A heteropolyacid-based ionic liquid as a thermoregulated and environmentally friendly catalyst in esterification reaction under microwave assistance

Article abstract of DOI:10.1016/j.catcom.2013.08.014

A new kind of heteropolyacid (HPA) ionic liquid [(CH₃) ₃NCH₂CH₂OH]H₂PW₁₂O ₄₀ (ChH₂PW) has been synthesized using choline chloride and H₃PW₁₂O₄₀ (HPW) as precursors. The catalyst exhibited a novel switchable property based on temperature. The separation of the catalyst would be explored by simply decreasing reaction temperature without appreciable loss. Excellent conversions (97%) for esterification have been obtained under microwave-accelerated conditions.

Full text of DOI:10.1016/j.catcom.2013.08.014

Catalysis Communications 42 (2013) 125–128
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A heteropolyacid-based ionic liquid as a thermoregulated and
environmentally friendly catalyst in esterification reaction
under microwave assistance
Xixin Duan a,d, Guiru Sun a, Zhong Sun a, Jianxin Li a, Shengtian Wang a, Xiaohong Wang a,
⁎
,
Shiwu Li b, Zijiang Jiang c
a
Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun 130024, PR China
Traffic College, Jilin University, Changchun 130025, PR China
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
b
c
d
Faculty of Chemistry and Biology, Beihua University, Jilin 132013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 June 2013
Received in revised form 8 August 2013
Accepted 13 August 2013
Available online 27 August 2013
A new kind of heteropolyacid (HPA) ionic liquid [(CH3)3NCH2CH2OH]H2PW12O40 (ChH2PW) has been synthe-
sized using choline chloride and H3PW12O40 (HPW) as precursors. The catalyst exhibited a novel switchable
property based on temperature. The separation of the catalyst would be explored by simply decreasing reaction
temperature without appreciable loss. Excellent conversions (97%) for esterification have been obtained under
microwave-accelerated conditions.
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Heteropolyacids
Ionic liquid
Esterification
Microwave
Biodiesel
1. Introduction
needs to be paid for designing new kinds of HPA ILs for esterification
or transesterification. Compared to the most ILs, choline chloride
Biodiesel, a mixture of the mono-alkyl esters from esterification of
fatty acid or transesterification of triglyceride molecules, is alternative
to conventional diesel fuel [1–3]. The routine procedure for biodiesel pro-
duction is esterification of free fatty acids (FFAs) and transesterification of
triglycerides [4,5] with short chain alcohols catalyzed by base or acid
catalysts [5–7]. Heterogeneous acid-catalyzed processes can be applied
to produce biodiesel from low-cost raw materials with high amount of
FFAs. By now, many approaches involving acid catalysis had been devel-
oped [8,9]. Compared to homogeneous catalysis, solid acid catalysts have
served as important functional materials for green and recyclable
catalysts.
Heteropolyacids (HPAs) and their salts have been reported as prom-
ising heterogeneous catalysts for esterification [10–15]. Wang Jun's
group [16] designed acidic ionic liquid (IL) functional HPAs and acted
as a “reaction-induced self-separating catalyst” for esterification. By
now, a series of HPA-IL hybrids had been employed as catalysts for es-
terification and transesterification [9,17]. Nevertheless, more attention
(ChCl) exhibits less expensive, low toxic to humans, environmentally
friendly and non-flammable characters [18], which has been used in
the conversion of carbohydrates into 5-hydroxymethylfurfural [19,20].
The combination of ChCl with Lewis acids had been investigated in
o-acetylation of cellulose and monosaccharides [21]. As we were
preparing our manuscript, Zhang's group [22] reported a series of
chorine IL catalysts, which were applied in biodiesel production as
efficient base catalysts. Therefore, it is beneficial to replace expen-
sive ILs with choline for sustainable synthesis of biodiesel.
To our knowledge, there is no report on the combination of ChCl
with HPAs on esterification. Microwave assisted synthesis of biodiesel
showed amazing accelerations, higher yields and high selectivity
[23–25]. Therefore, we explored an alternative to fulfill esterification
of FFAs catalyzed by ChCl functionalized HPA catalyst under micro-
waves. Its competitive advantages are (1) thermoregulated catalysis.
It could be used as a homogeneous catalyst in enhancement of temper-
ature. When temperature decreases, it precipitated and could be
recycled, just like a heterogeneous system. Thus, this catalyst combines
the advantages of homogeneous and heterogeneous catalysis resulting
in easy handling of the catalyst; (2) First example of a highly efficient
use of a ChCl functionalized HPA catalyst; (3) Providing hydrophobic
surroundings leading to water-tolerant property availability; (4) Very
⁎
Corresponding author. Fax: +86 431 85099759.
E-mail address: wangxh665@nenu.edu.cn (X. Wang).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.08.014

