A quick and green approach to prepare [Rmim]OH and its application in hydrophilic ionic liquid synthesis

Article abstract of DOI:10.1039/c1nj20361d

A quick and green process to prepare [Rmim]OH solutions is developed based on fast ion exchange between OH^- and halide ions (halide = Cl ^- or Br^-) in ethanol. Five ionic liquids were synthesized by neutralizing the obtained [Rmim]OH solution with acids to verify the feasibility of the whole process. The halogen content in the finally obtained ionic liquids and the experimental factors influencing the content are mainly discussed. Moreover, two other alcohols besides ethanol, propanol and iso-propanol, were further examined as solvents. The results show that this process, together with the alkylation and hydroxide anion route, is simple and efficient for preparing various ionic liquids, and that the process is a complementary synthetic route to existing strategies to prepare various ILs.

Full text of DOI:10.1039/c1nj20361d

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PAPER
A quick and green approach to prepare [Rmim]OH and its application in
hydrophilic ionic liquid synthesisw
Jian Gao, Jianguo Liu,* Bo Li, Wenming Liu, Yun Xie, Yuchen Xin, Ying Yin,
Xiao Jie, Jun Gu and Zhigang Zou*
Received (in Montpellier, France) 22nd April 2011, Accepted 10th May 2011
DOI: 10.1039/c1nj20361d
A quick and green process to prepare [Rmim]OH solutions is developed based on fast ion
exchange between OHꢀ and halide ions (halide = Clꢀ or Brꢀ) in ethanol. Five ionic liquids were
synthesized by neutralizing the obtained [Rmim]OH solution with acids to verify the feasibility of
the whole process. The halogen content in the finally obtained ionic liquids and the experimental
factors influencing the content are mainly discussed. Moreover, two other alcohols besides
ethanol, propanol and iso-propanol, were further examined as solvents. The results show that this
process, together with the alkylation and hydroxide anion route, is simple and efficient for
preparing various ionic liquids, and that the process is a complementary synthetic route to
existing strategies to prepare various ILs.
As halogen impurities usually have negative effects on ILs,
1. Introduction
several approaches have been proposed to remove halogen
Ionic liquids (ILs) are a group of salts that are liquid at
ambient temperature.1 In recent years, ILs have been
impurities or to prepare low halogen ILs. Deng et al. have
reported an electrolysis method to remove halide impurities.12
increasingly investigated in many fields such as fuel cells,2
The halide ions were firstly transformed to halogens and then
nanotechnology,3 catalysts4–6 and novel solvents7,8 due to
absorbed by ethylene to form a haloethane, which was then
their unique properties, such as low melting point, high ionic
removed from the final product. This process can reduce the
conductivity and low or zero vapor pressure.9
halogen impurity to some extent. However, the process needs a
Nowadays, dialkylimidazolium-based ILs are those most
complex system composed of an ethylene gasbag, a gas pump,
often studied.10,11 This type of IL can be obtained by the
an electrolysis instrument and maybe an exhaust gas treatment
traditional quaternization of alkylimidazoles with a suitable
system, etc. In addition, the yield of the IL will be reduced
alkylation agent. In this case, the range of ILs is limited and
after the several steps involved. Ion exchange with the corres-
the nature of the anion is determined by the alkylating reagent,
ponding acid has been proposed as a method to prepare ILs of
low halogen content.13,14 In this process, the halide-based
which is usually a haloalkane. Thus, anion metathesis, shown
in Scheme 1, is usually carried out with either silver salts in
precursor was reacted with the acid of the desired anion by
water or sodium salts in organic solvents when the desired
refluxing for several hours (4–48) under heating conditions.
anion cannot be obtained by direct alkylation. However, the
This process is cumbersome and inefficient for removing
halides.11 Thus, great efforts have been devoted to developing
first method is hampered by the high price of silver salts and
the second is restricted by high halogen impurities.
alternative synthetic routes. For example, a process was
introduced to prepare imidazolium tetrafluoroborate of high
quality by reacting Me3OBF4 with N-alkylimidazoles in
ether.15 Excellent reviews of existing ion exchange procedures
for IL synthesis can be found in the literature.11,16
Scheme 1 The traditional two-step method for ionic liquid synthesis,
R = alkyl, X = halogen, M = metal ion or H+, Y = the desired anion.
