Selective conversion of chitin to levulinic acid catalyzed by ionic liquids: Distinctive effect of: N-acetyl groups

Article abstract of DOI:10.1039/c9gc02669j

Selective and green conversion of chitin to levulinic acid (LA) has been realized by the catalysis of the ionic liquid (IL) 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ([C₃SO₃Hmim]HSO₄) up to a yield of 67.0%. The relationship between the structure of IL and the yield of LA was investigated, demonstrating that both acidity and the hydrogen bonding ability of IL dictate its catalytic activity. Furthermore, by means of NMR, IR, SEM, GPC analyses and determination of the degree of acetylation (DA), two approaches of deacetylation-depolymerization to glucosamine (GlcN) and direct depolymerization to N-acetylglucosamine (GlcNAc) were proposed as chitin hydrolysis mechanisms, which occur only at the outer surface while leaving the interior crystalline chitin intact. The N-acetyl groups in chitin construct strong intramolecular and intermolecular hydrogen bond networks that contribute to the integration of the crystalline structure and building up of barriers to shield the accessibility of glycosidic linkages with the acidic catalyst.

Full text of DOI:10.1039/c9gc02669j

Showcasing research presented by Prof. Li Liu et al. at
Dalian University of Technology, P. R. China.
As featured in:
Volume 22
Number
January 2020
Pages 1-272
1
Selective conversion of chitin to levulinic acid catalyzed by
7
Green
a greener sustainable future
ionic liquids: distinctive effect of N-acetyl groups
Chemistry
Cutting-edge research for
rsc.li/greenchem
The conversion of chitin into levulinic acid has been realized
by catalysis of ionic liquids (ILs) up to a yield of 67% in a
selective and recyclable fashion. A two-approach mechanism
was proposed in the presence of H-bonding networks
mainly contributed by the N-acetyl groups. The IL-catalyzed
methodology provides a green and simple paradigm for the
utilization of renewable chitin resources.
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Selective conversion of chitin to levulinic acid
catalyzed by ionic liquids: distinctive effect of
N-acetyl groups†
Cite this: Green Chem., 2020, 22, 62
Wuxin Hou, Qingyang Zhao and Li Liu
*
Selective and green conversion of chitin to levulinic acid (LA) has been realized by the catalysis of the
ionic liquid (IL) 1-methyl-3-(3-sulfopropyl)imidazolium hydrogen sulfate ([C3SO3Hmim]HSO4) up to a
yield of 67.0%. The relationship between the structure of IL and the yield of LA was investigated, demon-
strating that both acidity and the hydrogen bonding ability of IL dictate its catalytic activity. Furthermore,
by means of NMR, IR, SEM, GPC analyses and determination of the degree of acetylation (DA), two
approaches of deacetylation–depolymerization to glucosamine (GlcN) and direct depolymerization to
N-acetylglucosamine (GlcNAc) were proposed as chitin hydrolysis mechanisms, which occur only at the
outer surface while leaving the interior crystalline chitin intact. The N-acetyl groups in chitin construct
strong intramolecular and intermolecular hydrogen bond networks that contribute to the integration of
the crystalline structure and building up of barriers to shield the accessibility of glycosidic linkages with
the acidic catalyst.
Received 30th July 2019,
Accepted 21st October 2019
DOI: 10.1039/c9gc02669j
rsc.li/greenchem
ZnCl2 aqueous solution and obtained 5-hydroxymethylfurfural
(HMF) in 9% yield.10 Zang and co-workers adopted FeCl2 as
Introduction
As the second most abundant biomass next to cellulose, chitin the catalyst to improve the yield of HMF further to 19.3%.11 Jin
primarily originates from crustacean shells, such as those of and Yan hydrolysed chitin in NaOH solution using CuO and
crab and shrimp. There is a well-established protocol for chitin oxygen, resulting in an acetic acid (AcOH) yield of 38.1%.12
extraction from the shell waste, including demineralization,
Levulinic acid (LA) is widely recognized as one of the top
deproteinization and decolorization. Alternatively, greener twelve platform chemicals starting from biomass, which acts
technologies are being developed such as the plasma-based as a versatile building block towards the synthesis of solvents,
process and ionic liquid (IL) extraction.1–5
fuel additives, flavor substances, herbicides, pharmaceutical
In spite of enormous economic and environmental interest, agents, or resin and polymer precursors.13–23 Chitin conversion to
conversion of renewable chitin biomass has been rather LA has been initially reported under microwave irradiation.
limited due to its poor solubility. Corresponding to plant Kerton and co-workers hydrolyzed chitin by using Lewis acids at
biomass-based refinery, shell biorefinery was proposed by Yan 200 °C for 30 min, whereas SnCl4·5H2O provided LA in 22.2%
et al., suggesting that chitin holds great potential to be yield.24 Dibó and Mika hydrolyzed chitin in the presence of H2SO4
converted into chemicals.6 The recent decade has been dedi- at 190 °C for 20 min, improving the LA yield to 37.8%.25 However,
cated to improving the efficiency and selectivity of chitin con- the aforementioned acidic catalysts are difficult to recycle, and
version. For instance, by use of a simple biphasic reactor, highly selective conversion of chitin to LA remains challenging.
