A new functionalized ionic liquid for efficient glucose conversion to 5-hydroxymethyl furfural and levulinic acid

Article abstract of DOI:10.1016/j.molcata.2015.06.030

The conversion of glucose to 5-hydroxymethyl furfural (5-HMF) and levulinic acid (LA) using ionic liquid is a promising method for producing liquid fuels from renewable resources. In this study, three types of acidic functionalized ionic liquids (FILs) were prepared and used as catalysts in the conversion of glucose to 5-HMF and LA. The prepared FILs were characterized using CHNS elemental analysis and ¹H and ¹³C NMR. The acidity of the FILs was examined using pyridine-FTIR, Hammett and acid-base titration methods. The FIL with high acidity and with both Br?nsted and Lewis acid sites present seemed suitable for 5-HMF and LA production. Among the tested FILs, 1-sulfonic acid-3-methyl imidazolium tetrachloroferrate ([SMIM][FeCl₄]) demonstrated the highest catalytic performance. The yields of 5-HMF and LA reached as high as 18% and 68%, respectively after 4 h at 150°C. The catalyst was reused five times without significant loss of activity. Furthermore, for the kinetic analysis performed for glucose conversion, the activation energy and pre-exponential factor for the reaction were 38 kJ mol^-1 and 925 min^-1, respectively. The experimental results demonstrated the potential of FIL as a catalyst for biomass transformation to platform chemicals under mild process condition.

Full text of DOI:10.1016/j.molcata.2015.06.030

Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A new functionalized ionic liquid for efficient glucose conversion to
5-hydroxymethyl furfural and levulinic acid
Nur Aainaa Syahirah Ramli, Nor Aishah Saidina Amin∗
Chemical Reaction Engineering Group (CREG), Energy Research Alliance, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor,
Johor Bahru, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
The conversion of glucose to 5-hydroxymethyl furfural (5-HMF) and levulinic acid (LA) using ionic liquid
is a promising method for producing liquid fuels from renewable resources. In this study, three types of
acidic functionalized ionic liquids (FILs) were prepared and used as catalysts in the conversion of glu-
cose to 5-HMF and LA. The prepared FILs were characterized using CHNS elemental analysis and 1H and
13C NMR. The acidity of the FILs was examined using pyridine-FTIR, Hammett and acid-base titration
methods. The FIL with high acidity and with both Brønsted and Lewis acid sites present seemed suitable
for 5-HMF and LA production. Among the tested FILs, 1-sulfonic acid-3-methyl imidazolium tetrachlo-
roferrate ([SMIM][FeCl4]) demonstrated the highest catalytic performance. The yields of 5-HMF and LA
reached as high as 18% and 68%, respectively after 4 h at 150 ◦C. The catalyst was reused five times without
significant loss of activity. Furthermore, for the kinetic analysis performed for glucose conversion, the
activation energy and pre-exponential factor for the reaction were 38 kJ mol−1 and 925 min−1, respec-
tively. The experimental results demonstrated the potential of FIL as a catalyst for biomass transformation
to platform chemicals under mild process condition.
Received 7 January 2015
Received in revised form 13 June 2015
Accepted 22 June 2015
Available online 29 June 2015
Keywords:
Glucose conversion
Functionalized ionic liquid
5-hydroxymethyl furfural
Levulinic acid
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
cal compound with numerous potential uses such as textile dye,
animal feed, coating material, antifreeze, solvent, food flavouring
Concerns about increasing petroleum oil prices compel the
chemical industry to explore alternatives to fossil resources for
basic chemical productions [1]. Biomass resources are promising
alternatives for sustainable supply of fuels and valuable chem-
icals [2–4]. Carbohydrates derived from biomass are abundant,
relatively inexpensive, and are renewable energy sources. Car-
bohydrate such as glucose is a compound from which various
bio-based chemicals can be derived [1]. Among those chemicals,
5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA) have
received significant attention as platform chemicals for synthesiz-
ing a broad range of chemicals and bio-based fuels [5–7].
A versatile intermediate compound, 5-HMF, has been identi-
fied as the petroleum-based building block and biomass-based
carbohydrate chemical. 5-HMF is very useful not only for the
production of furan derivatives, fine chemicals, promising liquid
transportation fuels, and polymers, but also for the production
of important molecules such as 5-hydroxy-4- keto-2-pentenoic
acid and LA [8]. Besides, LA is also an important basic chemi-
agent, pharmaceutical compound and resin [9]. In the last decade,
an enormous amount of work in the development of sustainable
methods for the preparation of 5-HMF and LA from different feed-
stocks including monosaccharides [10,11], disaccharides [12–14],
polysaccharides [15–18], and raw lignocellulosic biomass [19–22]
has been reported.
The conversion of carbohydrates to 5-HMF and LA has been con-
ducted in water and various organic solvents in the presence of
various catalysts, including mineral acids [18,23–25], metal chlo-
rides [5,26,27], and solid acid catalysts [17,28,29]. Basically, in the
catalytic conversion of glucose, glucose isomerizes to fructose and
dehydrates to form 5-HMF, which will then catalytically rehydrate
to form LA [27,30]. The isomerization of glucose to fructose is cat-
alyzed by Lewis acid sites [31], whereas Brønsted acid sites are
required for 5-HMF rehydration to LA [31]. Moreover, Lewis acid
sites are also able to catalyze the decomposition of glucose to form
humins [31,32]. The glucose conversion reaction scheme to 5-HMF
and LA is depicted in Fig. 1.
