Bifunctional temperature-sensitive amphiphilic acidic ionic liquids for preparation of biodiesel

Article abstract of DOI:10.1016/j.cattod.2012.07.007

Several water-stable bifunctional temperature-sensitive amphiphilic Bronsted-acidic ionic liquids with an alkane sulfonic acid group and a polyether group were synthesized through polyethylene glycol monomethylether as raw material. The properties and structures of the ionic liquids were experimentally characterized. Esterification of oleic acid with methanol to biodiesel was investigated in various bifunctional temperature-sensitive amphiphilic acidic ionic liquids. The amphiphilicity, temperature-sensitive, polymerization degree, acidity and activity correlation for the ionic liquids were studied. It was found that their structures were consistent with the designed structure and their purities were high. Among all these ionic liquids, the ionic liquid - MPEG-350-ILs showed the best catalytic activity that is near to that of concentrated sulfuric acid, which is ascribed to its strong Bronsted acidity and amphiphilicity. The catalytic activity of each ILs is dependent on the polymerization degree of polyether cation. The catalytic activity decreased with lengthening polyether chain of ionic liquids. These ionic liquids exhibited temperature-sensitive property. The produced biodiesel could be separated via simple decantation and the ionic liquids could be reused.

Full text of DOI:10.1016/j.cattod.2012.07.007

Catalysis Today 200 (2013) 74–79
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Bifunctional temperature-sensitive amphiphilic acidic ionic liquids for
preparation of biodiesel
Qin Wu, Hualin Wan, Hansheng Li∗, Haoran Song, Tonghua Chu
School of Chemical Engineering and the Environment, Beijing Institute of Technology, 100081, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Several water-stable bifunctional temperature-sensitive amphiphilic Bronsted-acidic ionic liquids with
an alkane sulfonic acid group and a polyether group were synthesized through polyethylene glycol
monomethylether as raw material. The properties and structures of the ionic liquids were experimentally
characterized. Esterification of oleic acid with methanol to biodiesel was investigated in various bifunc-
tional temperature-sensitive amphiphilic acidic ionic liquids. The amphiphilicity, temperature-sensitive,
polymerization degree, acidity and activity correlation for the ionic liquids were studied. It was found
that their structures were consistent with the designed structure and their purities were high. Among
all these ionic liquids, the ionic liquid – MPEG-350-ILs showed the best catalytic activity that is near to
that of concentrated sulfuric acid, which is ascribed to its strong Brønsted acidity and amphiphilicity.
The catalytic activity of each ILs is dependent on the polymerization degree of polyether cation. The cat-
alytic activity decreased with lengthening polyether chain of ionic liquids. These ionic liquids exhibited
temperature-sensitive property. The produced biodiesel could be separated via simple decantation and
the ionic liquids could be reused.
Received 31 March 2012
Received in revised form 10 July 2012
Accepted 23 July 2012
Available online 18 August 2012
Keywords:
Ionic liquids
Biodiesel
Acid catalysis
Polyether
Temperature-sensitivity
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
heterogeneous acid catalysts are environmentally friendly, they
also have some drawbacks, such as low catalytic activity and deac-
With crude oil prices increasing rapidly, fossil fuels limited and
environment pollution intensified, it is increasingly necessary to
develop alternative clean and renewable energy sources. Biodiesel,
as an alternative diesel fuel, is a promising non-toxic, biodegradable
and renewable fuel [1–3].
tivation [7–10].
Room temperature ionic liquids have attracted much interest
as relatively clean and promising catalysts and alternative solvents,
which possess important attributes, such as wide liquid range, neg-
ligible vapor pressure, high catalytic activity, excellent chemical
and thermal stability, potential recoverability, design possibilities
and ease of separation of the products from reactants. Some organic
reactions involving alkylation [11–14], esterification [15–19], nitra-
tion [20], hydrogenation, epoxidation, or Diels-Alder have been
performed in room temperature ionic liquids with excellent yields
and selectivity [21]. Nevertheless, an example of direct using ionic
liquids as catalyst for the esterification of waste oils to biodiesel is
absent from the literature except our group’s [22]. A new method
for esterification of waste oils to biodiesel catalyzed by bifunc-
tional temperature-sensitive amphiphilic acidic ionic liquids is
proposed.
