A Novel and Eco-friendly Method for the Preparation of Ionic Liquids

Article abstract of DOI:10.1055/s-2003-42420

A novel and eco-friendly method is described for the direct preparation of a series of ionic liquids containing alkylimidazolium-based cations and hexafluorophosphate-based (or tetrafluoroborate-based) anions in a one-pot procedure by using ionic liquids themselves as solvents.

Full text of DOI:10.1055/s-2003-42420

2
626
SHORT PAPER
A Novel and Eco-friendly Method for the Preparation of Ionic Liquids
A
D
N
ovel andEco-f
a
riendlyMe
n
thodfor the
P
-
reparati
Q
onof Ionic Liquid
i
s
an Xu,* Bao-You Liu, Shu-Ping Luo, Zhen-Yuan Xu, Yin-Chu Shen
Lab of Green Chemistry and Synthetic Technology, Zhejiang University of Technology, Hangzhou 310014, P. R. China
Fax +86(571)88320609; E-mail: greenchem@zjut.edu.cn
Received 4 July 2003; revised 25 August 2003
quired, all of them remain much to be improved. Quite re-
Abstract: A novel and eco-friendly method is described for the di-
rect preparation of a series of ionic liquids containing alkylimidazo-
lium-based cations and hexafluorophosphate-based (or tetra-
fluoroborate-based) anions in a one-pot procedure by using ionic
liquids themselves as solvents.
cently, Varma and Namboodiri disclosed an improved
preparation of RTILs by using microwaves.9 Although
the microwave-assisted method avoided using organic
solvents as the reaction medium, the procedure was still
difficult to achieve on large-scale preparation.
,10
Key words: ionic liquid, alkylimidazolium, hexafluorophosphate,
tetrafluoroborate
Accordingly, we have recently been interested in the de-
velopment of not only environmentally benign but also
facile approaches to the preparation of RTILs and disclose
As the use of conventional volatile solvents required in herein our study results of a method using RTILs them-
various organic synthetic processes is of ecological con- selves as reaction medium, which can be carried out di-
cern, the room temperature ionic liquids (RTILs), also re- rectly and effectively from three components of N-
ferred to as designer solvents, have drawn powerful methylimidazole (MIm) [or 1,2-dimethylimidazole
attention due to their unique chemical characters.1 Such (DMIm)], alkyl halide, and hexafluorophosphate (or tet-
characters as negligible vapor pressure, thermal stability, rafluoroborate) salt in a one-pot procedure to prepare
favorable solubility of many and various chemicals, high alkylimidazolium hexafluorophosphate and alkylimida-
recovery and recycling potential, have made RTILs zolium tetrafluoroborate based RTILs. The method is in
emerge as a new green alternative to volatile organic sol- accordance with the principles of green chemistry in three
vents in the applications of organic synthesis, catalysis, aspects: 1) usage of an environmental benign solvent and
,2
3
–6
biocatalysis, polymerization, and so on. As more and inexpensive materials, 2) elimination of an additional pro-
more synthesis can be proceeded in these novel solvent cess required in the two-step procedure to separate and
systems with good yield and high selectivity, RTILs have purify the formed intermediate of quaternary alkylimida-
shown great promise in commercial applications.7
zolium halide (QImX), since QImX reacted with hexaflu-
orophosphate (or tetrafluoroborate) salt directly in situ to
generate the desired ionic liquid and 3) avoiding the te-
dious process for the desired product isolation from sol-
vent, since two of them were the same materials.
However, despite their successful applications as reaction
medium in synthetic processes, a practical method for the
preparation of RTILs, especially in large-scale produc-
tion, is still not available. The conventional procedures for
their preparation usually include two steps: with alkylim- Initially the method was evaluated to the preparation of
idazolium hexafluorophosphate based RTILs as example, BMImPF (Scheme 1) and the results are presented in
6
firstly, alkylimidazolium halides are prepared; secondly, Table 1. As can be seen, the reactivity of halides is in the
the obtained halides react with hexafluorophosphoric acid order of n-butyl iodide > n-butyl bromide > n-butyl chlo-
8
or its corresponding salts. A large excess of alkyl halides/ ride (entries 1, 5 and 7); n-butyl chloride was relatively
volatile organic solvents, such as acetone, toluene and less reactive and required longer reaction time, while the
CH CN, is always used as the reaction medium in the two higher reactivity of n-butyl bromide and n-butyl iodide
3
steps, and in some cases expensive silver salts are re- provided good yields with shorter reaction time. Hence, n-
4
5
BMImPF6
PF6-
+
n-C4H9X + KPF6
N
N
N
N
H C
3
H C
CH CH CH CH
2 2 2 3
3
10
2
6
7
8
9
(X=Cl, Br or I)
Scheme 1
SYNTHESIS 2003, No. 17, pp 2626–2628
x
x.
x
x
.
2
0
0
3
Advanced online publication: 07.10.2003
DOI: 10.1055/s-2003-42420; Art ID: F05703SS
Georg Thieme Verlag Stuttgart · New York
©

