Synthesis, characterization and catalytic performance of a novel picolinic acid-12-molybdophosphoric acid hybrid catalyst

Article abstract of DOI:10.5935/0103-5053.20150356

A novel 12-molybdophosphoric acid (HPM)-based complex (H₂ PI)₂(H ₃ O)[PMo₁₂O ₄₀] [Mo₂ O₅ (H₂ O)₂ (PI)₂ ]?11H₂ O (PI-HPM) was prepared by modification with picolinic acid (HPI) and characterized by the methods of Fourier transform infrared (FTIR) spectroscopy, thermogravimetry (TG), X-ray powder and single crystal diffraction. The complex retained the classical Keggin structure of bulk HPM, there were some strong hydrogen bonds existing between the [PMo₁₂ O₄₀ ]^3- polyanion, the [Mo₂ O₅ (H₂ O)₂ (PI)₂ ] coordination moiety, the protonated HPI and the lattice water molecules. Then PI-HPM was employed as heterogeneous catalyst for esterification reaction to evaluate its acid-catalytic activity. The complex exhibited high activity and good durability in reaction mixtures, indicating that it was a promising heterogeneous acid catalyst for esterification that including the conversion of oleic acid to oleates.

Full text of DOI:10.5935/0103-5053.20150356

http://dx.doi.org/10.5935/0103-5053.20150356
J. Braz. Chem. Soc., Vol. 27, No. 6, 1007-1013, 2016.
Printed in Brazil - ©2016 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
Synthesis, Characterization and Catalytic Performance of a Novel Picolinic
Acid‑12‑Molybdophosphoric Acid Hybrid Catalyst
Lijun Liu, Honghong Wang, Shuwen Gong,* Jing Lu* and Qian Zhang
Institute of Organic Functional Molecules and Materials, School of Chemistry and Chemical
Engineering, Liaocheng University, 252000 Liaocheng, China
A novel 12-molybdophosphoric acid (HPM)-based complex (H2PI)2(H3O)[PMo12O40]
[Mo2O5(H2O)2(PI)2]·11H2O (PI-HPM) was prepared by modification with picolinic acid (HPI) and
characterized by the methods of Fourier transform infrared (FTIR) spectroscopy, thermogravimetry
(TG), X-ray powder and single crystal diffraction. The complex retained the classical Keggin
structure of bulk HPM, there were some strong hydrogen bonds existing between the [PMo12O40]3–
polyanion, the [Mo2O5(H2O)2(PI)2] coordination moiety, the protonated HPI and the lattice water
molecules. Then PI-HPM was employed as heterogeneous catalyst for esterification reaction to
evaluate its acid-catalytic activity. The complex exhibited high activity and good durability in
reaction mixtures, indicating that it was a promising heterogeneous acid catalyst for esterification
that including the conversion of oleic acid to oleates.
Keywords: heteropoly acid, picolinic acid, heterogeneous catalysis, acid catalyst, esterification
Introduction
polar solvents.17,18 Therefore, supported HPAs and salts
of HPAs are developed to overcome the abovementioned
problems.2-4,17-20 Although these technologies can obviously
improve the separation problem of HPAs from reaction
system, the acid strength of supported HPAs and salts of
HPAs was commonly lower than that of bulk HPAs.21 The
supported HPAs materials also suffered from the leaching
of HPAs active species in polar reaction media. Therefore,
it is still desirable to design new HPAs catalysts suitable
for application in polar reaction mixtures.
In previous works, it was reported that there is strong
electronic interaction between the metal oxygen cluster and
the organic segment for organic compound modified hybrid
HPAs catalysts.22-24 So we considered modifying HPAs by
heterocyclic aromatic compounds containing carboxylic
acid group.25 The initial idea was to combine the HPAs
with aromatic compound basis on the existing electronic
interaction, which was different from supporting and
salification. The combination would decrease the solubility
and improve the durability of HPAs in polar mixtures. At
the same time, the carboxylic acid group may be favorable
to improve the acidity of the complex. In this paper, a
novel picolinic acid (HPI)-12-molybdophosphoric acid
(HPM) hybrid complex was prepared and characterized.
The prepared complex was applied in esterification as
heterogeneous catalyst to evaluate acid-catalytic activity.
