Br?nsted acidic ionic liquids for cellulose hydrolysis in an aqueous medium: Structural effects on acidity and glucose yield

Article abstract of DOI:10.1039/c8ra01950a

The conversion of cellulose into valuable chemicals has attracted much attention, due to the concern about depletion of fossil fuels. The hydrolysis of cellulose is a key step in this conversion, for which Br?nsted acidic ionic liquids (BAILs) have been considered promising acid catalysts. In this study, using BAILs with various structures, their acidic catalytic activity for cellulose hydrolysis assisted by microwave irradiation was assessed using the Hammett acidity function (H₀) and theoretical calculations. The glucose yields exceeded 10% when the H₀ values of the BAIL aqueous solutions were below 1.5. The highest glucose yield was about 36% in 1-(1-octyl-3-imidazolio)propane-3-sulfonate (Oimps)/sulfuric acid (H₂SO₄) aqueous solution. A long alkyl side chain on the imidazolium cation, which increased the hydrophobicity of the BAILs, enhanced the glucose yield.

Full text of DOI:10.1039/c8ra01950a

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Brønsted acidic ionic liquids for cellulose hydrolysis
in an aqueous medium: structural effects on acidity
and glucose yield†
Cite this: RSC Adv., 2018, 8, 14623
Shiori Suzuki,‡ Yuko Takeoka,
Masahiro Rikukawa
and Masahiro Yoshizawa-
*
Fujita
The conversion of cellulose into valuable chemicals has attracted much attention, due to the concern about
depletion of fossil fuels. The hydrolysis of cellulose is a key step in this conversion, for which Brønsted acidic
ionic liquids (BAILs) have been considered promising acid catalysts. In this study, using BAILs with various
structures, their acidic catalytic activity for cellulose hydrolysis assisted by microwave irradiation was
assessed using the Hammett acidity function (H0) and theoretical calculations. The glucose yields
exceeded 10% when the H0 values of the BAIL aqueous solutions were below 1.5. The highest glucose
yield was about 36% in 1-(1-octyl-3-imidazolio)propane-3-sulfonate (Oimps)/sulfuric acid (H2SO4)
aqueous solution. A long alkyl side chain on the imidazolium cation, which increased the hydrophobicity
of the BAILs, enhanced the glucose yield.
Received 5th March 2018
Accepted 7th April 2018
DOI: 10.1039/c8ra01950a
rsc.li/rsc-advances
ILs are known as green solvents for cellulose-related applica-
tions. In addition, there is a high degree of exibility in their
1. Introduction
design, and various types of ILs with tailored functionalities
have been discovered. Recently, Brønsted acidic ionic liquids
(BAILs) have attracted the attention of researchers.10–12 BAILs
are ILs functionalised to have acidic catalytic ability in addition
to cellulose-dissolution ability, thus they behave as both the
solvent and catalyst in the cellulose hydrolysis (Scheme 1). The
use of BAILs in place of conventional acid catalysts has several
advantages, including freedom from the need to neutralise and
separate the acid catalysts aer the reaction. Namely, there is no
acidic waste through the whole process.13 The rst application
of BAILs to cellulose hydrolysis was reported by Amarasekara
and Owereh in 2009.14 In that work, BAILs functionalised with
SO3H groups could work as solvents as well as acidic catalysts
for cellulose hydrolysis under moderate conditions at 70 ꢀC for
30 min, to yield 62% of the total reducing sugars (TRS) aer 1 h
of preheating without water. Those authors showed that
a higher concentration of the SO3H active sites in the BAILs
accelerated the reaction and lowered the operating tempera-
ture, reducing the energy cost. The same research group also
Cellulose is the most abundant natural polymer consisting of
glucose units linked via b-1,4-glycosidic bonds to form linear
molecular chains.1 The hydrolysis of cellulose to fermentable
sugars is an essential step for the production of bioethanol,
which is expected to be a source of renewable energy. Through
the hydrolysis step, cellulose is rstly degraded to oligosac-
charides, and then glucose is yielded in further depolymerisa-
tion reactions. Generally, glucose can isomerise to fructose
under Lewis acid catalysts, and the subsequent dehydration
generates 5-hydroxymethylfurfural (HMF) and other valuable
chemicals. However, this series of chemical conversion
processes of cellulose is difficult, due to the strong inter- and
intramolecular hydrogen bond network of the cellulose chains.
Therefore, the degradation of cellulose in the rst step requires
drastic pretreatments at high pressure and temperature before
the hydrolysis step.2 More efficient, simpler, and greener tech-
nologies are needed to produce cellulosic ethanol and other
useful substances.
Ionic liquids (ILs), which contain excellent hydrogen bond
acceptors such as Cl and acetate (OAc) anions, have been uti-
lised to dissolve cellulose3 and convert it into valuable
substances.4–6 Due to their thermal stability and recyclability,7–9
Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho,
Chiyoda-ku, Tokyo 102-8554, Japan. E-mail: masahi-f@sophia.ac.jp
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra01950a
Scheme 1 Cellulose hydrolysis to glucose using Brønsted acidic ionic
liquids as both solvent and acidic catalysts.
‡ Now at Faculty of Natural System, Institute of Science and Engineering,
Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan.
