A novel Bronsted acidic heteropolyanion-based polymeric hybrid catalyst for esterification

Article abstract of DOI:10.1016/j.catcom.2012.04.014

A new SO₃H-functionalized HPA-based acidic polymeric hybrid was prepared by coupling task-specifically designed SO₃H-functionalized polymeric ionic liquid with Keggin-structured heteropolyanion, and characterized by ¹H NMR, FT-IR, TG, XRD, BET, Hammett indicator, melting point, and elemental analysis. Its catalytic performance for esterification of alcohols with carboxylic acids was studied under solvent-free conditions. The results demonstrate that the polymeric hybrid is a highly active and selective solid catalyst for various esterifications, and can be easily recovered and steadily reused.

Full text of DOI:10.1016/j.catcom.2012.04.014

Catalysis Communications 25 (2012) 41–44
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
A novel Brønsted acidic heteropolyanion-based polymeric hybrid catalyst
for esterification
a
b
Yan Leng a, , Pingping Jiang , Jun Wang
⁎
a
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A new SO3H-functionalized HPA-based acidic polymeric hybrid was prepared by coupling task-specifically
designed SO3H-functionalized polymeric ionic liquid with Keggin-structured heteropolyanion, and character-
ized by 1H NMR, FT-IR, TG, XRD, BET, Hammett indicator, melting point, and elemental analysis. Its catalytic
performance for esterification of alcohols with carboxylic acids was studied under solvent-free conditions.
The results demonstrate that the polymeric hybrid is a highly active and selective solid catalyst for various
esterifications, and can be easily recovered and steadily reused.
Received 19 February 2012
Received in revised form 6 April 2012
Accepted 12 April 2012
Available online 21 April 2012
Keywords:
© 2012 Elsevier B.V. All rights reserved.
Heteropolyacid
Polymeric ionic liquid
Acidic catalyst
Heterogeneous catalysis
Esterification
1. Introduction
functionalized ILs [11], ILs with acidic counteranions [12], and pro-
tonated N-alkylimidazolium ILs [13], were prepared and used as sol-
As one of the most fundamental reactions in organic synthetic
chemistry, esterification has been usually catalyzed by liquid min-
eral acids [1], such as H2SO4, HF, and HCl. However, these acids
are extremely corrosive and need to be neutralized with large
amounts of base at the end of a reaction. Furthermore, it is very dif-
ficult to separate and reuse the acidic mineral catalysts. To over-
come these problems, great efforts have been made toward the
development of solid acid catalysts [2–4], such as supported miner-
al acids, acidic resins, and zeolites. A substantial progress is the use
of silica-supported Wells–Dawson heteropolyacid (HPA) as the
heterogeneous catalyst for esterification of levulinic acid [5]. Thus
far, developing highly efficient and recyclable heterogeneous cata-
lysts for esterification is still an attractive topic.
Ionic liquids (ILs) have been attracting significant attention as al-
ternative solvents and catalysts for chemical reactions because of
their negligible volatility, remarkable solubility, and potential recy-
clability [6,7]. Many acid-catalyzed organic reactions based on ILs
have been reported, among which esterification is an important re-
search focus [8,9]. For example, Davis and co-workers were the first
to report the temperature-controlled liquid–solid separation esteri-
fication system with the SO3H-tethered IL as solvent/catalyst [10].
Since then, various acidic task-specific ILs, such as double SO3H-
vents and/or catalysts for esterification reactions. Although the
above catalytic systems give improved performances, large
a
amount of IL is usually needed and its recoverability is not as conve-
nient as a solid catalyst. For the purpose of more convenient isola-
tion of ILs, the supported ILs phase catalysis becomes a hot topic
[14,15], but the complex preparation of the catalysts is less
satisfactory.
In recent years, the use of heteropolyacid-based IL (HPA-IL) to
catalyze organic reactions is an ongoing research area [16]. Some
HPA-IL composites have been reported and employed as catalysts
for acid-catalyzed reactions [17,18]. We have prepared a family of
HPA-based acidic hybrids by combining SO3H-functionalized IL cat-
ions with heteropolyanions, revealing that the resulting ionic solids
were highly efficient and reusable catalysts in esterifications [19–21].