Among all the existing methods, the hydroxide anion route
by neutralizing [Rmim]OH with acids of the desired anion is
considered to be efficient for synthesizing many ILs.
Eco-materials and Renewable Energy Research Center, Department of
[Rmim]OH, which can only exist in solution,17 is key to the
Materials Science and Engineering, Department of Physics and
route. Basically, it can be obtained by reacting water with a
carbene prepared by deprotonating [Rmim]X.18 An electro-
chemical way to synthesize [Emim]OH has also been
reported.16 Moreover, the preparation of ILs via hydroxide
precursors obtained via classical ion exchange over a column
National Laboratory of Solid State Microstructures, Nanjing
University, 22# Hankou Road, Nanjing, Jiangsu Province, China.
E-mail: jianguoliu@nju.edu.cn, zgzou@nju.edu.cn;
Fax: +86 25 83686632; Tel: +86 25 83621219
w Electronic supplementary information (ESI) available: See DOI:
10.1039/c1nj20361d
c
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has been reported by Ohno’s group.19,20 However, a quick and
green approach to synthesize [Rmim]OH is still highly desired.
In this study, an alternative green and simple process to
prepare [Rmim]OH with a low halogen content using only
alcohols as solvents is described. Several ILs from [Rmim]OH
obtained by this method have been prepared and investigated.
stirring and while the precipitate of the potassium salt (KY)
formed. The crude product was obtained after precipitation,
and was filtered and all the solvents in the solution removed on
a rotary evaporator. It was further dried in vacuo at 50 1C for
10 h and 120 1C for 24 h to obtain the final ionic liquid.
The synthesis of [Emim]HSO4 was a little bit different in
order to avoid the side reaction of ethanol and H2SO4. 0.5 mol
H2SO4 per mol KOH was dissolved in water and added to the
[Rmim]OH solution. All the active protons of H2SO4 were
neutralized to generate [Emim]2SO4 and K2SO4. Then,
another 0.5 mol H2SO4 per mol of [Emim]OH was added to
obtain [Emim]HSO4.
2. Experimental section
2.1 Synthesis of an [Rmim]OH solution
All chemical reagents were used as received, except for
1-methyl imidazole (Mim), which was distilled before use.
All water used in this investigation was purified. [Rmim]X
was in practice [Rmim]Br unless otherwise stated.
All experiments were carried out at temperatures lower than
10 1C. According to the 1H-NMR results, the experiments
were prone to result in failure if the temperature was higher
than 30 1C. This may imply that a low temperature is helpful
to keep the [Rmim]OH stable. Thus, it is beneficial to carry out
the process at a low temperature.
Firstly, 1-methylimidazole was reacted with halogenated
alkanes (RX) to prepare alkylimidazolium halides ([Rmim]X).
Then, 0.1 mol of the obtained [Rmim]X and 0.15 mol of KOH
was dissolved in 30 and 45 mL ethanol, respectively. The KOH
solution was added dropwise to the [Rmim]X solution while
stirring, and a white precipitate was immediately observed.
The system was stirred for another 5 min after all the KOH
solution had been added to ensure that the reaction was
complete. An [Rmim]OH solution in ethanol was obtained
when the precipitation was removed by filtering.
1
The obtained samples were submitted for H-NMR (Bruker
DRX500) and elemental analysis (Elementar Vario MICRO). The
Br content of the ILs were determined by the AgNO3 potentio-
metric titration method, which will be discussed in Section 3.3.
For comparison, the traditional two-step method described
in the literature11,21,22 was carried out to prepare [Bmim]BF4.
The reactions of [Emim]Br with NaHSO4 and NH4H2PO4 were
also carried out by the two-step method. The detailed process is
presented in the ESI.w The halide content was measured by the
same AgNO3 potentiometric titration method.
