A green and novel procedure for the preparation of ionic liquid

Article abstract of DOI:10.1016/j.jfluchem.2007.09.004

A green and novel procedure is described for the preparation of a series of ionic liquid containing alkylimidazolium-based or N-alkylpyridinium-based cations and hexafluorophosphate-based or tetrafluoroborate-based anions in one-pot solvent-free conditions to give excellent yields with shortened time.

Full text of DOI:10.1016/j.jfluchem.2007.09.004

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Journal of Fluorine Chemistry 129 (2008) 108–111
www.elsevier.com/locate/fluor
A green and novel procedure for the preparation of ionic liquid
Dong Fang a,b, Jian Cheng a, Kai Gong a, Qun-Rong Shi a, Xin-Li Zhou a, Zu-Liang Liu a,
*
a School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing 210094, Jiangsu, PR China
b Jiangsu Provincial Key Laboratory of Coastal Wetland Bioresources & Environmental Protection, Yancheng 224002, Jiangsu, PR China
Received 13 July 2007; received in revised form 8 September 2007; accepted 10 September 2007
Available online 14 September 2007
Abstract
A green and novel procedure is described for the preparation of a series of ionic liquid containing alkylimidazolium-based or N-
alkylpyridinium-based cations and hexafluorophosphate-based or tetrafluoroborate-based anions in one-pot solvent-free conditions to give
excellent yields with shortened time.
# 2007 Elsevier B.V. All rights reserved.
Keywords: Green procedure; Ionic liquids; Preparation
1. Introduction
1 metal or ammonium salt of the desired anion and acid–base
neutralization reactions, but either way need the preparation of
the imidazolium or pyridinium halides via alkylation
(Menschutkin step) using a large molar excess of haloalkane
(10–400%) for as long as 72 h at refluxing condition, and then
RTILs was prepared with variable yields and much longer
reaction time. Currently, the disadvantage of ionic liquids
outweigh their excellent solvent properties and designer solvent
features, which will always constrain their applicability [12].
Kralisch also suggested spicing up the ionic liquid research
efficient for energetic, environmental and economic balances
[13]. There are some non-conventional synthetic methods using
microwave (MW) or ultrasonic irradiation (US) to improve
access to these RTILs. Seddon and Deetlefts published a MW-
assisted, solvent-free preparation of ionic liquids leading to
reasonable good yields [14]. Namboodiri and Varma [14–17]
synthesized imidazolium halides, and then RTILs under US or
MW solventless, respectively in an open test tube. Khadilkar
and Rebeiro [18] synthesized imidazolium and pyridinium
halides under MW solventless in a closed vessel. Leveque et al.
reported an improved preparation of ionic liquids by US
[19,20]. Xu et al. prepared the ionic liquids with target ionic
liquids as solvents, but the method for ionic liquids used as
solvents was not indicated [21]. Each line of work has achieved
great improvements in yield and reaction time; however, the
search for the new readily available and green scale-up
procedure is still being actively pursued [22]. We found that
BMImPF6, BMImBF4, APyBF4 and APyPF6 can be prepared
with stoichiometric amounts of 1-methylimidazole or pyridine,
Since the beginning of the Green Chemistry movement 10
years ago, the need for alternative solvents for reactions has
been one of the major issues we have faced. In recent years, air-
and water-stable ionic liquids (ILs) have emerged as a
powerful alternative to conventional molecular organic
solvents due to their particular properties, such as undetectable
vapor pressure, wide liquid range, as well as ease of recovery
and reuse, and making ILs a greener alternative to volatile
organic solvents [1–3].
The vast majority of ionic liquid chemistry based on
nitrogen-containing heterocycles focuses on the use of 1-alkyl-
3-methylimidazolium and N-alkylpyridinium cations. 1-Alkyl-
3-methylimidazolium hexafluorophosphate (AMImPF6),
1-alkyl-3-methylimidazolium tetrafluoroborate (AMImBF4),
N-alkylpyridinium tetrafluoroborate (APyBF4) and N-alkylpyr-
idinium hexafluorophosphate (APyPF6) are typically ionic
liquids used as solvent or catalysts to extraction [4,5],
polyborane reactions [6], hydroxylation of alkyl halides with
water [7], syntheses of tribenzohexadehydro[12]annulene [8],
fluorodediazoniation [9], regioselective nitration of aromatic
compounds [10], oxidation [11], etc. There are two basic
methods for the preparation of these ionic liquids: metathesis of
a halide salt (Finkelstein step) with, for instance, a silver, group
* Corresponding author. Tel.: +86 25 84318865; fax: +86 25 84318865.
E-mail address: liuzuliang302@163.com (Z.-L. Liu).
0022-1139/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2007.09.004