126
X. Duan et al. / Catalysis Communications 42 (2013) 125–128
efficient for the conversion of long chain FFAs; (5) The use of microwave
irradiation instead of conventional heating.
The X-ray diffraction pattern was used to confirm the structure of
ChH2PW. Compared with pure HPW, ChH2PW shows the similar diffrac-
tion peaks (Fig. S2) at 10°, 15–18°, 21–27° and 30–40° (JCPDS no. 76-
1815). This result indicates the original structure of HPW being attained
after forming HPAs.
2. Experimental
The 31P MAS NMR spectrum of ChH2PW (Fig. S3) shows one peak at
δ = −16.2 ppm, whereas H3PW12O40·6H2O gives peak at −15.6 ppm.
The shift of 31P MAS NMR is attributed to the introduction of organic
2.1. Preparation of the catalysts
H3PW12O406H2O was synthesized by the reported paper [26].
ChCl (0.467 g, 3.34 mmol) was added to the solution of HPW (10 g,
3.34 mmol) in 20 mL of distilled water with stirring at room temperature
for 8 h. A white precipitate was formed and washed with distilled water,
recrystallized twice by CH3CN and dried at 60 °C and then gave a white
product [(CH3)3NCH2CH2OH]H2PW12O40 (ChH2PW) with the yield of
71.2%. [(CH3)3NCH2CH2OH]2HPW12O40 abbreviated as (Ch)2HPW was
prepared by the similar procedure with the molar ratio of ChCl to
H3PW12O40 as 2:1. [C16H33N(CH3)3]H2PW12O40 (CTAH2PW) was pre-
pared by the previous report [27].
cations into HPW, which could confirm the formation of ChH2PW and
3−
no physical mixture of ChCl and PW
O
.
12 40
The transmission electron microscopy (TEM) image (Fig. 1) shows
that the catalyst consists of irregular particles with sizes from 10 to
20 nm. The result of energy-dispersive X-ray (EDX) measurement
suggested the molar ratio of C:P:W = 5:1:12, giving the formula as
[(CH3)3NCH2CH2OH]H2PW12O40. These results demonstrate the forma-
tion of ChH2PW (Scheme S1). Also, the elemental analyses of Ch2HPW
are: W, 72.8; P, 1.11; C, 2.10; H, 0.65; and N, 0.55%, respectively.
The formation of micellar assembly of ChH2PW had been deter-
mined using specific conductivity to define the critical micelle concen-
tration (CMC) of ChH2PW (Fig. S4). It gave nearly a portion of two
straight lines with break points of specific conductivity versus concen-
tration plot [28]. Therefore, ChH2PW is a kind of amphiphilic molecules,
which could form micellar assembly in water or polar-solvent.
2.2. Esterification reaction
Esterification reactions were carried out in the presence of ChH2PW
at different reaction conditions. This reaction mixture was then irradiat-
ed by microwave (A 2450 MHz microwave generator) under reflux
(65 °C) for the specified reaction time. After the esterification reaction
completed, the upper layer were esters and the excess alcohol and the
lower layer was the catalyst, which was recovered by decantation and
washing with methanol and removing water under a vacuum at 60 °C
for further use. The mixture was concentrated using a rotary evaporator
to remove the excess alcohol. The conversion of free fatty acid into ester
was calculated by measuring the acid value of the product and the yield
of ester was detected by gas chromatography (GC).
The Brønsted acidity of ChH2PW was determined by titration
[29]. The acidity capacity of ChH2PW is about 0.62 mmol/g, which is
lower than that of HPW (1.79 mmol/g), but higher than that of ChCl
(0.02 mmol/g). This could be attributed to the organic fragment instead
of the proton of H3PW12O40
.
3.2. The catalytic activity of ChH2PW
In esterification of palmitic acid and methanol by microwave assis-
tance, different catalysts gave various performances, while the catalytic
activity (Fig. 2) was in the range of HPW ~ ChH2PW N ChCl. HPW as a
homogeneous catalyst, presented the conversion of 98.5% and TOF of
47.2 h−1. ChH2PW exhibited the similar performance as homogeneous
HPW. This result is very important that there are only a few successful
examples at present, in which the catalytic activity of heterogeneous
HPAs is comparable to those of the homogeneous analogs [30]. Com-
pared to other HPAs supported on silica, our results showed the higher
activity with lower usage of methanol and short reaction time [31] due
to the unique properties of ChH2PW.
3. Results and discussion
3.1. Catalyst characterization
The FT-IR spectrum of ChH2PW was given in Fig. S1a. The four bands
at 1080, 978, 893 and 808 cm−1 are due to vas(P–Oa), vas(W–Od),
vas(W–Ob) and vas(W–Oc), respectively, corresponding to the charac-
teristic bands of Keggin structure [18]. It indicated that the catalyst
well retained the Keggin structure after the organic cation replaced
the proton in HPW. The peaks at 3519, 3037 and 1471 cm−1 were at-
tributed to C\H, and C\N, respectively, showing the existence of the
organic group.
The higher activity of ChH2PW comes from the acid center of
H2PW12O−40 and thermoregulated property of choline functionalized
Fig. 1. The TEM image (left) and EDX pattern (right) of ChH2PW.