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studied the effect of the cation of BAILs on the acidic catalytic subsequently conducted to further examine the structural
ability for cellulose hydrolysis. They demonstrated that BAILs effects of the BAILs on the cellulose hydrolysis to yield glucose.
with imidazolium cations had higher acidic catalytic ability
than those with pyridinium and triethanol ammonium
cations.15 Parveen and Upadhyayula investigated the correlation
between the acidity of BAILs with different acidic functional
groups (SO3H, COOH, and OH) and the TRS yields in cellulose
2. Experimental
2.1 Materials
Microcrystalline cellulose (Avicel® PH-101, ꢁ50 mm particle
size) was purchased from Sigma-Aldrich, Co. LLC. and dried
under vacuum until constant weight before use. 1-Methyl-
imidazole, 1-butylimidazole, 1,3-propanesultone, and 1-
iodooctane were purchased from Wako Pure Chemical
Industries, Ltd. and puried through distillation before use.
Imidazole (>98.0%), chlorosulfonic acid (>97.0%), tri-
uoromethanesulfonic acid (TFS, >98.0%), methanesulfonic
acid (MeS, >98.0%), benzenesulfonic acid (BzS, >98.0%),
glacial acetic acid (AcOH, $99%), and 4-nitroaniline (>99.0%)
were also purchased from Wako Pure Chemical Industries,
Ltd. Potassium hydroxide (KOH, >86.0%), triuoroacetic acid
(TFAc, >99.0%), trichloroacetic acid (TClAc, 99%), sulfuric acid
(H2SO4, >96.0%), and hydrochloric acid (HCl, 35.0–37.0%)
were purchased from Kanto Chemicals Co., Inc. Bis(tri-
uoromethanesulfonyl)imide (HTFSI) was purchased from
Morita Chemical Industries, Co., Ltd.; and phosphoric acid
was purchased (>99.9%) from Sigma-Aldrich, Co. LLC. Other
chemicals were also commercially available and used as
received unless otherwise stated.
hydrolysis
with
the
combined
use
of
1-butyl-3-
methylimidazolium chloride ([Bmim]Cl). Their results showed
that BAILs with SO3H groups had the highest acidity and cata-
lytic activity, with 85% TRS yield at 100 C aer 90 min.16
ꢀ
Aer gradual development for a decade, some BAILs have
shown higher acidic catalytic activity than Brønsted acids for
cellulose hydrolysis under certain conditions.11,14,17 Amar-
asekara and Wiredu demonstrated the superior catalytic activity
of a BAIL, 1-(1-methyl-3-imidazolio)propane-3-sulfonate/hydro-
chloric acid (Mimps/HCl), compared to H2SO4 in cellulose
hydrolysis by a conventional heating method. In their report,
while measurable catalytic activity of H2SO4 was observed for
glucose generation (<17%), Mimps/HCl performed better
(glucose generation <22%) at a relatively low temperature range
from 140 ꢀC to 170 ꢀC. However, the glucose yield still remained
at 22% even aer 3 h of treatment at 170 ꢀC, so further
improvements are required.15 On the other hand, Kuroda et al.
reported that microwave heating effectively assisted the hydro-
lysis reaction using a BAIL catalyst, leading to 40% glucose yield
aer only 12 min at 160 ꢀC,18 owing to the characteristic strong
microwave absorption of ILs.19,20 Thus, it was suggested that
microwave heating of the BAILs might lead to higher glucose
yield.
2.2 Apparatus
1
The H NMR spectra were recorded in DMSO-d6 on a ECX-300
Beyond the acid hydrolysis reaction, a large variety of BAILs
have been investigated as acid catalysts for multiple chemical
reactions.14,15,21,22 The Hammett acidity function (H0) has been
widely used as an index to evaluate the Brønsted acidity of
BAILs. According to the reported H0 values,16,23,24 a shorter
spacer length between the acidic functional group and cation
and a larger number of acidic protons increase the proton
donor ability of BAILs. It was also reported that the HSO4 anion
showed higher Brønsted acidity in water than the Cl anion.24
However, the detailed effect of the BAIL structure (e.g. anion
species and side chain length of the cations) on H0 has not been
claried. Moreover, there is no conrmation that the H0 directly
indicates the acidic catalytic activity of BAILs, especially in
terms of the glucose yield through cellulose hydrolysis.
Based on this background, a comprehensive study of the H0
of BAILs and the corresponding glucose yields would enable
a better understanding of the factors contributing to their acidic
catalytic activity and also the designing of BAILs for future
applications. In this study, BAILs with different structures of the
anion species and the alkyl spacer, and side chain lengths in the
cations were synthesised and used as acid catalysts for cellulose
hydrolysis assisted by microwave irradiation in an aqueous
medium. The detailed relationship between the acidic catalytic
activity of the BAILs and the glucose yield was rstly investi-
gated using the H0 determined from UV-vis spectroscopy. In
spectrometer (JEOL, 7.05 T) operating at 300 MHz, and the
chemical shis are given in d (ppm) downeld from TMS (d ¼
0.00).
Cellulose hydrolysis assisted by microwave irradiation was
carried out in a 2 mL borosilicate glass vial with a poly(ether
ether ketone) (PEEK) cap and a Teon-coated silicone septum
(Anton Paar Japan, Co. Ltd.). Each reaction solution was heated
in microwave irradiation equipment (MONOWAVE 300; Anton
Paar Japan, Co. Ltd.) under the following conditions: frequency
of 2.45 GHz (single mode), maximal setting output of 100 W,
preheating time of 1 min, inner thermometer, ruby sensor
(optic bre), outer thermometer, and IR sensor.
Both the glucose assay and determination of H0 were con-
ducted using a UV-vis spectrometer (UV-PC3100; Shimadzu Co.)
and 10 mm micro quartz cells with a polytetrauoroethylene
(PTFE) stopper.