In this work, several water-stable bifunctional Bronsted-
acidic ionic liquids with an alkane sulfonic acid group and a
polyether group were synthesized through polyethylene glycol
monomethylether (MPEG) as raw material. They were character-
ized by nuclear magnetic resonance (NMR), infrared (IR) spectrum,
elementary analysis and differential thermal analysis (TG/DTA).
Esterification of oleic acid with methanol to biodiesel was inves-
tigated in various bifunctional acidic ionic liquids.
Biodiesel, consisting of fatty acid methyl esters (FAME), is usu-
ally produced by transesterification of vegetable oil or animal fat
with methanol in the presence of catalyst. Conventionally, the cat-
alyst is homogeneous base one [4–6]. Although the raw materials
used to produce biodiesel are available from a wide variety of
sources, waste oils are valuable for the production of biodiesel
in China. However, waste oils contain large amount of fatty acids
and cannot use base catalyst, which cause serious saponification.
Esterification of fatty acid with methanol to biodiesel is usually
catalyzed by acidic catalysts, such as homogeneous acid, or het-
erogeneous acid catalysts. However, homogeneous catalysts, such
as sulfuric acid, are corrosive and are not easily recovered, lead-
ing to the release of environmentally unfriendly effluents, which
inevitably leads to a series of environmental problems. Although
∗
Corresponding author. Tel.: +86 10 68918979; fax: +86 10 68918979.
E-mail addresses: hsliin126@126.com (H. Li), wuqin bit@126.com (Q. Wu).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.07.007

Q. Wu et al. / Catalysis Today 200 (2013) 74–79
75
A stoichiometric amount of concentrated sulfuric acid was
added dropwise to the zwitterions at 0 ◦C, and the mixture was
stirred at 50 ◦C for 3 days, resulting in the formation of the
[BsIm(CH2CH2O)nCH3][HSO4] ionic liquids. The ionic liquids were
washed repeatedly with ethyl acetate and ether to remove unre-
acted material and dried under vacuum [23].
(OCH2CH2) OH
SOCl2
CH3
CH3
n
N
N
Na
(OCH2CH2)
Cl
n
2.2. Characterization of the ionic liquids and their intermediates
O
(OCH2CH2)
N
N
CH3
The structures of the ionic liquids and their intermediates
were analyzed by IR spectroscopy, NMR and elementary analy-
sis. IR spectra were recorded on a Perkin-Elmer Spectrum One
FTIR spectrometer using KBr windows suitable for Fourier trans-
form infrared (FTIR) transmittance technology to form a liquid film.
1H NMR spectra were obtained on a JEOL JNM-ECA600 nuclear
magnetic resonance spectrometer. Chemical shifts are reported in
parts per million (ppm, ı). The C, H, N and S contents of the ionic
liquids were obtained on Elementar Analysensysteme GmbH (Var-
ioELV5.19.6) instrument. The thermal decomposition temperature
was determined by Perkin-Elmer TGA7 TG-DTA, with a heating rate
of 10 ◦C/min from room temperature to 600 ◦C.
n
S
O
O
N
N
N
N
C4H8SO3
(OCH2CH2)
CH3
CH3
n
H2SO4
H
C4H8SO3
HSO4
(OCH2CH2)
n
Scheme 1. Synthetic procedure of bifunctional acidic ionic liquids.
2.3. Esterification of oleic acid with methanol catalyzed by ionic
liquids and analysis
2. Experimental
The oleic acid was used as mimic of waste oil. Methanol and
oleic acid were obtained from Beijing Chemical Co., Ltd.
2.1. Preparation of bifunctional temperature-sensitive
amphiphilic acidic ionic liquids
The esterification reaction was carried out in a three-neck flask,
equipped with a condenser tube, magnetic stirring, and an oil bath
thermostat. The oleic acid was added to flask and heated. When
the temperature of oleic acid reached 68 ◦C, the methanol and cat-
alyst were added to flask. The reaction was allowed to proceed for
4 h with magnetic stirring and heating at the desired temperature.
After the reaction, the reactor was cooled to room temperature and
two phases were formed. The lower phase consisted of the pro-
duced methyl esters and oleic acid, and the upper phase contained
the ionic liquid catalyst and excess methanol. The lower phase con-
taining the methyl esters was simply separated from the upper
phase by decantation.
