Esterification of oleic acid in [Bmim]BF4/[Hmim]HSO4 + TX-100/cyclohexane ionic liquid microemulsion

Article abstract of DOI:10.1039/c4ra09408e

Esterification of oleic acid was carried out in a 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄)/Triton X-100 + 1-hexyl-3-methylimidazolium hydrogen sulfate ([Hmim]HSO₄)/cyclohexane microemulsion. A pseudo ternary phase diagram of the designed systems was drawn to investigate the phase behavior of the microemulsion, with the surfactant [Hmim]HSO₄ acting as a catalyst. The effects of various reaction parameters were explored. The results showed that the maximum yield of lauryl oleate reaches 91.17% and its selectivity reaches 98.55% under optimum reaction conditions. The reaction was carried out with 8 wt% catalyst at 373 K for 6 h. The molar ratios of [Bmim]BF₄ to the surfactant and of oleic acid to lauryl alcohol were 0.24 and 0.2, respectively. Comparison reactions between different alcohols and oleic acid were also performed, and the results showed that long alkyl chain alcohols promote the reaction rate. UV-vis absorption spectra demonstrated that the generated water enters the [Bmim]BF₄ microdomain of the ionic liquid microemulsions. A possible mechanism of the reaction was also presented. All the results indicate that the [Bmim]BF₄/TX-100 + [Hmim]HSO₄/cyclohexane microemulsion is a very efficient catalyst system for esterification reactions. This journal is

Full text of DOI:10.1039/c4ra09408e

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Table of Contents
[Hmim]HSO4-based microemulsion is an efficient catalyst system for esterification; it
provides large interface areas, the resultant water enters [Bmim]BF4 microdomain.

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Esterification of oleic acid in [Bmim]BF4/[Hmim]HSO4+TX-100
/cyclohexane ionic liquid microemulsion
Dongyu Jiang, Li Chen, Aili Wang and Zongcheng Yan*
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou 510640, China
Corresponding author. Tel: +86+20+87111109; Fax: +86 20 87111109
E-mail address: zcyan@scut.edu.cn
Table of Contents
[Hmim]HSO4-based microemulsion is an efficient catalyst system for esterification; it
provides large interface areas, the resultant water enters [Bmim]BF4 microdomain.
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Abstract: Esterification of oleic acid was carried out in 1-butyl-3-methylimidazolium
tetrafluoroborate ([Bmim]BF4)/Triton X-100+1-hexyl-3-methylimidazolium hydrogen
sulfate ([Hmim]HSO4)/cyclohexane microemulsion. A pseudo ternary phase diagram
of the designed systems was drawn to investigate the phase behavior of the
microemulsion, with the surfactant [Hmim]HSO4 acting as a catalyst. The effects of
various reaction parameters were explored. Results showed that the maximum yield of
lauryl oleate reaches 91.17% and its selectivity reaches 98.55% under optimum
reaction conditions. The reaction was carried out with 8 wt.% catalyst at 373 K for 6 h.
The molar ratios of [Bmim]BF4 to surfactant and of oleic acid to lauryl alcohol were
0.24 and 0.2, respectively. Comparison reactions between different alcohols and oleic
acid were also performed, and results showed that long alkyl chain alcohols promote
the reaction rate. UV–vis absorption spectra demonstrated that the generated water
enters the [Bmim]BF4 microdomain of the ionic liquid microemulsions. A possible
mechanism of the reaction was also presented. All results indicate that the
[Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane microemulsion is a very efficient
catalyst system for esterification reactions.
Keywords: Ionic Liquid; Oleic acid; Microemulsion; Esterification
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1. Introduction
Esterification reaction, as a dehydration reaction, is of major importance in synthetic
organic chemistry.1, 2 The traditional catalysts used in esterification are sulfuric acid
and phosphoric acid, but they have several drawbacks such as side reactions, low
reaction activity, low selectivity, equipment corrosion, and environmental pollution.
Generally, direct esterification is carried out in organic solvents and needs either of
two methods to shift the equilibrium between reactants and products.3–5 One method
is by removing the generated water azeotropically or using dehydrating agents, and
the other is by overdosing one reactant.
