Heteropolyacid immobilized on polymer/magnetic zeolite nanocomposite as a new and recyclable catalyst for the selective oxidation of alcohols

Article abstract of DOI:10.1007/s13738-017-1237-3

A new magnetic oxidation catalyst was prepared using immobilization of tungstophosphoric acid on poly(N-vinylimidazole) entrapped magnetic nanozeolite and characterized by FTIR, TGA, XRD, SEM, TEM, EDX, VSM, and ICP-OES. The resulting heterogeneous catalyst displays high catalytic performance for selective oxidation of alcohols compared to the other catalysts. The catalyst could be reused eight times without any loss of catalytic activity.

Full text of DOI:10.1007/s13738-017-1237-3

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-017-1237-3
ORIGINAL PAPER
Heteropolyacid immobilized on polymer/magnetic zeolite
nanocomposite as a new and recyclable catalyst for the selective
oxidation of alcohols
1
1
2
Maryam Maleki · Seyed Meysam Baghbanian · Mahmood Tajbakhsh
Received: 26 July 2016 / Accepted: 14 November 2017
©
Iranian Chemical Society 2017
Abstract
A new magnetic oxidation catalyst was prepared using immobilization of tungstophosphoric acid on poly(N-vinylimidazole)
entrapped magnetic nanozeolite and characterized by FTIR, TGA, XRD, SEM, TEM, EDX, VSM, and ICP-OES. The result-
ing heterogeneous catalyst displays high catalytic performance for selective oxidation of alcohols compared to the other
catalysts. The catalyst could be reused eight times without any loss of catalytic activity.
Keywords Magnetic nanozeolite · Poly(N-vinylimidazole) · Tungstophosphoric acid · Selective oxidation · Alcohols
Introduction
To solve these problems, an acceptable method is immo-
bilization of tungsten as homogeneous catalyst on the
organic (polymers [69–75]) and inorganic (silica [76–79],
mesoporous silica [80–83], magnetic nanoparticles [72, 84,
85], and graphene oxide [86]) supports. Among inorganic
supports, magnetic nanozeolite clinoptilolite (MNZ) is an
attractive support because of the excellent properties such as
inexpensive, good thermal and chemical stability, and easy
separation without the need for any ꢁltration or centrifuga-
tion processes [87–89, 92].
Oxidation reaction of diꢀerent primary and secondary alco-
hols to carbonyl compounds is one of the critical reactions
and key transformations at both laboratory and commercial
scales [1]. The oxidation of alcohols using several kinds of
reagents has already been investigated [2]. Among them,
hydrogen peroxide as an inexpensive and commercially
accessible reagent, is an signiꢁcance oxidant due to it can
be safely stored and employed and its low standard redox
potential in some acidic media [2, 3]. Several metals such
as manganese, rhenium, molybdenum, and ruthenium have
been applied as homogeneous catalysts in oxidation reac-
tions [4–63]. Among them, tungsten is the most considered
to selective oxidation of alcohols due to its selectivity and
eꢂciency in this reaction. As a result, several catalysts based
on tungstate have been reported for oxidation of alcohols
using H O but most of them are homogeneous and have
The supported ionic liquid technology is a fundamental,
new method to achieve liquid, including solid materials that
do not evaporate, made through surface modiꢁcation of a
porous solid [90]. In supported ionic liquid catalysis, the
focus has been mainly on the immobilization homogeneous
transition metals inside the thin ILs. The main drawback of
homogeneous catalysis is the recovering of the dissolved
catalyst from the reaction mixture that commonly can be
by distillation or extraction. This problem has restricted the
applications of homogeneous catalysts in multi-step reac-
tions that can be circumvented by the supported ionic liquid
technology [90].
2
2
problems such as separation and recovery catalyst, lack
of selectivity, and limit their use in industrial applications
[
64–68].
In order to solve the above problems and introduce a
new catalyst for the oxidation of alcohols, herein we wish
to report a convenient method for selective oxidation of
primary alcohols to their corresponding aldehydes with
12-tungstophosphoric acid immobilization onto poly(N-
vinylimidazole)-modiꢁed magnetic nanozeolite (MNZ@
PVImW) as a catalyst.
