed better catalytic performance in the dehydration of
fructose to HMF than ionic liquids with only Brønsted or Lewis acidity.29
BrønstedꢀLewis acidic Nꢀmethylꢀ2ꢀpyrrolidonium 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 Brønstedꢀ and
Lewisꢀacidic 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

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(ZnCl2 or SnCl4), Brønsted acids (H2SO4), or their mixtures may promote the
degradation of chitosan and its derivatives. BrønstedꢀLewis acidic ionic liquids combine
the characters of BrønstedꢀLewis solid acids and ionic liquids. However, the use of
BrønstedꢀLewis acidic ionic liquids for the degradation of chitosan to HMF has not
been reported. Due to the presence of Brønstedꢀ and Lewisꢀacidic 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
BrønstedꢀLewis acidic ionic liquids which efficiently catalyze the degradation of
chitosan to HMF and can be recycled by simple separation. Hence, we reason that a
BrønstedꢀLewis 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). Nꢀmethylimidazole and 1,4ꢀbutane 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

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purification.
The average molecular weight (Mw) of the chitosan used was approximately 350K
(deacetylation degree DD ¼ = 87.2%), based on viscosity measurements.
Lowꢀmolecularꢀweight chitosan (LMWC 350K) was chosen for studying the reaction
conditions.
2.2 Synthesis of Brønsted-Lewis acidic ionic liquids
Brønstedꢀacidic [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 Brønstedꢀacidic were illustrated in Fig.
1. Sulfuric acid (50 wt.%) was added to an equimolar amount of Nꢀmethylimidazole
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 nonꢀionic
residues and then vacuumꢀdried 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 vacuumꢀdried for 24 h at
80 °C. By adjusting the amount of added FeCl2, we prepared eight mixedꢀtype
BrønstedꢀLewis 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

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were synthesized using similar procedures.
N
N
SO3H
HSO4
Cl
N
Cl
N
N
H
H
[HSO3-b-N(C2H5)3][Cl]
[Hmim][Cl]
[Hmim][HSO4]
Figure 1. The structures of Brønstedꢀacidic.
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 50ꢀmL stainless steel vessel with
a Teflon lining, which was sealed by a screw cap. BrønstedꢀLewis acidic ionic liquid
catalysts of different structures and amounts were loaded into the reactor, which was
then immersed into an oil bath preꢀheated 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 iceꢀwater 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 ꢁm PTFE filter. The HMF yield was determined by
highꢀperformance 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

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Quantitative and qualitative analyses of HMF were performed by HPLC and gas
chromatography/mass spectrometry (GCꢀMS). The concentration of HMF was
determined with the help of HPLC (LC3000, Beijing Chuangxin Tongheng Science and
Technology Co., Ltd.) using a reverseꢀphase 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 was used. The column was maintained at 30 °C, and the volume used for each
injection was 2 ꢁL. In addition, we used GCꢀMS to determine the product structure. The
GCꢀMS instrument (Bruker SCIONSQ/436ꢀGC) was equipped with a 30 m × 0.25 mm
× 0.25 ꢁm HPꢀ5 MS capillary column. Helium was used as a carrier gas at a linear
velocity of 1.0 mL/min. The injector temperature was kept at 220 °C. The starting oven
temperature of 50 °C (2 min) was increased at 15 °C /min 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/s. GCꢀMS results are shown in Fig.2.

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Figure 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 ovenꢀdried at 80 °C for 12 h in a draught
drying cabinet. The thus obtained [Hmim][HSO4] – 0.5 FeCl2 was directly used in the
next run by adding a fresh chitosan sample and deionized water under the same reaction
conditions.

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3. Results and discussion
3.1 FT-IR characterization of Brønsted-Lewis acidic ionic liquids
So far, research on BrønstedꢀLewis acidic ionic liquids is still in its infancy. Due to
the acidity being an important property of BrønstedꢀLewis acidic ionic liquids, the
corresponding characterization methods have attracted attention and include
potentiometric titration, the Hammett indicator method, and FTꢀIR spectroscopy.36
Among the above methods, only FTꢀIR 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 BrønstedꢀLewis acidic ionic liquids by FTꢀIR spectroscopy.
Pyridine is widely used as an FTꢀIR 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 PyꢀLewis acid complex, respectively.38,39 In the FTꢀIR spectra, the
absorption peak of [PyH]+ is close to 1540 cm−1, while the PyꢀLewis 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
BrønstedꢀLewis 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(Supporting Information), compared with the spectra of pure pyridine and
[Hmim][HSO4] – FeCl2, two new absorption peaks at 1551 and 1443 cm−1 appeared in
the FTꢀIR spectra of pyridine/[Hmim][HSO4] – 0.5FeCl2 and pyridine/[Hmim][HSO4] –
0.7FeCl2, which indicated that these ionic liquids were both Brønstedꢀ and Lewisꢀacidic.
Analogously, the strength of Lewis acids can usually be characterized using