A point to be mentioned is that the KOH must be added in
solution. If the KOH is added in a pure state, a brown odorous
liquid results and none of the expected [Rmim]Y can be
obtained after the liquid is neutralized. This was observed by
1H-NMR spectroscopy. This failure maybe due to the high
KOH concentration in the region close to the KOH solid,
which breaks down the [Rmim]+ cation.
3. Results and discussion
3.1 Synthesis of [Rmim]OH
2.2 Synthesis of various ILs with [Rmim]OH
Ethanol
½Rmimꢁþ þ Xꢀ þ Kþ þ OHꢀнRmimꢁþ þ OHꢀ þ KX #
Fig. 1 illustrates the full process to prepare ILs via the
hydroxide anion route. The corresponding acid with an equal
molar quantity to KOH was dissolved in water and added
slowly to the above-obtained [Rmim]OH solution while
ð3Þ
Eqn (3) shows the [Rmim]OH synthesis process. The main idea
is to take advantage of the fast ion exchange procedure of
OHꢀ and halide anions in ethanol.
The mechanism for ion exchange is based on the fact that the
solubility of the starting materials are higher than that of MX in
the solvent. This mechanism has been involved in the synthesis of
many ILs, such as [Bmim]BF4 and [Emim]BF4, by the two-step
method.22 However, the ion exchange procedure in the two-step
method is quite slow, while in eqn (3) the reaction procedure is so
quick that the white precipitate of KX can be observed once the
KOH solution is added to the [Rmim]X solution.
As shown by eqn (3), the halogen content in [Rmim]OH can
be decreased further if an excess of KOH is added. The ratio of
KOH to [Rmim]OH to prepare [Rmim]OH was kept at 1.5
unless otherwise stated, and the unreacted KOH was precipi-
tated in the following step.
3.2 Synthesis of ILs of [Rmim]Y via [Rmim]OH
Ethanol
Ð
½Rmimꢁþ þ OHꢀ þ HY
½RmimꢁþYꢀ þ H2O ð4Þ
The synthesis process of ILs via [Rmim]OH is depicted in
Fig. 1 The full process for IL synthesis via [Rmim]OH.
eqn (4). The target ILs can be obtained when the precursor
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[Rmim]OH solution, obtained via eqn (3), is neutralized with
the acid composed of the desired anion; H2O was removed
from the product by evaporation.
Table 1 Found and calculated halogen contents for the samples
EmHSO4
0.039
0.22
EmDHP
0.038
0.49
EmBF4
0.063
0.37
BmBF4(Cl)a
0.00064
0.027
Calc./wt%
Found/wt%
Fig. 2 shows a picture of the five ILs (BmBF4, EmHSO4,
EmBF4, EmDHP and BmDHP) synthesized via the precursors
[Emim]OH or [Bmim]OH, obtained from [Rmim]Br and the
corresponding acid with the desired anion. All five ILs were
submitted for 1H-NMR and elemental analysis. The 1H-NMR
and elemental analysis results together imply that the obtained
samples were the products supposed. Detailed information is
presented in the ESI.w As shown in Fig. 2, [Bmim]BF4,
[Emim]HSO4 and [Emim]BF4 are brown or yellow, although
this kind of IL is believed to be colorless. This implies that
chromophoric impurities exist in the obtained samples.
However, no evidence has been found that the chromophoric
impurities affect either the chemical or the physical properties
of these ILs.23
BmBF4
0.075
0.66
BmDHP
0.055
0.69
Calc./wt%
Found/wt%
a
This sample was prepared with [Bmim]Cl as the precursor.
The halide content of [Bmim]BF4 obtained via the two-step
method was found to be 5.7% for Br and 1.8% for Cl,
while the reported data is 3–4% Br12,25 and 1.5% Cl,11
or greater than 5% for halides.26 Although the Cl content
can be lowered to about 300 ppm by extraction with
chilled water several times, it has been described as
cumbersome.11 Also, this cannot be carried out for ILs that
cannot form a biphasic system. Thus, the experiments,
together with some previously reported work, mean that the
samples obtained via the traditional two-step method are more
likely to suffer higher halide contents than those via the present
method.