X. Duan et al. / Catalysis Communications 42 (2013) 125–128
127
120
100
80
60
40
20
0
excess water to the mixture. It can be seen that the conversion was
related to water content (Fig. 4). When water content was below 0.2%,
the conversion was reduced to 72.8%, but the conversion was signifi-
cantly reduced to 1.46% for 1% of water. This result is comparable to
the other solid HPA catalyst Cs2.5H0.5PW12O40 [32] and higher than
H2SO4 [33] and SO24−/ZrO2 [34]. The reason is that ChH2PW is composed
of choline hydrophobic tail and a HPA hydrophilic head group, whose
hydrophobic tail would help water molecules be eliminated from the
catalytic sites. Compared to the other amphiphilic HPA molecules such
as CTAH2PW (CTA represents cetyltrimethyl ammonium), the longer
organic groups led to higher water-tolerance.
Conversion
TOF
96.5
98.5
97.0
89.8
52.5
43.1
47.8
47.1
46.6
25.2
The influence of chain length of acid and alcohol on esterification
with ChH2PW had been checked. The conversions for different alcohols
(methanol, ethanol, and butanol) are shown in Fig. S5. The type of
alcohols affects the reaction very obviously, which is perhaps because
of the different dissociation degree of three alcohols. It is known that
for alcohols, the acidity decreased as the alkyl group substitution
increased [35] resulting in weak dissociation degree. Therefore, metha-
nol can be easier to form alkoxide ions compared with other alcohols,
which is helpful for esterification. Therefore, the different acids reacting
with methanol give the highest conversion compared to other alcohols
and the different lengths of acid did not show significant influence on
esterification. As a heterogeneous acid catalyst, ChH2PW could form
micellar assembly in methanol during the reaction. The acid sites from
HPAs are fixed on the surface of the assembly, and the palmitic acid
molecules are easy to access to the catalytic sites. Therefore, no steric
effects were observed in esterification of different carboxylic acids.
Main parameters affecting esterification were investigated in Fig. S6.
The optimism reaction conditions are the molar ratio of methanol/acid/
catalyst 520:40:1, reaction time 50 min and temperature 65 °C.
As for heterogeneous catalysts, their regeneration is the most impor-
tant parameter in application. This thermoregulated ChH2PW can be
easily achieved by decreasing the temperature (Fig. 3c). The catalytic
activity remains efficient after six runs, showing only a slight decrease
in activity (Fig. 5). The leaching of ChH2PW was about 1.1 wt.% for
each cycle. And the total amount of ChH2PW leaching through six runs
is 6.5% of the starting amount. In order to determine the leaching of
ChH2PW, the UV–vis spectroscopy of the sample in methanol was
done. The two characteristic bands at 221 nm and 260 nm could be