The FT-IR spectra of the samples, which were previously
dried in vacuo for more than 24 h, were recorded on a Nicolet
6700 system with a MCT-A detector (Thermo Fisher Scientic,
Inc., Tokyo, Japan) equipped with an ATR unit. The FT-IR
measurements were conducted under a spectral resolution of
4 cmꢂ1
.
2.3 Synthesis of ILs
addition, IR spectroscopy analysis and theoretical calculations SO3H-functionalised BAILs were synthesised according to the
using density functional theory (DFT) optimisation were literature,25 and as briey explained below.
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2.3.1 Synthesis of Imds/HCl. Imidazole (6.81 g, 0.100 mol)
RSC Advances
For 1-(1-octyl-3-imidazolio)propane-3-sulfonate (Oimps),
imidazole (20.0 g, 0.294 mol) and KOH (25.2 g, 0.451 mol) were
dissolved in acetonitrile and stirred at r.t. for 24 h. 1-Iodo-
octane (35.2 g, 0.146 mol) was added dropwise to the solution.
The mixture was reuxed at 80 ꢀC for 12 h. Aer the removal of
excess solvent using a rotary evaporator, the concentrated
solution was washed with dichloromethane repeatedly to obtain
a pale yellow liquid, namely 1-octylimidazole. The crude
product was puried through distillation to obtain a colourless
clear liquid. The puried 1-octylimidazole (9.01 g, 50.0 mmol)
was dissolved in acetonitrile, and 1,3-propanesultone (6.11 g,
50.1 mmol) was added dropwise to the solution. The mixture
was stirred at r.t. for 3 days to obtain a white solid. This crude
product was washed with diethyl ether repeatedly. The puried
product was dried under vacuum for 24 h. Yield 70%, 1H NMR:
(300 MHz; DMSO-d6, d/ppm relative to TMS) d: 9.19 (1H, s), 7.80
(2H, tt, J ¼ 8.93 Hz), 4.31 (2H, t), 4.16 (2H, t), 2.40 (2H, t), 2.09
(2H, dt), 1.78 (2H, m), 1.25 (10H, m), 0.86 (3H, dt). Elemental
analysis calcd for C14H26N2O3S1: C, 55.6; H, 8.7; N, 9.3; S, 10.6,
found: C, 55.6; H, 8.7; N, 9.2; S, 10.5.
2.3.4 Synthesis of [Hmim]Cl. 1-Methylimidazole (2.36 g,
14.4 mmol) and HCl (2.4 mL, 14.4 mmol) were mixed and
stirred at 60 ꢀC for 5 h to synthesise 1-methylimidazolium
chloride ([Hmim]Cl). The crude product was puried by
stirring with activated carbon in methanol at r.t. for 1 h. The
resultant solution was ltrated to remove the activated
carbon, and methanol was evaporated from the ltrate. The
puried [Hmim]Cl was dried in vacuo for 24 h to obtain
a white solid. Yield 68%, 1H NMR: (300 MHz; DMSO-d6, d/
ppm relative to TMS) d: 3.85 (3H, dt), 7.78 (1H, t), 7.67 (1H, t),
9.14 (1H, s).
was dissolved in 1,2-dichloroethane, and chlorosulfonic acid
(12 mL, 0.200 mol) was added dropwise to the solution at ice
bath temperature. Aer stirring for 12 h, the mixture was stood
for 5 min to obtain a bi-layer solution. The resultant solution
was washed with 1,2-dichloroethane repeatedly to obtain
a white solid, namely 1,3-disulfonic acid imidazolium chloride
(Imds/HCl). The crude product was washed with diethyl ether
repeatedly, and the white solid was collected by vacuum ltra-
tion. The puried product was dried under vacuum for 24 h.
Yield 59%, 1H NMR: (300 MHz; DMSO-d6, d/ppm relative to
TMS) d: 10.70 (1H, br s), 9.10 (1H, d, J ¼ 4.12 Hz), 7.70 (2H, d).
2.3.2 Synthesis of Mims/HCl. 1-Methylimidazole (8.20 g,
0.100 mol) was dissolved in 1,2-dichloroethane, and chlor-
osulfonic acid (7.2 mL, 0.120 mol) was added dropwise to the
solution at ice bath temperature. Aer stirring for 20 min, the
mixture was stood for 5 min to obtain a bi-layer solution. The
resultant solution was washed with 1,2-dichloroethane repeat-
edly to obtain a viscous colourless liquid, namely 1-methyl-3-
sulfonic acid imidazolium chloride (Mims/HCl). The crude
product was puried through precipitation into diethyl ether,
and the white solid was collected by vacuum ltration. The
puried product was dried under vacuum for 24 h. Yield 81%,
1H NMR: (300 MHz; DMSO-d6, d/ppm relative to TMS) d: 9.06
(1H, s), 7.71 (1H, t), 7.66 (1H, t), 3.88 (3H, dt).
2.3.3 Synthesis of Mimps, Bimps, and Oimps/HX. BAILs
derived from three kinds of zwitterions were prepared accord-
ing to the literature.11,26 An equivalent molar amount of acid
(HX) was slowly added to the corresponding zwitterion (syn-
thesised according to the literature27,28). The mixture was stirred
ꢀ
at 80 C for 3 days to obtain a viscous liquid.