Nevertheless, catalytic stabilities of theses catalysts need to be improved.
Considering that ionic polymers containing IL units are potentially capa-
ble of acting the bifunctions of IL and polymer, such as high thermal sta-
bility, easily shaping, corrosion resistance, and variety of available
structures [22,23], we think it rational to design and prepare a HPA-
based polymeric hybrid catalyst by combining SO3H-functionalized IL
polymer with HPA. The thus obtained catalyst should possess a more
structural stability because of the involvement of a polymeric frame-
work. Herein, we describe the synthesis of a new HPA-based polymeric
hybrid by coupling poly(SO3H-functionalized IL) with Keggin HPA
(Scheme 1), as well as its catalytic performance for various esterifica-
tion reactions.
⁎
Corresponding author. Tel.: +86 510 85917090; fax: +86 510 85917763.
E-mail address: lengyan1114@126.com (Y. Leng).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.04.014

42
Y. Leng et al. / Catalysis Communications 25 (2012) 41–44
*
*
O
O
n
*
*
S
3
O
60 oC methanol AIBN 24h
N
N
N
N
H3PW12O40
3-
0 oC 12 h
PW12O40
N
N
N
N
-
-
SO3
SO3
SO3H
VMPS
Poly(VMPS)
Poly(VMPS)-PW
Scheme 1. Synthesis of the acidic polymeric hybrid Poly(VMPS)-PW.
2. Experimental
2.3. Procedure for esterification reactions
2.1. Materials and characterization techniques
The typical procedure for esterification reaction of acetic acid with
n-butanol is as follows: acetic acid (30 mmol), n-butanol (36 mmol),
and catalyst Poly(VMPS)-PW (0.2 g, 0.06 mmol) were added to a flask
with a water segregator. The reaction was refluxed at the heating tem-
perature 110 °C of the oil bath for 1.5 h with vigorous stirring. The product
mixture was analyzed by gas chromatography (GC) with a FID detector.
Under the employed conditions, only ester was detected as the product,
without finding any GC signals for by-products.
For other esterification reactions, the tests were carried out according
to the above procedure. Exceptionally, for the esterification of citric acid
with n-butanol, the reaction mixture was analyzed by liquid chromatog-
raphy (LC). For the reactions without using n-butanol as a reactant, tolu-
ene was added as the water-carrying agent. For the recycling tests, the
catalyst (Poly(VMPS)-PW or MimPS-PW) was recovered by filtration,
washing with acetone, and drying under vacuum at 60 °C for 2 h. The re-
covered catalyst was directly used for the next run.
All chemicals were of analytical grade and used as received. FT-IR
spectra were recorded on a Nicolet 360 FT-IR instrument (KBr discs) in
the 4000–400 cm−1 region. 1H NMR spectra were measured with a
Bruker DPX 300 spectrometer at ambient temperature in D2O using
TMS as internal reference. Elemental analyses (C, H, N) were performed
on a CHN elemental analyzer (FlashEA 1112). TG analysis was carried
out with a STA409 instrument in dry air at a heating rate of 10 °C/min.
Melting point was measured on the X-4 digital microscopic melting-
point apparatus. XRD patterns were collected on the Bruker D8 Advance
powder diffractometer using Ni-filtered Cu Kα radiation source at 40 kV
and 20 mA from 5 to 80° with a scan rate of 4°/min. The BET surface
area was measured at the temperature of liquid nitrogen using a
Micromeritics ASAP2010 analyzer. The samples were degassed at
150 °C to a vacuum of 10− 3 Torr before analysis. The acid strength
was measured with Hammett indicators, including crystal violet
(pKa=0.8), dicinnamalacetone (pKa= −3.0), and anthraquinone
(pKa=−8.20).