Ionic liquids are salts with melting point below 100 ◦C that are
prepared from combinations of different cations and anions. Ionic
liquids have received great attention in various fields owing to their
potential as eco-friendly solvents due to several unique proper-
∗
Corresponding author. Fax: +60 7 5588166.
E-mail address: noraishah@cheme.utm.my (N.A.S. Amin).
http://dx.doi.org/10.1016/j.molcata.2015.06.030
1381-1169/© 2015 Elsevier B.V. All rights reserved.

114
N.A.S. Ramli, N.A.S. Amin / Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
CH2OH
OH
H+
O
HOH2C
CHO
OH
O
OH
CH2OH
HOH2C
isomerization
OH
OH
fructose
OH
dehydration
OH
5-HMF
glucose
O
H+
H3C
COOH
rehydration
LA
Fig. 1. Reaction scheme of glucose conversion to 5-HMF and LA.
ties such as low vapour pressure, non-flammability, high thermal
stability and easy to separate from reactions [33,34]. With these
promising properties, ionic liquids are being used increasingly as
solvents in the dissolution of biomass and in the dehydration of car-
bohydrates to produce various useful chemicals including 5-HMF
and LA [34–37]. In this regard, imidazolium based ionic liquids are
well known for their solubilizing competency [36,37]. As such, ionic
liquids containing [BMIM]+ are powerful solvents [38].
Nevertheless, the concern towards the exorbitant price of ionic
liquid is prevalent. The high price limits the use of ionic liquids in
large scale reactions. Thus, the quantity of ionic liquid for reaction
purposes should be reduced. As such, the practicability of aque-
ous ionic liquid [39] and combination of ionic liquid with other
compounds such as ammonia and oxygen [40,41] could be imple-
mented to reduce the quantity of ionic liquid. Besides, the capability
to reuse ionic liquid is a good step towards reducing the overall cost
for any processes involving ionic liquids.
Lewis acidic groups were synthesized by altering the anions of
cations of the FIL. The acidic properties of the FILs were examined
accordingly. Next, the performance of the FILs was evaluated for
glucose conversion to main dehydration products (5-HMF and LA).
The factors influencing the glucose conversion involving reaction
temperature, reaction time, glucose loading, and FIL loading were
examined. The reusability of FIL was also addressed. In addition,
a simple kinetic analysis was performed for glucose conversion to
determine the reaction rate constant and activation energy of glu-
cose conversion using the selected FIL. The findings from this study
are expected to improve the knowledge of catalytic transformation
of renewable feedstocks to platform chemicals such as 5-HMF and
LA.
2. Materials and methods
2.1. Materials
Functionalized ionic liquids (FILs) are defined as ionic liquids
synthesized and designed through the inclusion of different types
of functional groups on the cations and anions. To date, FILs have
exhibited enormous potential to be applied to various processes
including catalytic conversion of renewable feedstocks to 5-HMF
and LA [42–49]. The potential of sulfonic acid (SO3H) FIL for replac-
ing conventional acid was first reported in 2002 [50]. SO3H FILs,
which have strong Brønsted acidity, are flexible and recyclable,
and could be used as both solvents and catalysts. Besides, SO3H
FILs have been used in the conversion of cellulose and glucose to
5-HMF and LA [43–45,47].
Many studies have demonstrated that metal salts (CrCl2, CrCl3,
SnCl4, GeCl4) are effective catalysts for hydrolysis of carbohydrates
[51–53]. Tao et al., found that the addition of MnCl2 in SO3H FIL
was effective for the production of 5- HMF from cellulose, whereas
LA was obtained as a side product [43]. Thus, the incorporation of
metal metals with the SO3H FIL system can also be used for the
production of 5-HMF and LA, as the incorporated metals will act
as the Lewis acid sites to catalyze the glucose conversion reaction.
Meanwhile, in the research conducted by Zhang et al. [47], a com-
plex FIL was used as a catalyst for the conversion of carbohydrates
to 5-HMF. The complex FIL was prepared by grafting CrCl3·6H2O on
the polymeric heteropolyacid- SO3H FIL.
All chemicals were obtained from Merck (Germany) and
used as received without any further purification. The chemi-
cals used for the preparation of FILs were 1-methylimidazole,
chlorosulfonic acid, dichloromethane, iron (III) chloride hexahy-
drate (FeCl3·6H2O), and 1-butyl-3-methyl imidazolium chloride
([BMIM][Cl]). Glucose, 5-HMF, LA, sodium hydroxide (NaOH),
3,5-dinitrosalicylic acid, potassium sodium tartrate tetrahydrate,
sodium sulfite, ethyl acetate, and sulfuric acid (H2SO4) were used
for the catalytic tests and product analysis. The chemicals involved
in the characterizations of FIL’s acidity were deuterated dimethyl
sulfoxide (DMSO-d6), chloroform (CDCl3), pyridine, 2,6-dichloro-
4-nitroaniline, 2-propanol, phenolphthalein, and NaOH.