The synthetic procedure of bifunctional temperature-sensitive
amphiphilic acidic ionic liquids is illustrated in Scheme 1.
2.1.1. Synthesis of CH3(CH2CH2O)n−1CH2CH2Cl
The pyridine, toluene and different polymerization degree of
polyethylene glycol monomethylether, molecular weight of which
is 350, 550 and 750, were dried, respectively. The polyethylene gly-
col monomethylether, pyridine and toluene were added to flask
under an environment of highly purified argon. The thionyl chloride
was added dropwise to flask, and the mixture was stirred at 45 ◦C
for 10 h. The product was purified with toluene through extraction,
filtration and distillation.
The conversion of oleic acid was analyzed by the acid–base titra-
tion. The base is water solution of potassium hydroxide.
3. Results and discussion
2.1.2. Preparation of CH3(OCH2CH2)nIm
The sodium was slowly added to methanol. The sodium methox-
ide solution produced, which was colorless transparent liquid, was
purified through filtration.
The imidazole was added to sodium methoxide solution, and the
mixture was stirred in the oil bath pan for 8 h, resulting in the for-
mation of the sodium imidazole. The sodium imidazole was purified
through filtration.
A stoichiometric amount of CH3(CH2CH2O)n−1CH2CH2Cl was
added to the methanol solution of sodium imidazole, and the mix-
ture was stirred in the oil bath pan for three days, resulting in the
formation of the CH3(OCH2CH2)nIm. The CH3(OCH2CH2)nIm was
washed repeatedly with methanol, acetone, tetrahydrofuran and
toluene, leached and distilled accordingly. Finally, the product was
dried at 70 ◦C under vacuum.
3.1. Characterizations of the ionic liquid intermediates
3.1.1. IR analysis of the CH3(CH2CH2O)n−1CH2CH2Cl
IR spectra of different polymerization degrees of MPEG and
MPEG-Cl are shown in Fig. 1(a–c). The band at 3373 cm−1 was
assigned to the O H stretching vibration. The 1646 cm−1 band
was assigned to the O H flexural vibration. The C H vibration
was observed at 2876 cm−1, 1458 cm−1, 1351 cm−1 and 1250 cm−1
.
The C O stretching vibration was observed at 1118 cm−1. The
940 cm−1 band was assigned to the C
O
C vibration. As shown
from Fig. 1(a), for the MPEG-350-Cl, the absorption peak inten-
sity of O H was weakened and a new band at 650 cm−1 was
founded, which was assigned to the
C Cl stretching vibra-
tion. It was shown that the MPEG-350-Cl had been synthesized
successfully.
2.1.3. Preparation of bifunctional acidic ionic liquids
Similarly, as seen from Fig. 1(b, c), the MPEG-550-Cl and MPEG-
A stoichiometric amount of 1,4-butane sultone was added drop-
wise to the R-(OCH2CH2)nIm at 0 ◦C and the mixture was stirred at
50 ◦C for 3 days. The zwitterions BsIm(CH2CH2O)nCH3 produced,
all of which were reddish brown thick liquids, were washed three
times with ethyl acetate and ether to remove unreacted material
and dried for 12 h in a vacuum.
750-Cl had been synthesized respectively. In addition, the O
H
peak intensity of MPEG-Cl had no obvious change compared with
that of MPEG in the Fig. 1(b, c), the reason is that the OH content
and peak intensity of MPEG decreases with the molecular weight
of MPEG increasing.

76
Q. Wu et al. / Catalysis Today 200 (2013) 74–79
a
a
MPEG-350-Cl
MPEG-350
MPEG-350-Im
MPEG-350-Cl
4000
3500
3500
3500
3000
3000
3000
2500
2000
1500
1000
500
Wavenumber(cm-1)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
b
b
MPEG-550-Cl
MPEG-550
MPEG550-Im
MPEG550-Cl
4000
2500
2000
1500
1000
500
Wavenumber (cm-1)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
c
c
MPEG-750-Im
MPEG-750-Cl
MPEG-750-Cl
MPEG-750
4000
2500
2000
1500
1000
500
Wavenumber (cm-1)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 1. IR spectra of different polymerization degrees of MPEG and MPEG-Cl.
Fig. 2. IR spectra of different polymerization degrees of MPEG-Im and MPEG-Cl.
