Heteropolyanion-based ionic liquids: Reaction-induced self-separation catalysts for esterification

Article abstract of DOI:10.1002/anie.200803567

(Figure Presented) It comes out in the wash: In the esterification of citric acid with n-butanol, heteropolyanion-based ionic liquid (IL) catalysts show high catalytic activity, self-separation, and easy reuse. The good solubility in reactants, nonmiscibility with ester product, and high melting point of the IL catalysts enable the reaction-induced switching from homogeneous (b in the picture) to heterogeneous (c) with subsequent precipitation of the catalyst (d).

Full text of DOI:10.1002/anie.200803567

Communications
DOI: 10.1002/anie.200803567
Catalysis
Heteropolyanion-Based Ionic Liquids: Reaction-Induced Self-
Separation Catalysts for Esterification**
Yan Leng, Jun Wang,* Dunru Zhu, Xiaoqian Ren, Hanqing Ge, and Lei Shen
Organic esters are important intermediates in chemical and
pharmaceutical industries, and they are mostly produced by
acid-catalyzed esterification reactions.[1] Various mineral
acids, such as H2SO4, HF, and H3PO4, have been used as
catalysts for esterification. However, these acids are
extremely corrosive and contaminative and need to be
neutralized at the end of the reaction. Furthermore, it is
very difficult to separate and reuse the mineral acid cata-
lysts.[2] To overcome the above-mentioned problems, the use
of heterogeneous catalysts such as resins,[3] supported mineral
acids,[4] heteropolyacids,[5] and zeolites[6] has attracted much
attention. However, these catalysts have disadvantages as
well, for example, the tendency to deactivation, operation
loss, and high mass transfer resistance, which limit their
practical application in esterification.
removing water by-product from the reaction system. Hetero-
polyacids (HPAs) and their salts have been extensively
investigated as solid acid catalysts.[19] The combination of
organic cations with heteropolyanions (or polyoxometalate
(POM) anions) can cause the formation of HPA salts, which
may be a kind of novel IL material. In fact, only a few
examples of these IL compounds (e.g., [(n-C4H9)4N]2M6O19,
[Cnmim]3PW12O40, [(n-C4H9)4N]4S2M18O62 (M = Mo, W; n = 2,
5; mim = methylimidazolium)) have thus far been de-
scribed,[20–22] and they were used as electrochemicals rather
than as catalysts. These considerations prompted us to design
new heteropolyanion-based ILs containing the acidic func-
tional group as “task-specific” catalysts for esterification
reactions.
We synthesized a series of HPA salts containing organic
In view of both the advantages and disadvantages of
homogeneous and heterogeneous catalysts, and to improve
catalyst recovery, multiphase systems, such as phase-transfer
catalysis,[7–9] thermoregulated phase-transfer catalysis,[10] and
liquid–liquid biphasic catalysis,[11] have been studied. Ionic
liquids (ILs) have been revealed as green reaction media
owing to their negligible volatility, excellent thermal stability,
remarkable solubility, and the variety of structures avail-
able.[12,13] Many acid-catalyzed organic reactions based on ILs
have been reported, among which esterifications are a hot
topic.[14–17] By using ILs, especially Brønsted acidic ILs, as
catalysts, the esterification system is homogeneous at the
early stage of the reaction, and at the end it often forms a
liquid–liquid biphase.[18] Consequently, ILs can be reused by
decantation of the ester usually in the upper level of the
resulting mixture. Davis and co-workers observed temper-
ature-controlled liquid–solid separation by using an alkane
sulfonic acid IL as solvent/catalyst for esterification.[14] Other
advantages of ILs for esterification are the high conversion
rate and selectivity. However, these results are not entirely
satisfactory because the present systems still suffer from the
high content of ILs (20–300 mol%) needed in the reaction
media, relatively long reaction times, and the need of
cations—[MIMPS]3PW12O40,
[TEAPS]3PW12O40—and investigated their catalytic behavior
[PyPS]3PW12O40,
and
in various esterification reactions. These new compounds
were fully characterized by FT-IR, H and 13C NMR spec-
1
troscopy, ESIMS, and melting-point determination (see the
Supporting Information). The results demonstrate that they
contain three organic cations and an inorganic heteropolyan-
ion. These compounds are not conventional ILs because their
melting points are above 1008C. Their solid nature could be
due to the extended hydrogen bonding networks between the
anion and cation (IR data, Supporting Information). The use
of propane sulfonate (PS) functionalized ILs for the esterifi-
cation reaction is considered to be an effective approach.[14]
Furthermore, the heteropolyanion existing in the compounds
gives them high melting points, which coincidently meets the
requirements of a solid catalyst. They not only served as
highly efficient and reusable catalysts but also circumvented
the above-mentioned limitations of conventional ILs for
[*] Y. Leng, Prof. Dr. J. Wang, D.-R. Zhu, Dr. X.-Q. Ren, H.-Q. Ge, L. Shen
State Key Laboratory of Materials-Oriented Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing University of Technology, Nanjing 210009 (China)
Fax: (+86)25-8317-2261
E-mail: junwang@njut.edu.cn
[**] We thank the National Natural Science Foundation of China (grant
nos. 20476046, 20771059) and the “Qinglan” Project of Jiangsu
Province for Young Researchers.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803567.