Maskal and Nikitin converted chitin into 5-(chloromethyl)
ILs have brought new vitality in biopolymer research start-
furfural (CMF) in 45% yield, together with levulinic acid (LA) ing from cellulose,26 due to their structural designability and
in 29% yield.7 Kerton and Yan developed the synthesis routes recyclability.27–37 Applications of ILs in chitin research are
of 3-acetamido-5-acetylfuran (3A5AF) from chitin monomer mostly focused on chitin dissolution.5,38–43 Zhao and co-
N-acetylglucosamine (GlcNAc, 60%)8 and chitin (7.5%),9 workers used HCl as the catalyst and 1-butyl-3-methyl-
respectively. Hou and co-workers hydrolysed chitin in 67 wt% imidazolium chloride ([Bmim]Cl) as the solvent to convert
chitin into total reducing sugars (TRS) of 25%.42 Reports of ILs
as catalysts for chitin conversion have been rather limited.
One example is N-methyl imidazolium hydrogen sulfate
School of Chemistry, Dalian University of Technology, Dalian 116024, China.
E-mail: lliu@dlut.edu.cn
([Mim]HSO4) used to convert chitin into HMF with a yield of
19.3%.43 To the best of our knowledge, selective conversion of
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9gc02669j
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chitin to LA by the catalysis of ILs has not been reported so
All the results were repeated at least three times. The
far. On the basis of our recent research on IL-catalyzed conver- analytical error was below 5% through comparison with stan-
sions of cellulose,44,45 lignocellulose,46 and chitosan,47 we dard samples of known concentrations. The solid residues
further explored the selective conversion of chitin to LA cata- were filtered, dried under high vacuum and weighed, whereas
lyzed by acidic ILs.48
Different from cellulose and chitosan, hydrolysis of chitin to chitin.
the yield of solid in weight percentage was calculated relative
comprises two major chemical reactions including the clea-
It should be noted that all the reactions were carried out in
vage of the glycosidic linkage on the main chain (depolymeri- a similar manner whilst varying the following parameters:
zation) and deacetylation on the side chain.7,8,10,12,49,50 The reaction temperature and time, water amount, IL dosage and
existing mechanisms of chitin hydrolysis mainly embrace (i) chitin input. The amount of water was optimized using itera-
selective depolymerization without deacetylation51–54 and (ii) tive methods, in combination with reaction temperature and
simultaneous depolymerization and deacetylation.55 Under time. Under the optimized conditions derived as above, the
both circumstances, the chitin structures fall apart, and the effects of IL dosage and chitin input were investigated.
polymeric chains are cleaved into oligomers at random. We
IL recycling and LA separation
report here a different mechanism of chitin hydrolysis, demon-
strating the distinctive effect of N-acetyl groups on the mecha-
nism in comparison with chitosan.
For the first cycle, chitin (250 mg), deionized water (6.000 g),
and [C3SO3Hmim]HSO4 (1.000 g) were heated in a Teflon-lined
autoclave at 180 °C for 5 h. After the solid residues were fil-
tered, the aqueous IL solution was extracted by methyl isobutyl
ketone (MIBK) 60 mL × 3 and then dried under high vacuum.
Over 95% of the IL can be recovered after extraction.
Afterwards, the IL was adjusted to 1.000 g (3.31 mmol) by
adding fresh IL of no more than 5% for subsequent cycles,
whereas 1-naphthalenesulfonic acid was used as the standard
to determine the amount of recycled IL by 1H NMR. IL re-
cycling was also tested for acidity loss after supplementation
with 1 equiv. of H2SO4. The LA product was isolated by evapor-
ation of MIBK, which was confirmed by NMR and mass spec-
trometry (MS) (see the ESI†).
Experimental
Materials and equipment
Chitin was purchased from TCI (Shanghai, China). Chitosan
was supplied by Beijing Solarbio Science & Technology Co., Ltd
(Beijing, China). Six ionic liquids including 1-methyl-3-(3-sul-
fopropyl)imidazolium hydrogen sulfate ([C3SO3Hmim]HSO4),
1-methyl-3-(3-sulfopropyl)imidazolium
methanesulfonate
([C3SO3Hmim]CH3SO3), 1-methyl-3-(3-sulfopropyl)imidazolium
phenylsulfonate ([C3SO3Hmim]PhSO3), 1-methyl-3-(3-sulfopro-
pyl)imidazolium 1-naphthalenesulfonate ([C3SO3Hmim]1-NS),
1-methyl-3-(3-sulfopropyl)imidazolium dihydrogen phosphate
([C3SO3Hmim]H2PO4) and 1-methyl-3-(3-sulfopropyl)imidazo-
lium chloride ([C3SO3Hmim]Cl) were synthesized according to
the literature,46 and characterized by NMR prior to use (see the
ESI†). The NMR spectra were recorded on a Bruker Avance II
400 MHz spectrometer. The Fourier transform infrared (FT-IR)
spectra were recorded on a Bruker Equinox 55 infrared spectro-
meter. The scanning electron microscopy (SEM) images were
obtained by using a NOVA NanoSEM 450 microscope.
Gas chromatography (GC)-MS analysis
The qualitative analysis of LA was conducted by GC-MS using
an Agilent 7890A gas chromatograph equipped with an Agilent
J&W 123-0732 capillary column (30 m × 0.32 mm × 0.25 mm),
as well as an MS triple quadrupole detector. The GC tempera-
ture program was applied: an initial temperature of 100 °C for
2 min, followed by heating to 210 °C at a rate of 10 °C min−1
and finally at 210 °C for 5 min. Mass spectrometric measure-
ment was performed by using electron impact ionization (EI)
at 70 eV and a scan range of m/z 30–500.