3. Results and discussion
3.1. Characterizations of Poly(VMPS)-PW
2.2. Catalyst preparation
The solid-state polymeric hybrid possesses a high melting point of
above 200 °C, and it was observed to be insoluble in the low polar sol-
vents, especially insoluble in various alcohol/carboxylic acid reaction
media. These properties therefore enable Poly(VMPS)-PW to be a solid
catalyst for esterification reactions.
The IR spectra of H3PW12O40 and Poly(VMPS)-PW are illustrated in
Fig. 1. In the wavenumber region of 800–1100 cm−1, H3PW12O40 gave
four characteristic bands at 1080 cm−1 (P–Oa), 984 cm−1 (W=Od),
889 cm−1 (W–Ob–W) (corner-sharing), and 804 cm−1 (W–Oc–W)
(edgesharing), which are assigned to a Keggin structure. For the hybrid
2.2.1. 1-vinyl-3-propane sulfonate imidazolium (VMPS)
In a 50 mL round-bottomed flask, 1-vinylimidazole (0.1 mol, 9.02 g)
was added to 1,3-propane-sultone (0.1 mol, 12.2 g) slowly at 0 °C,
followed by the stirring of the mixture at room temperature for about
12 h until it turned into solid, which was washed with ether and dried
in vacuo at room temperature. 1H NMR (300 MHz, CHCl3) δ (ppm)=
2.25 (m, 2 H, –CH2), 2.85 (m, 2 H, –CH2), 4.43 (t, 2 H, –CH2), 5.32 (d,
1H, –CH), 5.68 (d, 1H, –CH), 7.04 (m, 1H, –CH), 7.60 (d, 2 H, –CH2),
9.05 (s, 1H, –CH) (Fig. S1, Supplementary Material (SM) ).
2.2.2. Poly(VMPS)
Poly(VMPS) was prepared according to the literature [24,25]. In detail,
the obtained VMPS (2.5 g) and azobisisobutyronitrile (AIBM) (0.05 g)
were dissolved in methanol (10 mL) under nitrogen atmosphere, and
the mixture was refluxed at 60 °C for 24 h with stirring. Then the solvent
was removed by distillation, and the residue was washed with ethanol to
give a white solid.
a
1080
889
898
984
b
804
815
2.2.3. Poly(VMPS)-PW and control samples
The obtained Poly(VMPS) (0.88 g, monomer molar quantity was
4.17 mmol) was dissolved in methanol at 60 °C with stirring, an aqueous
solution of H3PW12O40 (PW) (4 g, 1.39 mmol) was added to the solution
of Poly(VMPS), followed by the stirring of the mixture for 24 h. On com-
pletion, methanol was removed in vacuum to give the final product
Poly(VMPS)-PW as a solid. Poly(VMPS)-HSO4 was prepared by reacting
Poly(VMPS) and H2SO4 with the monomer molar ratio of 1:1 at 0 °C.
MimPS-PW (MimPS: 1-methyl-3-propane sulfonate imidazolium) was
prepared according to our previous literature [19].
1043
1185
1200
1080
978
3436
3500
900
600
Wavenumbers (cm-1
)
Fig. 1. FT-IR spectra of (a) H3PW12O40 and (b) Poly(VMPS)-PW.

Y. Leng et al. / Catalysis Communications 25 (2012) 41–44
43
Table 1
Poly(VMPS)-PW, the four peaks appeared distinctively, with the simulta-
neous occurrence of the additional two bands at 1185 and 1043 cm−1 at-
tributable to the asymmetric and symmetric stretching vibrations of
S=O, respectively. This result indicates that the structures of both organic
cation and heteroplyanion for the hybrid catalyst are well reserved. The
slight shifts of the Keggin bands for Poly(VMPS)-PW compared with
H3PW12O40 are resulted from the strong ionic interaction between the
polymeric cations and heteropolyanions. It is thus suggested that the
combination of SO3H-functionalized IL polymer with H3PW12O40 pro-
duces a new HPA-based polymeric hybrid via ionic linkages. For
H3PW12O40, the broad and asymmetric peak at 3436 cm−1 for O–H vi-
brations was significantly enhanced in the case of Poly(VMPS)-PW,
due to the combination of organic polycations with heteropolyanions.
