Efficient selective dehydration of fructose and sucrose into 5-hydroxymethylfurfural (HMF) using dicationic room temperature ionic liquids as a catalyst

Article abstract of DOI:10.1016/j.catcom.2012.02.007

Dicationic room temperature ionic liquids (RTILs), with short oligo ethylene glycol linkers have been found to be highly active catalyst for the selective dehydration of fructose and sucrose into 5-hydroxymethylfurfural (HMF). The bis (N-methylimidazolium) cations containing short oligo ethylene glycol linkers and mesylate (CH₃SO₃^-) anions based ILs were employed in catalytic amount for sugar dehydration reactions. As a result, 92.3% of HMF yield was obtained from fructose in 40 min with one equivalent of [TetraEG(mim)₂][OMs]₂ at 120°C. While, 67.2% of HMF was achieved from dehydration of sucrose at 120°C in 150 min using two equivalents of [TetraEG(mim)₂][OMs]₂. Among those dicationic RTILs, [TetraEG(mim)₂][OMs]₂ RTIL was observed to be the most efficient catalyst, demonstrated by its ability to achieve 100% conversion and highest yield of HMF. In addition, effect of reaction time, molar ratio, temperature, and co-catalyst effect were discussed. The use of dicationic RTILs as a catalyst in sugar dehydration greatly reduces the equimolar ratio of catalyst and reaction time required for conversion of sugar into HMF.

Full text of DOI:10.1016/j.catcom.2012.02.007

Catalysis Communications 21 (2012) 96–103
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Efficient selective dehydration of fructose and sucrose into 5-hydroxymethylfurfural
(HMF) using dicationic room temperature ionic liquids as a catalyst
b
Arvind H. Jadhav a, Hern Kim a, , In Taek Hwang
⁎
a
Department of Environmental Engineering and Energy, Energy and Environment Fusion Technology Center, Myongji University, Yongin, Kyonggi-do 449-728, Republic of Korea
Korea Research Institute of Chemical Technology, Yuseong P.O. Box 107, Daejon 305-600, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2012
Received in revised form 6 February 2012
Accepted 7 February 2012
Available online 15 February 2012
Dicationic room temperature ionic liquids (RTILs), with short oligo ethylene glycol linkers have been found to
be highly active catalyst for the selective dehydration of fructose and sucrose into 5-hydroxymethylfurfural
(HMF). The bis (N-methylimidazolium) cations containing short oligo ethylene glycol linkers and mesylate
(CH3SO3−) anions based ILs were employed in catalytic amount for sugar dehydration reactions. As a result,
92.3% of HMF yield was obtained from fructose in 40 min with one equivalent of [TetraEG(mim)2][OMs]2
at 120 °C. While, 67.2% of HMF was achieved from dehydration of sucrose at 120 °C in 150 min using two
equivalents of [TetraEG(mim)2][OMs]2. Among those dicationic RTILs, [TetraEG(mim)2][OMs]2 RTIL was ob-
served to be the most efficient catalyst, demonstrated by its ability to achieve 100% conversion and highest
yield of HMF. In addition, effect of reaction time, molar ratio, temperature, and co-catalyst effect were dis-
cussed. The use of dicationic RTILs as a catalyst in sugar dehydration greatly reduces the equimolar ratio of
catalyst and reaction time required for conversion of sugar into HMF.
Keywords:
Dicationic room temperature ionic liquids
5-Hydroxymethylfurfural
Dehydration
Fructose
Sucrose
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
and catalysts or reagents, which limits the use of large scale reaction
for above methods. Moreover, the presence of acidic resin catalysts at
Finding ways to make fuels and chemicals from renewable sources
is among the most active research areas in chemistry. Biomass re-
sources are promising alternatives for the sustainable supply of fuel
and valuable chemicals [1–4]. Especially, carbohydrates derived
from biomass, which are abundant, relatively inexpensive and renew-
able promising energy sources [5]. Fructose and sucrose have been
considered as superior carbohydrate compounds from which various
furan based chemicals can be derived [5,6]. Among those chemicals,
5-hydroxymethylfurfural (HMF) has received significant attention
as a platform chemical for synthesizing a broad range of chemicals
and bio-based fuels [1–4]. In last decade, the enormous work in the
development of sustainable methods for the preparation of HMF
from biomass has been reported [7].
The dehydration of simple sugars to form HMF has been conducted
in water and in various organic solvents such as dimethyl sulfoxide,
dimethylformamide, and dimethylacetamide in presence of different
acidic catalysts [8–10]. Strongly acidic ion-exchange resin [11], zeolites
[12] supported heteropolyacid [13,14], and inorganic salts [15,16] have
also been employed for the dehydration of fructose. Recently, biphasic
water/organic systems using transition metal ions, mineral acids as cat-
alysts showed efficient dehydration reaction of sugars [17–20]. Many of
these reactions require drastic reaction conditions, excess of solvent
high temperature have favored the subsequent dehydration of HMF to
other by-products such as levulinic acid and formic acid which ulti-
mately reduces the yield of HMF [21,22]. Nowadays ionic liquids (ILs)
are the most convenient and popular solvents and catalysts for selective
preparation of HMF from sugars and these processes have received in-
creasing attention [11,17–21].