More interestingly, the single crystal of the modified
Acid catalyzed reaction is one of the most important
reactions, including esterification, acetalization,
etherification, nitration reaction and so on.1-5 Conventional
acid catalysts that are widely used in industries are
mineral acids, such as H2SO4, H3PO4, HCl, CF3COOH and
ClSO2OH.6,7 These liquid acids have excellent catalytic
activities, but they suffer from several drawbacks, such
as the corrosiveness, the existence of side reactions and
difficulty of separation from the reaction mixtures.7,8 Due
to stringent and growing environmental regulations, the
chemical industry needs the development of environment-
friendly synthetic methodologies. Heterogeneous acids,
which have low corrosion, reusability and high acidity,
are considered to be the suitable substitutes for liquid
acids.9,10 Taking esterification reaction as an example,
some heterogeneous acids have gained attention, which
included ion exchange resins, zeolites, sulfated oxides,
biomass carbon-based solid acid and heteropoly acids
(HPAs).6,11-17
HPAs are typical strong Brønsted acids with low
volatility, low corrosiveness and flexibility.11,17 However,
as a heterogeneous catalyst, the disadvantages of HPAs
lie in their low surface area (1-10 m2 g-1) and solubility in
*e-mail: gshw76@163.com; lujinglcu@163.com

1008
Synthesis, Characterization and Catalytic Performance of a Novel Picolinic
J. Braz. Chem. Soc.
complex was obtained, which helped to reveal the
combination between HPM and HPI.
the metal content was determined by inductively
coupled plasma (ICP) analysis on a PerkinElmer Optima
2000DV ICP spectrometer. Anal. calcd. for single crystal
(C24H49Mo14N4O67P, 2839.80): C, 10.14; H, 1.73; N, 1.97;
Mo, 47.33; P, 1.09; elemental analysis for PI-HPM powder:
C, 9.97; H, 1.76; N, 2.01; Mo, 46.85; P, 1.04.
Experimental
Synthesis of catalysts
For HPM and PI-HPM, the capacity of acid sites was
determined by acid-base titration. An amount of ca. 0.05 g
of each sample was suspended in 15 mL distilled water
for 2 h at room temperature. Then the sample was titrated
with 0.1 mol L-1 NaOH. The acidity was determined by the
following equation:
All solvents and reagents were purchased commercially
and used as received without any further purification.
The aqueous solution of HPI and HPM were prepared;
0.002 mol (0.246 g) HPI (Aladdin Industrial Corporation)
were dissolved in 5 mL distilled water, and 0.001 mol
(1.825 g) HPM (Sinopharm Chemical Reagent Co., Ltd.)
were dissolved in 10 mL distilled water. Then the solution
of HPI was slowly dropped to the solution of HPM under
vigorous stirring at room temperature.After stirring for 2 h,
the resulting green precipitate was filtered. The prepared
sample was dried at 110 oC and collected as investigated
catalyst that was referred to as picolinate (PI)-HPM.
When the green precipitate was filtered, the filtrate was
placed carefully and allowed to diffuse. After two weeks,
the green single crystal of PI-HPM was obtained.
CNaOHVNaOH
mHPM or PI-HPM
Acidity =
(1)
where CNaOH andVNaOH are the concentration and volume of
NaOH, respectively, and mHPM or PI-HPM is the mass of the acid.
Catalytic tests
Esterification reactions of carboxylic acids besides
oleic acid with alcohols were carried out in a 100 mL
round-bottomed flask equipped with a reflux condenser
at atmospheric pressure. In a typical run, 0.1 mol (6.0 g)
acetic acid, 0.3 mol (13.8 g) ethanol and 0.1 g catalyst
were added into the round bottom flask. After stirring for a
certain reaction time at the reflux temperature, the catalyst
was filtered and the product was determined using a gas
chromatograph with flame ionization detector (FID).
Esterification reactions of oleic acid with alcohols
were carried out in a 50 mL gas-tight batch reactor
with a magnetic stirrer, and an oil bath was used to
maintain the reaction temperature. In a typical run,
0.07 g PI-HPM (5 wt.% (0.5 mol%) of oleic acid) were
added to the mixture of oleic acid (5 mmol, 1.41 g) with
methanol (50 mmol, 1.6 g), after stirring for 6 h at desired
Characterization
X-Ray powder diffraction (XRD) analysis was carried
out using a XD-3 diffractometer (Beijing Purkinje General
Instrument Co. Ltd.) with Cu Kα (1.542 Å) radiation.
