Sonochemical synthesis and characterization of imidazolium based ionic liquids: A green pathway

Article abstract of DOI:10.1016/j.molliq.2015.08.009

The emerging importance of sonochemistry as "green" process and ionic liquids (ILs) as "green" materials with their wide applications are rapidly increasing. Imidazolium-based ionic liquid derivatives are synthesized using a facile and green ultrasound-assisted procedure. Their structures were characterized by IR, ¹H-NMR, ¹³C-NMR and Mass spectroscopy. The main advantages of the present procedure as compared with conventional method are mainly, its milder conditions, shorter reaction time, higher yields and selectivity without the need for a transition metal or base catalyst.

Full text of DOI:10.1016/j.molliq.2015.08.009

Journal of Molecular Liquids 211 (2015) 934–937
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Sonochemical synthesis and characterization of imidazolium based ionic
liquids: A green pathway
Garima Ameta a, Arpit Kumar Pathak a, Chetna Ameta a, Rakshit Ameta b, Pinki B. Punjabi a,
⁎
a
Photochemistry Laboratory, University College of Science, Mohanlal Sukhadia University, Udaipur, 313002 Raj., India
b
Deparment of Chemistry, PAHER University, Udaipur, 313003 Raj., India
a r t i c l e i n f o
a b s t r a c t
Article history:
The emerging importance of sonochemistry as “green” process and ionic liquids (ILs) as “green” materials with
their wide applications are rapidly increasing. Imidazolium-based ionic liquid derivatives are synthesized using
a facile and green ultrasound-assisted procedure. Their structures were characterized by IR, 1H-NMR, 13C-NMR
and Mass spectroscopy. The main advantages of the present procedure as compared with conventional method
are mainly, its milder conditions, shorter reaction time, higher yields and selectivity without the need for a
transition metal or base catalyst.
Received 23 May 2015
Received in revised form 31 July 2015
Accepted 4 August 2015
Available online xxxx
Keywords:
Green synthesis
© 2015 Published by Elsevier B.V.
Ultrasonic irradiation
Imidazolium-based ionic liquids
Spectroscopy
1. Introduction
influence of ultrasound on a chemical reaction is attributed to the for-
mation of cavitations, which are induced by the ultrasonic waves and
Ionic liquids have been studied extensively as green solvents be-
cause of their tempting properties such as negligible vapor pressure,
large liquidus range, high thermal stability, higher ionic conductivity,
large electrochemical window and ability to solvate compounds of
widely varying polarity [1–3]. Utilization of ionic liquids as solvent is
one of the goals of green chemistry because these provide a cleaner
and more sustainable chemistry and are receiving increasing interest
as environmental friendly solvents for many synthetic and catalytic pro-
cesses [4–8]. An intriguing characteristic is that their the physico-
chemical properties of ILs may be finely tuned as per the requirement
by suitable choice of cations and anions. Therefore, ionic liquids have
been recognized as “designer-solvents” [9]. The unique properties of
ionic liquids, such as very low volatility, nonflammability and stability
make them suitable for use in a variety of different areas, such as organic
synthesis, (bio)catalysis, electrochemistry, analytical chemistry, separa-
tion technology, nanotechnology, renewable resource utilization and its
use as functional fluids (e.g., lubricants, heat transfer fluids, corrosion
inhibitors) [10].
their collapse can release very high local temperature and pressure. En-
hancement in reaction rates and yield of product in both heterogeneous
and homogenous systems have been well documented [13,14]. Conven-
tionally, the synthesis of ionic liquids via quaternization of tertiary
amine species has been performed in stirred tanks using batch or semi
batch processes. Although looks quite simple, but these processes are
excessively time- and energy-consuming [15]; therefore, sustainable
nonconventional techniques for process intensification have been the
subject of considerable recent attention. Synthesis of ionic liquids
through ultrasonic waves is an emerging green technology [16], as it
claims a drastic decrease in the reaction time. An efficient single-stage
synthesis of several imidazolium-based ionic liquids with fluorinated
anions using low-frequency ultrasound has been described [17]. Since
then, a number of reports on successful ultrasound-assisted ionic
liquids' syntheses have confirmed the benefit of their application at
the laboratory scale [18].
Herein, we report the sonochemical synthesis and characterization
of some new 1,3-dibutylimidazolium based ionic liquids.
It is worth mentioning that, application of ultrasound has received
significant attention in the chemical and pharma industries during the
last two decades. Its goal is to meet the needs of green chemistry as a
promising pathway to the design, manufacture and use of chemical
products in order to reduce or eliminate chemical hazards by choosing
the safest and the most efficient way for their synthesis [11,12]. The
2. Materials and methods
All chemicals used in this study were of analytical reagent grade and
used as received. Imidazole (Himedia, India), n-BuLi (Sigma-Aldrich,
India), n-butyl bromide (CDH, India), and dichloromethane (Fisher Sci-
entific, India) were used as received. Sodium tetrafluoroborate, silver
sulphate, sodium trifluoromethane sulphonate and p-toluene sulphonic
acid (Himedia) were also used as supplied. Ultrasonic cleaner 392
⁎
Corresponding author.
E-mail address: pb_punjabi@yahoo.com (P.B. Punjabi).
http://dx.doi.org/10.1016/j.molliq.2015.08.009
0167-7322/© 2015 Published by Elsevier B.V.