For 1-(1-methyl-3-imidazolio)propane-3-sulfonate (Mimps),
1-methylimidazole (3.20 g, 39.0 mmol) was dissolved in aceto-
nitrile, and 1,3-propanesultone (4.76 g, 39.0 mmol) was added
dropwise to the solution. The mixture was stirred at r.t. for 3
days to obtain a white solid. Then, the crude product was
washed with diethyl ether repeatedly, and further puried
through recrystallisation with methanol twice. The puried
product was dried under vacuum for 24 h. Yield 58%, 1H NMR:
(300 MHz; DMSO-d6, d/ppm relative to TMS) d: 9.12 (1H, s), 7.78
(1H, t), 7.71 (1H, t), 4.30 (2H, t), 3.86 (3H, s), 2.41 (2H, t), 2.09
(2H, dt). Elemental analysis calcd (%) for C7H12N2O3S1: C, 41.2;
H, 6.0; N, 13.7; S, 15.7, found: C, 41.1; H, 5.7; N, 13.7; S, 15.5.
For 1-(1-butyl-3-imidazolio)propane-3-sulfonate (Bimps), 1-
butylimidazole (4.85 g, 39.1 mmol) was dissolved in acetoni-
trile, and 1,3-propanesultone (4.77 g, 39.1 mmol) was added
dropwise to the solution. The mixture was stirred at r.t. for 3
days to obtain a white solid. Then, the crude product was
washed with diethyl ether repeatedly, and further puried
through recrystallisation with ethanol twice. The puried
product was dried under vacuum for 24 h. Yield 82%, 1H NMR:
(300 MHz; DMSO-d6, d/ppm relative to TMS) d: 9.21 (1H, s),
7.82 (2H, tt), 4.30 (2H, t), 4.18 (2H, t), 2.42 (2H, t), 2.11 (2H, dt),
1.79 (2H, dt), 1.27 (2H, dq), 0.91 (3H, t). Elemental analysis
calcd for C10H18N2O3S1: C, 48.8; H, 7.4; N, 11.4; S, 13.0, found:
C, 48.9; H, 7.3; N, 11.4; S, 13.2.
2.4 Determination of Hammett acidity function (H0)
The Brønsted acidity of the BAILs in water in terms of H0 was
determined by UV-vis spectroscopy following the concept re-
ported in the literature,9,29 using 4-nitroaniline as a basic indi-
cator to receive the dissociative protons.
Upon increasing the acidity of the BAILs, the absorbance of
the unprotonated form of the indicator (denoted as I) decreases,
whereas its protonated form (IH+) could not be observed
because of its low molar absorptivity. Therefore, the [I]/[IH+]
ratio can be determined from the absorbance aer the addition
of BAILs. Then, H0 can be calculated using eqn (1), and it can be
regarded as the relative proton donating ability of the BAILs in
water.
ꢀ
ꢁ
ðIÞ
H0 ¼ pKðIÞ þ log
(1)
ðIHþÞ
aq
The H0 values of the BAILs were determined under the
prescribed concentrations of 4-nitroaniline (3 mg Lꢂ1, pK(I)aq
¼
pKa ¼ 0.99) and BAIL (50 mmol Lꢂ1) in aqueous solution. The
maximal absorbance of the I form of the indicator is observed at
380 nm in water. Aer the addition of BAILs into 4-nitroaniline
aqueous solutions, the absorbance of the solutions decreased as
shown in Fig. 1.
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2.7 Glucose analysis using a mutarotase/GOD method assay
The glucose yield aer cellulose hydrolysis was determined
through the mutarotase/glucose oxidase (GOD) method.30 The
clear hydrolysate solution (20 mL) was transferred into a sample
tube. At time 0, the enzymatic reaction was started by adding
4 mL of mutarotase/GOD assay reagent (Glucose CII test Wako
kit; Wako Pure Chemical Industries, Ltd.) to the sample tube.
The reaction solution was incubated in a water bath at 37 ꢀC for
4 min to obtain a pink solution. Then, the absorbance of this
solution at 505 nm was immediately measured by a UV-vis
spectrometer. A reagent blank was prepared by the same treat-
ment, using 20 mL of deionised water with 4 mL of assay
reagent. The glucose concentration was calculated by employ-
ing a calibration curve obtained using the standard glucose
solutions. The glucose yield was calculated according to eqn (2):
Glucose ðmgÞ
YieldGlu% ¼
ꢃ 100%
(2)
Cellulose ðmgÞ ꢃ 1:1
where the value of 1.1 was the molecular weight ratio between
glucose (C6H12O6, 180 g molꢂ1) and one anhydrous glucose unit
in cellulose (C6H10O5, 162 g molꢂ1).
3. Results and discussion
3.1 Acidic catalytic activity of SO3H-functionalised BAILs for
cellulose hydrolysis
Firstly, the acidic catalytic activity of the SO3H groups in the
BAILs and the effect of the spacer length between the SO3H
group and imidazolium cation were investigated. Table 1
summarises the chemical structures of the BAILs (Imds/HCl,
Fig. 1 UV-vis spectra of 4-nitroaniline in water before and after the
addition of BAILs with (a) various anions and (b) cations with different
alkyl side chain lengths.
Mims/HCl, and Mimps/HCl),
a zwitterion (Mimps), and
a protic IL([Hmim]Cl), and the corresponding glucose yields.
Both Mimps, which has a sulfonic acid group without an acidic
proton, and [Hmim]Cl showed very low glucose yields (below
4%), suggesting poor acidic catalytic ability for cellulose
hydrolysis, even though [Hmim]Cl has a proton in the
2.5 Molecular geometries
The minimum energy geometries of the BAILs were calculated
by DFT optimisation at the B3LYP/6-311G++(d,p) level using the
Gaussian 09 program series. The exact minimum was veried
through the vibrational analysis of each optimised structure to
ensure the absence of negative frequencies.16,23,24
Table 1 Acid catalytic abilities for cellulose hydrolysis and chemical
structures of SO3H-functionalised BAILs with different spacer lengths
between the SO3H groups and imidazolium cations, and a zwitterion
and a protic IL as references
Entry
Chemical structure
Glucose yield (%)
2.6 Microwave-assisted hydrolysis of cellulose
An aqueous solution of BAIL (1 mol Lꢂ1) was prepared by adding
the appropriate amount of deionised water. The accuracy of the
solution concentration was checked by titration with a stand-
ardised 0.5 mol Lꢂ1 NaOH aqueous solution using phenol-
phthalein as an indicator.