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Angew. Chem. Int. Ed. 2009, 48, 168 –171

Angewandte
Chemie
esterification. They could be used as homogeneous catalysts
for many esterification reactions because of their good
solubility in reaction media, and, moreover, at the end of
the reaction, the catalysts precipitated and could be recycled
without any regeneration being required, just like a hetero-
geneous system. Thus, these catalysts combine the advantages
of homogeneous and heterogeneous catalysis.
The esterification of citric acid with n-butanol [Eq. (1)]
was first carried out using [MIMPS]3PW12O40 as the catalyst
Figure 1. Photographs of the esterification of citric acid with n-butanol
over [MIMPS]3PW12O40. a) [MIMPS]3PW12O40 (light brown solid at
bottom), citric acid (white solid in the middle), and alcohol (liquid in
the upper level) before mixing; b) homogeneous mixture during the
reaction; c) heterogeneous mixture near completion of the reaction;
d) at the end of the reaction; the catalyst has precipitated.
phase-separation catalytic procedure, which is induced and
controlled by the reaction, so we refer to this catalyst as a
“reaction-induced self-separation catalyst”. The outstanding
feature of the catalyst is its excellent solubility in water or
strong polar solvents but nonmiscibility with apolar esters. In
above reaction systems, the solid catalyst [MIMPS]3PW12O40
can be dissolved completely in polycarboxylic acids at
reaction temperature or in polyols at room temperature, but
it is insoluble in the product (ester). Thus, it is homogeneous
at the early stage of the esterification reaction. With the
consumption of the polycarboxylic acids or polyols, the
system becomes heterogeneous, inducing a spontaneous
self-separation of the catalyst. So at the end of reaction, the
solid catalyst could be easily recovered by simple filtration. In
contrast, in the formation of butyl acetate, butyl laurate, and
dodecyl acetate, the reactions were heterogeneous (liquid–
solid biphasic) for the entire reaction (Table 1, entries 5, 7,
and 10) because of the insolubility of the catalyst in
monocarboxylic acids or alcohols. However, high yields
were also obtained in these heterogeneous systems possibly
because of the pseudoliquid phase
(Table 1, entry 1). To our surprise, a yield of 95.4% and a
selectivity of 98% for tributyl citrate were obtained at 1308C
for 3 h, and, more notably, during the reaction process,
switching from homogeneous to heterogeneous catalysis was
clearly observed. At the beginning of the reaction, the catalyst
itself was dissolved completely in the medium to form a
homogeneous mixture (Figure 1a,b), but near the completion
of the reaction, the system became turbid (Figure 1c), and the
catalyst precipitated at the end of the reaction (Figure 1d).
This unusual behavior stimulated us to investigate further
other esterification reactions with this catalyst. Similarly, the
progression from a monophase catalysis to a biphase separa-
tion was observed also in the syntheses of dibutyl succinate,
butyl lactate, and dibutyl oxalate, and excellent catalytic
activities and selectivities were obtained as well (Table 1,
entries 2–4, 6, 8, and 9; each reaction temperature is optimal
for that reaction). These findings manifest a solid–liquid–solid
behavior of the HPA salt.[23] In each
Table 1: Results of various esterification reactions over the [MIMPS]3PW12O40 catalyst.[a]
case, this novel catalyst for esterifi-
cation not only shows very high
catalytic activity but also can be
recovered very conveniently.