,
General procedure for chitin and chitosan conversion
Gel permeation chromatography (GPC) analyses
A mixture of chitin (or chitosan), deionized water and IL was
heated in a Teflon-lined autoclave that was preheated to a
given temperature under magnetic stirring. The reaction
mixture was analyzed by H NMR with IL as the internal stan-
dard, whereas H2, CH3 and Hc/d were used for integrations of
The molecular weights were analyzed by a GPC system
equipped with a Waters 2414 refractive index detector, a
Waters 1525 HPLC pump, and an Agilent PLgel 5 μm MIXED-C
column, using LiCl/DMF (0.1 mol L−1) as the eluent at a flow
rate of 1.0 mL min−1 at 35 °C. The samples (2 mg mL−1) were
dissolved in the same mobile phase solution through rigorous
stirring for one day. The raw data were calibrated using narrow
polydispersity polystyrene (PS) standards.
1
LA, AcOH and IL, respectively (Fig. S2†), according to our pre-
1
vious procedure.47 The H NMR spectra were attained at 296
0.5 K, using 12 μs pulse width and 1 s delay time. A total of 32
scans were collected. All spectra were referenced relative to
residual HOD at 4.79 ppm. The yields of LA and AcOH were
calculated from the following equations:
Measurement of the degree of acetylation (DA)
The chitin and chitosan samples were dissolved in 8 wt% of
NaOH/4 wt% of urea (D2O, −30 °C)56 and 1% DCl/D2O solu-
tions,47 respectively. According to a published protocol using
YLA ðmol%Þ ¼ ðmol of LAÞ=ðmol of chitinÞ ꢀ 100%
YAcOH ðmol%Þ ¼ ðmol of AcOHÞ=ðmol of chitinÞ ꢀ 100%
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the 1H NMR method,55 the DA values were calculated from the
following equation:
DA% ¼ 2HAc=H2 to 6 ꢀ 100%
wherein HAc is the integration of the peak ascribed to the
acetyl group and H2
is the integration of the peaks
6
to
assigned to the protons on C2 to C6.
Quantification of GlcNAc and glucosamine (GlcN) intermediates
The yields of GlcNAc and GlcN were calculated from the follow-
ing equations:
YGlcNAc ðmol%Þ ¼ ðmol of GlcNAcÞ=ðmol of chitinÞ ꢀ 100%
YGlcN ðmol%Þ ¼ ðmol of GlcNÞ=ðmol of chitinÞ ꢀ 100%
GlcNAc and GlcN intermediates were quantified by using a
Dionex ICS-5000 ion chromatography system equipped with an
IPAD detector and a CarboPac PA20 column (30 °C), whereas
an aqueous solution of NaOH (20.0 mM) at a flow rate of
0.45 mL min−1 was used as the mobile phase. Identification
and quantification were done by analysis of the standard
samples with known concentrations.
■
Fig. 1 (a) The effects of reaction temperature and time: ( ) 170 °C,
(
Results and discussion
) 180 °C, and ( ) 190 °C (250 mg of chitin, 1.000 g of [C3SO3Hmim]
The effects of reaction conditions on chitin conversion
HSO4, and 6.000 g of H2O); (b) chitin conversion to LA at different reac-
tion temperatures (5 h, 250 mg of chitin, 1.000 g of [C3SO3Hmim]HSO4,
and 6.000 g of H2O).
In comparison with chitosan, chitin conversion is more
difficult and higher temperature is required. At 170 °C which
is the optimum temperature for chitosan, chitin conversion to
LA was incomplete and only 40.4% of LA yield was obtained
after 5 h. As shown in Fig. 1a, as the reaction temperature
Water plays a vital role in promoting the conversion of
increased from 170 to 190 °C, the yield of LA was enhanced chitin to LA. As depicted in Fig. 2a, LA could not be obtained
initially, and reached plateaus of 56.5% and 53.8% at 180 and in the absence of water. When the amount of water was
190 °C, respectively. During the conversion, a dark colored increased from 20.1 wt% to 82.7 wt%, the LA yield ascended
solid known as humins was inevitably formed, and the yield of from 20.5% to 56.5%. In contrast, the yield of solid decreased
solid residues in weight percentage was calculated relative to from 21.8% to 4.0%, indicating that the dilution effect inhibits
chitin input. When the temperature was further increased up humin formation and thus promotes the production of LA.
to 210 °C (Fig. 1b), the yield of LA continued to decrease down When the amount of water was further increased from
to 48.4% whereas the yield of humins increased slightly to 82.7 wt% to 92.3 wt%, the yields of LA and solid both pla-
6.2%. Hence, higher temperature favors the formation of teaued until 86.5 wt%, and then the yield of LA descended
humins57 but decreases the yield of LA. To suppress the for- markedly to 32.2% and the yield of solid increased to 42.4%,
mation of humins, a lower reaction temperature was adopted. suggesting that excessive dilution, i.e., too low IL concen-
However, a much higher yield of solid (21.8%) was obtained at tration, is not efficient for the complete conversion of chitin to
170 °C, which was ascribed to the relatively slower and incom- LA.
plete chitin conversion at too low temperatures. Therefore, the
reaction temperature was kept at 180 °C for chitin conversion which LA could not be produced (Fig. 2b). Increasing the IL
to LA.
dosage led to an increase in LA yield, whereas the yield of LA
IL is essential to catalyse chitin conversation to LA, without
Notably, the decreased yield of LA can be ascribed to either was maximized at 56.5% in the presence of 13.8 wt% of IL and
the incomplete conversion of chitin feedstock or the increase plateaued out with an IL dosage above 13.8 wt%. Meanwhile,
of humins. Under both circumstances, the yield of solid the yield of solid decreased and stabilized at ca. 4% with an IL
increased. Characterisation of the solid residues by IR spec- dosage above 13.8 wt%, confirming that too low IL concen-
troscopy was helpful to distinguish whether the residues were tration is insufficient for complete chitin conversion. This is
unconverted chitin polymers or humins predominantly; the consistent with the water effect. Therefore, 13.8 wt% of IL was
process is elucidated in the characterisation section.
adopted in the conversion system.