2.2. Preparation of FILs
Three types of FILs, namely 1-butyl-3-methylimidazolium
tetrachloroferrate
([BMIM][FeCl4]),
1-sulfonic
acid-3-
methylimidazolium chloride ([SMIM][Cl]), and 1-sulfonic
acid-3-methylimidazolium tetrachloroferrate ([SMIM][FeCl4])
were prepared for catalytic conversion of glucose to 5-HMF and
LA. Firstly, [BMIM][FeCl4] catalyst was prepared according to a
previous method [57]. Equimolar [BMIM][Cl] and FeCl3·6H2O were
mixed under vigorous stirring, and the mixture was stirred for 24 h
at room temperature. Two distinct layers were formed, in which
the darker layer contained [BMIM][FeCl4], and the upper layer
mostly consisted of water. The aqueous upper layer was separated
by centrifugation, and the [BMIM][FeCl4] layer was then dried at
80 ◦C in order to remove the remaining water.
CrCl2 and CrCl3 are the mostly used metal salts for the conver-
sion of carbohydrates; either the salts are used alone as a catalyst
or combined with ionic liquid in the reaction system [27,47,53–55].
However, high price, toxicity and environmental pollution derived
from CrCl2 and CrCl3 have necessitated the search for non-toxic
and low cost metal salts. Due to the low cost and non-toxic prop-
erties of FeCl3 [56], FeCl3·6H2O was introduced into the group
of imidazolium based SO3H FIL. In the current study, several FILs
([BMIM][FeCl4]), [SMIM][Cl]), [SMIM][FeCl4]) with Brønsted and/or
Meanwhile, [SMIM][Cl] was prepared by mixing 1-
methylimidazole in dry CH2Cl2 (50 mL) before SO3HCl was
added dropwise at room temperature. The molar ratio of 1-

N.A.S. Ramli, N.A.S. Amin / Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
115
The acid strength of the FIL was determined by the Hammett
method using UV–visible spectroscopy. A solution of 2,6-dichloro-
4-nitroaniline, acted as an indicator, was added to the FIL, and the
mixture was stirred for 15 min. The UV spectra were recorded on
an Agilent 8453 UV–vis spectrophotometer (370 nm) at room tem-
perature. The acidity of FIL was evaluated from the determination
of Hammett acidity function (Eq. (2)):
Cl
+
FeCl3.6H2O
FeCl4
N
N
+
+
6H2O
N
+
N
ꢀ
ꢃ
(a)
I
[ ]
ꢁ
ꢂ
Ho = pK(I)aq + log
(2)
IH+
Where pK(I)aq is the pKa value of the indicator, and [I] and [IH+] are
the molar concentrations of unprotonated and protonated forms of
the indicator in the solvent, respectively.
Cl
+
SO3HCl
N
+
N
CH2Cl2
N
+
N
SO3H
The acid values of the FILs were determined by the acid-base
titration method. Approximately, 0.1 g of FIL sample was dissolved
in 10 mL of 2-propanol as a solvent and phenolphthalein as an indi-
cator. The mixture was then titrated with 0.1 M NaOH and shaken
vigorously. The titration was stopped when the end point has been
reached once the clear mixture turns pink. The amount of NaOH
used for the titration was recorded. In addition, a blank titration
was also performed. The acid value, expressed as the amount of
NaOH required to neutralize 1 g of FIL, was calculated using Eq. (3).
(b)
Cl
+
FeCl3.6H2O
N
+
N
SO3H
FeCl4
(A − B) × M × 40
N
+
N
6H2O
+
Acid value (g/g) =
(3)
W
SO3H
(c)
Where W is the sample mass (g), A is the volume of NaOH required
for titration of the sample (mL), B is the volume of NaOH required
for titration of the blank (mL), and M is the molarity of the NaOH
solution.
Fig. 2. The addition of ions involved in the preparation of (a) [BMIM][FeCl4], (b)
[SMIM][Cl], and (c) [SMIM][FeCl4].
2.4. Performance testing of FILs
methylimidazole and SO3HCl was 5:5.2 mol. The reaction mixture
was stirred for 20 min, allowed to stand for 5 min, and then CH2Cl2
was decanted. The precipitate was washed three times with dry
CH2Cl2 and dried at 80 ◦C to give [SMIM][Cl]. This procedure is in
accordance with the method described in the literature [58]. Mean-
while, the preparation of [SMIM][FeCl4] involved the mixing of the
prepared [SMIM][Cl] and FeCl3·6H2O. Equimolar [SMIM][Cl] and
FeCl3·6H2O were mixed under vigorous stirring, and the mixture
was stirred for 24 h at room temperature. [SMIM][FeCl4] was dried
at 80 ◦C to remove water. The addition of ions involved during the
preparation of [BMIM][FeCl4], [SMIM][Cl], and [SMIM][FeCl4] is
illustrated in Fig. 2.
The FILs performance was tested by conducting glucose con-
version to 5-HMF and LA. The catalytic conversion of glucose was
carried out in a closed 100 mL Schott bottle equipped with a ther-
mocouple. In a typical reaction, the reactor was loaded with glucose
(0.1 g) dissolved in distilled water (10 mL) and an FIL catalyst (10 g).
The solution was heated to the desired temperature at a constant
stirring speed of 200 rpm. After the reaction was completed, the
mixture was cooled down to room temperature. All samples were
filtered before further analysis using high performance liquid chro-
matography (HPLC). Randomly selected experimental runs were
repeated to test the reproducibility of the data.