Ionic liquids (ILs) have received much attention as green media for reactions and
separations because of their unique physicochemical properties.6 A concept relating to
ionic liquid microemulsions (ILMs) was put forward to expand the application of
ILs.7–9 The IL-based microemulsions show potential in the synthesis of metal
nanomaterial10, biodiesel production11, and enzymatic reaction12. Microemulsion is a
thermodynamically stable, homogeneous, isotropic, and optically transparent solution
formed by two or more immiscible liquids, which are stabilized by a surfactant and
occasionally, a co-surfactant film at the liquid–liquid interface.13,14 Moreover,
microemulsion is an excellent media for organic reactions because it can provide a
very large interfacial area and overcome the incompatibility of reagents.15-18 Research
studies on esterification catalyzed by microemulsions have been reported.19–22 The
results of these studies showed that esterification occurs in the interface area, and the
resultant water enters the water microdomain of the microemulsion, making the
esterification equilibrium shift towards product generation. Esterification in
traditional microemulsions needs more than 24 h at low temperature of 318 K to reach
equilibrium. The product yield decreases with increasing reaction temperature
because traditional microemulsions should be kept at low temperature to remain
stable. However, ILMs have relatively higher temperature-independence compared
with traditional aqueous systems.23 The microstructure of ILMs remains stable over a
wide range of temperature, increasing the range of application for the ILM systems in
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many fields. ILMs acting as media in esterification may have more advantages owing
to the unique features of the ionic liquids and microemulsions.
In this study, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4)/Triton
X-100+1-hexyl-3-methylimidazolium hydrogen sulfate ([Hmim]HSO4)/cyclohexane
microemulsion was formulated and investigated to determine its phase behavior.
Pseudo ternary phase diagrams were used to design a more reasonable system for
esterification. Esterification reactions between oleic acid and different alcohols were
carried out in these ILM system. [Hmim]HSO4 was used as a surfactant-type
Brønsted-acid catalyst. Based on the pseudo ternary phase diagrams of ILMs, the
effects of ILM composition and reaction parameters were investigated. The UV–vis
spectrograms obtained indicated that the resultant water could enter the [Bmim]BF4
microdomain of the ILMs. A possible mechanism of the reaction was also
demonstrated. This work may provide an efficient method for preparing oleates.
2. Results and Discussion
2.1 Pseudo ternary phase diagrams
The phase diagram of [Bmim]BF4/[Hmim]HSO4/cyclohexane system at 298 ± 1 K is
shown in Fig. 1(a). The blank region was the single-phase microemulsion, and the
shadow region marked the multi-phase, which was a cloudy region. The fact that
[Bmim]BF4, [Hmim]HSO4, and cyclohexane can formulate the microemulsion proves
the surfactant function of [Hmim]HSO4. However, the formation of ILMs requires
large amounts of the surfactant [Hmim]HSO4, which is expensive. Therefore, in view
of this economic problem, we used a combination of [Hmim]HSO4 and TX-100
surfactants to reduce the dosage of [Hmim]HSO4. Fig. 1(b) shows the phase diagram
of [Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane system at 298 ± 1 K. The pseudo
ternary phase diagram showed the consistent formation of single-phase
microemulsion over [Bmim]BF4 or cyclohexane content range of 0–100 wt.%. The
single-phase region area of [Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane
microemulsion is larger than that of [Bmim]BF4/[Hmim]HSO4/cyclohexane
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microemulsion. In this research, [Hmim]HSO4 acted as a surfactant-type
Brønsted-acid catalyst, and the [Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane
microemulsion was chosen as the media for the preparation of lauryl oleate.