3.1.2. IR analysis of the CH3(OCH2CH2)nIm
addition, the 685 cm−1 band peak disappeared or weakened.
It was shown that the imidazole group had substituted Chlo-
rine group and the CH3(OCH2CH2)nIm had been synthesized
successfully.
Fig. 2(a–c) shows the IR spectra of different polymeriza-
tion degrees of MPEG-Im and MPEG-Cl. For the MPEG-Im,
the bands at 3130 cm−1 and 1511 cm−1 were assigned to
the C H and C N ring stretching vibration of imidazole. In

Q. Wu et al. / Catalysis Today 200 (2013) 74–79
77
CH3][HSO4] ionic liquids. As can be seen from Fig. 3(a), for the
BsIm-MPEG-350, the bands at 1245 and 1167 cm−1 were assigned
a
to the O
assigned to the C
S
O stretching vibrations. The 1040 cm−1 band was
S
O stretching vibrations. These results showed
the existence of sulfonic acid group. For the MPEG-350-ILs, the
peak intensity of sulfonic acid group increased obviously com-
pared with the BsIm-MPEG. The 596 cm−1 band was assigned to
the SO4H group vibration further indicative of the existence of
main group. In addition, the wide peak at 3300–3500 cm−1 was
assigned to the O H group of water because the MPEG-350-Ils
is super water-absorbable. As can be seen from Fig. 3(b, c), the
similar results were obtained. Therefore, the IR data are consistent
with the structures for the ionic liquids shown in Scheme 1.
MPEG-350-ILs
BsIm-MPEG-350
MPEG-350-Im
3.2.2. NMR analysis of ionic liquids
The MPEG ionic liquids were analyzed by 1H NMR spectroscopy.
The NMR spectral data of the ionic liquids are as follows.
MPEG-350-ILs: 1H NMR (600 MHz, CDCl3): ı 4.22 (t, 2H), 2.01
(m, 4H), 1.53 (s, 2H), 8.7988 (m, 1H), 7.35 (s, 2H), 3.66 (m, 29H),
3.38 (m, 3H).
4000
3500
3500
3500
3000
3000
3000
2500
2000
1500
1000
1000
1000
500
500
500
Wavenumber (cm-1)
MPEG-550-ILs: 1H NMR (600 MHz, D2O): ı 4.25 (t, 2H), 1.86 (m,
4H), 1.58 (s, 2H), 8.68 (s, 1H), 7.31 (s, 2H), 3.53 (d, 46H), 3.21 (s, 3H).
MPEG-750-ILs: 1H NMR (600 MHz, CDCl3): ı 4.14 (m, 2H), 2.05
(t, 4H), 1.26 (s, 2H), 8.61 (t, 1H), 7.33 (s, 2H), 3.65 (m, 61), 3.38 (s,
3H).
The NMR spectral data of the ionic liquids agreed with their
designed structures (Scheme 1). Therefore, the synthesis methods
for the ionic liquids were reliable.
b
MPEG-550-ILs
3.2.3. Elementary analysis of ionic liquids
The N, C, S and H elementary analysis values of ionic liquids are
listed in Table 1. For the MPEG-350-ILs, the atomic number ratio
of carbon to hydrogen is 21.71:42.42. The atomic number ratio of
carbon to nitrogen is 21.71:2. The atomic number ratio of carbon to
sulfur is 21.71:2. As shown in Table 1, the experimental atomicity
ratio of N, C, S to H is near to the theoretical value. In addition,
as can be seen from Table 1, the similar results were obtained for
the MPEG-550-ILs and MPEG-750-ILs. It was demonstrated that the
purity of the ionic liquids was high.
BsIm-MPEG-550
MPEG-550-Im
4000
2500
2000
1500
Wavenumber (cm-1)
c
3.3. TG of ionic liquids
Fig. 4(a–c) displays the TG-DTA curve of the three ionic liquids.
As can be seen from Fig. 4(a), for the MPEG-350-ILs, there are four
weight loss regions from 30 to 600 ◦C. From room temperature to
165 ◦C, the weight loss was 6% due to the removal of the water
and the organic solvent. From 165 to 230 ◦C, the weight loss was
54%, which was ascribed to the decomposition of polyether group.