G. Ameta et al. / Journal of Molecular Liquids 211 (2015) 934–937
935
(Systronics) was used for ultrasonic irradiation with a frequency of
40 kHz and a nominal power of 115 W.
N–C stretching — 1260 cm−1 and 3080 cm−1, C_C stretching —
1645 cm−1 and C_N stretching — 1632 cm−1
.
3. Experimental
4.1.2. 1,3-Dibutylimidazolium tetrafluoroborate ([BBIM] BF4)
C–H stretching — 2850 cm−1, CH2 bending — 1446 cm−1, CH3
bending — 1368 cm−1, _CH stretching — 3061 cm−1, N–C stretching —
1287 cm−1 and 3095 cm−1, C_C stretching — 1653 cm−1, C_N
stretching — 1664 cm−1 and B–F stretching — 1453 cm−1, 712 cm−1
All reactions were carried out in an ultrasonic bath. The synthesis of
ionic liquids involves three steps (Scheme 1).
and 481 cm−1
.
Step I — 1-butylimidazole was synthesized by N-alkylation of imidaz-
ole (0.1 mol) with n-butyl lithium (0.1 mol) in ethanol under
the influence of ultrasound for 3 h at room temperature. The
progress of the reaction was monitored by TLC.
4.1.3. 1,3-Dibutylimidazolium sulphate ([BBIM] SO4)
C–H stretching — 2868 cm−1, CH2 bending — 1462 cm−1, CH3 bend-
ing — 1380 cm−1, _CH stretching — 3133 cm−1, N–C stretching —
1272 cm−1 and 3051 cm−1, C_C stretching — 1646 cm−1, C_N
stretching — 1639 cm−1, S_O stretching — 1167 cm−1 and S–O
Step II — Reaction of 1-butylimidazole (0.1 mol) with 1-bromobutane
(0.1 mol) was carried out in the presence of ultrasonic waves
for 2.5 h at 10 ° C to form 1,3-dibutylimidazolium bromide
(ionic liquid) by quaternization method. The bromide salt
was dissolved in dichloromethane and then extracted with
ether. The muddy solution was kept at room temperature to
become transparent as some precipitates settle down. Then,
the solution was filtered and the purification was carried
out three times. Finally, the salt was obtained after pumping
out the trace solvent in vacuum.
Step III — Anion metathesis is the methodology of choice for the prep-
aration of water and air stable ionic liquids based upon 1,3-
dialkylimidazolium cations. This method involves the treat-
ment of the halide salt with the silver/sodium/potassium
salts of BF−4 , SO24−, TfO− and Tos− or free acid of the appropri-
ate anion.
stretching — 949 cm−1
.
4.1.4. 1,3-Dibutylimidazolium p-toluene sulphonate ([BBIM] Tos)
C–H stretching — 2875 cm−1, CH2 bending — 1461 cm−1, CH3
bending — 1383 cm−1, _CH stretching — 3109 cm−1, N–C stretching —
1333 cm−1 and 2963 cm−1, C_C stretching — 1601 cm−1, C_N
stretching — 1719 cm−1, C_C stretching (conjugation) — 1629 cm−1
,
C–H in plan — 1399 cm−1, C–H bend (para) — 817 cm−1, S_O
stretching — 1383 cm−1 and S–O stretching — 753 cm−1
.
4.1.5. 1,3-Dibutylimidazolium tetrafluoromethane sulphonate ([BBIM] TfO)
C–H stretching — 2939 cm−1, CH2 bending — 1466 cm−1, CH3
bending — 1371 cm−1, _CH stretching — 3112 cm−1, N–C stretching —
1256 cm−1 and 2966 cm−1, C_C stretching — 1639 cm−1, C_N
stretching — 1632 cm−1, S_O — 1371 cm−1, S–O — 851 cm−1 and C–
F stretching (trifloromethyl— 2 strong broad bands) — 1166 cm−1 and
In the presence of ultrasound, reaction of 1,3-dibutylimidazolium
bromide ([BBIM] Br) was carried out either with sodium tetrafluorobo-
rate, silver sulphate, sodium trifluoromethyl sulphonate (sodium
triflate) or p-toluene sulphonic acid to give 1,3-dibutylimidazolium tet-
rafluoroborate (Step III case I), 1,3-dibutylimidazolium sulphate (Step