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ization to humins on the basis of complete conversion of
chitin.
A similar dilution effect was observed for cellulose in which
the LA yield decreased slightly from 63.0% to 52.4% with the
cellulose input increasing from 3.4 wt% to 13.6 wt%.45
However, in the case of chitin, the yield of LA decreased
abruptly from 56.5% (at a chitin input of 3.4 wt%) to 29.6% (at
a chitin input of 12.5 wt%), disclosing the overloading of
chitin feedstock in addition to the dilution effect. As discussed
in the subsequent section, this hypothesis could be supported
by our results on IL recycling.
Recycling of the IL
Compared with inorganic acid catalysts, IL is characteristic of
its recyclability. Upon chitin conversion, MIBK proved to be
the optimum extractant for the LA product, with an extraction
efficiency of up to 98%.58 The recyclability of IL was demon-
strated through phase separation.
Unexpectedly, as illustrated in Fig. 3, continued cycles show
a downward trend of LA yield from 56.5% to 48.5%, 46.0%,
40.4%, and 34.9%. It is assumed that the reduced catalytic
activity could be ascribed to NH3 elimination during chitin
conversion leading to a decrease in IL acidity. Although NH3
elimination was proposed during chitin conversion in the
literature,11,12,24,25 attempts to detect NH3 in the reaction
mixture have been unsuccessful. During our investigation,
characteristic triplet peaks appeared on the 1H NMR spectra of
the reaction mixture (Fig. S2†), which are attributed to NH4+ by
comparison with the standard NH4HSO4. Thus, NH3 elimin-
ation has been unambiguously confirmed during chitin con-
version. Furthermore, to offset the acidity loss, 1 equiv. of
H2SO4 was supplemented to the reused IL, whereas the yield of
LA did not decrease noticeably over five cycles (Fig. 3). For the
same reason, the aforementioned overloading phenomenon at
higher chitin input could be attributed to the larger amount of
NH3 formed during chitin conversion, causing a significant
Fig. 2 The effects of (a) H2O amount (180 °C, 5 h, 250 mg of chitin,
and 1.000
g of [C3SO3Hmim]HSO4), (b) [C3SO3Hmim]HSO4 dosage
(180 °C, 5 h, 250 mg of chitin, and 6.000 g H2O), and (c) chitin input
(180 °C, 5 h, 1.000 g of [C3SO3Hmim]HSO4 and 6.000 g of H2O) on the
▶
yields of ( ) LA and ( ) solid.
As depicted in Fig. 2c, less chitin input resulted in a
remarkable increase of LA yield up to 67.0% with a chitin
input of 0.7 wt%. Conversely, the yield of solid kept decreasing
with less chitin input. Compared to modulating the para-
meters of water and IL to improve the LA yield, a decrease of
chitin input further inhibits the yield of humins to below 2%,
thereby demonstrating a more pronounced dilution effect than
that by increasing the amount of water. This is attributed to
the fact that intermediate concentration could be lowered
down more efficiently without the consequences of too low IL
Fig. 3 The reuse of IL. Reaction conditions: 180 °C, 5 h, 250 mg of
concentration, which suppressed the intermolecular polymer- chitin, 1.000 g of [C3SO3Hmim]HSO4, and 6.000 g of H2O.
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decrease in IL acidity and resulting in a steep decline in LA
yield.
The effect of IL structures on chitin conversion
To gain insight into the catalytic mechanism of IL, six ILs were
applied for chitin conversion, whose Brønsted acidities were
evaluated by using the Hammett acidity function (H0) with
4-nitroaniline as the basic indicator.45 As shown in Table 1,
the acidities of ILs were found to follow the sequence of
−
−
−
HSO4 > PhSO3 ∼ CH3SO3 > 1-NS > H2PO4−. As a result, the
stronger the acidity of IL, the higher the LA yield was achieved,
highlighting the essential role of acid catalysis in the process
of chitin conversion.
Surprisingly, when [C3SO3Hmim]Cl was tested, 54.0% yield
of LA was achieved, which is abnormally high and inconsistent
with the acidity sequence. This could be attributed to the
stronger H-bonding ability of Cl−, which contributes to the
breaking of the H-bonding network amongst chitin, enhancing
the accessibility of the acidic catalyst to chitin and thereby
improving the catalytic activity of IL.45
However, for cellulose, [C3SO3Hmim]Cl gave rise to an even
higher LA yield than [C3SO3Hmim]HSO4.45 It can be learnt
from our results on IL recycling during chitin conversion that
deamination causes a decrease in IL acidity. Therefore, it was
speculated that the yield of LA catalysed by 3.31 mmol of
[C3SO3Hmim]Cl may not reach its maximum yield. After
increasing the dosage of [C3SO3Hmim]Cl to 4.97 mmol, the
yield of LA for chitin conversion indeed increased to 61.5%
and then plateaued out. Hence, taking the acidity loss into
consideration, the Cl− proved to have a definitely prominent
effect on chitin conversion that could surpass [C3SO3Hmim]
HSO4, despite the fact that the acidity of [C3SO3Hmim]Cl is
relatively lower than [C3SO3Hmim]HSO4.