It has been claimed on a number of occasions that ILs represent a
new paradigm in green chemistry [23]. ILs have received great atten-
tion in various fields owing to their potential as eco-friendly solvents
[17–21], due to several unique properties such as extremely low
vapor pressure, non-flammability, high thermal stability and easy to
separate from reactions [24]. Because of these promising properties,
ILs have been used as solvent in the dissolution of biomass and in
the dehydration of sugars to produce various useful chemicals includ-
ing HMF [25]. Especially, various imidazolium ILs can act as solvent in
the dehydration of fructose, such as 1-ethyl-3-methylimidazolium
chloride ([EMIM][Cl]) and 1-butyl-3-methylimidazolium chloride
([BMIM][Cl]) with chromium (II) chloride were efficiently catalyze
the dehydration of fructose [26,27]. The ILs have crossed the barrier
of solvent and entered very successfully into the area of catalyst. Several
special ILs for instance 1H-3-methylimidazolium chloride ([HMIM][Cl])
[15], N-methylmorpholinium methyl sulfonate ([NMM][CH3SO3]) and
[BMIM][Cl] are applied as catalyst and solvent for fructose dehydration
[28,29].
In addition, various tailor made ILs especially, dicationic ILs are be-
coming popular green solvent for various chemical conversions [30].
⁎
Corresponding author. Tel.: +82 31 330 6688; fax: +82 31 336 6336.
E-mail address: hernkim@mju.ac.kr (H. Kim).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.02.007

A.H. Jadhav et al. / Catalysis Communications 21 (2012) 96–103
97
The numerous applications of dicationic ILs have been investigated in
the field of science such as organic reactions and enzymatic treatment
as well as in chemical analysis [31–33]. It was also reported that dica-
tionic ILs have been found capable of dissolving cellulose and carbo-
hydrates to a great extent [32]. For example lignocellulose material
can dissolve in the presence of dicationic [C2(mim)2][Cl]2 ILs [34]. In
addition, dicationic [C4(mim)2][Cl]2 IL perform the pyrolysis of cellu-
lose significantly to levoglucosenone as the main anhydrosugar
product at considerably lower temperature [35]. This reported meth-
od of controlled pyrolysis of cellulose to anhydrosugar by using
[C6(mim)2][Cl]2, [C6(mim)2][Br]2 dicationic ILs [35] encourages us to
synthesize the structurally modified dicationic ILs of imidazolium
core moiety and utilize it in sugar dehydration significantly.
It is well known that, the dehydration of sugars are possibly car-
ried out in hydrophilic ILs [15], the dicationic ILs synthesized and uti-
lized in this method are hydrophilic in nature and liquid at room
temperature. The imidazolium cations with short oligo ethylene gly-
col chains as a linkers and methane sulfonate (CH3SO3−) group as an-
ions based RTILs were able to dissolved fructose and sucrose. To the
best of our knowledge, no reports have been found on dicationic
RTILs as catalysts for the fructose and sucrose dehydration reactions.
Here, we introduce an efficient strategy for selective conversion of
fructose and sucrose into HMF by using dicationic RTILs as catalysts.
In addition, the effect of short oligo ethylene glycol chain length on
dehydration of fructose and sucrose has been studied. These dicatio-
nic ILs are able to show 100% conversion of sugars to HMF at 120 °C
efficiently showing catalytic performance only at one equimolar
quantity which leads to green process that emphasizes atom econo-
my and elimination of hazardous waste.
reaction mixture was quenched with water and extracted with
dichloromethane (3×50 ml). The organic layer was washed with
water (3×50 ml) dried over sodium sulfate and concentrated to dry-
ness under reduced pressure on rotary evaporator to afforded diethy-
lene glycol dimesylate precursor; yield 92.6%; thick liquid; 1H NMR
(500 MHz, CDCl3): δ 4.46 (t, 2×2H), 3.86 (t, 2×2H), 3.19 (2×3H);
IR (500–4000 cm−1): 2987, 2864 [υ(C\H)]; 1356, 1174 [υ(S_O)];
1114 [υ(C\O\C)]; 767 [υ(S\O)]. The same procedure was followed
for the synthesis of tri and tetra ethylene glycol dimesylate precursor.
2.3.2. Triethylene glycol dimesylate
Yield 92.9%; thick liquid; 1H NMR (500 MHz, CDCl3): δ 4.43
(t, 2×2H), 3.72 (t, 2×2H), 3.63 (t, 2×2H), 3.03 (s, 2×3H); FT-IR
(500–4000 cm−1): IR (500–4000 cm−1): 2961, 2827 [υ(C\H)];
1338, 1133 [υ(S_O)]; 1098 [υ(C\O\C)]; 799 [υ(S\O)].
2.3.3. Tetra ethylene glycol dimesylate
Yield 94.2%; thick liquid; 1H NMR (500 MHz, CDCl3): δ 4.37
(t, 2×2H), 3.63 (t, 2×2H), 3.60 (s, 4×2H), 3.04 (s, 2×3H); FT-IR
(500–4000 cm−1): 2982, 2887 [υ(C\H)]; 1341, 1162 [υ(S_O)];
1090 [υ(C\O\C)]; 726 [υ(S\O)].