This is indicative of the existence of the extended hydrogen bonding
network between organic polycations and heteropolyaions.
The XRD pattern (Fig. S2, SM) for neat H3PW12O40 displayed the
featured diffraction peaks for a HPA crystal, indicating an excellent
long range order of the Keggin anions. However, for the polymeric hy-
brid Poly(VMPS)-PW, all these peaks disappeared. This phenomenon
demonstrates the loss of the long-range crystal order for the hybrid,
mostly due to the rearrangement of the Keggin anions during the
self-assembly of heteropolyanions and polymeric cations.
Esterification of acetic acid with n-butanol by Poly(VMPS)-PW and the control
catalysts.a.
Entry
Catalyst
Phenomenon
Conb [%]
Selc [%]
1
2
3
4
5
Poly(VMPS)-PW
MimPS-PW
Poly(VMPS)-HSO4
H3PW12O40
Liquid–solid
Liquid–liquid
Liquid–solid
Homogeneous
–
97.4
94.5
52.3
88.2
37.7
100
100
100
100
100
Without catalyst
a
Reaction conditions: catalyst (0.2 g), acetic acid 30 mmol, molar ratio of acetic
to n‐butanol 1:1.2, 110 °C for 1.5 h.
b
Conversion of acetic acid.
Selectivity for butyl acetate.
c
the hybrid as an insoluble solid catalyst for esterification, resulting
in the solid–liquid biphasic system.
In contrast, when Poly(VMPS)-HSO4 was used as the catalyst, it
was attached onto the bottom surface of the flask reactor as a gelati-
nous liquid, which greatly lowered the mass transfer and gave a low
conversion 52.3% (entry 3). The conversion was only slightly higher
than that of the non-catalyst system (entry 5), implying that the het-
eropolyanion in the polymeric hybrid is indispensable for both the
solid nature and the high activity of Poly(VMPS)-PW. Pure H3PW12O40
,
The elemental analysis for Poly(VMPS)-PW found C 8.29%, N 2.44%, S
2.76 and H 1.43% by weight percentage, which is in well agreement
with the calculated value C 8.15%, N 2.38%, S 2.72% and H 1.28%. Thermal
stabilities of Poly(VMPS) and Poly(VMPS)-PW were investigated by TG
analyses in Fig. 2. The decomposition temperature of Poly(VMPS) was
about 280 °C, which is remarkably lower than 310 °C for Poly(VMPS)-
PW. This fact strongly indicates that Poly(VMPS)-PW is not a physical
mixture of Poly(VMPS) with HPA, but a ionic hybrid. The drastic weight
loss of nearly 20% in the temperature range 310–500 °C corresponds to
the thermal decomposition of the organic moiety Poly(VMPS) in the
catalyst. All these results suggest that Poly(VMPS)-PW has the structure
illustrated in Scheme 1.
a homogeneous catalyst, presented a relative high conversion of 88.2%
due to the well-known strong acidity arising from the three protons in
the cation position (entry 4), which, however, is still lower than that of
Poly(VMPS)-PW. It is thus suggested that the SO3H functional groups in
the polymeric cations are responsible for the high yield of esterification.
Poly(VMPS)-PW has a very low specific surface area 1 m2⋅g−1
with a negligible pore volume. However, its catalytic activity is higher
than that of the homogeneous H3PW12O40 and the heterogeneous
heteropolyanion-free Poly(VMPS)-HSO4. These facts allow to infer a
bulk-type catalysis mode for Poly(VMPS)-PW because of the specific
secondary structures of heteropolyanion-based salts [26], wherein
the medium strong acid sites of SO3H groups (−3.0bH0 b0.8) in the
bulk of Poly(VMPS)-PW must have fully played the roles as active
centers for esterification.