Fig. 2 also shows that [Emim]H2PO4 and [Bmim]H2PO4 are
solid at room temperature. According to a previous report, the
ꢀ
melting points of ILs with H2PO4 as the anion are higher
than 100 1C.2 These salts were investigated as ionic liquids,24
although they are not room temperature ionic liquids.
One point to be mentioned is that the above discussions
ꢀ
focus on ILs with BF4 as the anion. Other anions may be
difficult to obtain via the traditional two-step method. For
example, [Emim]Br can be used to react with NaBF4 in
acetone to prepare [Emim]BF4. However, if [Emim]HSO4
was desired, the experiment showed that the expected anion
exchange between the [Emim]Br and NaHSO4 cannot in
practice take place in acetone because the Br content was
found to be 32.7%, according to our experiments, almost the
same content as for the original [Emim]Br (41.8%). The
reaction between [Emim]Br and [NH4]H2PO4 was also tried
in acetone for 24 h and the Br content was also found to be
unacceptably high (18.2%). This failure may be due to the
inadequate solubility difference between MY and MX in
acetone, as showed in eqn (2). In some cases, however, the
solubility of KX can be even higher than that of MY, making
it impossible for the expected reaction to take place. Although
other solvents may be found suitable for the above reactions,
the solubility of both MY and MX must first be evaluated.
This means that a solvent suitable for one reaction may not be
suitable for another. With the method used in this study, once
[Rmim]OH was obtained, many various ILs could be obtained
by an acid–base reaction between the [Rmim]OH solution and
various acids with desired anions. Thus, the process presented
here is more flexible in the selection of anions.
Fig. 2 The ILs obtained (from left to right): BmBF4, EmHSO4,
EmBF4, EmDHP and BmDHP (Bm = [Bmim], Em = [Emim],
DHP = H2PO4ꢀ).
3.3 Halogen (Cl, Br) content in the ILs
Table 1 presents the halogen contents of the obtained ILs.
Halogen impurities are usually known to have negative
effects11 on ILs, and it is very important to eliminate them
in the field of IL synthesis. Therefore, the halogen content of
the final product is a very important indicator to assess the
merit of the synthesis processes of ILs.
As demonstrated in Table 1, the halogen contents were in
the range 0.17–0.69%, measured by the potentiometric
titration method. The low halogen contents are a result of
the low solubility of KX in ethanol. In particular, the Cl
content was 0.027% for [Bmim]BF4 obtained via the [Bmim]Cl
precursor, as shown in Table 1. The low Cl content is a result
of the lower solubility of KCl than KBr in ethanol, which is
0.03 g/100 g ethanol for KCl and 0.46 g/100 g ethanol for KBr.
While in eqn (2) of the traditional two-step method shown
in Scheme 1, the solubility of MY is not much greater than
that of MX in most cases.22 As a result, there is no great
driving force for the equilibrium to shift to the right. This leads
to the incomplete conversion of the halide precursor to the
target IL and considerable precursor [Rmim]X remaining in
the final product, even after a long reaction time.11,18
The halogen content in the ILs was determined by the
AgNO3 potentiometric method. This method is often applied
to the quantitative analysis of halides.27 It is considered a
standard procedure to determine the chloride content of
compounds28 and has also been used to determine bromide
contents.29 The main idea is to determine the end-point of
precipitation titrations by the potentiometric method.28,30
Principally, this method is very similar to the Vollhard method
for halide determination. The only difference is that the
end-point is determined according to the alternation in
potential difference indicated on a pH meter, meaning that
human error can be reduced to a minimum. In the Vollhard
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method, the end-point is determined visually with thiocyanate
as a visual indicator, and experience is greatly needed.
In fact, the theoretical value of the halogen content can
be even lower, as shown in Table 1. The Br content was
calculated according to the solubility of KX in ethanol.
The calculation process is summarized in eqn (5) when
KOH : [Rmim]Br is larger than 1. The calculation was carried
out assuming that water was completely absent from the
ethanol.