X-Ray single crystal diffraction data was collected on a
Bruker SMART 1000 CCD diffractometer, equipped with
graphite-monochromated Mo Kα radiation with a radiation
wavelength of 0.71073 Å by using the ω-scan technique.
The structure was solved by direct method and refined by
full-matrix squares on F2 using SHELXTL program. All
non-hydrogen atoms were refined anisotropically. Positions
of the hydrogen atoms were geometry placed and refined
isotropically as a riding mode. The crystal data of the
complex was deposited in the Cambridge Crystallographic
Data Centre with the CCDC number 1412295. The
refinement crystal data and the selected bond parameters
are listed in Tables S1 and S2 (Supplementary Information).
Fourier transform infrared (FTIR) spectra were
recorded on a Thermo Nicolet 6700 FTIR spectrometer in
KBr matrix in the range of 400-2000 cm-1.
o
temperature (40, 60, 80 or 100 C). The products were
identified by comparison with the authentic samples
and finally by gas chromatography-mass spectrometry
(GC-MS). The conversion of oleic acid was determined
using an Agilent 6890 gas chromatograph equipped with
a HP-5MS capillary column and FID detector; methyl
caprylate was selected as internal standard.
Stability test of the PI-HPM was carried out by running
four consecutive experiments under the same reaction
conditions (mole ratio of methanol to oleic acid: 10:1,
reaction temperature: 80 oC, catalyst amount: 10 wt.% of
the weight of oleic acid and reaction time: 6 h). Between
the catalytic experiments, the catalyst was separated from
the reaction mixture by filtration, washed with methanol
and dried at 110 oC.
Thermal analysis was carried out using a PYRTSI
thermal analyzer. The samples with mass of about 10 mg
were placed in alumina crucible. The measurements were
o
performed with a heating rate of 10 C min-1 in dynamic
nitrogen atmosphere with the flow rate of 50 mL min-1.
Elemental analysis (C, H and N) was performed
on a PerkinElmer 2400 elemental analyzer, whereas

Vol. 27, No. 6, 2016
Liu et al.
1009
Results and Discussion
Catalyst characterization
Single crystal X-ray diffraction revealed that the
PI-HPM crystallizes in triclinic crystal system, P-1 space
group. In PI-HPM, there exists one classical Keggin-type
[PMo12O40] metal oxygen cluster, one [Mo2O5(H2O)2(PI)2]
coordination moiety, two protonated isolated HPI and
twelve lattice water molecules, where one additional proton
had to reside (Figure 1). The [PMo12O40] polyanion shows a
classical Keggin-type structure. It was formed by a central
PO4 tetrahedron surrounded by four Mo3O13 groups, in which
three MoO6 octahedra shared edges. One PO4 tetrahedron
and twelve MoO6 octahedra from four Mo3O13 groups
shared corners and made up the Keggin-type [PMo12O40]
cluster. For the [Mo2O5(H2O)2(PI)2] coordination moiety,
each molybdenum cation was coordinated by two terminal
oxygen atoms, one oxygen atom from one water molecule,
one bridging oxygen atom, and finally chelated by one PI
anion through its nitrogen and carboxylate oxygen atoms.
These two molybdenum centers were linked to each
other by the bridging oxygen atom to form a dinuclear
coordination moiety. As shown in Figure 2, there are some
strong hydrogen bonds existing between the [PMo12O40]3–
polyanion, the [Mo2O5(H2O)2(PI)2] coordination moiety,
the protonated HPI and the lattice water molecules,
and the parameters of the hydrogen bonds are listed in
Table 1.
Figure 1. The crystallographic unique unit of PI-HPM.
Figure 2. The complicated hydrogen bonds in the PI-HPM complex.
The XRD pattern of PI-HPM is presented in Figure 3.
For comparison, the XRD patterns of raw HPM and HPI are
also displayed. Pure HPM shows the characteristic pattern
in the Keggin polyanion that matched with literature very
well.18,26 The diffraction peaks of PI-HPM were observed
mainly in the four ranges of 2q = 6-10, 16-20, 25-30 and
33-35o, so it can be deduced that PI-HPM still maintained
the Keggin structure of raw HPM.27 But PI-HPM exhibited
very different diffraction peaks from that of raw HPM and
no diffraction peaks of HPI were detected, suggesting that
PI-HPM was a new crystal phase but not the simple mixture
of HPM and HPI. This conclusion agreed well with the
result of single crystal diffraction study.