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acetonitrile as an FTꢀIR spectroscopy probe.39–41 The nitrile group reacts with the Lewis
acid to produce a CNꢀLewis 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(Supporting Information), 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
CNꢀLewis 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 BrønstedꢀLewis acidic ionic liquids to
characterize their thermal stability. The thermogravimetric analysis is illustrated in Fig.
S8(Supporting Information). 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 Lewisꢀacidic sites FeCl2 were
successfully combined with the Brønstedꢀacidic sites [Hmim][HSO4]. Furthermore, the

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decomposition temperatures of BrønstedꢀLewis acidic ionic liquids [Hmim][HSO4] –
0.5FeCl2 and [Hmim][HSO4] – 0.7FeCl2 are 337.00 °C and 338.04 °C, respectively.
Therefore, BrønstedꢀLewis 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 BrønstedꢀLewis 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 BrønstedꢀLewis acidic
ionic liquids was 0.5 (x = 0.5), and all reactions utilized deionized water as a clean,
nonꢀcorrosive, nonꢀflammable, renewable, readily available, cheap, and environmentally
friendly solvent. As shown in Table 1, different types of BrønstedꢀLewis 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 (stirꢀreflux)
with the same [Hmim][HSO4] – 0.5FeCl2 loadings and volumes of deionized water
under reflux in a 50ꢀmL roundꢀbottom 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

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BrønstedꢀLewis 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 Brønstedꢀacidic and Lewisꢀacidic ionic liquids
exhibited a significant effect on the dehydration of chitosan to HMF due to their unique
advantages.
To examine the influence of the molar fraction of FeCl2, the bestꢀperforming
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
BrønstedꢀLewis acidic ionic liquids for the conversion of chitosan to HMF was equal to
0.5.
Table 1. Effect of different BrønstedꢀLewis acidic ionic liquids on the conversion of the
chitosan into HMF under hydrothermal conditionsa

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HMF Yield(%)
180 ◦C
20.41
35.72
12.93
13.46
44.11
1.61
Entry
Catalyst
160 ◦C
200 ◦C
13.80
16.17
3.01
1
[Hmim][HSO4]
Trace
14.51
4.94
2
[Hmim][HSO4]ꢀ 0.5 ZnCl2
[Hmim][HSO4]ꢀ 0.5 Zn(CH3COO)2
[Hmim][HSO4]ꢀ 0.5 SnCl2
[Hmim][HSO4]ꢀ 0.5 FeCl2
3
4
Trace
10.18
0.52
9.53
5
29.16
Trace
2.51
b
6
[Hmim][HSO4]ꢀ 0.5 FeCl2
7
[Hmim][HSO4]ꢀ 0.5 CrCl3
[Hmim][HSO4]ꢀ 0.5 NiCl2
[Hmim][HSO4]ꢀ 0.5 CuCl2
[Hmim][HSO4]ꢀ 0.5 CuCl
[Hmim][HSO4]ꢀ 0.5 FeCl3
[Hmim][HSO4]ꢀ 0.5 CeCl3
[Hmim]Cl
Trace
5.24
13.62
33.47
4.36
8
12.90
Trace
Trace
3.55
9
Trace
Trace
2.96
10
11
12
13
14
15
16
17
18
19
20
21
22
23
a
4.88
15.60
12.08
3.74
5.27
8.11
Trace
Trace
11.25
15.79
8.94
Trace
Trace
14.21
19.78
10.02
6.53
[Hmim]Clꢀ 0.5 FeCl2
[HSO3ꢀbꢀNꢀ(C2H5)3]Cl
[HSO3ꢀbꢀNꢀ(C2H5)3]Clꢀ 0.5 ZnCl2
[HSO3ꢀbꢀNꢀ(C2H5)3]Clꢀ 0.5 FeCl2
FeCl2
2.69
15.95
24.62
14.39
12.12
1.48
9.83
ZnCl2
Trace
Trace
6.34
Trace
Trace
26.57
Trace
2.69
NiCl2
3.57
c
[Hmim][HSO4], FeCl2
30.5
c
[Hmim]Cl, FeCl2
Trace
8.34
1.03
c
[HSO3ꢀbꢀNꢀ(C2H5)3]Cl, ZnCl2
14.46
Experiment conditions: chitosan, 50 mg; deionized water, 20 g; concentration of
BrønstedꢀLewis acidic ionic liquid aqueous solution, 1.25 wt%; treatment time, 4 h;
temperature, 160 ◦C, 180 ◦C and 200 ◦C.