Imds/HCl
35 ꢄ 0.3
Mims/HCl
30 ꢄ 2.3
Cellulose (10 mg) was added to a 1 M BAIL aqueous solution
(1 mL) in a 4 mL borosilicate glass vial. The vial was rmly
closed and heated for 15 min at 160 C by microwave irradia-
tion. The resultant solution was immediately cooled to ice bath
temperature to quench the reaction, and neutralised by drop-
wise addition of 0.5 mol Lꢂ1 NaOH aqueous solution. The
turbid solution was ltrated to remove the brownish residues.
Then, the ltrate was collected for glucose assay.
Mimps/HCl
Mimps
24 ꢄ 0.3
3.6 ꢄ 0.3
ꢀ
[Hmim]Cl
1.5 ꢄ 1.0
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RSC Advances
imidazolium cation. The difference between the pKa values (i.e., Mimp showed proton donation to the basic indicator as shown
DpKa) of HCl (pKa ¼ ꢂ7.0) and 1-methylimidazole (pKa ¼ 7.06) is in Fig. 1(a), indicating that they hardly work as acidic catalysts.
above 10, which may cause the originally active proton to stick
Next, to determine the relationship between the H0 of the
on the imidazolium cation.31 On the other hand, all of the SO3H- BAIL aqueous solutions and the acidic catalytic activity, these
functionalised BAILs showed higher glucose yields (over 20%), BAILs were used for cellulose hydrolysis, and the glucose yields
suggesting that the existence of the active proton in the SO3H were compared. Fig. 2(a) shows the relationship between the H0
group was key for the acidic catalytic activity of the BAILs.
of Mimps/HX and the glucose yield. Mimps/H2SO4 with the
BAILs with shorter spacer lengths and two SO3H groups lowest H0 showed the highest glucose yield (32 ꢄ 2.2%) among
showed higher glucose yields. The activity order of the BAILs for the BAILs used in this study. With increasing H0, the glucose
cellulose hydrolysis (Imds/HCl > Mims/HCl > Mimps/HCl) yield tends to become lower. This result implies that the anion
agrees with that for the hydration reaction of phenyl acety- species of BAILs certainly affects their proton donor activity,
lene.24 Both Imds/HCl and Mims/HCl showed excellent acidic which is related to the acidic catalytic activity in terms of
catalytic ability. However, the relatively low thermal and glucose yield.
chemical stability of the corresponding zwitterions may be
Consistent with their high H0 values, Mimps/AcOH and
a concern (Fig. S1 and S2†). The lack of alkyl spacer may provide Mimps showed little acidic catalytic ability, and the glucose
the ready desorption of SO3H groups with the release of chlor- yields were 3.3 ꢄ 0.9% and 3.6 ꢄ 0.3%, respectively. Both
osulfonic acid, especially under basic conditions.25 Flexible Mimps/H3PO4 (4.7 ꢄ 2.0%) and Mimps/TClC (5.3 ꢄ 1.5%) also
alkyl spacers improve the stability of the zwitterions themselves, worked slightly as acidic catalysts for cellulose hydrolysis,
and also assist the approach of the SO3H group to the glycoside although they had medium acidity. BAILs with the H2PO4 anion
bond of cellulose. Moreover, it has been reported that BAILs have also been reported to show poor acidic catalytic ability in
with a propyl spacer (C3) exhibit higher TRS yields in cellulose cellulose hydrolysis with conventional heating, due to their low
hydrolysis than those with a butyl spacer (C4).14,15,21 According acidity.21 These results imply that, to function as an acid catalyst
to these results, it was concluded that the SO3H-functionalised for cellulose hydrolysis, a BAIL needs to have a H0 lower than 1.5
BAILs with a C3 spacer are the most suitable for investigating
the detailed structural effects on the acidic catalytic activity of
BAILs in cellulose hydrolysis.
3.2 Structural effects of BAILs on acidity and glucose yield
3.2.1 Anion species. The H0 value in water was investigated
for different BAILs with a Mimps cation and various anions
(Xꢂ), abbreviated as Mimps/HX (Table 2). The HSO4 anion with
an active proton in itself showed the lowest H0 value, and the Cl
anion was the second lowest one. Neither Mimps/AcOH nor
Table 2 Calculation and comparison of the H0 values of BAILs with
various anions and cations with different alkyl chain lengths in water at
room temperature
Zwa
HXb
Amax
I (%)
IH+ (%)
H0
Nonec
Mimps
—
H2SO4
HCl
TFS
BzS
0.314
0.172
0.186
0.208
0.210
0.212
0.220
0.222
0.223
0.258
0.311
0.307
0.186
0.197
0.197
0.223
100
55
59
66
67
68
70
71
71
82
99
98
59
63
62
71
0
—
45
41
34
33
32
30
29
29
18
1
1.07
1.15
1.28
1.30
1.31
1.36
1.37
1.38
1.65
2.96
2.63
1.14
1.22
1.21
1.38
MeS
HTFSI
TFAc
TClAc
H3PO4
AcOH
—
H2SO4
HCl
H2SO4
HCl
2
Bimps
Oimps
41
37
38
29
a
Zw: zwitterion. b HX: acid used to prepare the BAILs labelled as Zw/HX,
where X corresponds to the anion species. c Indicator: 4-nitroaniline.