0.00
0.00
1.00
1.00
w
(
0.25
0.25
T
0.75
0.75
X-100:
w
)
(
4
)
4
C
6
BF
C
6
m
BF
im
im
m
0.50
0.50
im
0.50
0.50
im
H
Bm
SO
4
(
H
Bm
multi-phase
w
SO
4
w(
)
=4:1)
multi-phase
0.75
0.75
0.25
0.25
1.00
0.00
1.00
0.00
0.00
0.00
0.25
0.50
0.75
1.00
0.25
0.50
0.75
1.00
w(Cyclohexane)
w(Cyclohexane)
(a)
(b)
Fig. 1 Phase diagrams of [Bmim]BF4/[Hmim]HSO4/cyclohexane system (a) and
[Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane system (b) at 298 ± 1 K
2.2 Catalytic activity of [Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane
microemulsion
To investigate the catalytic properties of different catalysts, esterification of oleic acid
and lauryl alcohol was introduced. Table 1 presents the comparison of the catalytic
activities of different catalysts. Results showed that [Hmim]HSO4 in the ILM system
revealed much higher catalytic activity than other catalysts. By contrast, [Bmim]BF4
had almost no catalytic activity. The yield decreased to 9.06% when the reaction was
carried out in the [Bmim]BF4/TX-100/cyclohexane system because the
microemulsion separated the reactants from the [Bmim]BF4. Esterification can’t gain
good product yield in microemulsion without catalyst. After formulating
[Hmim]HSO4-based microemulsion, the esterification carried out in the ILM system
attained better product yield and selectivity than when [Hmim]HSO4 is used as a
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direct catalyst. In addition, [Hmim]HSO4-based microemulsion revealed much higher
catalytic activity than [Hmim]HSO4 and cyclohexane without formulation of ILMs.
These results can be ascribed to the large interface area of the ILMs.24 The reaction
was also studied in the absence of catalyst, obtaining 12.35% yield and 85.35%
selectivity. These results indicate the significant role of [Hmim]HSO4-based ILM
systems as a catalyst media for esterification.
Table 1 Catalytic activity of the catalystsa
Entry Catalyst
Without catalyst
Reaction media
Yield/(%)b Selectivity/(%)
1
/
12.35
13.68
82.56
9.06
85.35
90.72
94.82
89.03
97.88
98.55
2
3
4
5
6
[Bmim]BF4
/
[Hmim]HSO4
Without catalyst
[Hmim]HSO4
[Hmim]HSO4
/
[Bmim]BF4/TX-100 /cyclohexane
cyclohexane
84.25
91.17
[Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane
a Reaction conditions: oleic acid: lauryl alcohol = 1:5 molar ratio, T = 373 ± 1 K, reaction time: 6
h, catalyst amount: 8 wt.%. b Determined by HPLC using external standard method
2.3 Effect of the R values of ILMs
A
series of reverse ILM samples with different
R
values (R
=
n([Bmim]BF4)/n(TX-100+[Hmim]HSO4)) was prepared to characterize the effect of [Bmim]BF4
concentration of the ILMs, expressed as R values, on esterification. The results are
shown in Table 2. The yield of lauryl oleate attained a maximum value at R = 0.24. In
general, esterification occurred at the interface of the microemulsion.12 The large
interface area provided more opportunity of contact between the reactants and
allowed the completion of the reaction. The interface area increased with increasing R
values, varying from 0.12 to 0.24, and the lauryl oleate maximum yield was achieved
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at R = 0.24. Esterification is a dehydration reaction, and the generated water can
easily access the [Bmim]BF4 microdomain in the ILM system because of the
hydrophily of [Bmim]BF4.25 With the increase in R values, the product yield
decreased. An excess in [Bmim]BF4 caused the microemulsion to become unstable,
hence decreasing the phase interface area. This phenomenon led the esterification
equilibrium to move toward the reverse direction, thereby facilitating ester hydrolysis
and reducing product yield.
Table 2 The effect of ILM with different R values on the esterificationa
Entry
R value
0.12
Yield of product/(%)b
Selectivity/(%)
98.07
1
2
3
4
5
6
74.13
81.68
91.05
88.56
87.08
86.72
0.18
98.91
0.24
98.02
0.30
97.89
0.36
97.55
0.48
96.91
a Reaction conditions: oleic acid: lauryl alcohol = 1:5 molar ratio, T = 373 ± 1 K, reaction time: 6
h. b Determined by HPLC using external standard method
2.4 Effect of reaction conditions
The effects of reaction conditions including the reaction temperature and time, the
molar ratio of oleic acid to lauryl alcohol, and the catalyst amount were investigated.