*
Seyed Meysam Baghbanian
s.baghbanian@gmail.com; s.m.baghbanian@iauamol.ac.ir
1
2
Department of Chemistry, Ayatollah Amoli Branch, Islamic
Azad University, Amol, Iran
Faculty of Chemistry, University of Mazandaran,
Babolsar 47415, Iran
Vol.:(0
1
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Journal of the Iranian Chemical Society
Experimental
Synthesis of MPS‑coated magnetic nanozeolite (MNZ@
MPS)
Reagents and analysis
Chemical modiꢁcation of MNZ surface with MPS before
polymerization causes grafting of polymer chains onto MNZ
by covalent bonding. In other words, modiꢁcation of MNZ
by methacrylate groups causes polymerization easier on the
surface of MNZ, and all of them are grafted by a copolymer
shell [93]. As a result, MNZ (1 g) was dispersed in 15 mL of
absolute ethanol, and then 2 mL of ammonium solution was
added to the ꢃask. After that, 10 mmol (excess) of the MPS
solution was added slowly, and the mixture was stirred at
70 °C for 12 h. MNZ@MPS was magnetically separated and
washed several times with ethanol and dried under vacuum
at 65 °C.
FeCl ·6H O, FeCl ·4H O, 3-methacryloxypropyltrimeth-
3
2
2
2
oxy-silane (MPS, 98%), 1-bromododecane, 1-vinylimi-
dazole, and ammonia (30%) were obtained from Aldrich.
1
9
,4-dibromobutane and 2,2′-azobisisobutyronitrile (AIBN,
7%) were obtained from Merck. The raw zeolite material
was an Iranian commercial clinoptilolite (CP) (Afrandtooska
Company) obtained from deposits in the region of Semnan,
Iran. The X-ray powder diꢀraction (XRD) of the catalyst was
performed on a Philips PW 1800 X-ray diꢀractometer with a
CuKα source (LEF tube, 40 kV, 50 mA). Thermogravimetric
analyses (TGA) were taken under a nitrogen atmosphere using
a PerkinElmer analyzer. Scanning electron microscope (SEM)
picture was observed using a KYKY-EM 3200 microscope
Synthesis of 1,4‑butanediyl‑3,3′‑bis‑L‑vinylimidazolium
dibromide (BVD)
(
acceleration voltage 26 kV). Energy dispersive spectroscopy
EDS) diagrams were recorded by a VEGA\\TESCAN-LMU
(
scanning electron microscope. Transmission electron micros-
copy (TEM) experiments were conducted on a JEOL-2100
microscope operated at 200 kV. Fourier-transform infrared
spectra of the samples were recorded on a Rayleigh WQF-510
FT-IR spectrometer using KBr disks. Magnetic measurements
were performed using vibration sample magnetometer (VSM,
MDK, and Model 7400, Kashan, Iran) analysis. Elemental
analyses for C, H, and N were performed using a Heraeus
CHN-O-Rapid analyzer. A simultaneous ICP-OES (Varian
Vista-Pro, Springvale, Australia) coupled to a V-groove nebu-
lizer and equipped with a charge-coupled device (CCD) was
applied for determination of the trace metal ions. The com-
position of reaction mixture was analyzed by GC using an
Agilent gas chromatograph ꢁtted with a Factor Four HP-5MS
capillary column.
Typically, 1,4-butanediyl-3,3′-bis-L-vinylimidazolium dibro-
mide (BVD) was synthesized based on a reported method
[94]. 1-Vinylimidazole (2.82 g, 30 mmol) was dissolved in
15 mL ethanol, and 1,4-dibromobutane (3.24 g, 15 mmol) in
15 mL ethanol was added dropwise to the 1-vinylimidazole
solution. The mixture was stirred at 60 °C for 10 h. After
cooling of the reaction ꢃask to ambient temperature, the
reaction mixture was added dropwise into 500 mL of dry
diethyl ether. The white powder was separated and dried at
room temperature.
1,4-Butanediyl-3,3′-bis-L-vinylimidazolium dibromide
1
(Mp = 148 °C). H NMR (DMSO-d6, δ, ppm): 9.55 (2H),
8.28 (2H), 7.93 (2H), 7.47 (2H), 5.92 (2H), 5.42 (2H), 4.30
1
3
(4H), 1.82 (4H). C NMR (DMSO-d6, δ, ppm): 134.9,
129.4, 123.8, 120.1, 109.2, 49.3, 26.4.