Fig. 2 Correlation between the glucose yields and H0 values of BAILs
with various anions (left) and different alkyl chain lengths of the cation
(right).
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at the least. Detailed discussion will be given below in Section index for evaluating the acidic catalytic activity of BAILs in
3.4 on estimating the acidic catalytic activity of BAILs using water, and this value depends on both the anion species and
theoretical studies.
alkyl side chain length of the cations. While there seems to be
Notably, the glucose yield of Mimps/HCl (23.7 ꢄ 0.3%) was a certain relationship between H0 determined by the structures
lower (despite its second lowest H0) than that of BAILs with of the BAILs and glucose yields, a few exceptions were observed.
sulfonic acid anions, including BzS (28.7 ꢄ 0.3%) and MeS (28.0 For example, the glucose yield was relatively low for BAILs with
ꢄ 0.6%). It was reported that BAILs with Cl anions are superior the Cl anion despite their low H0 values, as shown in Fig. 2.
acidic catalysts compared to those with HSO4 in cellulose Moreover, the relationship between glucose yield and H0 value
hydrolysis.10,11,24 However, the Cl anion-based BAILs signi- for BAILs with various anion species (Fig. 2(a)) was different
cantly have been shown to accelerate not only cellulose hydro- when changing the alkyl side chain length of the cations
lysis, but also the subsequent dehydration reaction of the (Fig. 2(b)). Based on these specic observations, it can be said
generated glucose to produce HMF.10,21 These serial reactions that the glucose yield does not depend solely on the H0 value.
might cause a relatively low glucose yield. Under hydrothermal
From a different viewpoint, the hydrogen bond network
conditions at 120–160 ꢀC, glucose is reversibly transformed rst between the cation and anion is also an important factor when
into fructose, and subsequently into HMF through dehydration evaluating the acidic catalytic activity of BAILs. To conrm the
reactions.32 Generally, the isomerisation of glucose to fructose cation–anion interaction in the BAILs, the FT-IR spectrum of
occurs under a Lewis acid catalyst, but in the presence of Cl Bimps/H2SO4 as a representative BAIL was measured and
anion-based BAILs, the reactions go through the complexation compared to that of the pristine zwitterion (Bimps), as shown in
of the BAILs with the open-chain sugars.33 While it is unclear Fig. 3. Considering the high hydrophilicity of the BAIL and
whether similar dehydration reactions occur under other BAILs zwitterion, both samples were carefully dried in vacuo just
with different anions, a high Brønsted acidity is expected to before the ATR mode FT-IR measurements. The stretching
lower the glucose yields through further conversion to HMF or vibration bands of the SO3 group generally appear as two peaks
other derivatives. The amount of residue aer cellulose hydro- at around 1200 and 1050 cmꢂ1. The corresponding peaks
lysis using Mimps/HCl was only one-fourth as compared with appeared at 1195 and 1144 cmꢂ1 in the spectrum of Bimps, and
that of Mimps/H2SO4 as shown in Table S2,† suggesting the shied to 1167 and 1035 cmꢂ1 in that of Bimps/H2SO4,
superior acidic catalytic ability of Mimps/HCl for this series of respectively. In addition, another peak derived from the
reactions.
stretching vibration of the N]C bond on the imidazolium ring
3.2.2 Alkyl side chain length of the cations. The effect of was recorded at 1645 cmꢂ1 for Bimps, and this peak shied to
alkyl side chain length of the imidazolium cations on the H0 1705 cmꢂ1 in the presence of H2SO4. These peak shis strongly
value in water was subsequently examined by UV-vis measure- imply the existence of an interaction between the Bimps cation
ments. The changing absorbance of the basic indicator in water and HSO4 anion. In particular, the evident shis of the peaks
is shown in Fig. 1(b). Table 2 also summarises the H0 values of associated with the SO3 group clearly indicate that the proton
BAILs with methyl, butyl, and octyl groups on the cations in derived from the used Brønsted acid was transferred onto the
combination with Cl or HSO4 anions. Clearly, BAILs with an SO3 group in the zwitterion to afford the SO3H group in the
octyl group in the cation have higher H0 values regardless of the BAILs. In addition, the shi of the N]C bond stretching peak
anion species.
strongly implies an environmental change around the imida-
The relationship between the H0 value and glucose yield for zolium cation, due to some interactions with the counter anion,
BAILs with different alkyl side chain lengths is depicted in presumably via hydrogen bonding.
Fig. 2(b). The glucose yields gradually improved with increasing
alkyl side chain length, despite the concomitant increase in H0
value, which decreases the glucose yield when changing the
anion species, as shown in Fig. 2(a). In a previous study of
a similar cellulose conversion process, it was demonstrated that
in dichloroethane, BAILs with longer alkyl side chains have
higher H0 and lower HMF yields.34 This might be derived from
the suppressed dehydration reaction of glucose by decreasing the
proton donor activity of the BAILs. Thus, cellulose hydrolysis was
semi-selectively promoted in BAILs with long alkyl side chains,
increasing the glucose yield. The increased weight of the residual
aer cellulose hydrolysis using BAILs based on Bimps and Oimps
cations also supports this conclusion (see Table S2†).