Fig. 4 Time course of the solid residues. Reaction conditions: 180 °C,
250 mg of chitin, 6.000 g of H2O, 1.000 g of [C3SO3Hmim]HSO4.
ture was maintained. After reaction for 4 h, the solid residues
turned from grey into dark and were stabilized at 11 mg, sug-
gestive of mainly leftover humins, which was further sup-
ported by IR and SEM analyses.
According to the FT-IR spectroscopy results (Fig. 5), the
α-chitin feedstock was characteristic of the amide I band split
at 1660 and 1626 cm−1, which have been assigned to the
stretching vibrations of CvO forming an H-bond to N–H on
the adjacent chain and the CvO bifurcation with an
additional H-bond to the primary O–H on the same chain,
respectively.39,53,56 Meanwhile, 1558 cm−1 was ascribed to the
amide II band (stretching vibrations of C–N and bending
vibrations of N–H).
After reaction for 0.5 h, the unchanged amide I bands at
1660 (CvO⋯H–N) and 1626 cm−1 (CvO⋯H–O and CvO⋯H–
N) convinced that the chitin structure was well preserved at the
early stage. Furthermore, starting from the fibrous surface
(Fig. 6a), a smooth surface shows up on the SEM image after
0.5 h (Fig. 6b and c), indicating the limited accessibility of gly-
cosidic bonds in the interior of the chitin structure at the
micrometer level. This was distinctly different from the porous
surface morphology in the cases of cellulose and chitosan, in
Characterisation of the solid residues
In the process of chitin conversion, insoluble solid residues
were obtained, which could provide real-time monitoring of
the reaction pathway. As depicted in Fig. 4, the solid residues
gradually decreased and levelled off at 4 h. The 1H NMR
spectra (Fig. S3†) remained basically the same as that of chitin
feedstock until 3 h, suggesting that the chitin skeletal struc-
Table 1 Conversion of chitin to LA catalyzed by ILsa
Amax [I]b (%) [IH+]b (%) H0
YLA (mol%)
c
IL
—
0.426 100
0
41
34
34
17
15
26
—
0
[C3SO3Hmim]HSO4
[C3SO3Hmim]CH3SO3 0.283
[C3SO3Hmim]PhSO3
[C3SO3Hmim]1-NS
[C3SO3Hmim]H2PO4
[C3SO3Hmim]Cl
0.252
59
66
66
83
85
74
1.15 56.5
1.28 50.8
1.28 51.6
1.68 49.0
1.74 5.4
0.283
0.352
0.364
0.316
1.44 54.0
a Reaction conditions: 180 °C, 5 h, 250 mg of chitin, 3.31 mmol IL,
6.000 g of H2O. b IH+ and I refer to the protonated and unprotonated
forms of 4-nitroaniline, respectively. c H0 = pK(I)aq + log([I]/[IH+]) (for
4-nitroaniline, pK(I)aq = 0.99).
Fig. 5 IR spectra of the solid residues. Reaction conditions: 180 °C,
250 mg of chitin, 6.000 g of H2O, 1.000 g of [C3SO3Hmim]HSO4.
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Fig. 7 (a) Mw, Mn, and PDI during chitin conversion, (b) Mn vs. conver-
sion for chitin, DA curves of chitosan (c) and chitin (d). Conditions:
250 mg of chitin or chitosan, 6.000
[C3SO3Hmim]HSO4, 180 °C.
g of H2O, and 1.000 g of
Fig. 6 SEM images of the solid residues after the conversion of chitin at
180 °C for (a) 0 h, (b) 0.5 h, (c) 1 h, (d–e) 3 h, (f–g) 4 h, (h–i) 5 h.
Reaction conditions: 250 mg of chitin, 6.000 g of H2O, 1.000 g of
[C3SO3Hmim]HSO4.
be explained since the emerging low-MW parts affect Mn more
remarkably than Mw.
which the glycosidic bonds were accessible throughout the
structure.
Unexpectedly, even for the solid residues at 3 h whereas
only 24.6 mg of solid residues were left and the conversion was
close to completion, high molecular weights of 213 000 (Mw)
and 35 000 (Mn) were determined. Thereby, it can be inferred
that chitin depolymerization does not proceed at random. Due
to the massive hydrogen bond networks, the interior of the
chitin crystal structure was retained, while the depolymeriza-
tion occurs only at the outer surface. Throughout the conver-
sion process of chitin feedstock, the inclination of Mn decrease
becomes steeper gradually, supportive of chitin depolymeriza-
tion in a stepwise manner (Fig. 7b).
After reaction for 3 h, the IR spectra show two new bands at
1699 and 1612 cm−1
, corresponding to the stretching
vibrations of the CvO and CvC groups, respectively, which
are the characteristics of humins as reported.45,59 However, 3 h
is solely the starting time point for humin formation but not
the ending time point for chitin conversion. The amide I
bands at 1660 (CvO⋯H–N) and 1626 cm−1 (CvO⋯H–O and
CvO⋯H–N) gradually weakened after 4 h and vanished after
5 h along with the amide II band at 1558 cm−1, suggesting that
5 h is more likely the ending time point for chitin conversion.