2.4. Typical procedure for the synthesis of dimesylate RTILs
2.4.1. Diethylene glycol-bis (3-methylimidazolium) dimesylate
[DiEG(mim)2][OMs]2
A mixture of diethylene glycol dimesylate (5.0 g, 19.06 mmol) and
N-methylimidazole (3.12 g, 38.12 mmol) in acetonitrile (50 ml) was
refluxed magnetically for 4 days in a two necked round bottom flask
equipped with water condenser. The reaction progress was moni-
tored by TLC. After completion of reaction, the reaction mixture was
allowed to cool at room temperature. The solvent was evaporated
under reduced pressure on rotary evaporator at 55 °C. The reaction
mixture was washed with ethyl acetate (3×15 ml) to remove
unreacted starting materials and resulting quaternized diethylene
glycol-bis (3-methylimidazolium) dimesylate was obtained; yield
89%; thick liquid; 1H NMR (500 MHz, CDCl3): δ 9.79 (s, 2×H), 7.82
(s, 2×H), 7.50 (s, 2×H), 4.51 (t, J=4.8 Hz, 2×2H), 4.06 (s, 2×3H),
3.85 (t, J=4.8 Hz, 2×2H), 2.67 (s, 2×3H); 13C NMR (125 MHz,
2. Experimental
2.1. Materials
N-methylimidazole (99%), methanesulfonyl chloride (99%),
triethyl amine (99%), diethylene glycol (99%), triethylene glycol
(99%), tetra ethylene glycol (99%), 5-hydroxymethylfurfural (99%),
D-fructose (99%), sucrose (99%), FeCl3, CuCl2, NiCl2.6H2O, CoCl2.6H2O,
sodium sulfate, etc. were purchased from Sigma Aldrich. All the sol-
vents were purchased from commercial sources and were distilled
from the relevant agents prior to use.
CDCl3):
δ 138.0, 123.5, 123.3, 65.4, 58.2, 49.4, 39.8; FT-IR
(500–4000 cm−1): 3094 [υ(Ar\H)]; 2956, 2889 [υ(C\H)]; 1617
[υ(C_N)]; 1569, 1452, 1423 [υ(C_C)]; 1180, 1034 [υ(S_O)]; MS
m/z [M+H]+ calcd for C13H24N4O4S: 332.14; found: 332.15.
2.2. Characterization
All synthesized dicationic RTILs were characterized by 1H NMR
and 13C NMR spectroscopy on Bruker spectrometer 500 MHz and
125 MHz, respectively using CDCl3 as a solvent. The reported chemical
shifts were against TMS as reference for 1H and 13C NMR. Low resolu-
tion mass spectra were obtained using electrospray ionization on
Waters Micromass ZQ LC/MS 2000 mass spectrometer. FT-IR spectra
were recorded on Varian 2000 (Scimitar series) spectrophotometer.
TLC analysis was performed on silica-gel Poly Gram SIL G/UV 254 plates
to monitor the reaction progress.
2.4.2. Triethylene glycol-bis (3-methylimidazolium) dimesylate
[TriEG(mim)2][OMs]2
Yield 93%; thick liquid; 1H NMR (500 MHz, CDCl3): δ 9.51
(s, 2×H), 7.56 (s, 2×H), 7.53 (s, 2×H), 4.38 (t, J=4.8 Hz, 2×2H),
3.94 (s, 2×3H), 3.75 (t, 2×2H), 3.52 (t, J=4.8 Hz, 2×2H), 2.64
(s, 2×3H); 13C NMR (125 MHz, CDCl3): δ 137.7, 125.5, 124.1, 76.8,
71.1, 69.0, 49.6, 39.8; FT-IR (500–4000 cm−1): 3095 [υ(Ar\H)];
2942, 2819 [υ(C\H)]; 1569 [υ(C_N)]; 1569, 1455, 1391 [υ(C_C)];
1180, 1064 [υ(S_O)]; MS m/z [M+H]+ calcd for C15H28N4O5S:
376.17; found: 376.15.
2.3. Typical procedure for synthesis of precursors
2.4.3. Tetra ethylene glycol-bis (3-methylimidazolium) dimesylate
[TetraEG(mim)2][OMs]2
2.3.1. Diethylene glycol dimesylate
A solution of diethylene glycol (5.0 g, 47.11 mmol) in 50 ml of
dichloromethane was taken in round bottom flask and the reaction
mixture was cooled at 0 °C then triethyl amine (9.53 g, 94.22 mmol)
was added in it dropwise and the reaction mixture was stirred for
10 min. Then methanesulfonyl chloride (10.79 g, 94.22 mmol) was
added in the reaction mixture over a period of 5–10 min. The reaction
mixture was stirred at room temperature for 12 h. The reaction pro-
gress was monitored by TLC, complete disappearance of starting ma-
terial was observed after 12 h. After completion of the reaction, the
Yield 87%; thick liquid; 1H NMR (500 MHz, CDCl3): δ 9.66
(s, 2×H), 7.62 (s, 2×H), 7.58 (s, 2×H), 4.50 (t, 2×2H), 4.02
(s, 2×3H), 3.90 (t, J=4.8 Hz, 2×2H), 3.65 (t, J=4.8 Hz, 2×2H),
3.59 (t, 4×2H), 2.76 (s, 2×3H); 13C NMR (125 MHz, CDCl3): δ
138.0, 123.3, 123.2, 77.5, 70.3, 69.1, 49.5, 39.8, 36.3; FT-IR
(500–4000 cm−1): 3096 [υ(Ar\H)]; 2973, 2829 [υ(C\H)]; 1612
[υ(C_N)]; 1568, 1526, 1458 [υ(C_C)]; 1183, 1073 [υ(S_O)]; 1136
[υ(C\N)]; MS m/z [M+H]+ calcd for C17H32N4O6S: 420.20; found:
420.19.