The bond-valence calculations revealed that all Mo are
in their +6 oxidation states, P is +5 and each of the oxygen
atoms on the [PMo12O40] cluster are –2. Therefore, the total
charge of this cluster is –3. For the [Mo2O5(H2O)2(PI)2]
unit, the overall charge is 0. The free picolinic acid
groups were each protonated, each yielding (H2PI)+.
The additional charge (1 H+) should be residing on one
of the non-coordinated 12 lattice water molecules. The
molecular formula is therefore (H2PI)2(H3O)[PMo12O40]
[Mo2O5(H2O)2(PI)2]·11H2O.
The single crystal particles that were obtained through
solvent evaporation were collected and analyzed by XRD;
Table 1. Hydrogen bond parameters in the PI-HPM
D–H···A
d(H..A) / Å
1.922
1.702
2.077
2.338
2.091
2.164
2.191
∠DHA / degree
161.12
d(D..A) / Å
2.75
D–H···A
d(H..A) / Å
2.411
1.889
2.316
1.866
2.059
2.604
1.954
∠DHA / degree
152.53
d(D..A) / Å
3.19
N2–H···O30
O29–H···O34
O30–H···O22
O31–H···O19
O33–H···O31
O35–H···O23
O36–H··· O9
O27–H···O32
O30–H···O35
O31–H···O6
O34–H···O35
O34–H··· O33
O35–H···O7
O36–H··· O33
158.55
2.483
2.912
3.179
2.646
2.982
2.946
139.25
2.593
3.041
2.678
2.892
3.333
2.704
167.16
143.47
169.91
159.29
122.46
166.4
161.47
144.43
147.9
146.67

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Synthesis, Characterization and Catalytic Performance of a Novel Picolinic
J. Braz. Chem. Soc.
pyridinium ion band at 1536 cm-1, which was absent in the
IR spectrum of HPM.23,29
(e)
(d)
(c)
Thermal analyses were carried out on bulk HPM, HPI
and PI-HPM to determine the thermal stability and the
results are reported in Figure 5. The thermogravimetry
(TG) of HPM showed a weight loss of about 8.6% up to
o
a temperature of 130 C, indicating the loss of free and
physical adsorbed water.21 The gradual weight loss of about
2.4% up to 716 oC corresponded to the mass loss due to the
reaction between acidic protons and structural oxygen from
HPM, releasing water, followed by sharp decomposition
to MoO3 and POx species.30,31 PI-HPM exhibited similar
weight loss at about 130 and 720 oC like raw HPM, which
also can be attributed to the loss of free and adsorbed water
(the calculated amount is 7.6%) and decomposition of raw
HPM, respectively. In contrast with HPM and HPI, the
quick weight loss of HPI at about 160 oC did not occur in
the TG profile of PI-HPM, but a new weight loss around
(b)
(a)
10
20
30
40
2θ / degree
Figure 3. XRD patterns of (a) HPM; (b) HPI; (c) PI-HPM precipitate;
(d) PI-HPM crystal and (e) PI-HPM after fourth use.
the pattern is also shown in Figure 3. It can be seen that
the crystals show similar pattern to that of the PI-HPM
precipitate, which indicated that these crystals have same
structure as PI-HPM precipitate.
The FTIR spectra of HPM, HPI and PI-HPM are
illustrated in Figure 4. The four bands for bulk HPM at
1062, 960, 868 and 783 cm-1 are due to the stretching
o
310 C appeared and the percentage of weight loss was
about 17.1% (from 91.7% at 310 oC to 74.6% at 550 oC),
which was close to the theoretical value content of HPI in
the PI-HPM (the calculated amount is 17.3%). Therefore, it
can be considered that the PI-HPM was thermally stable in
temperatures less than 310 oC. PI-HPM decomposed in the
temperature range of 310-550 oC, indicating that PI-HPM
was less thermally stable than pure HPM.
mode of P–O in central tetrahedral, Mo=Oterminal
,
Mo‑O‑Mo(corner-sharing) and Mo–O–Mo(edge-sharing) in its Keggin
structure, respectively.23,26,28 There are four bands (1062,
961, 872 and 797 cm-1) within the range of 700-1100 cm-1 in
the spectra of PI-HPM that were similar with HPM, which
revealed that the Keggin structure of the HPM was well
maintained after modification with HPI. The characteristic
bands of HPI located at 1655, 1354, 1298 and 765 cm-1
were also observed for PI-HPM, verifying the presence
of HPI in the complex, which was consistent with the
result of single crystal X-ray diffraction. And the pattern
of PI-HPM showed clear evidence for the presence of the
100
(a)
(b)
80
60
40
20
0
(d)
(c)
-100
0
100 200 300 400 500 600 700 800 900
Temperature / °C
(c)
Figure 5. TG curves of (a) HPM; (b) PI-HPM and (c) HPI.