General procedure
Synthesis of 12‑tungstophosphoric acid immobilization
onto poly(N‑vinylimidazole) modiꢀed magnetic
nanozeolite (MNZ@PVImW)
Synthesis of magnetic nanozeolite (MNZ)
For the synthesis of magnetite nanozeolite, ꢁrst, 1 g of
nanozeolite [91] was added to 50 mL of distilled water in
a round bottom ꢃask, ultra-sonicated for 15 min at room
MNZ@MPS (0.5 g), 1-vinylimidazole (VIm) (1 mL), and
BVD (0.3 g) were dissolved in methanol (50 mL) and ultra-
sonicated for 20 min at room temperature. Then, the mixture
deoxygenated bubbling argon for another 20 min. Afterward,
0.008 g of AIBN was added into the mixture, and the ꢃask
was placed in an oil bath at 60 °C for 24 h. The precipitates
(MNZ@PVIm) were magnetically separated from the solu-
tion, washed several times with methanol, and dried under
vacuum at 50 °C. Next, the MNZ@PVIm (1 g) was dispersed
in 50 mL deionized water with continuous ultrasonication
and stirring for 30 min. Then, a solution of H3PW12O40
0.015 gr in 50 mL deionized water was added to the above
mixture and stirred for 24 h at room temperature. The brown
solid product (MNZ@PVImW) was magnetically separated,
temperature, and deoxygenated by bubbling of N gas for
2
3
0 min. After that, solutions of FeCl (1 M in water) and
3
3
+
2+
FeCl (2 M in HCl 2 M) with molar ratio of 4:1 (Fe /Fe )
2
were added dropwise to the nanozeolite (NZ) mixture and
stirred for 30 min [92]. Then, while the mixture was vigor-
ously stirred, 70 mL of ammonia solution was slowly added
into the mixture. When the color of solution changes from
white to dark, the prepared composite was separated using
a simple magnet, washed with distilled water several times,
and ꢁnally dried in oven at 50°C for 6 h.
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Journal of the Iranian Chemical Society
washed ꢁve times with deionized water (5 × 50 mL), twice
with ethanol (2 × 25 mL), and dried under vacuum at 50 °C.
was attached on the surface of MNZ by reaction of –OH
groups on the MNZ and the (MeO) –Si groups in the MPS to
3
form the active vinyl groups on the surface of MNZ. After-
ward, MNZ entrapped cross-linked poly(N-vinylimidazole)
was synthesized by copolymerization of 1-vinylimidazole
(VIm) monomer in the presence of MNZ@MPS and BVD as
cross-linked. Copolymerization begins by AIBN in metha-
nol at 60 °C, and MNZ@PVIm copolymer was precipitated.
In the next step, the generated copolymer was reacted with
HPW to produce of MNZ@PVImW catalyst (Fig. 1).
Catalytic oxidation of benzyl alcohols
In a typical procedure, benzyl alcohols (1 mmol), CH CN
3
(
3 mL), H O (4 mmol), and MNP@PILW (2 mg) were
2
2
added to a 50 mL round bottom ꢃask. The reaction mixture
was stirred at 40 °C for a deꢁned time. After the reaction,
the catalyst was separated from the mixture by a magnet.
The product was extracted by ethyl acetate (5 mL) and then
analyzed by gas chromatography (GC).
Characterization of the catalyst
General procedure for oxidation of benzyl alcohols in large
scale
FT-IR spectra of (a) NZ, (b) MNZ, (c) MNZ@PVIm, and
(d) MNZ@PVImW are shown in Fig. 2. In the spectrum of
−1
the NZ, the peak at 3853 and 3467 cm corresponds to OH
stretching of Si–OH and water, and the medium intensity band
A mixture of benzyl alcohols (10 mmol), CH CN (30 mL),
3
−1
H O (40 mmol), and MNP@PILW (20 mg) was stirred at
at 1637 cm corresponds to the bending vibration of H–O–H
2
2
−1
4
0 °C for a deꢁned time. After completion
groups. The strong band at 1095 cm is related to the asym-
metric stretching vibration of the Si–O and Al–O groups, and
of reaction (monitored by TLC), the catalyst was magneti-
−1
cally separated, washed with CH CN, and dried for another
the weak bands at 476 cm are due to bending vibrations of
O–Si–O and O–Al–O groups [91]. The IR spectrum of MNZ
revealed the changes in the framework conꢁgurations of zeolitic
structure after the incorporation of Fe O . Changes occurred
3
run. The product was extracted by ethyl acetate (50 mL) and
then analyzed by gas chromatography (GC).