Correspondingly, the SEM images show only a few isolated
spherical particles smaller than 1 μm after 3 h (Fig. 6d and e),
Chitin hydrolysis mechanism
which then grow both in size and amount after 4 h (Fig. 6f and g) Here a question arises: what is the mechanism of chitin hydro-
until mainly agglomeration without the continuous surface lysis like, in the presence of hydrogen bond networks? The
underneath after 5 h (Fig. 6h and i). Thereby, the SEM results are degree of acetylation (DA) signifies the deacetylation level in
consistent with the IR analyses, verifying that humins appeared a relation to depolymerization during the hydrolysis process,
little bit after 3 h and became widespread after 4 h, with chitin which can be calculated from the integrals on the 1H NMR
disappearing until 5 h.
spectra (Fig. S3†). For IL-catalyzed chitosan hydrolysis (Fig. 7c),
Hence, from the beginning to 3 h, the solid residues are the DA value is reduced quickly to zero after 20 min, whereas
basically unconverted chitin polymers, whereas humins could only 9.6% of chitosan feedstock has been converted,
be neglected. In order to demonstrate how chitin depolymeri- suggesting that deacetylation becomes prone to completion
zation proceeds straightforwardly, the molecular weights once the polymeric structure falls apart.47
(MWs) of the solid residues were analysed by GPC and calcu-
From chitosan to chitin, the DA value of the feedstock
lated. As shown in Fig. 7a, the polydispersity (PDI) broadens increases, giving rise to stronger intra- and intermolecular
during the reaction, implying that more and more low-MW hydrogen bonding interactions. Upon comparison, it can be
parts come into being. Moreover, the number-average mole- seen from the DA plot of chitin (Fig. 7d) that the DA values
cular weight (Mn) decreased from the beginning 83 000 to nearly remained constant until 3 h, with an average value of
35 000 at 3 h, verifying the ongoing depolymerization process. 76.1%. Not taking into account the inert acetyl groups in the
However, the weight-average molecular weight (Mw) decreased interior of the chitin crystal structure, the invariable DA values
slightly from the beginning 258 000 to 213 000 at 3 h. This can during chitin conversion suggest either depolymerization
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without deacetylation or equivalent deacetylation/depolymeri- intermediate verified the first approach of depolymerization
zation at the outer surface. without deacetylation. After 13 min, GlcN and acetic acid
Undoubtedly, the intermediates provide direct evidence of remained, whereby GlcNAc presumably deacetylated into GlcN
the depolymerization mechanism, which indicated that both as well and could be barely observed. GlcN increased to reach
the aforementioned approaches exist. For IL-catalyzed chitin the maximum at 30 min, then decreased and disappeared
hydrolysis, it is relatively difficult for depolymerization reaction after 4 h, whereas LA emerged after 30 min due to GlcN trans-
to occur, as chitin does not depolymerize at all at 80 °C for formation. As a deacetylation product, acetic acid increased
5 h. According to the 1H NMR spectrum, the characteristic steeply after the turning point at 12 min; however, it is syn-
resonance signal of acetic acid was observed alone, which was chronous with the depolymerization products of GlcN and LA,
reasonable taking into account that GlcNAc could deacetylate and levelled out after 4 h, justifying the second approach of
even at a lower temperature of 40 °C. Likewise, with the temp- equivalent deacetylation/depolymerization. Thus, for the IL-
erature increased to 180 °C, from the quantitative product ana- catalyzed chitin hydrolysis, there are two approaches of depoly-
lyses (Fig. 8) and 1H NMR spectra (Fig. S4†), the earliest merization: (i) direct depolymerization to the GlcNAc
product emerging at 10 min was acetic acid from deacetyla- monomer and (ii) deacetylation followed by depolymerization
tion, thus confirming that deacetylation occurs easier than to release the GlcN monomer.
depolymerization. Subsequently, both GlcNAc and GlcN inter-
Apparently, the depolymerization mechanism of chitin is
mediates came into being at 12 min. The detection of GlcNAc distinctly different from that of chitosan, highlighting the
effect of the N-acetyl group which is essential to form
stronger hydrogen bond networks. Novak and co-workers also
pointed out the disturbed crystal structure and the weakened
hydrogen bonds by N-deacetylation of chitin.60 It is well known
that the CvO groups in chitin can serve as acceptors for both
the primary O–H groups of the same chain and the N–H
groups of the adjacent chain.39,50,56 As proposed in Fig. 9,
these hydrogen bonds help to hold the crystalline structure
together and build up barriers to prevent the hydrolysis of the
β-(1–4)-linkage from the acidic catalyst at the molecular
level.50,53,61 For these acetyl groups protruding outwards, due
to easy accessibility of the acidic catalyst, the –GlcNAc unit
first deacetylates to transform into the GlcN unit. After
removal of the acetyl group to break its hydrogen bonding, the
β-(1–4)-linkage was exposed to a catalytic attack. Secondly, the
unshielded –GlcN unit dehydrates from the polymeric chain.
On the other hand, for those acetyl groups whose hydrogen
Fig. 8 Product analyses during chitin conversion. Conditions: 250 mg
of chitin, 6.000 g of H2O, and 1.000 g of [C3SO3Hmim]HSO4, 180 °C.
bonds are unable to block the hydrolysis of the β-(1–4)-linkage
Fig. 9 Two proposed approaches of the chitin depolymerization mechanism.