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b The experiment were refluxed in 50 mL roundꢀbottom flask at 180 ◦C in the oil bath.
c
The Brønstedꢀacidic and Lewisꢀacidic as two separate catalysts in reaction,
respectively.
50
40
30
20
10
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
The molar fraction FeCl2 of the [Hmim][HSO4]-FeCl2 ( x )
Figure 3. Effect of the molar fraction (x) on HMF production under hydrothermal
conditions. Reaction conditions: chitosan, 50 mg; deionized water, 20 g; concentration
of BrønstedꢀLewis 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 Figs. 4, 5, and 6,
respectively. By varying the temperature between 140 °C and 220 °C, we established

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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.
50
44.11
40
36.13
30
20
10
0
29.16
24.18
20.66
16.35
10.18
7.32
5.44
140
150
160
170
180
190
200
210
220
Temperature (
)
Figure 4. Effect of the temperature on HMF production under hydrothermal conditions.

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Reaction conditions: chitosan, 50 mg; deionized water, 20 g; concentration of
BrønstedꢀLewis acidic ionic liquid aqueous solution, 1.25 wt%; reaction temperature,
180 °C, reaction time, 4 h.
50
44.11
40
37.35
31.13
30
27.05
22.65
19.33
20
10
0
8.35
1
2
3
4
5
6
7
Time ( h )
Figure 5. Effect of reaction time on HMF production under hydrothermal conditions.
Reaction conditions: chitosan, 50 mg; deionized water, 20 g; concentration of
BrønstedꢀLewis acidic ionic liquid aqueous solution, 1.25 wt%; reaction temperature,
180 °C.

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50
40
30
20
10
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
The centration of [Hmim][HSO4]- 0.5 FeCl2 ( wt%)
Figure 6. Effect of the different concentration of [Hmim][HSO4] – 0.5 FeCl2 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] – 0.5 FeCl2 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 sideꢀreactions, such as rehydration of HMF to levulinic acid and
crossꢀpolymerization 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

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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 interꢀ 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
ringꢀopening reaction to obtain an openꢀchain aldose. In the following sections, we
present the calculated mechanistic details of pathways I and II to understand the
synergetic effects of BrønstedꢀLewis acidity on the conversion of chitosan to HMF.
In pathway I, the subsequent ketoneꢀenol tautomerism and dehydration afford the

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fiveꢀmembered 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
bondꢀbreaking process by weakening the proximal C1–O bond and making it more
amenable to hydrolysis.25 Therefore, the openꢀchain aldose intermediate is dehydrated
to an enamine, followed by electrolytic rearrangement, deaminization, and dehydration
to form a fiveꢀmembered furan ring. Finally, HMF can be generated by further
dehydration of the fiveꢀmembered 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.

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Cl
Fe
X
H
O
X
H
O
X
H
O
H
Im
H
Im
O
X
H
Im
Cl
X
H
X
open chain form
open chain form
O
O
O
X
H
O
O
O
O
H
O
O
Pathway II
NH2
Pathway I
NH2
HO
H
NH2
O
H
X
isomerization
ꢀH2O
H
H
H
Cl
Cl
Cl
(Xꢀ: HSO4ꢀ, Clꢀ.)
X
H
O
Fe
Hydrolysis
H
Cl
Fe
O
H2N
HO
NH2
X
H
OH
O
X
X
H
O
O
Cl
O
Im
Fe
OH
O
H
X
H
Im
H
H
X
Im
H
Cl
H
X
O
O
O
H
isomerization
HN
ꢀNH
O
3
H H
O
*
OH
N
O
*
O
O
HO
HO
O
H
NH
X
O
X
O
O
n
H
H
O
H
HO
O
H
X
OH
Im
H
ꢀNH
H
3
Chitosan
isomerization
O
OH
O
O
O
O
X
H
O
HO
H
OH
O
Im H
HOH2C
O
ꢀH2O
ꢀH2O
CHO
O
O
ꢀH2O
O
ꢀH2O
HO
H
HO
H
5-HMF
HO
HO
Scheme 1. Proposed mechanism of BrønstedꢀLewis acidic ionic liquid promoted
conversion of chitosan into HMF.
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 5ꢀhydroxymethylfurfural (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

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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 largeꢀscale industrial
synthesis.
50
44.11
40
37.31
31.32
29.51
30
20
10
0
1
2
3
4
Runs Numbers
Figure 7. Recycling of [Hmim][HSO4]ꢀ 0.5 FeCl2 for the synthesis of HMF from
chitosan under hydrothermal conditions. Reaction conditions: chitosan, 50 mg;
[Hmim][HSO4]ꢀ 0.5 FeCl2, 250 mg; deionized water, 20 g; reaction temperature, 180 °C;
reaction time, 4 h.
4. Conclusion

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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 5ꢀHMF. We expect that the
present work will boost the rapid development of chitin biomass towards their novel
applications.
Acknowledgments
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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