(b)
(a)
Catalytic activity
As acid catalyst, PI-HPM was applied to esterification
reaction to evaluate its catalytic activity and these results
are listed in Table 2. Acetic acid, n-propionic acid and
n-hexanoic acid were subjected to esterification reaction
with methanol, ethanol, n-propanol and n-butanol. It was
found that PI-HPM exhibited excellent catalytic activity in
these reactions and high conversions of carboxylic acids
2000 1800 1600 1400 1200 1000
Wavenumber / cm-1
800
600
400
Figure 4. FTIR spectra of (a) HPM; (b) HPI; (c) PI-HPM and (d) PI-HPM
after fourth use.

Vol. 27, No. 6, 2016
Liu et al.
1011
Table 2. Esterification reactions of carboxylic acids with alcohols over
PI-HPM catalyst
acid > n-propionic acid > n-hexanoic acid, which can
be explained on the basis of the strength of the reacting
acids.32 Furthermore, the decrease of conversion for the
esterification of acetic acid with different alcohols might
be due to the increase of steric hindrance with the growth
of alkyl chain of alcohols.
entry
1
Catalyst
PI-HPM
PI-HPM
PI-HPM
PI-HPM
Acid (A)
acetic acid
acetic acid
acetic acid
acetic acid
Alcohol (B) Conversion / %
methanol
ethanol
100a
96.3
83.7
68.5
91.4
80.7
78.9
62.4
100b
100
2
3
n-propanol
n-butanol
PI-HPM was also applied to the esterification of oleic
acid with different alcohols. From Table 2, it can be seen
that high conversion was obtained for the esterification
of oleic acid with methanol or ethanol. The esterification
of oleic acid with methanol was considered to be a main
method for the synthesis of biodiesel. For comparison,
esterification of oleic acid was carried out only in the
presence of HPI or HPM. A conversion of 20.7 and 89.7%
of oleic acid took place, respectively. After the reaction
ended, however, it was difficult to separate the HPI and
HPM due to the dissolution in reaction mixtures. Therefore,
PI-HPM not only exhibited higher catalytic activity than
HPM, but also overcame the difficulty of separation from
esterification mixtures, which was the very purpose of using
HPI to modify the HPM.Acid-base titration was performed
to explain why PI-HPM has good catalytic activity. The
results indicated that the capacity of acid sites increased
from 11.88 mmol g-1 of HPM to 11.98 mmol g-1 of PI-HPM.
For comparison, previous results for the esterification of
oleic acid or acetic acid over some reported typical solid
acid catalysts are also gave in Table 2.
4
5
PI-HPM n-propionic acid methanol
PI-HPM n-propionic acid ethanol
n-hexanoic acid methanol
6
7
PI-HPM
PI-HPM
PI-HPM
PI-HPM
PI-HPM
PI-HPM
fresh PI-HPM
HPI
8
n-hexanoic acid
oleic acid
oleic acid
oleic acid
oleic acid
oleic acid
oleic acid
oleic acid
oleic acid
oleic acid
acetic acid
ethanol
methanol
ethanol
9
10
11
12
13
14
15
16
1733
1832
n-propanol
n-butanol
methanol
methanol
methanol
methanol
ethanol
82.3
76.9
94.3c
20.7
89.7
89.1d
ca. 90e
57f
HPM
used PI-HPM
SnO2/WO3
ZSTA-15
n-butanol
aFor entries 1-8: reflux, time = 6 h, catalyst amount = 0.1 g, acid:alcohol
ratio = 1:3; bfor entries 9-12: time = 6 h, reaction temperature = 100 oC,
catalyst amount = 10 wt.% of oleic acid, acid:alcohol ratio = 1:10;
o
cfor entries 13-16: time = 6 h, reaction temperature = 80 C, catalyst
d
The effect of reaction parameters, including reaction
time, temperature, mole ratio and catalyst dosage on
esterification of oleic acid with methanol catalyzed by PI-
HPM were studied (Figure 6). The conversion of 94.3%
of oleic acid was achieved at the mole ratio of alcohol to
acid of 10:1, catalyst amount of 10 wt.% of oleic acid and
amount = 10 wt.% of oleic acid, acid:alcohol ratio = 1:10; conversion
was achieved after the fourth recycling of PI-HPM; ereflux, time = 12 h,
catalyst amount = 0.1 g, acid:alcohol ratio = 1:120; ftime = 4 h, reaction
temperature = 80 oC, catalyst amount = 0.025 g, acid:alcohol ratio = 1:16.