Entry Carboxylic acid
(A)
Alcohol (B)
Reaction conditions
Yield [%][c] Sel. [%][c]
[b]
T [8C] t [h] nA:nB Phenomenon
From the observations in
Table 1, we were interested to
know whether compounds similar
to [MIMPS]3PW12O40 also behaved
as reaction-induced self-separation
catalysts. Thus, we investigated the
1
2
3
4
citric acid
n-butanol
n-butanol
n-butanol
n-butanol
130
130
110
120
3
3
2
3
1:5 phase separa-
95.4
98.6
85.5
95.2
98
100
100
98
tion
succinic acid
lactic acid
1:4 phase separa-
tion
1:3 phase separa-
tion
1:4 phase separa-
tion
mandelic acid
catalytic
performance
of
[TEAPS]3PW12O40
and
5
6
lauric acid
oxalic acid
n-butanol
n-butanol
110
120
2
4
1:1.2 heterogeneous 91.4
100
98
1:3 phase separa-
tion
74.4
[PyPS]3PW12O40 for the esterifica-
tion of citric acid with n-butanol
(Table 2, entries 2 and 3). As
expected, [TEAPS]3PW12O40 and
[PyPS]3PW12O40 also acted as very
active reaction-induced self-separa-
tion catalysts. However, their cata-
lytic activities were slightly lower
than that of [MIMPS]3PW12O40. To
determine whether the incorpora-
7
8
acetic acid
acetic acid
n-butanol
glycerol
110
100
1.5
3
1:1.2 heterogeneous 94.5
100
89
8:1 phase separa-
81.2
tion
2.4:1 phase separa-
tion
9
acetic acid
acetic acid
ethylene
glycol
dodecanol
110
110
3
2
87.9
98
10
1:1.2 heterogeneous 94.6
100
[a] Reaction conditions: catalyst [MIMPS]3PW12O40 0.20 g (0.06 mmol). [b] nA:nB: molar ratio of
carboxylic acid to alcohol. [c] Yield or selectivity for the ester (entries 1–7 and entry 10 based on
carboxylic acid (30 mmol), entries 8 and 9 based on alcohol (30 mmol)).
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Communications
Table 2: Comparison of various catalysts for the esterification of citric
Table 3: Catalytic recycling of the [MIMPS]3PW12O40 catalyst for the
esterification of citric acid with n-butanol.[a]
acid with n-butanol.[a]
Entry Catalyst
Phenomenon
–
phase separation 89.2
phase separation 91.5
heterogeneous
heterogeneous
heterogeneous
Yield [%][b] Sel. [%][b]
Run
Yield [%][b]
Sel. [%][b]
1
2
3
4
5
6
7
8
9
10
without catalyst
[TEAPS]3PW12O40
[PyPS]3PW12O40
[MIM]3PW12O40
[TEA]3PW12O40
[Py]3PW12O40
[MIMPS]3SiW12O40 phase separation 89.3
[MIMPS]3PMo12O40 phase separation 97.6
[MIMPS]HSO4
H3PW12O40
62.5
95
98
98
98
98
98
98
98
98
98
1
2
3
4
95.4
91.6
86.5
84.5
98
98
98
98
73.8
68.3
69.6
[a] Reaction conditions: catalyst 0.20 g (0.06 mmol), citric acid
(30 mmol), molar ratio of citric acid to n-butanol 1:5, 1308C for 3 h.
[b] Yield or selectivity of tributyl citrate (based on citric acid).
homogeneous
homogeneous
76.2
96.8
run, 80.2 wt% of the catalyst was recovered and 84.5% yield
was obtained. In a control experiment, 0.16 g (0.2 g ꢀ 80.2%,
the same amount as that of the fourth run) fresh catalyst was
used, and 89.6% yield of tributyl citrate was obtained. This
result indicates a slight decrease in the catalytic activity of the
recovered catalyst.
The FTIR spectra for fresh [MIMPS]3PW12O40 and
recycled samples are compared in Figure S4 in the Supporting
Information. It is revealed from the observation of four
featured peaks of the Keggin anion at 1080, 978, 893, and
810 cmÀ1 that the Keggin structure of the heteropolyanion in
fresh catalyst is well retained after the protons in the HPAs
[a] Reaction conditions: catalyst (0.06 mmol), citric acid (30 mmol),
molar ratio of citric acid to n-butanol 1:5, 1308C for 3 h. [b] Yield or
selectivity of tributyl citrate (based on citric acid).
tion of the acidic PS functional group in the organic cations of
ILs is indispensible to achieve a high yield of esterification
and to realize the reaction-induced phase-separation catalysis
system, the same reaction was carried out on the three
counterpart catalysts without PS: [TEA]3PW12O40,
[MIM]3PW12O40, and [Py]3PW12O40. Entries 4–6 in Table 2
show that the counterparts without PS were insoluble in the
reactants and/or products, and the yields were only slightly
higher than that of the control experiment (without catalyst;
Table 2, entry 1), but much lower than those obtained using
the PS-functionalized catalysts. The catalytic activities were in
are substituted by the large organic cation. There were also
À1
=
three characteristic peaks at 1230, 1170 (S O), and 621 cm
(imidazole ring) for the cation. Moreover, the IR spectrum for
the recycled sample that was repeatedly used for four times
was consistent with that of the fresh one, indicating good
structural stability of the catalyst. However, the peak
intensities decreased to some extent, which might be indica-
tive of the slight deactivation of the recovered catalyst.