68 | Green Chem., 2020, 22, 62–70
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spatially, the corresponding –GlcNAc unit could be cleaved
directly without resorting to deacetylation first to break the
hydrogen bond barriers. Given that oligomers were not
detected in solution, if the internal β-(1–4)-linkage was cleaved
by either means rather than the terminal linkage, the oligo-
mers were assumed to be still attached to the hydrogen bond
networks, as reflected by the GPC analysis.
Such a depolymerization process is then continued on the
outer surface until the chitin feedstock is converted comple-
tely, whereas deacetylation–depolymerization or direct depoly-
merization affords GlcN and GlcNAc monomers, respectively,
leaving the interior crystalline chitin intact. Meanwhile,
3 P. Beaney, J. Lizardi-Mendoza and M. Healy, J. Chem.
Technol. Biotechnol., 2005, 80, 145–150.
4 M. Boric, H. Puliyalil, U. Novak and B. Likozar, Green
Chem., 2018, 20, 1199–1204.
5 J. L. Shamshina, Green Chem., 2019, 21, 3974–3993.
6 N. Yan and X. Chen, Nature, 2015, 524, 155–157.
7 M. Mascal and E. B. Nikitin, ChemSusChem, 2009, 2, 859–
861.
8 M. W. Drover, K. W. Omari, J. N. Murphy and F. M. Kerton,
RSC Adv., 2012, 2, 4642–4644.
9 X. Chen, S. L. Chew, F. M. Kerton and N. Yan, Green Chem.,
2014, 16, 2204–2212.
GlcNAc could deacetylate into GlcN. The resultant glucosam- 10 Y. Wang, C. M. Pedersen, T. Deng, Y. Qiao and X. Hou,
monium dehydrates and deaminizes to HMF and subsequently
LA through rehydration.50
Bioresour. Technol., 2013, 143, 384–390.
11 S. Yu, H. Zang, S. Chen, Y. Jiang, B. Yan and B. Cheng,
Polym. Degrad. Stab., 2016, 134, 105–114.
12 X. Gao, X. Chen, J. Zhang, W. Guo, F. Jin and N. Yan, ACS
Sustainable Chem. Eng., 2016, 4, 3912–3920.
13 D. W. Rackemann and W. O. S. Doherty, Biofuels, Bioprod.
Biorefin., 2011, 5, 198–214.
14 S. M. Sen, D. M. Alonso, S. G. Wettstein, E. I. Gurbuz,
C. A. Henao, J. A. Dumesic and C. T. Maravelias, Energy
Environ. Sci., 2012, 5, 9690–9697.
15 F. D. Pileidis and M.-M. Titirici, ChemSusChem, 2016, 9,
562–582.
16 S. Li, Y. Wang, Y. Yang, B. Chen, J. Tai, H. Liu and B. Han,
Green Chem., 2019, 21, 770–774.
17 Y. Shao, K. Sun, Q. Li, Q. Liu, S. Zhang, Q. Liu, G. Hu and
X. Hu, Green Chem., 2019, 21, 4499–4511.
18 Y. Bernhard, L. Pagies, S. Pellegrini, T. Bousquet,
A. Favrelle, L. Pelinski, P. Gerbaux and P. Zinck, Green
Chem., 2019, 21, 123–128.
Conclusions
We have demonstrated a selective route to convert chitin
biomass to LA up to 67% by the catalysis of ILs. The effect of
IL structures on chitin conversion has been investigated,
whereas both acidity and the hydrogen bonding ability of ILs
exert significant influences on the yield of LA. Herein, the
N-acetyl groups of chitin construct strong intramolecular and
intermolecular hydrogen bond networks that contribute to the
integration of the crystalline structure and building up of the
barriers to shield the accessibility of glycosidic linkages with
the acidic catalyst, resulting in two approaches of deacetyla-
tion–depolymerization to GlcN and direct depolymerization to
GlcNAc. While for chitosan, strong hydrogen bond networks
are not formed due to low DA values, hence the accessibility of
acidic catalysts to chitosan was enhanced, causing the poly-
meric structure ready to fall apart and be cleaved into oligo-
mers irregularly. The demonstration of the chitin depolymeri-
zation mechanism is deemed to promote the in-depth under-
standing and thereby offer new insight into the utilization of
renewable chitin resources.
19 J. J. Bozell, Science, 2010, 329, 522–523.
20 J. Q. Bond, D. M. Alonso, D. Wang, R. M. West and
J. A. Dumesic, Science, 2010, 327, 1110–1114.
21 S. Van de Vyver, J. Thomas, J. Geboers, S. Keyzer, M. Smet,
W. Dehaen, P. A. Jacobs and B. F. Sels, Energy Environ. Sci.,
2011, 4, 3601–3610.
22 R. Weingarten, W. C. Conner and G. W. Huber, Energy
Environ. Sci., 2012, 5, 7559–7574.
23 S. G. Wettstein, D. M. Alonso, Y. X. Chong and
J. A. Dumesic, Energy Environ. Sci., 2012, 5, 8199–8203.
24 K. W. Omari, J. E. Besaw and F. M. Kerton, Green Chem.,
2012, 14, 1480–1487.
Conflicts of interest
There are no conflicts to declare.
25 Á. Szabolcs, M. Molnár, G. Dibó and L. T. Mika, Green
Chem., 2013, 15, 439–445.