D. Fang et al. / Journal of Fluorine Chemistry 129 (2008) 108–111
109
Scheme 1. Novel procedure for RTILs.
alkylhalides and potassium, sodium or ammonium salt of
hexafluorophosphate or tetrafluoroborate in one-pot under
solvent-free condition (Scheme 1).
2.1.2. The synthesis of 1-butyl-3-methylimidazolium
tetrafluoroborate (BMImBF4)
1-Methylimidazol 0.05 mol, 1-bromobutane 0.05 mol and
sodium salt of tetrafluoroborate 0.05 mol were stirred in a
three-necked flask with a reflux condenser at 80 8C for 3.5 h.
Upon completion of the reaction, the mixture was diluted with
50 mL of acetonitrile, after elimination of the precipitated salt
(KBr or NaBr), the filtrate was then filtered through a pad of
celite to remove the residual halide salt and finally concentrated
by rotary evaporation to afford a colorless to pale yellow liquid
with a yield of 98%, without reaction with AgNO3 aq. The
BMImBF4 was then further dried under high vacuum at 80 8C
2. Experimental
Melting points were determined on a Thomas Hoover
apparatus and are reported uncorrected. The IR spectra were
run on a Nicolete spectrometer and expressed in cmÀ1 (KBr).
1H NMR spectra were recorded on Bruker DRX300 (300 MHz)
and 13C NMR spectra on Bruker DRX300 (75.5 MHz)
spectrometer. All chemicals (AR grade) were commercially
available and used without any further purification.
1
for 6 h. The compound was analyzed by H NMR, 13C NMR,
and FT-IR spectroscopy, and the spectral data tally with the
structure.
2.1. The general procedure for the synthesis of ionic liquids
1H NMR (300 MHz, acetone-d6): d 9.51 (s, 1H, CH), 7.89 (s,
1H, CH), 7.83 (s, 1H, CH), 4.42 (t, 2H, CH2), 4.09(s, 3H, CH3),
2.1.1. The synthesis of 1-butyl-3-methylimidazolium
hexafluorophosphate (BMImPF6)
1.92 (m, 2H, CH2), 1.38 (m, 2H, CH2), 0.93 (t, 3H, CH3). 13
C
1-Methylimidazol 0.05 mol, 1-bromobutane 0.05 mol and
potassium salt of hexafluorophosphate 0.05 mol were stirred in
a three-necked flask with a reflux condenser at 80 8C for 3.5 h.
Then, 10 mL water was added and bi-phase of water/ionic
liquids formed, the immiscible ionic liquids BMImPF6 layer
was separated from the water phase, washed with quantitative
fresh deionized water until the water phase will not react with
AgNO3 aq. and then with diethyl ether (3Â 15 mL). The ionic
liquids were dried in vacuum at 120 8C for 2 h, yielded 93% of
colorless to pale yellow liquids product. The compound was
analyzed by 1H NMR, 13C NMR, and FT-IR spectroscopy, and
the spectral data tally with the structure.
NMR (75 MHz, CDCl3): d 137.44, 124.34, 122.98, 49.72,
36.53, 32.48, 19.62, 13.37. IR (cmÀ1): 3144.76, 3071.25,
2960.55, 2935.58, 2873.65, 1571.90, 1465.51, 1382.92,
1337.38, 1170.21, 1062.29, 754.75.
The TGA analyses show that BMImBF4 is thermally stable
up to 340 8C.
2.1.3. The synthesis of N-butylpyridinium tetrafluoroborate
(BPyBF4)
The same procedure was used as for BMImBF4 except that
ammonium or sodium salt of tetrafluoroborate was used and the
reaction was carried out at 100 8C for 4 h and colorless liquid
was obtained with a yield of 92%.
1H NMR (300 MHz, acetone-d6): d 8.95 (s, 1H, CH), 7.75 (s,
1H, CH), 7.65 (s, 1H, CH), 4.32 (t, 2H, CH2), 4.02 (s, 3H, CH3),
1.90 (m, 2H, CH2), 1.40 (m, 2H, CH2), 0.94 (t, 3H, CH3). 13
C
NMR (75 MHz, CDCl3): d 137.02, 124.38, 123.00, 49.84,
36.20, 32.31, 19.58, 13.29. IR (cmÀ1): 3171.46, 3124.90,
2965.68, 2938.72, 2877.92, 1575.14, 1467.21, 1386.54,
1339.39, 1169.63, 844.79, 751.95.
2.1.4. The synthesis of N-alkylpyridinium
hexafluorophosphate (APyPF6)
The same procedure was used as for BMImPF6 except that
the reaction was carried out at 100 8C for 4 h, recrystallized
with 95% ethanol and white crystal product was obtained with a
yield of 93%, mp 64–64.5 8C.
The TGA analyses show that BMImPF6 is thermally stable
up to 360 8C.