In summary, a series of nonconventional IL compounds in
the solid state at room temperature composed of propane
sulfonate functionalized organic cations and heteropolyan-
ions were synthesized. They were used as “reaction-induced
self-separation catalysts” for various esterification reactions
with one of the reactants being polycarboxylic acid or polyol.
The good solubility in the polycarboxylic acid or polyol,
nonmiscibility with ester product, and high melting points of
the heteropolyanion-based IL catalysts result in the switching
from homogeneous to heterogeneous catalysis, which makes
the recovery and catalytic reuse of this kind of catalyst very
convenient. On the basis of the findings of this work, the
exploration of the catalysts in other reactions and altering the
composition of the catalysts is in progress.
the
order
of
[MIM]3PW12O40 > [Py]3PW12O40 >
[TEA]3PW12O40, which is in accordance with that of the PS-
bearing catalysts. Therefore, the PS group in ILs is responsible
for both the high catalytic activity and the reaction-induced
phase separation.
The influence of various inorganic anions in the IL
catalysts with the same MIMPS cation on the esterification of
citric acid with n-butanol is shown in Table 2, entries 7–9. It
can be seen that the two catalysts with SiW12O40 and PMo12O40
heteropolyanions demonstrated the reaction-induced phase-
separation catalysis with high activities. However, when
[MIMPS]HSO4 was used as the catalyst, a homogeneous
system resulted which gave a low yield. Although pure HPA
catalyst H3PW12O40 gave a very high yield of 96.8%, its good
solubility throughout the reaction makes its isolation from the
reaction mixture difficult.
From the results reported herein, we think that it is the
functional group PS in the IL catalysts rather than the
heteropolyanion that provides the acid site responsible for the
high activity in esterification. However, heteropolyanions are
able to endow the IL catalyst with high melting points, which
is responsible for the solid–liquid–solid phase transformation
and catalyst separation.
As it is very convenient to recover the catalyst at the end
of the reaction, the solid catalyst left could be readily reused
for the next run. Therefore, the recycled [MIMPS]3PW12O40
catalyst without any regeneration steps was investigated in
the esterification of citric acid with n-butanol. As shown in
Table 3, [MIMPS]3PW12O40 exhibited a 95.4% yield of
tributyl citrate for the first run, and the catalytic activity
slowly decreased with repeated use. At the fourth reaction
Experimental Section
General preparation of the catalysts: High-melting-point ionic liquid
catalyst [MIMPS]3PW12O40 was synthesized as follows: Methylimida-
zole (0.11 mol) and 1,3-propane sulfone (0.10 mol) were dissolved in
toluene (20 mL) and stirred for 24 h at 508C under a nitrogen
atmosphere. A white precipitate (MIMPS) formed, which was
filtered, washed with diethyl ether three times, then dried in a
vacuum. MIMPS (0.06 mol) was added to an aqueous solution of
H3PW12O40 (0.02 mol), and then the mixture was stirred at room
temperature for 24 h. Water was removed in vacuum to give the
product as a solid, which was characterized by FTIR, 1H and 13C NMR
spectroscopy, ESIMS, and melting-point determination.
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[TEAPS]3PW12O40, [PyPS]3PW12O40, [MIMPS]3SiW12O40, [MIM-
PS]3PMo12O40, [TEA]3PW12O40, [MIM]3PW12O40, and [Py]3PW12O40
were prepared accordingly.
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Typical procedure for esterification: Citric acid (30 mmol),
n-butanol (150 mmol), and [MIMPS]3PW12O40 (0.06 mmol) were
added to a flask with a water segregator. The reaction was heated
under reflux at 1308C with vigorous stirring for 3 h. After the
reaction, tributyl citrate was detected by liquid chromatography (LC),
and the compositions of the isolated other esters were detected by gas
chromatography (GC) (Table 1, entries 2–10). If there was no
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Received: July 22, 2008
Revised: October 21, 2008
Published online: December 3, 2008
Keywords: biphasic catalysis · heterogeneous catalysis ·
homogeneous catalysis · ionic liquids · phase-transfer catalysis
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