The reusability of the FIL was tested for glucose conversion
according to the following steps. After a completed run, 5 g of
distilled water was added to the reaction mixture to reduce the
viscosity of the solution, as well as to facilitate the extraction of 5-
HMF and LA. The liquid product was then separated from the FIL by
extracting the product with ethyl acetate (5 mL × 10) [60]. After the
extraction process, the remaining compounds in the solution were
FIL and water. The solution was then vaporized at 105 ◦C until the
water in this system was completely removed. The dried FIL was
used directly in the next run by adding fresh feedstock under the
same reaction conditions.
2.3. Characterization of FILs
The elemental analysis of the FILs was conducted using Elemen-
tal Analyzer Vario MICRO Cube. The results of 1H NMR and 13C NMR
(Fig. S1–S6) were recorded on NMR Bruker Avance II 400 MHz in
DMSO-d6 and CDCl3 solvents.
The acidic type (Brønsted and Lewis) of the FILs was determined
by FTIR using pyridine as a probe. The FILs were heated and mixed
with pyridine by a volume ratio of 5:2. The FTIR spectra of the FIL
were recorded on a PerkinElmer infrared spectrometer using KBr
pellets and pressed KBr discs (KBr pellets/film) in a resolution of
4 cm−1, and in a frequency range of 1350–1600 cm−1. The ratio of
Brønsted to Lewis acidity was determined based on the method in
the literature [59] shown in Eq. (1).
2.5. Product analysis
For glucose conversion analysis, dinitrosalicylic acid (DNS)
method was used throughout this study. DNS reagent was pre-
pared by mixing 2 g of NaOH in a 100 mL solution. About 80 mL
of NaOH solution was mixed with 1 g of 3,5-dinitrosalicylic acid,
30 g of potassium sodium tartrate tetrahydrate, and 0.83 g of
sodium sulfite to give the DNS reagent. 1 mL of supernatant from
the product sample was mixed with 4 mL of DNS reagent. The
resulting solution was heated in boiling water for 5 min. Next,
AB × CL
Ratio Brønsted to Lewis acidity =
(1)
AL × CB
Where AB and AL are the areas of Brønsted and Lewis acidity
from FTIR (cm−1), respectively, CB is the coefficient of Brønsted
acidity (188 cm/mmol), and CL is the coefficient of Lewis acidity
(142 cm/mmol).

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N.A.S. Ramli, N.A.S. Amin / Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
Table 1
Acidic properties of the FILs.
L
B
[BMIM][FeCl4]
FIL
Ho
Acid value (g NaOH/g FIL)
[BMIM][FeCl4]
[SMIM][Cl]
[SMIM][FeCl4]
3.58
3.40
3.36
0.28
0.45
0.94
[SMIM][Cl]
3.36, and 1.16 for [BMIM][FeCl4], [SMIM][Cl], and [SMIM][FeCl4],
respectively.
The acidic properties of the FILs, based on the Hammett and
acid-base titration methods, are summarized in Table 1. The Ho
values of the [BMIM][FeCl4], [SMIM][Cl], and [SMIM][FeCl4] are
3.58, 3.40, and 3.36, respectively. Since lower value of Ho corre-
sponds to greater acid strength [62], [SMIM][FeCl4] has higher acid
strength than the other two FILs. It is important to note that the
acidity amount of the FILs determined by acid- titration is con-
sistent with the Hammett test according to the following order:
[SMIM][FeCl4] > [SMIM][Cl] > [BMIM][FeCl4]. Based on the tests, it
is suggested that the acidity of [SMIM][FeCl4] is predominantly
contributed by the presence of sulfonic acid group.
[SMIM][FeCl4]
1560
1540
1520
1500
1480
1460
1440
1420
1400
Wavenumber (cm-1
)
Fig. 3. Pyridine-FTIR spectra of the FILs.
the absorbance of the mixture was measured at 540 nm using
UV–vis spectrophotometer. Meanwhile, the concentrations of 5-
HMF and LA in the liquid product were determined using HPLC
(Waters 2690) under the following conditions: column = Hi Plex H;
flow rate = 0.6 mL/min, mobile phase = 5 mM H2SO4, detector = UV
250 nm; retention time = 45 min; and column temperature = 60 ◦C.
Glucose conversion and product (5-HMF, LA) yields were calculated
according to the following Eqs. (4)–(5).
3.2. Glucose conversion with different FIL catalysts
FILs with different cations and anions were employed as cata-
lysts for glucose dehydration reaction. As indicated in Fig. 4, the
conversions of glucose in the reactions carried out for 1 to 5 h at
110 to 170 ◦C depended on the FIL catalysts. The FILs were effi-
cient for glucose conversion, and the acidity of the FIL influenced
the reaction to a great extent. Large number of acid sites is essen-
tial in all the reaction pathways involved including isomerization
and dehydration/rehydration reactions. The isomerization of glu-
cose into fructose has been reported in favour of Lewis acid sites
[10,32,63,64].
Glucose conversion(%)
Initial glucose amount(g) − final glucose amount(g)
=
× 100%
(4)
Initial glucose amount(g)
Product amount(g)
Initial glucose amount(g)
Product yield(%) =
× 100%
(5)
A
complete conversion of glucose was achieved using
[BMIM][FeCl4] (Fig. 4a). Meanwhile, [SMIM][Cl] exhibited poor
activity, with the highest glucose conversion registered was 80%
at 170 ◦C after 5 h reaction time (Fig. 4b). The poor performance
of [SMIM][Cl] for the conversion of glucose is probably due to
the absence of Lewis acid sites. Thus, glucose conversion by using
[SMIM][Cl] as a catalyst was attributed by the Brønsted acid sites.