98
A.H. Jadhav et al. / Catalysis Communications 21 (2012) 96–103
2.5. Typical reaction procedure for the dehydration of D-fructose and sucrose
Acetonitrile layer containing RTIL was separated by decantation pro-
cess. The same procedure was repeated five times and acetonitrile
was collected in round bottom flask. The collected acetonitrile layer
was dried over sodium sulfate and evaporated under reduced pres-
sure to obtain RTIL as thick liquid. Further RTIL was analyzed by
NMR and IR spectroscopy and reused for the next attempts.
Freshly dried dicationic RTILs (1.0 g) in vacuum oven for 30 min at
60 °C was charged into a 25 ml two necked round bottom flask
equipped with a magnetic stirrer and a condenser. This assembly
was placed in an oil bath preset at 120 °C temperature with stirring
for 30 min and then followed by the addition of appropriate equimo-
lar amount of fructose or sucrose into reaction vessel. The reaction
mixture was stirred vigorously and heated with a thermostatically
controlled oil bath for a specific time. The sample collections were
carried out at successive time interval and diluted with 5 ml milliQ
water and analyzed by HPLC.
3. Results and discussions
Scheme 1 illustrates that the synthetic route for dicationic RTILs,
[DiEG(mim)2][OMs]2, [TriEG(mim)2][OMs]2, and [TetraEG(mim)2]
[OMs]2 was synthesized by using appropriate precursors and N-
methylimidazole. The choice of the counter ion made this method
convenient for preparation of imidazolium ILs in good yield. Different
lengths of oligo ethylene glycol chains used in this method were pro-
tected with mesylate groups and developed three kinds of precursors.
Further, these precursors were reacted with two equivalent of N-
methylimidazole and afforded dicationic ILs. The detailed synthetic
procedures for these ILs are described in the Experimental section.
Three different kinds of dicationic RTILs were examined for dehy-
dration of fructose and sucrose and in these ILs two imidazolium cat-
ions are linked with short oligo ethylene glycol chains and mesylate
(CH3SO3−) groups as anions. In synthesized ILs, we changed the linker
chains of oligo ethylene glycol and investigated the effect on the de-
hydration of sugars. In this study, di, tri, and tetra ethylene glycol
chains were used as linkers between two imidazolium rings. The se-
lection of oligo ethylene glycol chains in cations makes the ILs more
hydrophilic in nature [36]. We believed that oxygen atoms in the
oligo ethylene glycol chains of cations act as hydrogen bond acceptor
and forms hydrogen bonding with sugars, and promote the sugar dis-
solution [37]. Literature reports that only short oligo ethylene glycol
chains can increase the hydrophilicity and chemical stability. Due to
these properties, short oligo ethylene glycol chains interact with
sugars and forms hydrogen bonds. While a longer oligo ethylene gly-
col chains are able to increase the hydrophobicity of the overall RTILs
[36,37]. Therefore, a suitable oligo ethylene glycol chain length in be-
tween the imidazolium cations is necessary for the hydrophilicity of
RTILs. On the other hand, mesylate (CH3SO3−) anions also form strong
hydrogen bonding with sugars [32].The hydrogen bond forming
mesylate anions, oxygen containing bis (imidazolium) cations are
beneficial to the sugar dissolution in the synthesized RTILs. Their ca-
pability in forming hydrogen bonds is considered as the main mech-
anism of sugar dissolution and formation of HMF. The synthesized
dicationic RTILs in this study possess Lewis acidic characters. It is
also reported that Lewis acidic ILs are more effective than the
Brønsted acidic ILs in terms of both the sugar conversion and the
HMF yield [38]. In this work, the dicationic RTILs showed an efficient
Lewis acidity for the dehydration of fructose and sucrose into HMF
and achieved the highest yield of HMF in the absence of any other cat-
alyst and solvent.
2.6. Analytical methods
HMF was analyzed by using a Waters HPLC with an UV detector at
284 nm and a SunFire™ C18 column (4.8 mm×150 mm) at 35 °C. A
mixture of methanol and water at a volumetric ratio of 1:4 was
used as the mobile phase. The flow rate was 0.6 ml/min. The fructose
and sucrose concentration were analyzed using a Water HPLC system
with RID-2414 detector and a Bio-Rad Aminex HPX-87H ion exclu-
sion column (300 mm×7.8 mm). The eluent used was 5.0 mM
H2SO4 with a flow rate of 0.6 ml/min. The amount of fructose and su-
crose was determined using the external standards. 1H and 13C NMR
spectra of HMF were recorded by using a Bruker NMR spectrometer
(500 MHz) with CDCl3 as the solvent. Varian 2000 (Scimitar series)
spectrophotometer was used for the IR spectroscopy.