3.2. Esterification of acetic acid with n-butanol over various catalysts
In order to investigate the scope of the Poly(VMPS)-PW catalyst
for esterifications, other acid and alcohol substrates were tested,
with the results shown in Table 2. It can be seen that good to excellent
conversions with 100% selectivity for corresponding esters were
obtained in all cases. The prolonged carbon chains in reactants almost
did not obstruct the high yields for esters (entries 3, 6 and 7). Though
the esterification of the aromatic acid was more difficult compared
with that of aliphatic acids, a good conversion 87.5% was obtained
as well (entry 8). Consequently, the hybrid catalyst Poly(VMPS)-PW
Table 1 lists the performances of various catalysts for the esterification
of acetic acid with n-butanol. The polymeric hybrid Poly(VMPS)-PW led
to a liquid–solid heterogeneous reaction system and presented an excel-
lent conversion 97.4% with perfect selectivity (entry 1, also see Figs. S3A-
S3D in SM for influences of reaction conditions). The nonpolymeric hybrid
MimPS-PW prepared by combining SO3H-functionalized IL with
H3PW12O40 also offered a high conversion 94.5% with 100% selec-
tivity. However, it caused a liquid–liquid biphasic reaction system
(entry 2) due to the relative low melting point (110 °C) and the
miscibility with the produced water [20]. This phenomenon indi-
cates that the featured structure of the polymeric IL-cation endows
Table 2
Results of various esterification reactions of carboxylic acid with alcohol over Poly(VMPS)-
PW catalyst.a.
100
Entry Carboxylic
acid (A)
Alcohol (B) Temperature Time
nA:
nB
Conc
[%]
Seld
[%]
b
[°C]
[h]
a
80
1
2
Acetic acid
Acetic acid
n-butanol
Ethylene
glycol
110
115
1.5
3
1:1.2 97.4
2.4:1 92.2
100
98
60
40
3
4
5
6
7
8
Acetic acid
Citric acid
Succinic acid
Lauric acid
Stearic acid
Benzoic acid
Dodecanol 110
2
3
3
2
3
4
1:1.2 95.2
1:5
1:4
1:1.2 92.0
1:1.2 88.6
1:1.2 87.5
100
98
100
100
100
100
n-butanol
n-butanol
n-butanol
n-butanol
n-butanol
130
130
120
120
130
98.3
96.4
20
b
a
Reaction conditions: catalyst Poly(VMPS)‐PW 0.20 g (0.06 mmol).
nA:nB: the molar ratio of mono‐carboxylic acid to mono‐alcohol is 1:1.2; for poly-
b
100
200
300
400
500
600
carboxylic acid or polyol substrates the molar ratios are the respective optimum values.
Temperature (OC)
c
The conversion of substrates (entry 1 and entries 3–8 based on carboxylic acid
(30 mmol), entry 2 based on alcohol (30 mmol)).
d
The selectivity for corresponding ester.
Fig. 2. TG pattern of (a) H3PW12O40 and (b) Poly(VMPS).

44
Y. Leng et al. / Catalysis Communications 25 (2012) 41–44
Poly(VMPS)-PW
MimPS-PW
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.04.014.
100
80
60
40
20
0
Acknowledgements
The authors thank the financial support from the National Natural
Science Foundation of China (No. 21136005) and the Fundamental
Research Funds for the Central Universities (No. JUSRP111A04).
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can be applied to a variety of esterification reactions with different
substrates.
The recycling performance of Poly(VMPS)-PW and MimPS-PW in
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In summary, we prepared a new SO3H-functionalized HPA-based
polymeric hybrid by pairing SO3H-functionalized polymeric IL-cations
with heteropolyanions. The obtained polymeric hybrid was revealed
to be a highly efficient solid catalyst for various esterification reactions,
presenting the advantages of easy recovery and steady reuse. The acidic
SO3H functional groups in the hybrid catalyst account for the excellent
catalytic activity, while both the polymeric framework of IL-cation and
the large heteropolyanion are responsible for the catalyst's solid nature
and insolubility.
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