It is verified that Lewis acid site has better activity in catalyzing the
isomerization of glucose than Brønsted acid site [65,66].
3. Results and discussions
3.1. Properties of FILs
The prepared [BMIM][FeCl4] and [SMIM][Cl] are viscous dark
and yellowish liquids, respectively, while [SMIM][FeCl4] is a stiff
solid at room temperature that melts at higher temperature around
70 ◦C. The elemental CHNS analysis results show similar calculated
and found values, which validated the prepared FILs. CHNSO analy-
sis (%): [BMIM][FeCl4]: Calculated C: 28.52, H: 4.46, N: 8.32; Found
C: 28.61, H: 4.45, N: 8.35. [SMIM][Cl]: Calculated C: 24.19, H: 3.53,
N: 14.11, S: 16.12; Found C: 24.26, H: 3.52, N: 14.15, S: 16.17.
[SMIM][FeCl4]: Calculated C: 13.31, H: 1.94, N: 7.76, S: 8.87; Found
C: 13.28, H: 1.94, N: 7.74, S: 8.91.
For the determination of the FILs’ acidity types, pyridine can
react separately with Brønsted and Lewis acids in the FILs [61].
Brønsted acid sites are observed at the absorption peak near
1540 cm−1 in the FTIR spectra, whereas the absorption peak of
Lewis acid sites are detected close to 1450 cm−1. By observing
these two peaks, the acidic type of the FILs can be determined. The
pyridine-FTIR spectra of the FILs are obtained as shown in Fig. 3. The
absorption peaks near 1540 and 1450 cm−1 appear in the pyridine-
FTIR spectra of [SMIM][FeCl4], suggesting that [SMIM][FeCl4]
comprised of both Brønsted and Lewis acid sites. Meanwhile,
[SMIM][Cl] and [BMIM][FeCl4] mostly contained Brønsted acid sites
and Lewis acid sites, respectively. The Brønsted acid sites in the
FIL could come from the sulfonic acid group in the imidazolium
The trend of glucose conversion using [SMIM][FeCl4] as a cat-
alyst was similar to [BMIM][FeCl4], suggesting that the Lewis acid
−
sites generated from FeCl4 enhanced the conversion of glucose
through isomerization reaction (Fig. 4c). It was observed that glu-
cose conversion increased considerably at elongated reaction time
and then remained relatively stable after 3 h, except at 110 ◦C,
for both [BMIM][FeCl4] and [SMIM][FeCl4]. For all FILs, low glu-
cose conversion rates were observed at low reaction temperature
(110 ◦C) compared to the conversion at higher temperatures, which
confirmed that elevated temperature contributes to the accelera-
tion of reaction rate and conversion efficiency.
The effect of reaction temperature and time on 5-HMF and LA
yields using three different FILs is illustrated in Fig. 5. Reaction time
and temperature played important roles in both glucose conver-
sion and product yields. The presence of LA was substantial at all
reaction conditions. This implies that 5-HMF from the dehydration
of glucose was converted to rehydration product, mainly LA. It was
also observed that the colour of the solution turned darker colour as
the reaction proceeded, which might be the evidence of decompo-
sition of the formed 5-HMF to LA. The dark coloured solution might
also be due to the presence of insoluble byproduct. The change in
the solution’s colour was clearly observed when using [SMIM][Cl]
−
cation, whilst the Lewis acid sites could be contributed by FeCl4
in the anion. The Brønsted to Lewis acid ratio of the FILs are 0.47,

N.A.S. Ramli, N.A.S. Amin / Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
117
out this study, which probably due to the tendency of formic acid
to decompose to CO2, H2, CO and H2O in heat and acidic mediums
as reported previously [68,69].
(a)
100
80
60
40
20
0
The performance of [BMIM][FeCl4] as a catalyst for glucose con-
version (Fig. 5a) demonstrated that LA yield was enhanced with
increasing temperature from 110 to 170 ◦C and at prolonged reac-
tion time. Meanwhile, the 5-HMF yield decreased after 3 h reaction
time at 170 ◦C, and after 4 h at 150 ◦C. Generally, the LA yield from
glucose using [BMIM][FeCl4] as a catalyst was lower than 5-HMF
yield throughout the reactions. This condition was due to the scarce
presence of Brønsted acid sites in [BMIM][FeCl4] which is essential
for the rehydration of 5-HMF to LA. Thus, 5-HMF was not produc-
tively rehydrated to LA. Fig. 5b shows the distributions of 5-HMF
and LA yields when [SMIM][Cl], a Brønsted FIL, was used for glu-
cose conversion reactions. The highest 5-HMF yield was recorded
at 110 ◦C after 5 h reaction time, while the highest LA yield was
observed at 170 ◦C after 5 h reaction time. The remaining 5-HMF in
the product was low since the produced 5-HMF rehydrated to LA
in the presence of Brønsted acid sites. Since glucose was not fully
converted in the reaction system with [SMIM][Cl] as a catalyst, low
LA yields were also generated. Meanwhile, when [SMIM][FeCl4]
was used as a catalyst, 5-HMF and LA yields surged up to 31%
and 67%, respectively (Fig. 5c). The high activity demonstrated by
[SMIM][FeCl4] is most likely because of its high acidity comprised
of both Lewis and Brønsted acid sites.