seen corresponding to the charge transfer of oxygen to tungsten, indi-
cating that ChH2PW dissolved in methanol with Keggin structure. It
can be concluded that the leaching of ChH2PW was attributed to the dis-
solution in methanol during the reaction. The IR spectrum of ChH2PW
22.9
7.0
PW(c)
PW(a)
PW(b)
ChCl(a)
HPW(a)
HPW(a)
2
2
2
2
ChH
ChH
ChH
(Ch)
Fig. 2. Esterification of palmitic acid with different catalysts. Reaction conditions: palmitic
acid 0.02 mol, the molar ratio of methanol/acid/catalyst 520:40:1, 50 min, 65 °C, (a) micro-
wave irradiation, 50 min; (b) conventional heating, 50 min; (c) conventional heating, 5.5 h.
HPA (Fig. 3). It can be seen that before the reaction, ChH2PW decanted
to the bottom of the reactor to form heterogeneous one with methanol
(Fig. 3a). With an increase in temperature to 65 °C, ChH2PW dispersed
around methanol phase (Fig. 3b). The morphology of this form was test-
ed by TEM (Fig. 1), showing that ChH2PW assembles as micellar spheres
with size around nanometers. In addition, the formation of micellar
assembly in methanol at 65 °C had been confirmed by the CMC test.
Therefore, choline functionalized HPAs could act as a temperature-
responsive catalyst in this reaction. The catalytic activity of ChH2PW
could be enhanced and exhibited a high conversion of 97%, which is
close to that of HPW. After the reaction, with a decrease in temperature
to the room temperature, the catalyst precipitated from the mixture to
be separated for reuse. In addition, microwave assistance (97% for
50 min) is more available for esterification and can reduce reaction
time compared to the conventional heating (96.5% for 5.5 h and 52.5%
for 50 min). Therefore, microwave assistance could promote the reac-
tion and shorten the reaction time.
Another purpose of this work is to seek a heterogeneous acid catalyst
to fulfill esterification of FFAs in crude feedstocks containing water. The
presence of water could promote the reverse reaction, therefore the
effect of water on catalytic activity had been done by adding some
100
80
60
40
ChH2 PW
20
CTAH2 PW
0
0.0
0.2
0.4
0.6
0.8
1.0
content of water (%)
Fig. 3. Photographs of the esterification of palmitic acid and methanol over ChH2PW.
(a) Before the reaction; (b) homogeneous mixture during the reaction; (c) after the
reaction.
Fig. 4. Effect of water on the esterification. Reaction conditions: palmitic acid 0.02 mol, the
molar ratio of methanol/acid/catalyst 520:40:1, 50 min, 65 °C.

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X. Duan et al. / Catalysis Communications 42 (2013) 125–128
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90
80
70
60
50
40
30
20
10
0
10
9
8
7
6
5
4
3
2
1
0
Science and Technology Department (20086035, 20100416, and
201105001).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2013.08.014.
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