110
D. Fang et al. / Journal of Fluorine Chemistry 129 (2008) 108–111
Table 1
especially when the reactant could not form uniform phase.
Hence, the KPF6 used in our work was powder instead of crystal
to facilitate the mix under vigorous stirring. However, for
crystal reactant, the reaction time should be prolonged (more
than 4 h) and the reaction temperature should be heightened
(110–120 8C) to carry out the reaction completely. In case of
pyridinium hexafluorophosphate (APyPF6), same reactant
could be used as for BMImPF6. But for pyridinium
tetrafluoroborate, only NH4BF4 and NaBF4 could be employed,
and no desired products could be obtained with KBF4, the
reason is still being investigated.
The preparing of room-temperature ionic liquid in one-pot solvent-free con-
ditions
Entry RTILs
R
X
Y
M
Time
(h)
T
Yielda
(8C) (%)
1
2
EMImPF6
C2H5
C4H9
Br PF6
Br PF6
K
4.0
3.5
3.5
3.5
4.0
3.5
3.5
3.5
4.0
3.0
3.5
3.5
70
80
91
93
90
86
94
98
94
91
90
93
90
89
90
92
91
88
BMImPF6
PMImPF6
HMImPF6
EMImBF4
K
3
C5H11 Br PF6
C6H13 Br PF6
K
80
4
K
80
5
C2H5
Br BF4
Cl BF4
K
70
6
BMImBF4 C4H9
PMImBF4
HMImBF4 C6H13 Br BF4
K
80
7
C5H11 Br BF4
K
80
8
K
80
9
EPyPF6
BPyPF6
PPyPF6
HPyPF6
EPyBF4
BPyBF4
PPyBF4
HPyBF4
C2H5
C4H9
Br PF6
Cl PF6
Na
Na
Na
Na
70
10
11
12
13
14
15
16
100
100
100
70
4. Conclusion
C5H11 Br PF6
C6H13 Br PF6
We have developed a very efficient, quick, and practical
method for the preparation of ionic liquids, in contrast to the
several-day time needed using conventional methods that
require a large excess of alkylhalides/organic solvent as
reaction media or concentrated acid. The synthesis can be
performed on flexible reaction scales compared to non-
conventional methods under microwave or ultrasonic irradia-
tion. It should greatly contribute to the realization of the
environmental benign process.
C2H5
C4H9
Br BF4 NH4 4.0
Cl BF4 Na
3.0
3.5
3.5
100
100
100
C5H11 Br BF4 Na
C6H13 Br BF4 Na
a
Isolated yield.
3. Results and discussion
The other ionic liquids could be prepared by the same
procedure using C2H5Br, C3H7Br, C5H11Br, C6H13Br, etc. as
alkylation’s reagents. Various RTILs were investigated and the
results were listed in Table 1.
Acknowledgements
This work was financially supported by the Educational
Committee of Jiangsu Province (07KJD530238), Jiangsu
Provincial Key Laboratory of Coastal wetland Bioresources
& Environmental Protection, (JLCBE07020) Nanjing Uni-
versity of Science & Technology (no. 2006001).
Using conventional heating methods, the first step in ionic
liquid synthesis is time-consuming; the second step is also a
long-time procedure. From Table 1, it can be seen that this
method needs 3.0–4.0 h totally, in contrast to the several-day
time needed using conventional methods; the reaction time
could be strongly decreased.
It is important to note that all of the halides used in this study
were efficiently converted to the corresponding ionic liquids.
The alkyl chlorides were relatively not less reactive and
provided excellent yields under the same reaction conditions as
the alkyl bromides (entries 6, 10, 14). In contrast to the reported
rate at which the quaternisation of 1-methylimidazole or
pyridine proceeds follows the conventional order: R–I > R–
Br > R–Cl. The ethyl bromide required relatively longer
reaction time; the reaction temperature was relatively lower for
its lower bp (entries 1, 5, 9, 13), however, the yield was not
decreased.
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