The highest LA yield using [SMIM][Cl] and [SMIM][FeCl4] was
obtained for glucose conversion reaction at 150 ◦C. Meanwhile, the
highest LA yield was achieved at 170 ◦C when [BMIM][FeCl4] was
employed. It is a known fact that elevated temperature contributes
to the acceleration of reaction rate and conversion efficiency. At
higher temperature, atoms donate or receive electrons more eas-
ily, increasing chemical reaction rate [70,71]. Nevertheless, in this
study, lower 5-HMF and LA yields were recorded after 3 h reac-
tion time at higher reaction temperatures (150–170 ◦C), possibly
due to the formation of other soluble and insoluble byproducts
[63,72]. The insoluble black solid residues observed in this study
were regarded as humins based on previous reports [44,73]. It
was observed that the formation of humins were obvious when
[BMIM][FeCl4] and [SMIM][FeCl4] were used as catalysts. This
might be due to the presence of Lewis acid sites in the FILs, since
Lewis acid sites are known to catalyze the formation of humins
from glucose decomposition reaction [32].
Collectively, the results indicated that strong acidity of the FIL is
an important aspect in glucose conversion for the production of 5-
HMF and LA. Among the three FILs, [SMIM][FeCl4] was identified as
the most effective FIL, where the medium strong acid sites of SO3H+
groups together with FeCl4− in the bulk of [SMIM][FeCl4] must have
fully played the roles as active sites for the dehydration reaction.
From the experimental results of glucose conversion at different
reaction temperatures and time, reaction conditions of 150 ◦C and
4 h were chosen for the subsequent experiments. Table 2 compares
the performances of various ionic liquids used [44,74,75] for glu-
cose conversion reaction in order to evaluate the competency of the
catalysts used in this study. It is evident that the FILs in this study
have improved or are comparable to the dehydration reaction with
respect to reaction time, reaction temperature, glucose conversion,
5-HMF yield, and LA yield. The relatively high glucose conversion,
as well as LA yield using [SMIM][FeCl4], could be well explained by
the use of water as the reaction medium and due to the high acidity
of the catalyst.
1
2
3
4
5
Reaction time (h)
100
80
60
40
20
0
(b)
1
2
3
4
5
Reaction time (h)
(c)
100
80
60
40
20
0
1
2
3
4
5
Reaction time (h)
Fig. 4. Effect of reaction temperature and time on glucose conversion using
[BMIM][FeCl4] (a), [SMIM][Cl] (b), and [SMIM][FeCl4] (c) as catalysts. ᭿170 ◦C,
ꢀ150 ◦C, ᭹130 ◦C, ×110 ◦C. (0.1 g glucose, 10 g FIL catalyst, 10 mL water).
and [SMIM][FeCl4] as the catalysts since the two FILs solutions are
yellowish.
The presence of water in the reaction system has facilitated
the 5-HMF rehydration. As reported previously, 5-HMF, unstable in
water, could be further converted to LA and formic acid [27]. Theo-
retically, formic acid produced from 5-HMF rehydration is in equal
molar ratio with LA [28,67]. However, the production of formic acid
is not reported since low formic acid yield was observed through-
3.3Effect of glucose and FIL loading on glucose conversion
catalyzed by [SMIM][FeCl4]
As [SMIM][FeCl4] was the most effective FIL in this study,
[SMIM][FeCl4] was chosen for further investigation on the influ-
ence of other reaction conditions. Thus, the effect of glucose loading
was studied using [SMIM][FeCl4] as a catalyst. The effect of this

118
N.A.S. Ramli, N.A.S. Amin / Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
50
40
30
20
10
0
30
25
20
15
10
5
(a)
(a)
0
1
2
3
4
5
1
2
3
4
5
Reaction time (h)
Reaction time (h)
15
12
9
30
25
20
15
10
5
6
3
(b)
(b)
0
0
1
2
3
4
5
1
2
3
4
5
Reaction time (h)
Reaction time (h)
75
60
45
30
15
0
40
30
20
10
0
(c)
(c)
1
2
3
4
5
1
2
3
4
5
Reaction time (h)
Reaction time (h)
Fig. 5. Effect of reaction temperature and time on 5-HMF and LA yields using [BMIM][FeCl4] (a), [SMIM][Cl] (b), and [SMIM][FeCl4] (c) as catalysts. ᭿170 ◦C, ꢀ150 ◦C, ᭹130 ◦C,
×110 ◦C (0.1 g glucose, 10 g FIL catalyst, 10 mL water).
parameter was investigated because process economics and envi-
ronmental friendliness can be improved if higher glucose loading is
used with a constant amount of FIL. Fig. 6 exhibits the conversion of
glucose and yields of 5-HMF and LA at various glucose loadings in
the range of 0.05–0.3 g. When the glucose loading increased from
0.05 to 0.2 g, the product yields did not decrease significantly. This
infers that excess reactive sites are available for extra conversion
of glucose under the applied reaction condition. Further increase
in the glucose weight (0.2–0.3 g) decreased the glucose conversion
from 97% to 91%, whereas 5-HMF and LA yields decreased by 5% and
14%, respectively. Additional increase in the glucose loading led to a
decrease in 5-HMF and LA yields, which probably due to the gener-
ation of the insoluble byproducts, humins, in the reaction system.