Fig. 2 presents the effects of the reaction temperature and time on product yield.
The yield of lauryl oleate increased and the equilibrium time became shorter as the
reaction temperature increased. High temperature gave rise to more frequent and
successful collisions for high conversion of reactants to ester products.26 Ionic liquid
microemulsions revealed relatively high temperature-independence resulting in
shorter esterification equilibrium time. In addition, ILM can remain stable at
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relatively high temperature. The lauryl oleate yield increased distinctly with
increasing temperature, which indicated a similar characteristic with esterification
catalyzed by acid.27 However, the color of the product became darker at temperatures
higher than 373 K, and therefore, the optimal reaction temperature for esterification in
ILMs was 373 K. In addition, the yield of lauryl oleate increased as the reaction time
increased. Within 6 h, the reaction approached equilibrium, and the yield of lauryl
oleate did not increase anymore when the reaction time was extended. Hence, the
optimal reaction time for esterification in ILMs was 6 h. Results show that the
participation of ILM system as a media in esterification reduces the reaction time
compared with results in previous studies.13–15,19 In addition, because large surface
areas were employed, [Hmim]HSO4 exhibited high activity under mild conditions.
100
90
80
353K
373K
393K
70
60
4
5
6
7
8
Time/h
Fig. 2 The effect of temperature and time on esterification, oleic acid: lauryl alcohol =
1:5 molar ratio, catalyst amount = 8 wt.%
Given that the esterification reaction is reversible, excess amounts of lauryl alcohol
use favor the conversion of oleic acid. Fig. 3 shows the product yields at decreasing
molar ratio of oleic acid to lauryl alcohol, varying from 1:1.1 to 1:7. Low molar ratios
are required to allow the reaction to proceed until completion.28,29 The yield increased
rapidly from 73.53% to 91.17% with the decrease in oleic acid to lauryl alcohol ratio
from 1:1.1 to 1:5. Further decreases in ratio led to only a slight increase in yield
probably because the ILM system was diluted with excess lauryl alcohol. Therefore,
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1:5 oleic acid to lauryl alcohol ratio was the most appropriate molar ratio for
esterification in the present work.
100
80
60
40
20
0
1:1.1
1:2
1:4
1:5
1:7
n(oleic acid):n(lauryl alcohol)
Fig. 3 Effect of molar ratio of oleic acid to lauryl alcohol on esterification, T = 373 ±
1 K, t = 6 h, catalyst amount = 8 wt.%
The amount of [Hmim]HSO4 used was also found to affect esterification; relevant
results are presented in Fig. 4. The greater the amount of catalyst used, the more the
active site promotes the transformation of the reactant.30 The yield increased with the
increment of [Hmim]HSO4 concentration. On the one hand, this result can be ascribed
to the availability of large surface area and acid sites, which favored the dispersion of
more active species.27 On the other hand, the amount of hydrogen ions may increase
with increasing [Hmim]HSO4 concentration, which causes rapid attainment of
equilibrium for the reaction.27 This increasing yield trend slowed down quickly when
the concentration of [Hmim]HSO4 exceeded 8 wt.% (based on reactants). Hence, the
appropriate concentration of [Hmim]HSO4 for esterification was 8 wt.%.
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100
90
80
70
60
5
6
7
8
9
10
11
12
13
Mass fraction of C6mimHSO4/%
Fig. 4 Effect of [Hmim]HSO4 concentration on esterification, oleic acid: lauryl
alcohol = 1:5 molar ratio, T = 373 ± 1 K, t = 6 h
To study the influence of different alcohols as raw materials, various alcohols
(methanol, ethanol, 1-butanol, 1-hexanol, 1-octanol, and lauryl alcohol) were selected,
and their esterification activities were compared under the same reaction conditions.
As shown in Fig. 5, the esterification conversions for most alcohols exceeded 90%
except for methanol and ethanol. Methanol and ethanol exhibited relatively poor
activities because of their very low boiling points. As the reaction temperature reaches
a certain degree, a mass of methanol or ethanol liquid was converted into vapor.