3.3 Estimation of acidic catalytic activity by considering the
hydrogen bond network between BAILs
3.3.1 Conrmation of cation–anion interaction using FT-
IR. The above results show that the H0 value is an important (lower) after drying in vacuo at 80 ꢀC for 24 h.
Fig. 3 ATR mode FT-IR spectra of Bimps/H2SO4 (upper) and Bimps
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3.3.2 Theoretical studies of the structural effect on the
hydrogen bond network in the BAILs. Despite the experimental
conrmation of the cation–anion interaction in the BAILs, the
FT-IR spectra only provide limited information about the
detailed hydrogen bond network involved in the BAILs. There-
fore, we further investigated the interaction between the ion
pair in the BAILs through the minimum energy geometries,
which were obtained through DFT geometry optimisations. The
RSC Advances
optimised structures of the BAILs are shown in Fig. 4 (see
Fig. S3† for a more detailed version with labelling numbers on
all atoms), and the geometry parameters are summarised in
Table 3.
The theoretical studies reveal the existence of hydrogen bond
networks between the ion pair in the BAILs. Their acidic cata-
lytic activity depends on the active proton in the SO3H group,
a weak acidic proton at the C2 position of the imidazolium
Fig. 4 Optimised molecular structures of BAILs with various anions and different alkyl chain lengths in the cations calculated with B3LYP/6-
311G++(d,p).
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Table 3 Geometry parameters of the BAILs used in this study calculated with B3LYP/6-311G++(d,p)
Zwa
HXb
H–O bond of –SO3H (A)
–SO3H/X (A)
(N)2C–H bond (A)
(N)2C–H/X (A)
Other H.B. in BAILs (A)
˚
˚
˚
˚
˚
Mimps
H2SO4
H
20–O19 ¼ 1.073
H
20–O28 ¼ 1.392
C1–H5 ¼ 1.091
H5–O27 ¼ 1.853
H5–O24 ¼ 2.625
H5–Cl23 ¼ 2.167
H5–O29 ¼ 1.878
H5–O28 ¼ 2.850
H5–O41 ¼ 1.984
H5–O40 ¼ 2.560
H5–O30 ¼ 1.828
H5–O28 ¼ 2.823
H5–N27 ¼ 1.978
H
H
—
25–O24 ¼ 0.980
25–O22 ¼ 1.905
HCl
TFS
H
H
20–O19 ¼ 1.008
20–O19 ¼ 0.996
H
H
20–Cl23 ¼ 2.051
20–O34 ¼ 1.713
C1–H5 ¼ 1.103
C1–H5 ¼ 1.093
H
H
H
14–O28 ¼ 2.221
BzS
H
H
H
26–O25 ¼ 1.004
20–O19 ¼ 1.008
20–O19 ¼ 0.993
H26–O39 ¼ 1.632
H20–O29 ¼ 1.614
H20–O30 ¼ 1.701
C1–H5 ¼ 1.086
C1–H5 ¼ 1.099
C1–H5 ¼ 1.092
14–O40 ¼ 2.483
14–O28 ¼ 2.176
MeS
HTFSI
H
H
H
H
H
H
H
H
H
H
26–O33 ¼ 2.148
10–O30 ¼ 2.308
14–O28 ¼ 2.226
14–O28 ¼ 2.218
32–O31 ¼ 0.963
29–O33 ¼ 2.386
14–O28 ¼ 2.878
24–O28 ¼ 2.627
25–O24 ¼ 0.980
25–O22 ¼ 1.913
TFAc
TClAc
H3PO4
H
H
H
H
H
26–O25 ¼ 1.034
26–O25 ¼ 1.033
24–O22 ¼ 1.640
20–O19 ¼ 1.663
26–O25 ¼ 1.494
H26–O29 ¼ 1.524
H26–O29 ¼ 1.523
H24–O23 ¼ 1.002
H20–O25 ¼ 1.000
H26–O29 ¼ 1.037
C1–H5 ¼ 1.097
C1–H5 ¼ 1.096
C1–H5 ¼ 1.095
H5–O28 ¼ 1.764
H5–O28 ¼ 1.766
H5–O33 ¼ 1.814
AcOH
H2SO4
C1–H5 ¼ 1.089
C1–H5 ¼ 1.089
H5–O28 ¼ 1.869
Bimps
Oimps
H
20–O19 ¼ 1.076
H20–O28 ¼ 1.385
H5–O27 ¼ 1.879
H5–O24 ¼ 2.635
H5–Cl23 ¼ 2.180
H5–O52 ¼ 1.885
H5–O49 ¼ 2.642
H5–Cl23 ¼ 2.186
HCl
H2SO4
H
H
20–O19 ¼ 1.009
45–O44 ¼ 1.073
H
H
20–Cl23 ¼ 2.038
45–O53 ¼ 1.392
C1–H5 ¼ 1.101
C1–H5 ¼ 1.089
—
H
50–O49 ¼ 0.979
H50–O47 ¼ 1.916
HCl
H
20–O19 ¼ 1.009
H20–Cl23 ¼ 2.042
C1–H5 ¼ 1.101
—
a
Zw: zwitterion. b HX: acid used to prepare the BAILs labelled as Zw/HX, where X is the corresponding anion species.