5-HMF can combine with fructose and cross-polymerize to form
humins, especially in an aqueous mixture system [44]. Besides,
the decrease in the glucose conversion at higher glucose loading

N.A.S. Ramli, N.A.S. Amin / Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
119
Table 2
Comparison of different ionic liquids for glucose conversion reaction.
Ionic liquid / co-solvent
Reaction condition
Glucose conversion (%)
Product yield (%)
5-HMF
Reference
min
◦C
LA
[SO3HMIM][CF3SO3]/MIBK
[EMIM][HSO4]
[C2OHMIM][BF4]/DMSO
[BMIM][FeCl4]/H2O
[SMIM][Cl]/H2O
360
240
60
240
240
240
120
100
180
150
150
150
97.4
95.0
NA
98.9
60.0
99.9
75.1
9
67.3
48.3
6.3
2.8
NA
NA
22.4
25.8
67.8
[44]
[74]
[75]
This study
This study
This study
[SMIM][FeCl4]/H2O
17.7
LA
5-HMF
Glucose
LA
5-HMF
Glucose
(a)
100
80
60
40
20
0
100
95
90
85
80
100
90
100
80
60
40
20
0
80
70
60
0.05
0.10
0.15
0.20
0.25
0.30
3g
5g
10g
13g
15g
Glucose loading (g)
Catalyst loading (g)
Fig. 6. Effect of glucose loading on glucose conversion and 5-HMF and LA yields
using [SMIM][FeCl4] as catalyst (10 g FIL catalyst, 10 mL water, 150 ◦C, 4 h).
LA
5-HMF
Glucose
(b)
100
80
60
40
20
0
100
95
90
85
80
implied that there were insufficient catalytic sites available for the
substrate glucose in the reaction system at the experimental con-
ditions [27]. From the analyses, 0.2 g of glucose was used for the
next testing.
FIL dosage is also an important parameter that determines glu-
cose conversion, as well as 5-HMF and LA yields. Different amounts
of [SMIM][FeCl4] (3–15 g/0.2 g of glucose) were employed in the
glucose dehydration reaction at 150 ◦C for 4 h. Fig. 7a shows that
varying the amount of [SMIM][FeCl4] catalyst affected the glucose
conversion and product yields. The glucose conversion increased as
the amount of [SMIM][FeCl4] increased from 3 to 10 g. A minimum
amount of 10 g [SMIM][FeCl4] was needed for a complete conver-
sion of glucose. Upon increasing the amount of−[SMIM][FeCl4], the
corresponding increase in the number of FeCl4 in the anions and
the SO3H+ groups in the cations seems to favour the overall reac-
tions. As the amount of Lewis acid sites increased, the conversion
of glucose conversion also increased. When [SMIM][FeCl4] dosage
was above 10 g, side effects such as aldol condensation occurred,
which led to the decrease of product yields. Besides, the large
decrease in the glucose conversion and product yields at higher
[SMIM][FeCl4] loading might be attributed to more viscous reaction
mixture. This condition will hamper the glucose conversion and
consequently inhibit the rehydration of 5-HMF to LA. The signifi-
cance of high viscosity on glucose conversion and product yields has
been discussed previously [76,77]. Since the difference of LA yield
between 5–10 g and 10–13 g of [SMIM][FeCl4] dosage was consider-
ably small, 5 g of [SMIM][FeCl4] was chosen as the optimum initial
amount of FIL to reduce the cost of reaction.
2:1
1.5:1
1:1
1:1.5
1:2
Water : catalyst loading
Fig. 7. Effect of FIL loading (a) and water: FIL loading (b) on glucose conversion and
5-HMF and LA yields using [SMIM][FeCl4] as catalyst (0.2 g glucose, 10 mL water,
150 ◦C, 4 h (a), 5 g FIL catalyst, 150 ◦C, 4 h (b)).
version and product yields between the sample with and without
[SMIM][FeCl4] verified the role of FIL as a catalyst in the glucose
conversion reaction.
Since water is used as a solvent in the reaction, the effect of
water loading was also examined in this study. Given the high
cost of ionic liquid, the addition of water that reduces the purity
of ionic liquid may offset the quantity of ionic liquid used. Water
lowers the viscosity of ionic liquid, increases the solubility of feed-
stocks and catalyst, and at the same time encourages the hydrolysis
reaction and mass transfer [55,78–80]. The effect of water to FIL
([SMIM][FeCl4]) ratio from 2:1 to 1:2 on the glucose conversion
and dehydration product yields are shown in Fig. 7b. A complete
conversion of glucose was consistently reported regardless of the
In order to confirm the role of FIL as a catalyst in the conver-
sion of glucose, an experiment using blank sample (sample without
[SMIM][FeCl4]) was also performed. The glucose conversion, 5-
HMF yield, and LA yield reached 47%, 5%, and 2%, respectively, at
150 ◦C after 4 h reaction. The huge difference of the glucose con-

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N.A.S. Ramli, N.A.S. Amin / Journal of Molecular Catalysis A: Chemical 407 (2015) 113–121
LA
5-HMF
Glucose
6
100
80
60
40
20
0
100
90
80
70
60
4
2
0
0
50
100
150
200
250
300
Reaction time (min)
Fig. 9. The kinetic profiles of glucose conversion catalyzed by [SMIM][FeCl4] at
different reaction temperatures. ᭿170 ◦C, ꢀ150 ◦C, ᭹130 ◦C, ×110 ◦C.