Furthermore, the lengths of the carbon chains in methanol and ethanol are too short;
hence, they cannot serve as co-surfactants. On the contrary, 1-butanol and 1-hexanol
are common co-surfactants.31 In traditional water–oil microemulsions, co-surfactants
can change the surface activity and HLB of surfactants, participate in a micelle, and
adjust the polarity of water and oil.32 As co-surfactants, 1-butanol and 1-hexanol could
easily stay in the interface area, providing more opportunities to contact with oleic
acid. Similarly, 1-octanol and lauryl alcohol can serve as co-surfactants, hence, they
produced
product
yields
exceeding
90%.
Thus,
[Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane microemulsions are excellent green
media for esterification.
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100
80
60
40
20
0
l
l
l
l
l
l
o
h
o
o
o
o
o
n
n
n
n
n
ta
a
a
a
-Hex
o
c
l
ta
-Bu
th
c
e
Eth
a
l
-O
M
1
1
ry
1
u
a
L
Fig. 5 Esterification of oleic acid with different alcohols, oleic acid: alcohol = 1:5
molar ratio, T = 373 ± 1 K, t = 6 h, conversion based on acid value titration
2.5 UV–vis analysis
The micropolarity of ILM was studied through UV–vis absorption spectra
measurement. The shift in the absorption maximum of methyl orange (MO) is a
sensitive measure of the local environment about ILMs. The absorption spectra of
MO in different solvents were studied, and the absorption maximum is red-shifted
with increasing polarities of pure solvents.33–35 As the water content in
microemulsions increases, the visible absorption maximum λmax of MO shifts to long
wavelengths (red shift), indicating great polarity.33–34 Fig. 6 shows the absorption
spectra of MO in ILMs before and after the reaction. The absorption maximum is
red-shifted from 395 nm to 425 nm. This red shift in absorption maximum clearly
indicates that the resultant water entered the [Bmim]BF4 microdomain.
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0.5
0.4
0.3
0.2
0.1
0.0
amax=395nm
bmax=425nm
b
a
400
450
500
550
600
Wavelength/nm
Fig. 6 Absorption spectra of MO in ILM before (a) and after the reaction (b) at 298 ±
1 K, MO = 4 × 10-5 M
2.6 Mechanism of reaction
Reverse ILM system was a special reaction medium and the proposed reaction
mechanism is showed in Scheme 1. Initially, [Bmim]BF4/TX-100
+[Hmim]HSO4/cyclohexane system provided a high interfacial area of contact to
promote the reaction rate. The esterification took place at the interface area in the ILM
systems. The [Bmim]BF4 microdomain in ILM system entrapped the generated water,
which accelerated the formation of ester and promoted the equilibrium process in
favor of the products. [Bmim]BF4, which is a hydrophilic ionic liquid, contacted with
water by hydrogen bonding.35 [Hmim]HSO4 could be used as a surfactant-type
Brønsted-acid catalyst, and its polar acid heads would congregate because of the
formation of ILMs, leading to a relatively high acidity in the interfacial area, and an
effective catalytic machinery for esterification.
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TX-100
DOI: 10.1039/C4RA09408E
BmimBF4
R OH
1
H2O
Oleic acid
Cyclohexane
Oleate
HmimHSO4
OH
R
OH
H
R OH
HO OH
C
1
O
OH
HO OH
C
C
OH
C
R
R
R
R
1
OH
OH
H
R
R
OH
O
H
2
C
O
C
O
+
R
R
1
1
OH
-
H
O
R
R
C
O
C
O
R
R
1
1
H
R: CH (CH ) C CH(CH ) CH
3
2 7
2 6
2
R : C H
1
12 25
Scheme 1. Proposed mechanism for esterification in ILMs
3. Experimental
3.1 Materials
[Bmim]BF4 and [Hmim]HSO4 were purchased from Centre of Green Chemistry and
Catalysis, Lanzhou Institute of Chemical Physics. All ionic liquids are >99.0 wt %
pure. TX-100 (AR), cyclohexane (AR), lauryl alcohol (AR), methanol (AR) and oleic
acid (AR) were obtained from Tianjin Kermel Chemical Reagent Science and
Technology Co. Ltd. Methyl orange (MO) was supplied by Beijing Chemical Reagent
Co. Ltd. Before use, TX-100 and ionic liquids were vacuum-dried at 70 °C for 6 h to
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remove excess water. Unless otherwise stated, all experiments were performed in
nitrogen atmosphere.