2.7. Separation procedure for HMF
After completion of reaction, the reaction mixture was transferred in
to the 250 ml round bottom flask and afterwards 50 ml of THF was
added into the flask. The reaction mixture was stirred vigorously for
2 h at room temperature. THF layer containing HMF was separated by
decantation process. The same procedure was repeated three times
and THF was collected in round bottom flask. The collected THF layer
was dried over sodium sulfate and evaporated under reduced pressure
to obtain pure HMF as thick pale yellow liquid. Further HMF was ana-
lyzed by NMR and IR spectroscopy. According to the NMR spectra, puri-
ty was 99%. 1H NMR of 5-Hydroxymethylfurfural (500 MHz, CDCl3): δ
7.7252 (s, 2H), 6.5244 (d, 1H), 7.23 (d, 1H), 9.5867 (s, H). 13C NMR
(125 MHz, CDCl3): δ 57.6795, 110.1014, 152.4123, 160.74, 177.8046.
FT-IR (500–4000 cm−1): IR (500–4000 cm−1): 3359, 3156, 3117,
2923, 2869, 1668, 1581, 1520, 1429, 1396, 1367, 1336, 1188, 1072,
and 1019.
2.8. Separation and purification of dicationic RTILs
After completion of one set of fructose dehydration reaction, the
reaction products were separated using THF, remaining reaction mix-
ture containing RTIL was transferred in to the 50 ml round bottom
flask and 15 ml of acetonitrile was added in it. The selection of aceto-
nitrile for the separation of RTIL from the reaction mixture was due to
these dicationic RTILs that showed good solubility in acetonitrile. The
mixture was stirred vigorously for 30 min at room temperature.
3.1. Dehydration of fructose and sucrose in dicationic RTILs
To study the efficient catalytic activity of dicationic RTILs, we car-
ried out the dehydration of sugars in three different RTILs at 120 °C
Scheme 1. The reaction pathway for the synthesis of imidazolium dicationic RTILs.

A.H. Jadhav et al. / Catalysis Communications 21 (2012) 96–103
99
100
80
60
40
20
0
HMF yield in these dicationic RTILs are due to increased hydrogen bond-
ing in ether linkage of the successive increasing oligo ethylene glycol
chains between the two imidazolium rings. As the length of the oligo
ethylene glycol chain is longer in RTILs, their catalytic activity and
hydrophilicity are correspondingly higher. From the above results we
concluded that activity order between the RTILs and HMF yield can be
[DiEG(mim)2][OMs]2b[TriEG(mim)2][OMs]2b[TetraEG(mim)2][OMs]2.
Here, the obtained conversion of fructose and yield of HMF with
[TetraEG(mim)2][OMs]2 RTIL are higher to that with reported
method using monocationic ILs at similar reaction temperature
[38]. Therefore, for the further investigations for fructose dehydra-
tion, one equivalent of [TetraEG(mim)2][OMs]2 with 40 min reac-
tion time at 120 °C was selected.
Fructose
Sucrose
A
B
C
The similar trend was observed in dehydration of sucrose in
which for 100% conversion of sucrose, two equivalents of RTILs
were needed with 150 min reaction time at 120 °C. In the pres-
ence of [DiEG(mim)2][OMs]2 and [TriEG(mim)2][OMs]2 sucrose
converted 100% with 54.4% and 61.2% HMF yield, respectively.
Fig. 1 shows dehydration of sucrose with highest yield of HMF
67.2% achieved in [TetraEG(mim)2][OMs]2 IL. While for detailed
investigation of sucrose dehydration reaction, two equivalents of
[TetraEG(mim)2][OMs]2 with 150 min reaction time at 120 °C
reaction temperature were used. The given Table 1 shows the
comparison between the reported results of dehydration of fruc-
tose with different heating modes in monocationic ILs and dicatio-
nic RTILs utilized in this study. The fructose conversion, yield and
selectivity of HMF by using dicationic ILs in conventional oil bath
studied in this method are quite efficient and higher then reported
methods in various heating modes.
Types of RTILs
Fig. 1. Dehydration of fructose and sucrose in dicationic RTILs. Reaction condition:
(a) fructose dehydration, one equimolar of [TetraEG(mim)2][OMs]2, 40 min reaction
time, 120 °C. (b) Sucrose dehydration, two equimolar of [TetraEG(mim)2][OMs]2,150 min
reaction time, 120 °C.
(Scheme 2). Initially the dehydration was performed by using one
molar ratio of sugars in RTILs. Visual observation indicated that the
fructose and sucrose were completely soluble in all RTILs under the
experimental conditions. [DiEG(mim)2][OMs]2, [TriEG(mim)2][OMs]2,
and [TetraEG(mim)2][OMs]2 ILs were able to convert 100% fructose
with one equimolar ratio in appropriate time. Fig. 1 shows that the
oligo ethylene glycol linkers between the two imidazolium cations
greatly affected the HMF yield. The reactivity of dicationic ILs were
initially investigated with fructose in [DiEG(mim)2][OMs]2 IL at 120 °C
for 40 min. This reaction showed that 100% conversion of fructose
with 69.8% HMF yield could be achieved. As the oligo ethylene
glycol chain increase from di to triethylene glycol, the HMF yield
increased from 69.8% to 77.2% at uniform reaction conditions.