Acknowledgements
26 R. P. Swatloski, S. K. Spear, J. D. Holbrey and R. D. Rogers,
J. Am. Chem. Soc., 2002, 124, 4974–4975.
27 A. Brandt, J. Grasvik, J. P. Hallett and T. Welton, Green
Chem., 2013, 15, 550–583.
This project was supported by the Fundamental Research
Funds for the Central Universities (DUT18JC16).
28 C. Li and Z. K. Zhao, Adv. Synth. Catal., 2007, 349, 1847–
1850.
Notes and references
1 H. K. No, S. P. Meyers and K. S. Lee, J. Agric. Food Chem., 29 H. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang, Science,
1989, 37, 575–579. 2007, 316, 1597–1600.
2 F. Shahidi and J. Synowiecki, J. Agric. Food Chem., 1991, 39, 30 R. Rinaldi, R. Palkovits and F. Schueth, Angew. Chem., Int.
1527–1532.
Ed., 2008, 47, 8047–8050.
This journal is © The Royal Society of Chemistry 2020
Green Chem., 2020, 22, 62–70 | 69

View Article Online
Paper
Green Chemistry
31 J. B. Binder and R. T. Raines, J. Am. Chem. Soc., 2009, 131, 47 W. Hou, L. Liu and H. Shen, Carbohydr. Polym., 2018, 195,
1979–1985. 267–274.
32 M. E. Zakrzewska, E. Bogel-Łukasik and R. Bogel-Łukasik, 48 A. C. Cole, J. L. Jensen, I. Ntai, K. L. T. Tran, K. J. Weaver,
Chem. Rev., 2011, 111, 397–417.
33 A. M. da Costa Lopes and R. Bogel-Łukasik, ChemSusChem,
2015, 8, 947–965.
34 M. Grilc, B. Likozar and J. Levec, Catal. Today, 2015, 256,
302–314.
35 A. S. Amarasekara, Chem. Rev., 2016, 116, 6133–6183.
D. C. Forbes and J. H. Davis, J. Am. Chem. Soc., 2002, 124,
5962–5963.
49 X. Chen, Y. Gao, L. Wang, H. Chen and N. Yan,
ChemPlusChem, 2015, 80, 1565–1572.
50 X. Chen, H. Yang and N. Yan, Chem. – Eur. J., 2016, 22,
13402–13421.
36 Z. Zhang, J. Song and B. Han, Chem. Rev., 2017, 117, 6834– 51 M. Yabushita, H. Kobayashi, K. Kuroki, S. Ito and
6880. A. Fukuoka, ChemSusChem, 2015, 8, 3760–3763.
37 L. T. Mika, E. Cséfalvay and A. Németh, Chem. Rev., 2018, 52 H. Kobayashi, K. Techikawara and A. Fukuoka, Green
118, 505–613. Chem., 2017, 19, 3350–3356.
38 H. Xie, S. Zhang and S. Li, Green Chem., 2006, 8, 630– 53 G. Margoutidis, V. H. Parsons, C. S. Bottaro, N. Yan and
633. F. M. Kerton, ACS Sustainable Chem. Eng., 2018, 6, 1662–1669.
39 Y. Wu, T. Sasaki, S. Irie and K. Sakurai, Polymer, 2008, 49, 54 E. Husson, C. Hadad, G. Huet, S. Laclef, D. Lesur,
2321–2327.
V. Lambertyn, A. Jamali, S. Gottis, C. Sarazin and
A. Nguyen Van Nhien, Green Chem., 2017, 19, 4122–4131.
55 X. Chen, H. Yang, Z. Zhong and N. Yan, Green Chem., 2017,
19, 2783–2792.
40 Y. Qin, X. Lu, N. Sun and R. D. Rogers, Green Chem., 2010,
12, 968–971.
41 P. S. Barber, C. S. Griggs, G. Gurau, Z. Liu, S. Li, Z. Li,
X. Lu, S. Zhang and R. D. Rogers, Angew. Chem., Int. Ed., 56 Y. Fang, R. Zhang, B. Duan, M. Liu, A. Lu and L. Zhang,
2013, 52, 12350–12353. ACS Sustainable Chem. Eng., 2017, 5, 2725–2733.
42 Z. Zhang, C. Li, Q. Wang and Z. K. Zhao, Carbohydr. Polym., 57 B. Girisuta, L. P. B. M. Janssen and H. J. Heeres, Ind. Eng.
2009, 78, 685–689. Chem. Res., 2007, 46, 1696–1708.
43 M. Li, H. Zang, J. Feng, Q. Yan, N. Yu, X. Shi and B. Cheng, 58 L. C. Nhien, N. V. D. Long, S. Kim and M. Lee, Ind. Eng.
Polym. Degrad. Stab., 2015, 121, 331–339. Chem. Res., 2016, 55, 5180–5189.
44 H. Ren, Y. Zhou and L. Liu, Bioresour. Technol., 2013, 129, 59 C. Falco, N. Baccile and M. M. Titirici, Green Chem., 2011,
616–619. 13, 3273–3281.
45 H. Ren, B. Girisuta, Y. Zhou and L. Liu, Carbohydr. Polym., 60 B. Bradic, D. Bajec, A. Pohar, U. Novak and B. Likozar,
2015, 117, 569–576. React. Chem. Eng., 2018, 3, 920–929.
46 L. Liu, Z. Li, W. Hou and H. Shen, Carbohydr. Polym., 2018, 61 M. Petrov, L. Lymperakis, M. Friak and J. Neugebauer,
181, 778–784.
Biopolymers, 2013, 99, 22–34.
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