Table 3
Fresh 1st reuse 2nd reuse 3rd reuse 4th reuse
Reusability
Reaction rate constant (k) of glucose conversion catalyzed by [SMIM][FeCl4] at dif-
ferent reaction temperatures.
Reaction temperature
(◦C)
Reaction rate constant,
Correlation
coefficient
Fig. 8. Reusability of [SMIM][FeCl4] for glucose conversion to 5-HMF and LA (0.2 g
k (min−1
)
glucose, 5 g FIL catalyst, 5 mL water, 150 ◦C, 4 h).
110
130
150
170
0.0052
0.0121
0.0195
0.0265
0.978
0.993
0.981
0.980
amount of water loaded in the reaction system. It appears that the
presence of water only affects the product yields. When the amount
of water was low compared to the dosage of [SMIM][FeCl4] (1:2),
the product yields were low due to the high viscosity of the reaction
mixture, which resulted in low mass transfer rate and affected the
occurrence of the dehydration /rehydration reaction. Fig. 7b also
indicates that the yield of 5-HMF decreased with the increase of
water to FIL ratio loading (1:1.5 to 1.5:1). The reason was that when
the dosage of water increased, the 5-HMF rehydration occurred
progressively and the yield of LA correspondingly increased. The LA
yield continued to increase with increasing water to FIL ratio until
1.5:1, beyond which the LA yield decreased when more water was
loaded into the reaction system. This observation is probably due
to the opposition between H+ in [SMIM][FeCl4] and water to cat-
alyze the rehydration reaction, which suppresses the intermediate
reactions. Previous studies on glucose conversion in ionic liquids
reported that the presence of water in the reaction system gives
a negative effect on the reaction [55,78,81]. However, this study
verified that the FIL used in the aqueous system registered a good
performance for glucose conversion to 5-HMF and LA. Thus, at the
optimum water:FIL ratio, the reaction system applied in this study
should have the potential to be economically viable.
The experiments were carried out at the specified temperature for 5 h with 10 g FIL
catalyst and 0.1 g glucose in 20 mL H2O.
Table 4
Kinetic parameters for glucose conversion catalyzed by [SMIM][FeCl4].
Parameter
Value
Reaction order, n
1.0
Activation energy, Ea (kJ mol−1
)
38.1
925.0
0.968
Pre- exponential factor, A (min−1
)
Correlation coefficient
dence of the glucose conversion reactions. The results show that the
value of reaction rate constant increased with increasing temper-
ature, indicating that higher temperature accelerates the reaction
rate of glucose conversion (Table 3). The activation energy, Ea and
pre-exponential factor, A, were obtained by applying the Arrhenius
plot using the values of the rate constants and are listed in Table 4.
A value of 38 kJ mol−1 was reported in this study for the activa-
tion energy of glucose conversion, which is lower with than those
reported by using mineral acid [23,82,83] and [C2OHMIM][BF4]
ionic liquid [75] as catalysts, and comparable to the activation
energy reported by using Cr-SO3H polymeric ionic liquid [47] as
a catalyst. This study signifies that the introduction of functional
groups (SO3H and FeCl3) in ionic liquid catalyst has accelerated the
reaction, consequently lowering the activation energy and increas-
ing the reaction rate.
3.4. FIL reusability
To confirm the reusability of the synthesized FILs, [SMIM][FeCl4]
was selected for the dehydration reaction over five cycles. The cata-
lyst was recovered after extracting the products using ethyl acetate.
The performance of [SMIM][FeCl4] reusability was conducted for
glucose conversion at 150 ◦C for 4 h using 5 g of [SMIM][FeCl4] and
water, and 0.2 g of glucose. As shown in Fig. 8, the catalytic activ-
ity of [SMIM][FeCl4] did not decrease significantly after five runs,
which demonstrated that [SMIM][FeCl4] was stable in this system.
The conversion of glucose maintained at around 90% until the fifth
run. The decrease by 6% and 8% in 5-HMF and LA yields, respectively,
were observed.
4. Conclusions
Efficient glucose conversion to 5-HMF and LA has been achieved
using acidic FIL as a catalyst. [SMIM][FeCl4] displayed the high-
est catalytic activity in the conversion of glucose, with 18% and
68% yields of 5-HMF and LA, respectively, which was achieved
after 4 h at 150 ◦C. The acidity of the solution registered a large
effect on glucose conversion and product yields, where the cat-
alyst with stronger acidity led to higher activity. The presence
of both Brønsted and Lewis acid sites seemed suitable for 5-
HMF and LA production. The significantly high activity of aqueous
[SMIM][FeCl4] reduced the quantity of catalyst needed for the
reaction. [SMIM][FeCl4] catalyst was reused and exhibited favor-
able catalytic activity over five successive cycles. In addition, the
kinetic analysis implied that the introduction of functional groups
3.5. Kinetic analysis of glucose conversion reaction
A kinetic analysis of the conversion of glucose catalyzed
by [SMIM][FeCl4] was carried out in a temperature range of
110–170 ◦C. The values of -ln (1-X) (where X is the conversion of
glucose) were plotted against reaction time (t) at different reac-
tion temperatures to determine the reaction rate constants (k)
(Fig. 9). The linearity of Fig. 9 supported the first order depen-

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Acknowledgements
The authors would like to extend their gratitude to Universiti
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