III case II), 1,3-dibutyl imidazolium trifluoromethyl sulphonate (triflate)
(Step III case III) and 1,3-dibutylimidazolium p-toluene sulphonate
(tosylate) (Step III case IV) ionic liquids, respectively in dry acetone.
These ionic liquids were dissolved in dichloromethane, which were fur-
ther extracted with ether for purification. The muddy solution was kept
at room temperature to become transparent. The solutions were then
filtered and the purification was carried out three times. Finally, the
salts were obtained after pumping out the trace solvent in vaccum.
Preparation of dry acetone — Dry MgSO4 was kept in an oven for 1 h
to evaporate H2O. 3 g of MgSO4 was mixed with 50 mL of acetone with
continuous stirring (5 h). Thereafter, it was allowed to stand overnight
to get dry acetone.
1256 cm−1
.
4.2. NMR spectra
1,3-Dibutylimidazolium bromide ([BBIM] Br) — 1H-NMR (CDCl3,
400 MHz): δ ppm = 0.90 (t, 9CH), 1.29 (m, 8CH), 2.01 (m, 7CH), 4.94
(t, 6CH), 7.51 (t, 5CH), 7.86 (t, 4CH), 8.86 (s, 2CH); 13C-NMR (CDCl3,
100 MHz): (δ ppm): 12.98 (9CH), 20.49 (8CH), 32.23 (7CH), 51.46
(6CH), 122.84 (4CH), 124.65 (5CH) and 138.23 (2CH).
1,3-Dibutylimidazolium tetrafluoroborate ([BBIM] BF4) — 1H-NMR
(CDCl3, 400 MHz): δ ppm = 0.83 (t, 9CH), 1.32 (m, 8CH), 1.91 (m,
7CH), 4.29 (t, 6CH), 7.74 (t, 5CH), 7.81 (t, 4CH), 8.69 (s, 2CH); 13C-NMR
(CDCl3, 100 MHz): (δ ppm): 13.11 (9CH), 19.99 (8CH), 31.32 (7CH),
55.39 (6CH), 121.53 (4CH), 124.60 (5CH) and 139.03 (2CH).
1,3-Dibutylimidazolium sulphate ([BBIM] SO4) — 1H-NMR (CDCl3,
400 MHz): δ ppm = 0.85 (t, 9CH), 1.23 (m, 8CH), 1.99 (m, 7CH), 5.12
(t, 6CH), 7.68 (t, 5CH), 7.97 (t, 4CH), 8.65 (s, 2CH); 13C-NMR (CDCl3,
100 MHz): δ ppm = 13.84 (9CH), 23.56 (8CH), 24.48 (7CH), 13.82
(6CH), 122.12 (4CH), 116.48 (5CH) and 138.94 (2CH).
4. Results and discussion
A Perkin-Elmer spectrum-2000 Fourier transform IR spectro-
photometer (USA) was used to obtain the IR spectra between 400
and 4000 cm−1. The samples were prepared in pellet form using
spectroscopic grade KBr. 1H-NMR and 13C-NMR spectra were recorded
by multinuclear FTNMR spectrometer Model Avance-II (Bruker). The
instrument was equipped with a cryomagnet of field strength 9.4 T.
Its 1H frequency was 400 MHz, while for 13C the frequency was
100 MHz. Waters, Q-Tof micro Mass spectrometer (LC–MS) was used
for mass spectral analysis of synthesized ionic liquids.
1,3-Dibutylimidazolium p-toluenesulphonate ([BBIM] Tos) — 1H-
NMR (CDCl3, 400 MHz): δ ppm = 0.86 (t, 9CH), 1.24 (m, 8CH), 2.24
(m, 7CH), 5.21 (t, 6CH), 7.68 (t, 5CH), 8.12 (t, 4CH), 9.03 (s, 2CH), 2.35
(eCH), 7.38 (cCH), 7.68 (bCH); 13C-NMR (CDCl3, 100 MHz): (δ ppm):
13.28 (9CH), 19.15 (8CH), 31.71 (7CH), 49.52 (6CH), 121.71 (4CH),
122.51 (5CH), 135.33 (2CH), 21.23 (eCH), 140.09 (dCH), 129.07 (cCH),
125.73 (bCH) and 141.03 (aCH).
1,3-Dibutylimidazolium tetrafluoromethanesulphonate ([BBIM]
TfO) — 1H-NMR (CDCl3, 400 MHz): δ ppm = 0.91 (t, 9CH), 1.31
(m, 8CH), 1.89 (m, 7CH), 4.24 (t, 6CH), 7.65 (t, 5CH), 7.71 (t, 4CH), 8.96
(s, 2CH); 13C-NMR (CDCl3, 100 MHz): (δ ppm): 13.04 (9CH), 19.20
(8CH), 32.35 (7CH), 49.46 (6CH), 121.63 (4CH), 122.39 (5CH), 136.97
(2CH) and 135.22 (aCH).
4.1. FTIR spectra
4.1.1. 1,3-Dibutylimidazolium bromide ([BBIM] Br)
IR (KBr, cm−1): C–H stretching — 2874 cm−1, CH2 bending —
1464 cm−1, CH3 bending — 1378 cm−1,_CH stretching — 3137 cm−1
,