From 230 to 376 ◦C, the weight loss was 25%, which was ascribed
to the decomposition of butane sulfonic acid group. The weight of
ionic liquid slowly decreased up to 376 ◦C due to the decompo-
sition of imidazole. The weight of ionic liquid sharply decreased
above 165 ◦C. Therefore, the thermal decomposition temperature
of MPEG-350-ILs was 165 ◦C. As can be seen from Fig. 4(b, c), the
similar change regularity was obtained for the MPEG-550-ILs and
MPEG-750-IL. The thermal decomposition temperatures of MPEG-
550-ILs and MPEG-750-ILs were 201 ◦C and 212 ◦C, respectively. It
was shown that these ionic liquids were stable below 165 ◦C.
MPEG-750-ILs
BsIm-MPEG-750
MPEG-750-Im
4000
2500
2000
1500
Wavenumber (cm-1)
Fig. 3. IR spectra of different polymerization degrees of MPEG-Im, BSIm-MPEG and
MPEG-ILs.
3.4. Solubility analysis of ionic liquids
3.2. Characterizations of the ionic liquid catalysts
The solubility of ionic liquids was studied at room temperature
in the different polarity solvents, such as water, methanol, ethyl
acetate and ether. In addition, the solubility of ionic liquids was
investigated in the biodiesel methyl oleate at room temperature
(see Table 2). As shown in Table 2, the solubility of MPEG ionic
3.2.1. IR analysis of the ionic liquids
Fig. 3(a–c) displays the infrared spectra of the
CH3(OCH2CH2)nIm, BsIm(CH2CH2O)nCH3 and [BsIm(CH2CH2O)n

78
Q. Wu et al. / Catalysis Today 200 (2013) 74–79
Table 1
Elementary analysis of ionic liquids.
Ionic liquids
N (wt%)
C (wt%)
S (wt%)
H (wt%)
N:C:S:H (mol:mol:mol:mol)
MPEG-350-ILs
MPEG-550-ILs
MPEG-750-ILs
3.989
5.818
4.608
32.85
36.47
40.24
10.55
9.468
8.318
7.005
7.204
7.664
0.104:1.00:0.120:2.54
0.139:1.00:0.0972:2.35
0.0982:1.00:0.0774:2.27
Table 2
100
Solubility of three ionic liquids in some common solvents.
MPEG-350-ILs
a
Ionic liquids
Solubility
Water
80
Methanol Ethyl acetate Ether
Methyl oleate
MPEG-350-ILs Miscible Miscible
MPEG-550-ILs Miscible Miscible
MPEG-750-ILs Miscible Miscible
Insoluble
Insoluble
Insoluble
Insoluble Insoluble
Insoluble Insoluble
Insoluble Insoluble
60
40
20
liquids increased with the increase of solvent polarity. It was shown
that the polarity of MPEG ionic liquids was strong. In addition, the
MPEG ionic liquids were not dissoluble in methyl oleate. It was
seen that the MPEG ionic liquid catalyst can be easily separated
from biodiesel via simple decantation and reused.
0
3.5. Esterification of oleic acid with methanol to biodiesel
100
200
300
400
500
600
Temperature (oC)
The effect of different polymerization degrees of ionic liquids
on the esterification reaction and the catalytic activity of concen-
trated sulfuric acid were studied (see Fig. 5). It can be seen that
the conversion of oleic acid increased significantly when the reac-
tion time was extended. The reaction had approached equilibrium
when the reaction time was extended to 4 h. The polymerization
degree of ionic liquids with a polyether group has a significant
effect on its catalytic activity. With an increase in the polymer-
ization degree of ionic liquid, the catalytic activity of ionic liquid
decreased. Among all the ionic liquids, the MPEG-350-ILs showed
the best catalytic performance, which is near that of concentrated
sulfuric acid catalyst. The polyether group of ionic liquids pos-
sesses the temperature-sensitive function. Under low temperature,
the hydrogen bond can be formed between oxygen of polyether
group and methanol and then the ionic liquids are hydrophilic.