3.2 Construction of pseudo ternary phase diagram
The pseudo ternary phase diagram of the microemulsions were determined at 298 ± 1
K by direct observation. In a typical experiment, mixtures of cyclohexane with
surfactant ([Hmim]HSO4 or mixture of TX-100 and [Hmim]HSO4 at 4:1 mass ratio)
were prepared by changing the mass ratio to 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and
1:9. [Bmim]BF4 was titrated into the mixtures under moderate agitation. The phase
boundary was evaluated by observing the phase transition of the solution from clear
transparent to turbid. Phase diagrams could be used to characterize the system further
and choose the proper reaction system for esterification.
3.3 Preparation of ILM and Esterification in the ILM
Cyclohexane (18.90 mmol) was added into the mixtures of TX-100 (3.40 mmol) and
[Hmim]HSO4, at 4:1 TX-100 to [Hmim]HSO4 mass ratio, in a round flask at 298 ± 1
K. After achieving thermal equilibrium, the mixtures were titrated with [Bmim]BF4
(0.88 mmol) under moderate agitation until an optically clear single-phase solution
was formed.
Esterification was carried out in the above ILMs under atmospheric pressure after
the addition of oleic acid (4.67 mmol) and lauryl alcohol (23.35 mmol). The reaction
was allowed to proceed for 4 h to 8 h with vigorous stirring and heating at the desired
temperature. After the reaction, the mixture was added into 80 ml methanol and
allowed to stand for a few hours. Afterwards, the mixture divided into two phases; the
lower phase mainly consisted of the produced lauryl oleate and the upper phase
remained clear.
3.4 Characterization
The selectivity means the percentage of yield to conversion. Yield of product was
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determined by High Performance Liquid Chromatography (HPLC, Agilent 1100,
American Agilent Company) analysis using external standard method. Conversion of
oleic acid was determined by acid value titration. The acid value was determined by
titration of KOH/ethanol standard solution (0.1 mol/L). The UV–visible absorption
spectra were measured by a Shimadzu UV-2450 spectrophotometer. A 4 × 10−5 M MO
solution was added to the substance.
4. Conclusions
In conclusion, the [Bmim]BF4/TX-100+[Hmim]HSO4/cyclohexane microemulsion
was found to be an excellent catalyst system for esterification. The designed
microemulsion system had two obvious predominances for esterification. First, a very
high yield (> 90%) was obtained after a short reaction time even at mild temperature
condition (373 K), which proved that esterification can perform well in the ILM
system without the use oxidative and corrosive acid, basic catalyst, or expensive
enzyme catalyst. Second, the reaction procedure was simple because removal of
generated water by distillation or desiccation is no longer necessary; the generated
water entered the [Bmim]BF4 domain of microemulsion. The superiority of
[Hmim]HSO4 in the ILM system for esterification was due to its two-fold functions;
on one hand, it acted as a Brønsted-acid catalyst to catalyze the reaction. On the other
hand, it served as a surfactant to enlarge the interface area and entrap the generated
water during esterification reaction. Compared with organic reaction using traditional
aqueous systems as catalyst and media, the ILM systems showed much more
superiority including excellent temperature-independence, high reaction rate,
enhanced equilibrium conversion, and controllable reaction region selectivity. We
believe that the ILM systems can be a promising alternative for traditional medium in
applications for etherification, thioetherification reactions, as well as other organic
dehydration reactions.
Acknowledgements
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RSC Advances
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DOI: 10.1039/C4RA09408E
This research was supported by the National Natural Science Foundation of China
(21376088), Project of Production, Education and Research, Guangdong Province and
Ministry of Education (2012B09100063, 2012A090300015), and the Fundamental
Research Funds for the Central Universities (2013ZZ0071). The authors would also
gratefully acknowledge the support from the Guangdong Provincial Laboratory of
Green Chemical Technology.
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