SHORT PAPER
A Novel and Eco-friendly Method for the Preparation of Ionic Liquids
2627
Table 1 Direct Preparation of BMImPF6
a
Entry
n-C H X
mol
MIm (mol)
KPF (mol)
BMImPF (g) Temp (°C)
Time (h)
Isolated Yield
%)
4
9
6
6
(
1
2
3
4
5
6
7
n-C H Cl
0.08
0.08
3.2
0.05
0.05
2.0
0.05
0.05
2.0
10.0
10.0
80
80
80
60
80
80
80
25
35
35
15
15
15
10
74
83
86
75
85
87
85
4
9
400.0
10.0
n-C H Br
0.05
0.05
2.0
0.05
0.05
2.0
0.05
0.05
2.0
4
9
10.0
400.0
10.0
n-C H I
0.05
0.06
0.05
4
9
a
Used as solvent.
butyl bromide may be the favorable choice, since it has
higher reactivity than n-butyl chloride and is more stable
in air and less expensive than n-butyl iodide. The increase
of temperature/time gave the corresponding increase in
yields (entries 1,2, 4 and 5), which reflected that the reac-
All starting materials were commercially available and were used
without further purification. IR spectra were recorded on a Bruker
Equinox 55 spectrometer. H NMR spectra were obtained on a
1
Bruker Avance 500 (TMS as internal standard).
tion process was also sensitive to the reaction temperature 1-n-Butyl-3-methylimidazolium Hexafluorophosphate; Gener-
and time. Anyway, the established reaction procedure was al Procedure
In a 50 mL round-bottomed flask, fitted with a reflux condenser and
conducted successfully and the higher yields were
a mechanical stirrer, were placed KPF (9.2 g, 0.05 mol), N-meth-
ylimidazole (4.1 g, 0.05 mol), n-butyl chloride (7.4 g, 0.08 mol) and
6
achieved under the conditions investigated, compared
1
1,12
with the literatures,
yield of 1-n-butyl-3-methylimidazolium chloride
BMImC) was obtained from N-methylimidazole and n-
in a two-step procedure, 64%
BMImPF (10.0 g); the stirrer was started and the mixture was heat-
6
ed up to 80 °C. After 35 h of stirring, the mixture was treated with
water (15 mL) to dissolve the formed KCl and then the aq phase was
(
butyl chloride in refluxing toluene under N for 24 hour, removed, the left BMImPF phase was washed with water (15 mL
2
6
and 91% yield of BMImPF was achieved from BMImC × 3), followed by drying on a Rotavapor under vacuum, to give
6
BMImPF (21.8 g), which contained 11.8 g of freshly formed
and sodium hexafluorophosphate after stirring for 24
hours at room temperature in acetone.
6
BMImPF (83.1% yield).
6
IR (film): 3170 [ (CH), aromatic], 2966 and 2878 [ (CH), aliphat-
Then, the method was extended to the preparation of
alkylimidazolium hexafluorophosphates and alkylimida-
zolium tetrafluoroborates based RTILs and the results are
summarized in Table 2. All the products listed in Table 2
–1
ic], 1573 and 1467 [ (C=C)], 842 [ (PF)] cm .
1H NMR (DMSO): = 9.09 (s, 1 H, H ), 7.75 and 7.69 (2 s, 2 H, H ,
2
4
H ), 4.16 (t, 2 H, J = 7.16 Hz, H ), 3.84 (s, 3 H, H ), 1.76 (m, 2 H,
5
6
10
H ), 1.26 (m, 2 H, H ), 0.90 (t, 3 H, J = 7.38 Hz, H ).
7
8
9
1
were characterized by IR and H NMR spectroscopy; the
spectroscopic data were identical with those of the authen- 1-n-Butyl-3-methylimidazolium Tetrafluoroborate; General
8
,12
tic sample or in the literature. As can be seen, the meth- Procedure
NaBF (220.0 g, 2 mol), N-methylimidazole (164.0 g, 2 mol), n-bu-
od was found to be general and applicable to the RTILs
bearing alkyl chains of varying lengths (R ) or different
alkylimidazoles (R). It did not matter what kind of
hexafluorophosphate salt or tetrafluoroborate salt was
employed (entries 1, 3 and 10–12). Yields of up to 87%
were obtained under mild conditions.
4
tyl bromide (274.0 g, 2 mol) and BMImBF (400.0 g) were charged
4
successively into a 1 L round-bottomed flask fitted with a reflux
condenser and a mechanical stirrer. The reaction proceeded at 80 °C
for 15 h. After the completion of the reaction, the mixture was trans-
ferred to a 2 L round-bottomed flask, treated with water (500 mL)
and extracted with CH Cl (300 mL × 3). Then the combined ex-
2
2
tracts were evaporated to remove the solvent, and gave BMImBF4
We believe that the method developed is a promising
green route for the facile and direct preparation of the
most extensively used RTILs of alkylimidazolium
hexafluorophosphates and alkylimidazolium tetrafluoro-
borates. There are many major advantages such as eco-
friendliness, simple procedure, and high yield achieved in
mild conditions. The present method is suitable for appli-
cations both in laboratory preparation and in large-scale
manufacture. The extension of this method for other
RTILs is still under way in our laboratory.
(
(
800.5 g), which contained 400.5 g of freshly formed BMImBF4
88.6% yield).
IR (film): 3162 [ (CH) aromatic], 2965 and 2877 [ (CH) aliphatic],
1
1H NMR (CDCl3): = 8.83 (s, 1 H, H ), 7.33 and 7.29 (2 s, 2 H, H ,
H ), 4.19 (t, 2 H, J = 7.45 Hz, H ), 3.96 (s, 3 H, H ), 1.85 (m, 2 H,
–
1
574 and 1467 [ (C=C)], 1060 [ (BF)] cm .
2
4
5
6
10
H ), 1.37 (m, 2 H, H ), 0.96 (t, 3 H, J = 7.38 Hz, H ).
7
8
9
Synthesis 2003, No. 17, 2626–2628 © Thieme Stuttgart · New York