PI-HPM: picolinic acid-12-molybdophosphoric acid hybrid complex;
HPI: picolinic acid; HPM: 12-molybdophosphoric acid hybrid complex;
ZSTA-15: zirconia-supported silicotungstic acid.
o
reaction time of 6 h at 80 C. Conversion of 100% was
were obtained. The desired result was also achieved, that
the PI-HPM can be easily separated by centrifugation
from the reaction system, indicating that the complex
presented good durability. The percentage of conversion
of these acids with methanol followed the order: acetic
obtained when temperature was increased to 100 oC.
After reaction, PI-HPM was separated from the
reaction mixture only by simple filtration and washed with
methanol. After drying the catalyst was reused with new
charge of oleic acid and methanol. The result suggested that
100
100
100
(a)
(b)
(c)
(d)
100
90
80
70
60
50
90
80
70
60
90
80
70
60
90
80
40
30
70
60
50
40
50
40
20
1
2
3
4
5
6
20
40
60
80
100
3:1
5:1
7:1
10:1
15:1
0
2
4
6
8
10 12 14 16
time / h
Temperature / oC
Molar ratio (methanol:oleic acid)
Catalyst dosage / wt.%
Figure 6. Effects of reaction parameters on the esterification of oleic acid. Reaction conditions: (a) reaction temperature = 100oC, catalyst amount = 10 wt.%
of oleic acid, acid:alcohol ratio = 1:10; (b) time = 6 h, catalyst amount = 10 wt.% of oleic acid, acid:alcohol ratio = 1:10; (c) time = 6 h, reaction
temperature = 100 oC, catalyst amount = 10 wt.% of oleic acid; (d) time = 6 h, reaction temperature = 100 oC, acid:alcohol ratio = 1:10.

1012
Synthesis, Characterization and Catalytic Performance of a Novel Picolinic
J. Braz. Chem. Soc.
PI-HPM catalyst was reusable with little loss in activity. A
conversion 89.1% was achieved after the fourth recycling
(Table 2, entry 16), suggesting that the PI-HPM was stable
and commendably compensated the disadvantage of raw
HPM.
To verify the recovery of PI-HPM catalyst, XRD and
IR measurements were performed after four recycles; the
results are shown in Figure 3e and Figure 4d, respectively.
The XRD patterns of catalyst before and after reaction were
almost identical, which indicated that there was no obvious
change for the catalyst structure after esterification. For
IR spectrum, the four Keggin characteristic bands in the
range of 700-1100 cm-1 were all observed.All these results
indicated the highly chemical and structural stability of
PI-HPM catalyst.
Compared with the IR spectrum of fresh PI-HPM,
however, new infrared bands at 2925, 2855 and 1740 cm-1
were observed for the recycled catalyst spectrum, which is
similar with the IR spectrum of methyl oleate, supported by
the Spectral Database for Organic Compounds (SDBS No.
8872). That is, the catalyst was attached to methyl oleate
after reaction, which might cause the slight reduction in
catalytic activity of the reused catalyst. Therefore, more
effective treatment method should be developed to optimize
the catalytic activity of reused catalyst.
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In this work, a novel HPM-based complex was prepared
by the modification of HPM with HPI, and the complex was
applied in esterification reaction as heterogeneous catalyst
to evaluate its acid-catalytic activity. The characterization
results suggested the complex still retained the Keggin
ion structure of HPM. The complex simultaneously
possessed high catalytic activity and good durability in
reaction mixtures, indicating that PI-HPM was an excellent
heterogeneous catalyst for esterification. Compared with
commercial mineral acid, PI-HPM was more benign from
the environmental protection point of view.
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Acknowledgments
The authors are grateful for the support of the National
Natural Science Foundation of China (21101086), the
Doctor Foundation of Shandong Province (BS2010CL011)
and the Foundation of Liaocheng University (318011406).
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Submitted: August 20, 2015
Published online: December 16, 2015