ꢀ
ꢁ
2ꢂ
ꢀ
ꢁ
L
KSP
MX
M
Brðwt%Þ ¼
ꢂ
ꢂ 100
ð5Þ
N
ðC ꢀ 1Þ
where L is the volume of ethanol used (litre), N is the amount
of IL (mole), KSP is the solubility product constant for KX in
ethanol (9 ꢂ 10ꢀ4 and 1 ꢂ 10ꢀ5 for KBr and KCl, respectively,
at 25 1C), C is the ratio of KOH to halide salt in moles, MX is
the molecular weight of X and M is the molecular weight of
the target IL.
Fig. 3 Br content as a function of the KOH : [Bmim]Br ratio.
The amount of ethanol used in the preparation can also
affect the amount of KX in the solution. According to eqn (5),
the increase in ethanol usage can lead to the increase in
halogen content for the final IL. This resulted in the Br content
difference for [Bmim]H2PO4 in Fig. 3 and Table 1, which
was 1.51 and 0.69 wt%, respectively. In the synthesis of
[Bmim]H2PO4 for Fig. 3, 1.4 L ethanol per mole of product
was used, while 0.7 L ethanol was used in the experiments for
Table 1. Additionally, more water enters the system as more
ethanol is used, leading to a further increase of the halogen
content in the final product. Therefore, ethanol usage should
be limited if the low halogen content ILs are needed. However,
[Rmim]OH is prone to degradation since [Rmim]OH cannot
exist in high concentration.16 Therefore, an appropriate
amount of ethanol must be applied. A concentration of
OHꢀ lower than 1.2 mol Lꢀ1 is suitable to avoid degradation
while keeping the halogen content spontaneously low enough
according to our experiments.
Taking [Emim]HSO4 listed in Table 1 as an example, a
total of 75 mL of ethanol was used for a 0.1 M IL. The
KOH : [Emim]Br ratio was 1.5. According to eqn (5), the
calculated Br content should be:
ꢀ
ꢁ
2ꢂ
ꢀ
ꢁ
0:075
0:1
9 ꢂ 10ꢀ4
ð1:5 ꢀ 1Þ
80
Brðwt%Þ ¼
ꢂ
ꢂ 100 ¼ 0:039%:
208
The calculated values for the samples are also presented in
Table 1. The difference between the calculated and found
values for the halogen content may be caused by small
amounts water in the ethanol. No water in the ethanol was
considered during the calculation, but ethanol with a water
content of 0.3% was employed in the experiments. Since the
solubility of KBr in water is high, 69 g/100 g water, even a tiny
amount of water in the ethanol can cause a significant rise in
the halogen content by introducing soluble KBr into the
[Rmim]OH precursor. This is why the found value of the
halogen content is higher than the calculated one. The calcula-
tion implies that the halogen content can be even lower if more
water is removed from the applied ethanol.
Solvent is another factor influencing the halogen content of
the final IL. The principle of this method is to utilize the fast
ion exchange between OHꢀ and halide ions in solvents. Thus,
any solvent in which the solubility of [Rmim]X and
[Rmim]OH is high while that for KX is low should be a
suitable solvent, in theory. However, other issues, such as
toxicity, cost, viscosity, availability and ease of separation
from the final product, should be taken in consideration in real
applications. Therefore, methanol should not be considered
due to its high toxicity and the high solubility of KBr in it
(2.1 g/100 g methanol). For butanol, its high boiling point of
118 1C makes it difficult to separate from the product by
conventional evaporation. Propanol, with a boiling point of
97 1C, and iso-propanol, with a boiling point of 83 1C, can also
fulfil the restriction of solubility. To test the feasibility of the
two solvents, [Emim]H2PO4 was prepared using ethanol,
propanol and iso-propanol as the solvent, respectively. The
Br content of the final products were 0.65, 0.51 and 1.66% for
samples obtained from ethanol, propanol and iso-propanol,
respectively. This indicates that the Br content for the product
obtained from iso-propanol was two-times higher than the
samples obtained from ethanol and propanol as the solvent.