cation, and in the case of the HSO4 anion also the proton within described in Table 2. Mimps/H2SO4 has a signicant hydrogen
the anion itself.23 However, the pKa value of the acidic proton in bond (H25–O22 ¼ 1.905 A) between the HSO4 anion and SO3H
˚
the imidazolium ring is expected to be signicantly higher than group (Fig. 4), and this strong hydrogen bond network would
that of the SO3H group.35 Thus, the active proton in the SO3H cause a longer H–O bond length in the SO3H group. However,
group might be the main contributor to the acidic catalytic Mimps/HSO4 has another acidic proton in the HSO4 anion. The
activity of the BAILs. The H–O bond distance in the SO3H group oxygen of the HSO4 anion forms comparatively strong hydrogen
is shortened with weaker cation–anion interaction in the BAILs, bonding with the imidazolium ring hydrogen (H5–O27 ¼ 1.853,
which allows better accessibility to the substrate due to the H5–O24 ¼ 2.625), which makes the proton in the HSO4 anion
higher mobility of the acidic proton in the SO3H group. more labile and acidic.23 Therefore, in the particular case of
Therefore, BAILs with a shorter H–O bond distance in the SO3H BAILs with HSO4 anions, the active proton in the HSO4 anion
group are considered to have higher acidic catalytic activity. In might contribute to the acidic catalytic activity, in addition to
the calculation results for BAILs with the Mimps cation, Mimps/ the proton of the SO3H group that is the main source of acidic
˚
HTFSI showed the shortest bond distance (H20–O19 ¼ 0.993 A), catalytic activity for other BAILs.
and the other BAILs with relatively short H–O distances can be
Hydrogen bond networks were also conrmed in BAILs with
˚
ranked as Mimps/TFS (H20–O19 ¼ 0.996 A) < Mimps/BzS (H26
–
other sulfonic acid anions. The distance between the anion and
˚
˚
O25 ¼ 1.004 A) < Mimps/HCl (H20–O19 ¼ 1.008 A) ꢁ Mimps/MeS SO3H group becomes shorter with increasing degree of
˚
(H20–O19 ¼ 1.008 A). Mimps/HTFSI was therefore expected to hydrogen bonding. Such cation–anion interactions may inhibit
have the highest acidic catalytic activity according to the theo- the accessibility of the proton of the SO3H group to the glycoside
retical calculations. However, its actual glucose yield (24.0 ꢄ bonds of cellulose for hydrolysis. Therefore, the actual acidic
2.1%) was lower than that of BAILs with sulfonic acid anions catalytic activities of Mimps/TFS and Mimps/BzS as shown in
(Fig. 2(a)). One explanation is that in this study, the cellulose Fig. 2(a) were less than that of Mimps/H2SO4, as the latter also
hydrolysis experiments were conducted in an aqueous medium, has the advantage of another active proton in the anion,
and the DFT calculations failed to fully consider the effect of regardless of the strength of the hydrogen bond networks.
water molecules. Thus, Mimps/HTFSI may not fully realise its
Another hydrogen bond between the anion and the weak
acidic catalytic ability in water because of the hydrophobicity of acidic proton at the C2 position of the imidazolium cation was
the TFSI anion,36,37 which may also be reected in its relatively observed. When the proton is closely covered by the anion, its
high H0 value (Table 2).
accessibility is also reduced.9 Moreover, the shorter (N)2C–H
Despite it having the lowest H0, the H–O bond distance in the distance due to the strong hydrogen bonding also makes the
SO3H group of Mimps/H2SO4 (H20–O19 ¼ 1.073 A) was longer acidic proton less labile.24 However, these hydrogen bond
˚
˚
than that of Mimps/TFAc (H26–O25 ¼ 1.034 A) and Mimps/TClAc networks are all absent in Mimps/HCl. The Cl anion is located
˚
˚
(H26–O25 ¼ 1.033 A), which showed much higher H0 values as in the middle between the SO3H proton (H5–Cl23 ¼ 2.167 A) and
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RSC Advances
the one at the C2 position of the imidazolium cation (H20–Cl23 spectroscopy. The strong hydrogen bond network in BAILs
˚
¼ 2.051 A). Hence, in addition to the SO3H group being the might inhibit the acidic catalytic activity, especially in the case
main acid functional site, the weak acidic proton on the imi- of sulfonic acid anions, while the long alkyl chain had no effect.
dazolium ring also contributes to the acidic catalytic ability of The moderate decrease of the Brønsted acidity of the BAILs in
Mimps/HCl. These structural advantages may provide superior water is key for improving the glucose yield.
acidic catalytic ability and accelerate both cellulose hydrolysis
and glucose dehydration.
Conflicts of interest
Despite the long alkyl chains in their cations, signicant
hydrogen bond networks were not observed in either Bimps/
HCl or Oimps/HCl. On the other hand, Bimps/H2SO4 (H25–O22
The authors declare no conict of interest.
˚
˚
¼ 1.913 A) and Oimps/H2SO4 (H50–O47 ¼ 1.916 A) showed
strong hydrogen bond networks, and the strength was slightly Acknowledgements
weaker for the longer alkyl side chain. It should be noted that
the bond distances of both O–H in the SO3H groups (Zw/HCl:
This work was supported by a Sophia University Special Grant
for Academic Research.
˚
˚
1.008–1.009 A, Zw/H2SO4: 1.073–1.076 A) and C–H at the C2
˚
position of the imidazolium cations (Zw/HCl: 1.101–1.103 A,
˚
Zw/H2SO4: 1.089–1.091 A) were quite constant. In general, N-
Notes and references
methyl group substitution increases the electron density of the
C2 carbon on the imidazolium ring, owing to the electron
donating nature of the methyl group. However, the effect of the
alkyl side chain length on the acidic catalytic ability of BAILs
seemed to be slight, considering the small changes in the
(N)2C–H bond distance in Table 3. This suggests that the alkyl
side chain length of the imidazolium cations hardly affects the
acidic catalytic activity of BAILs, if not considering the water
effect.
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