2
628
D.-Q. Xu et al.
SHORT PAPER
Table 2 Direct Preparation of RTILsa
RTIL
Y-
R'
+
R'Br
+
MY
N
N
N
CH3
N
CH3
R
R
Entry
R
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
R
MY
RTILb
Product
Isolated Yield (%)
1
2
3
4
5
6
7
8
9
CH (CH ) CH
KPF6
BMImPF6
BMImPF6
BMImPF6
PeMImPF6
HMImPF6
HeMImPF6
OMImPF6
MPMImPF6
MBMImPF6
BMImBF4
BMImBF4
BMImBF4
HMImBF4
OMImBF4
MPMImBF4
MBMImBF4
BDMImPF6
HDMImPF6
BDMImBF4
HDMImBF4
BMImPF6
BMImPF6
BMImPF6
PeMImPF6
HMImPF6
HeMImPF6
OMImPF6
MPMImPF6
MBMImPF6
BMImBF4
BMImBF4
BMImBF4
HMImBF4
OMImBF4
MPMImBF4
MBMImBF4
BDMImPF6
HDMImPF6
BDMImBF4
HDMImBF4
87
89
88
93
92
93
93
89
92
90
88
89
94
92
88
91
93
95
94
93
2
2 2
3
3
3
3
3
3
3
CH (CH ) CH
NH PF
2
2 2
4
6
CH (CH ) CH
NaPF6
KPF6
KPF6
KPF6
KPF6
KPF6
KPF6
KBF4
2
2 2
CH (CH ) CH
2
2 3
CH (CH ) CH
2
2 4
CH (CH ) CH
2
2 5
CH (CH ) CH
2
2 6
CH(CH )CH CH
3
3
2
CH(CH )(CH ) CH
3
3
2 2
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
CH (CH ) CH
2 2 2
3
3
3
3
3
CH (CH ) CH
NH BF
2
2 2
4
4
CH (CH ) CH
NaBF4
NaBF4
NaBF4
NaBF4
NaBF4
KPF6
2
2 2
CH (CH ) CH
2
2 4
CH (CH ) CH
2
2 6
CH(CH )CH CH
3
3
2
CH(CH )(CH ) CH
3
3
2 2
CH3
CH3
CH3
CH3
CH (CH ) CH
2 2 2
3
3
3
3
CH (CH ) CH
KPF6
2
2 4
CH (CH ) CH
NaBF4
NaBF4
2
2 2
CH (CH ) CH
2
2 4
a
All the preparations were run with MIm (or DMIm) (2 mol), R Br (2 mol) and MY (2 mol) in 400 g RTIL at 80 °C for 15 h.
Used as solvent.
b
(
8) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton. Trans.
References
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2
Synthesis 2003, No. 17, 2626–2628 © Thieme Stuttgart · New York