When [TetraEG(mim)2][OMs]2 IL was applied for the dehydration of
fructose, it was observed that fructose dehydrated more rapidly and
selectively into HMF in comparison with di and tri ethylene glycol ILs.
This reaction was providing 92.3% of HMF with 100% conversion of
fructose. The dicationic [TetraEG(mim)2][OMs]2 IL represented itself
as the most effective catalyst for the achievement of highest HMF
yield with 100% conversion of sugar. The substantial differences in the
3.2. Effect of reaction time on dehydration
The dehydration reaction time of sugar was varied to study the
rate of HMF formation as a function of time. As plotted in Fig. 2(a),
the yield of HMF improved from 12.3% to 49.1% upon increasing the
duration of the reaction period from 5 min to 20 min in dicationic
[TetraEG(mim)2][OMs]2 IL catalyzed dehydration of fructose at
120 °C. Further increase in reaction time to 40 min under similar con-
dition resulted in maximum HMF yield of 92.3% with 100% fructose
conversion. The same behavior of increasing HMF yield with an
Scheme 2. The dehydration reaction of D-fructose and sucrose with dicationic RTILs.

100
A.H. Jadhav et al. / Catalysis Communications 21 (2012) 96–103
Table 1
Comparisons of reported results of monocationic ILs with dicationic RTILs utilized in this study for dehydration of fructose.
Sr. no.
Ionic liquids
Temperature (°C)
HMF yield (%)
Selectivity (%)
Conversion (%)
Heating mode
Molar proportionality
Reference
1
2
3
4
5
6
7
8
9
[Bmim][Cl]
[Bmim][Cl]
[NMM][CH3SO3]
[Hmim][Cl]
[Bmim][Cl]
120 °C
155
90
120
175
140
120
120
120
63.1
98
74.8
7.3
67.6
98
93.4
99
Oila
MIb
Oila
Oila
MIb
Oila
Oila
Oila
Oila
10
10
10
10
10
10
1
[38]
[29]
[28]
[38]
[29]
[38]
In this work
In this work
In this work
–
–
11.1
100
60.4
69.8
77.2
92.3
65.7
97
100
100
100
100
97
[Bmim][Cl]
60.4
69.8
77.2
92.3
[DiEG(mim)2][OMs]2
[TriEG(mim)2][OMs]2
[TetraEG(mim)2][OMs]2
1
1
a
Oil bath heating.
Micro wave heating.
b
increasing reaction time was observed for sucrose dehydration in
similar reaction condition with [TetraEG(mim)2][OMs]2 RTILs as
shown in Fig. 2(b). The yield of HMF from sucrose improved from
7.0% to 35.5% when increasing the reaction time period from 15 min
to 75 min. Afterwards a similar trend of slow increasing HMF yield
up to 67.2% in 150 min was observed in sucrose dehydration.
probably the optimal reaction times under the given experimental
conditions.
3.3. Effect of temperature on dehydration
It is important to note that the color of the reaction mixture grad-
ually turned from pale yellow to deep brown when the dehydration
of sugar was continued for a longer reaction time. Perhaps, the color
change of the reaction mixture is associated with the decomposition
of HMF into side products as reported in the literature [39]. In our
results, after 40 min in fructose dehydration and 150 min in sucrose
dehydration the color of the reaction mixture changes from pale yel-
low to dark brown rapidly. However the conversion or decomposition
of HMF into other by-product was observed in both the sugars. There-
fore, 40 min in fructose and 150 min in sucrose dehydration were
The dehydration reactions of sugars were studied at 80 °C–130 °C
in order to determine the efficiency of dicationic [TetraEG(mim)2]
[OMs]2 RTIL, as well as the effect of reaction temperature on the for-
mation of HMF. As shown in Fig. 3, the reaction temperature had a
large effect on the dehydration of sugars into HMF. The reaction tem-
perature plays an important role in both the sugar conversion and
HMF yield. When the reaction temperature was 80 °C, the conversion
rates of sugars with the formation rate of HMF were relatively low.
Only 23.0% and 18.1% HMF yield were obtained from fructose and su-
crose, respectively. The gradual increase in HMF yield was observed
with the increasing reaction temperature. When the reaction temper-
ature increased up to 100 °C, the conversion of sugars and the forma-
tion of HMF became much faster. Finally, 100% sugar conversion
reached at 120 °C and resulted 92.3% and 67.2% HMF yields from fruc-
tose and sucrose, respectively. The increasing HMF yield with increas-
ing reaction temperature is due to the less viscous environment
leading to a high rate of molecular transport so the efficiency of disso-
lution of sugars is usually very high [40]. Therefore, raising tempera-
tures is an alternative strategy to reduce the viscosity of RTILs and can
also accelerate the speed of molecular transformation, enhancing the
yield of HMF. On the other hand, temperatures at 130 °C gave rise to
reported by-products [39] and insoluble polymer leading to lower
HMF selectivity in both the sugar dehydrations. The results indicate
that harsh reaction conditions may not only promote the dehydration
reaction of sugars, but also accelerate HMF polymerization and for-
mation of related by-products. The use of RTILs as catalyst and solvent
a
100
80
60
40
Selectivity
Yield
20
0
Conversion
0
10
20
30
40
50
Time (min)
b
100
100
80
60
40
20
0
Fructose
Sucrose
80
60
40
20
0
Yield
Conversion
Selectivity
80
90
100
110
120
130
0
30
60
90
120
150
Temperature (oC)
Time (min)
Fig. 2. Effect of reaction time on dehydration of fructose and sucrose.Reaction condi-
tion: (a) fructose dehydration, one equimolar [TetraEG(mim)2][OMs]2, 120 °C reaction
temperature. (b) Sucrose dehydration, two equimolar [TetraEG(mim)2][OMs]2, reac-
tion time, 120 °C reaction temperature.