936
G. Ameta et al. / Journal of Molecular Liquids 211 (2015) 934–937
Scheme 1. Ultrasound-assisted synthesis of 1,3-dibutylimidazolium based ionic liquids.
4.3. Mass spectra
1,3-Dibutylimidazolium tetrafluoroborate ([BBIM] BF4) — 181.2
(C11H21N2), 124.1 (C7H12N2), 67.8 (BF3)
1,3-Dibutylimidazolium sulphate ([BBIM] SO4) — 181.2 (C11H21N2),
124.1 (C7H12N2)
1,3-Dibutylimidazolium bromide ([BBIM] Br) — 181.2 (C11H21N2),
124.1 (C7H12N2)

G. Ameta et al. / Journal of Molecular Liquids 211 (2015) 934–937
937
introduction of such an efficient synthetic protocols should lower
down the cost of ionic liquids; thus, encourages a wider use of these ne-
oteric solvents.
Acknowledgments
Fig. 1. The chemical structure of 1, 3-dibutylimidazolium ionic liquid, where X = Br−, BF4−
,
One of the authors (Dr. Garima Ameta) is thankful to University
Grants Commission (F.15-1/2011-12/PDFWM-2011-12-GE-RAJ-
9578(SA-II)), New Delhi for providing financial assistance in the
form of Post Doctoral Fellowship. We are thankful to Prof. Suresh C.
Ameta for giving valuable suggestions during the progress of the
work. We are also thankful to the Director, SAIF and CIL, Chandigarh,
India for providing spectral data.
SO24−, TfO and Tos.
1,3-Dibutylimidazolium p-toluenesulphonate ([BBIM] Tos) — 181.2
(C11H21N2), 124.1 (C7H12N2), 171.2 (C7H7O3S), 156.1 (C6H4O3S), 15
(CH3), 91 (C7H7)
1,3-Dibutylimidazolium tetrafluoromethanesulphonate ([BBIM]
TfO) — 181.2 (C11H21N2), 124.1 (C7H12N2), 149 (CF3O3S)
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Table 1
1, 3- Dibutylimidazolium-based ionic liquids.
S.N.
1
Structural formula
Reaction time in Ultrasound (Hours)
2.5
Yield (%)
95
Viscosity η/cp
Conductivity (S/m)
0.23
613
[BBIM] Br
2
3.0
79
373
0.17
[BBIM] BF4