With an increase in the temperature, the hydrogen bond is bro-
ken and then the ionic liquids are oleophilic. It is indicative of
homogeneous catalytic reaction under high temperature and
100
MPEG-550-ILs
b
80
60
40
20
0
100
200
300
400
500
600
Temperature (oC)
100
80
100
MPEG-750-ILs
c
80
60
40
20
0
H2SO4
MPEG-350-ILs
MPEG-550-ILs
MPEG-750-ILs
60
40
20
0
0
50
100
150
200
250
100
200
300
400
500
600
Temperature (oC)
Time (min)
Fig. 5. Conversion of oleic acid versus reaction time in the different ionic liquids or
concentrated sulfuric acid. Reaction conditions: catalyst:n(methanol):n(oleic acid))
0.017:10:1; 68 ◦C.
Fig. 4. TG curve of the three ionic liquids.

Q. Wu et al. / Catalysis Today 200 (2013) 74–79
79
100
80
60
40
20
0
synthesized through polyethylene glycol monomethylether as
raw material. They were characterized by infrared (IR) spectrum,
nuclear magnetic resonance (NMR), elementary analysis, solubility
analysis and differential thermal analysis (TG/DTA). Esterification
of oleic acid with methanol to biodiesel was investigated in var-
ious bifunctional temperature-sensitive amphiphilic acidic ionic
liquids. It was found that their structures were consistent with the
designed structure and their purity was high. These ionic liquids are
stable below 165 ◦C by analyzing TG/DTA. These bifunctional acidic
ionic liquids showed excellent catalytic performance for esterifica-
tion to biodiesel. The catalytic activity of each ILs is dependent on
the polymerization degree of polyether cation. The catalytic activ-
ity decreased with lengthening polyether chain of ionic liquids.
The ionic liquid – MPEG-350-ILs showed the best catalytic activity
that is near that of concentrated sulfuric acid, which is ascribed to
its strong Brønsted acidity and amphiphilicity. These ionic liquids
exhibited temperature-sensitive property. The produced biodiesel
could be separated via simple decantation and the ionic liquids
could be reused.
1
2
3
run
4
5
Fig. 6. The reuse of ionic liquid.
Acknowledgments
bi-phase separation under room temperature. Therefore, the poly-
merization degree of polyether in the ionic liquids determined if
the ionic liquid and reactant could become emulsion or microemul-
sion, which is a pseudo-homogeneous catalysis system. When the
polymerization degree of ionic liquids increased, the hydrophilic-
ity of ionic liquid with polyether group increased. The emulsion or
microemulsion reaction system had been become difficultly. For
the low polymerization degree of ionic liquids catalyst, the hydro-
gen bond of polyether group and methanol was weak and then
the hydrophilicity of ionic liquid with polyether group decreased.
Under high temperature, the hydrophilicity of ionic liquid with
polyether group changed into the lipophilicity. The alkane sul-
fonic acid group of ionic liquids is hydrophilic. Therefore, ionic
liquid with an alkane sulfonic acid group and a polyether group
is amphiphilic and the emulsion or microemulsion reaction system
had been become easily and the better catalytic performance had
been exhibited [24–27].
In addition, when reaction time was less than 2.5 h, with an
increase in the reaction time, the MPEG-350-ILs showed a larger
increase in the conversion of oleic acid compared with the others.
However, when the reaction time was extended to 4 h, the conver-
sion of oleic acid from MPEG-350-ILs as the catalyst was slightly
better than that from ILs 550 and 750, which indicated that the rate
of esterification reaction using IL 350 was reduced and the reaction
had probably approached equilibrium.
This work is supported by the National Nature Science Founda-
tion of China (20706006, 20976013), the Excellent Young Scholars
Research Fund of Beijing Institute of Technology (2007Y0509) and
the Fundamental Research Foundation of Beijing Institute of Tech-
nology (20081042006).
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In order to investigate the potential reusability of the ionic liq-
uids in the esterification reaction, a series of recycle experiments
were conducted with a methanol to oleic acid molar ratio of 10 and
ionic liquid to oleic acid molar ratio of 0.057 at 68 ◦C for 1.5 h. After
each recycle reaction, the ionic liquid catalyst and excess methanol
phase were separated by simple decantation. The methanol was
removed through reduced pressure distillation. As shown in Fig. 6,
the ionic liquid MPEG-350 was used five times and its catalytic
activity changed very little. This indicates that ionic liquid MPEG-
350 as the catalyst for the esterification is recyclable and stable.
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4. Conclusions
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Several water-stable bifunctional Bronsted-acidic ionic liquids
with an alkane sulfonic acid group and a polyether group were