Fig. 3. Effect of temperature on dehydration of fructose and sucrose.Reaction condition:
(a) fructose dehydration, one equimolar [TetraEG(mim)2][OMs]2, 40 min reaction
time. (b) Sucrose dehydration, two equimolar [TetraEG(mim)2][OMs]2, 150 min reac-
tion time.

A.H. Jadhav et al. / Catalysis Communications 21 (2012) 96–103
101
in dehydration of sugar significantly reduced the temperature re-
quired for the dehydration reaction.
3.4. Effect of molar ratio of [TetraEG(mim)2][OMs]2 RTIL on dehydration
The dehydration reaction of sugars into HMF was studied at vari-
able mole ratio of RTILs for optimizing the reaction condition and
maximizing the HMF yield. The concentration of [TetraEG(mim)2]
[OMs]2 was varied from 0.5 equivalence to 2.5 equivalence while
other reaction parameters, reaction time and reaction temperature
were kept constant. As shown in Fig. 4(a) the yield of HMF increased
from 44.2% to 92.3% with an increasing the [TetraEG(mim)2][OMs]2
mole ratio from 0.5 to 1.0 in fructose dehydration. When we used
more than one equivalent of RTIL the selectivity with yield of HMF de-
creased and other reported side products were formed [39]. The most
important feature of synthesized dicationic ILs which is showing
100% sugar conversion with 92.3% HMF yield from fructose in one
equimolar ratio of [TetraEG(mim)2][OMs]2 using conventional oil
bath heating. In sucrose dehydration, the effect of mole ratio showed
a slightly different effect than fructose dehydration reaction. Results
showed that only 22.3% HMF was formed from sucrose with 0.5 equi-
molar RTIL. The yield of HMF increased from 36.7% to 67.2% with 100%
conversion of sucrose upon doubling the RTILs from 1.0 to 2.0 molar
ratios as shown in Fig. 4(b). The sucrose dehydration results with
two equimolar of [TetraEG(mim)2][OMs]2 RTIL indicate that, the pre-
sent catalytic system is able to convert fructose and glucose into HMF.
To gain more insights in sucrose dehydration, we performed dehy-
dration of glucose in which 1:1 molar ratio of glucose and dicationic
[TetraEG(mim)2][OMs]2 RTIL at 120 °C, reaction was performed.
The reaction obtained 100% conversion of glucose with 72.7% HMF
yield in 135 min reaction time. The above result indicated that,
this catalytic system is also efficiently converting glucose into
Fig. 5. Effect of metal halides (Co-catalyst) on dehydration of fructose and sucrose.R-
eaction condition: (a) fructose dehydration, one equimolar [TetraEG(mim)2][OMs]2
,
40 min reaction time, 120 °C reaction temperature. (b) Sucrose dehydration, two equi-
molar [TetraEG(mim)2][OMs]2, 150 min reaction time, 120 °C reaction temperature.
HMF with good yield. The results of sucrose, glucose, and fructose
dehydration indicate that the dehydration of sucrose is a two step
reaction in which sucrose firstly hydrolyzed to fructose and glucose,
and these two hexose dehydrated to produce HMF [13,41]. There-
fore, high equimolar molar ratios of RTILs improved HMF yield
and sugar conversion, meaning that kinetic of sucrose dehydration
is fast with high equimolar ratio of RTILs. In sucrose dehydration re-
action, above 2.0 equivalent of [TetraEG(mim)2][OMs]2 IL showing
the polymerization of HMF and formation of other reported by-
products was observed [39].
3.5. Effect of metal halides (co-catalyst) on dehydration
From the above results it can concluded that these dicationic RTILs
were an efficient catalyst system at 120 °C. By using [TetraEG(mim)2]
[OMs]2 RTIL 100% sugar conversion and 92.3% and 67.2% HMF yield
obtained from fructose and sucrose, respectively. However at 100 °C,
only 54% and 41% HMF yield were obtained from fructose and sucrose
in [TetraEG(mim)2][OMs]2 IL under this condition. The dehydration of
sugar into HMF has been reported at low temperature [20,42]. There-
fore to increase the HMF yield and reactivity of [TetraEG(mim)2]
[OMs]2 at 100 °C, 1 equivalent of additional commercially available
metal halides as co-catalysts were used such as FeCl3, CuCl2·6H2O,
CoCl2·6H2O, and NiCl2·6H2O in the sugar dehydration reaction re-
spectively. The effect of co-catalyst on dehydration reaction of fructose
and sucrose is outlined in Fig. 5, and summarized in Table 2. All
selected co-catalyst shown 100% sugar conversion in appropriate re-
action condition but only NiCl2·6H2O was shown to be the most effi-
cient co-catalyst with 81.2% and 62.2% HMF yield from fructose and
sucrose. At present, the results above indicated that to some extent,
the structure of dicationic RTILs affected the interaction between
metal chloride and RTILs, thereby, influenced the dehydration of
a
100
Yield (%)
80
Conversion
60
40
20
0
0.5
1
1.5
2
b
100
80
60
40
20
0
Yield
Conversion
Table 2
Comparative study of metal halides in [TetraEG(mim)2][OMs]2 RTIL and other reported
solvent system for sugar dehydration reaction.