This may be attributed to the usage of a large amount of
iso-propanol, since its ability to dissolve KOH (28 g/100 g
iso-propanol) is quite low compared to ethanol and propanol
Besides halogen impurity in the final product, potassium
residual was also checked in the final product using atomic
absorption spectroscopy. The K content values were found to
be at 100 ppm grade. More details about K detection are
presented in the ESI.w
3.4 Factors influencing the halogen content in the ILs
Fig. 3 shows the Br content in the final IL as a function of the
KOH : [Bmim]Br ratio during the experiments. The Br content
reduced from 5.25 to 1.4 wt% as the ratio increased from 1 : 1
to 1.5 : 1, and the content decreased from 0.72 to 0.68 wt% as
the ratio increased from 2 : 1 to 2.5 : 1, respectively. However, a
Br content of 0.75 wt% was observed when the ratio increased
to 3 : 1. But, according to eqn (5), the increase in the ratio
should lead to a drop of halogen content. This contradiction
can also be explained with the water content in the ethanol.
When more KOH was added for a higher KOH : [Bmim]Br
ratio, more ethanol must be applied to dissolve the KOH,
which brings more water into the process.
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(38 g/100 g propanol). Thus, more water was introduced into
the system using isopropanol to dissolve a given amount of
KOH, resulting in an increase of the Br content. What’s more,
as the solubility of KOH in iso-propanol is relatively low,
more iso-propanol must be used to dissolve the required
KOH, elevating the price of the process. The results indicate
that ethanol and propanol are both good solvents for this
method. However, ethanol is the optimistic choice due to its
low toxicity, low cost and ease of separation from the product.
The solubility of KOH in propanol and iso-propanol was
measured, as described in the ESI.w
complementary synthetic route to the previous existing
strategies, as mentioned in the Introduction, to prepare
various ILs.
Acknowledgements
This work was financially supported by the National Basic
Research Program of China (2007CB613300), the Natural
Science Foundation of China (20906045), Doctoral Funding
(20090091120033) and the Scientific Research Foundation for
Returned Scholars of Ministry of Education of China.
Although IL synthesis by neutralizing [Rmim]OH with
the desired acid has been reported,17–19 the preparation
of [Rmim]OH in this study is different. Compared to the
reported processes, it is not only an alternative process
for [Rmim]OH solution preparation, but also has several
advantages.
References
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eqn (4) can finish in minutes. Thus, the time needed for the
reaction takes only around 0.5 h. Taking the separation time
into consideration, the whole process composed of eqn (3) and
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21,32
eqn (2) in Scheme 1 alone takes 12 h31 or 24 h,
or even
3 d.22 Taking the separation time in consideration, this process
will take a much longer time. What’s more, even after a 24 h
reaction, the halide content is still 3–4%.
The whole process is green. Only ethanol and water are
necessary as solvents. What’s more, the ethanol can even be
recycled after de-watering. This is in accordance with the
concept of green chemistry. While in the traditional two-step
method, the second step is usually carried out in volatile or
toxic solvents such as acetonitrile,32 dichloromethane33 or
acetone,34 making it environment unfriendly.35
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Another advantage is that the synthesis process is easy to
carry out. Only operations such as mixing, stirring and filter-
ing are used. Thus, this process can be easily carried out at low
cost with instruments common to most labs. Thus, this process
has the potential to be adopted widely in the field of IL
synthesis.
Furthermore, many other precursors with various imid-
azolium cations can be prepared by this method and then be
used to synthesize the final IL, since [Rmim]X can be obtained
simply by the direct alkylation of 1-methylimidazole or
1,2-dimethylimidazole with the corresponding 1-chloroalkane
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In summary, a quick and green method to prepare [Rmim]OH
solutions is described. In this method, the quick exchange
procedure of Xꢀ and OHꢀ in ethanol is fully utilized, and the
halogen can be removed by the precipitation of KX from the
ethanol. Furthermore, based on [Rmim]OH, several ILs were
synthesized. The results of 1H-NMR and elemental analysis of
the final product showed that the obtained samples were the
products supposed. Therefore, this method coupled with
[Rmim]X preparation and the hydroxide anion route is a
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