Entry
Metal
halides
Temperature
(°C)
Conversion
of sugar (%)
HMF yield
(%)
Reference
1
2
3
4
5
6
7
8
FeCl2
FeCl3
CuCl2.2H2O
AlCl3
FeCl3
CuCl2.6H2O
CoCl2.6H2O
NiCl2.6H2O
90
90
–
1
42
65
69
72.2
63.8
74.2
81.2
[41]
[41]
[50]
[50]
In this study
In this study
In this study
In this study
–
100
100
100
100
100
100
–
–
100
100
100
100
0.5
1
1.5
2
2.5
Fig. 4. Effect of molar ratio on dehydration of fructose and sucrose.Reaction condition:
(a) fructose dehydration, 40 min reaction time, 120 °C reaction temperature. (b) Su-
crose dehydration, 150 min reaction time, 120 °C reaction temperature.
Reaction condition: 1:1:1 equimolar of [TetraEG(mim)2][OMs]2, metal halide, fructose,
40 min reaction time, 120 °C reaction temperature.

102
A.H. Jadhav et al. / Catalysis Communications 21 (2012) 96–103
slight longer reaction time to convert 100% fructose into HMF. Therefore,
it can be concluded that the dicationic [TetraEG(mim)2][OMs]2 RTIL was
able to utilize at least five times for the dehydration of fructose.
4. Conclusions
In summary, new dicationic RTILs have been found which can cat-
alyze the selective dehydration of fructose and sucrose into HMF effi-
ciently. Such type of dicationic RTILs was used for the first time in the
dehydration of fructose and sucrose into HMF and displayed high cat-
alytic activity. The process showed a fructose conversion of 100% with
92.3% of HMF yield was obtained at 120 °C in relatively short reaction
time of 40 min by using [TetraEG(mim)2][OMs]2 RTIL. Same RTIL used
for the sucrose dehydration at 120 °C, achieved 100% sucrose conver-
sion with 67.2% of HMF yield in 150 min reaction time. At 100 °C by
using NiCl2·6H2O as a co-catalyst, 100% conversion of fructose and
sucrose with 81.2% and 62.2% of HMF yield was achieved, respective-
ly. Results showed that the synthesized dicationic RTILs are capable of
producing HMF from fructose and sucrose. The hydrogen bond form-
ing mesylate anions, short oligo ethylene glycol chains containing
imidazolium cations are beneficial to sugar dehydration. From the re-
sults it is concluded that these synthesized dicationic RTILs have recy-
clability and efficient catalytic ability to produce HMF from fructose
and sucrose up to five cycles. Presently, further development and
modification of these RTILs are ongoing in our laboratory to improve
catalytic ability for biomass conversion to fine chemicals and organic
transformations.
Fig. 6. Proposed interaction of metal halides with [TetraEG(mim)2][OMs]2 RTIL.
Acknowledgment
This study was supported by a grant (#10035574) from the “Plat-
form Chemical Process Technology from Lignocellulosic Biomass Con-
version” R&D Program funded by the Ministry of Knowledge Economy
and by a grant support by priority Research Centers Program through
the National Research Foundation of Korea (NRF) funded by the Minis-
try of Education, Science and Technology (2011-0022968), Republic of
Korea.
Fig. 7. Recycling of dicationic RTILs.Reaction condition: one equimolar [TetraEG(mim)2]
[OMs]2, 40 min reaction time, 120 °C reaction temperature.
sugars efficiently at 100 °C. In case, NiCl2·6H2O showed the good cat-
alytic ability to produce HMF with [TetraEG(mim)2][OMs]2, we be-
lieve that the NiCl2·6H2O molecule has suitable size to form
interaction with the oligo ethylene glycol chains. Literature reveals
that, NiCl2·6H2O used as efficient catalyst system as its highly active
complex formation with oligo ethylene glycol substrate [43–49]. At
present, Fig. 6 shows the proposed interaction with the NiCl2·6H2O
molecule and oligo ethylene glycol chains from dicationic RTILs. The
complex formation step between metal halides and oligo ethylene
glycol chains, produces highly active catalytic system and was able
to show efficient catalytic activity at lower reaction temperature.
From the obtained results, we concluded that, this is the reason behind
the highest catalytic activity shown by NiCl2·6H2O catalyst system
among the other metal halide used in this report as a co-catalyst.
Therefore, the present catalytic system achieved an efficient yield of
HMF from fructose and sucrose at 100 °C reaction temperature.
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