3
4
compared with NZ include the OH stretching vibrations at
−
1
−1
Results and discussion
3138 cm , the Fe–O–Fe stretching vibrations at 598 cm ,
−1
and the weak stretching vibration of Fe–O at 476 cm . The
results show that Fe O nanoparticles are formed into the NZ
Production of the catalyst system
3
4
[
89, 92, 95]. The spectrum of the MNZ@PVIm revealed the
−1
The MNZ was prepared by co-precipitation of FeCl and
OH stretching vibrations at 3425 cm and the C–H stretch-
3
−
1
FeCl in the presence of natural nanozeolite. Next, the MPS
ing vibrations of alkyl chains at 2925 cm . Additionally,
2
Fig. 1 Preparation of MNZ@PVImW
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Journal of the Iranian Chemical Society
Fig. 2 FT-IR spectra (a) NZ, (b)
MNZ, (c) MNZ@PVIm and (d)
MNZ@PVImW
MNZ@PVIm shows stretching vibration of a carbonyl group
−1
at 1712 cm and the C=N and C=C stretching vibrations of
−1
the imidazole ring at 1647 and 1501 cm , respectively, that
prove successful presence of PVIm on MNZ. For the spec-
trum of MNZ@PVImW, new peaks were observed at 805 and
−
1
−1
−1
8
95 cm (W–O–W), 954 cm (W=O), and 1091 cm (P–O)
3
−
that can be related to [PW O ] anion [96, 97]. All the results
1
2 40
achieved from FT-IR spectra approve that MNZ entrapped into
3−
the copolymers and the [PW O ] ions were successfully
12
40
entrapped into the polymeric matrix.
The thermal gravimetric analysis (TGA) of MNZ and
MNZ@PVImW are exhibited in Fig. 3. In TGA curves of
MNZ and MNZ@PVImW, a mass loss observed between
room temperature and 200 °C could be related to the removal
of surface adsorbed water. In the case of MNZ@PVImW, the
second step weight loss (41%) starting from 250 °C to 600 °C
was likely due to the thermal decomposition of the copolymer.
Patterns of XRD for NZ, MNZ, and MNZ@PVImW are
investigated in Fig. 4. As shown in Fig. 4a, the obtained
XRD pattern exhibited reflection peaks of NZ at 2θ =
Fig. 3 TGA curves of (a) MNZ and (b) MNZ@PVImW
of zeolite with additional broad peaks at 2θ = 35.81°,
5
7.18°, and 62.74° related to the 311, 511, and 440 crystal-
lographic planes of the face-centered cubic (FCC) iron oxide
nanocrystals which fully comply with the standard Fe O
3
4
(
orange balls) (JCPDS 19-0629) [98]. Figure 4c shows the
2
1.74°, 27.46° and 35.92° found and identiꢁed as natural
XRD pattern of MNZ@PVImW with a broad peak, and low
reꢃection intensity at 8–10°, 17–22°, and 24–40° can be
assigned to the diꢀraction characteristic peaks of crystalline
H PW O Keggin structure [99].
zeolite of clinoptilolite when compared with the standard
pattern (green stars) (JCPDS 00-025-1349). The XRD pat-
terns of MNZ (Fig. 4b) have the same characteristic peaks
3
12 40
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Journal of the Iranian Chemical Society
Fig. 4 XRD patterns of (a) NZ, (b) MNZ, and (c) MNZ@PVImW
The surface morphology of MNZ@PVImW was studied
by SEM image (Fig. 5a). As can be seen, all the particles
have similar structures. TEM image of MNZ@PVImW
shows (Fig. 5b) that MNZ dispersed into the polymeric
3
−
matrix and the [PW O ] ions was entrapped into the
1
2
40
copolymer. EDX analysis from the obtained MNZ@PVImW
Fig. 5 a SEM, b TEM images, and c EDX analysis of MNZ@
(
Fig. 5c) showed the presence of the expected elements in
PVImW
the structure of the catalyst, namely oxygen (27.68%), iron
(
(
42.54%), aluminum (3.34%), silicon (16.83%), phosphorus
0.67%), and tungsten (8.49%).
Figure 6 shows the magnetization of MNZ and MNZ@
PVImW at room temperature (300 K). The MNZ and
MNZ@PILW showed an “S”-shaped hysteresis loop with-
out remanence which indicates the particles have good
superparamagnetic properties. The saturation magnetiza-
tion of MNZ and MNZ@PILW was 47.7 and 27.6 emu/g,
respectively. These results show that the magnetization of
MNZ@PVImW decreased considerably with the formation
copolymer on MNZ. However, the typical superparamag-
netic behavior of MNZ@PVImW implies that the catalyst
can be magnetically separated.
The total loading amount of tungstate ions in MNZ@
PVImW was calculated by ICP-OES to be 9 wt%
(
0.04 mmol/g).
Fig. 6 Magnetization of (a) MNZ and (b)MNZ@PILW
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Journal of the Iranian Chemical Society
Table 1 Results of oxidation of
benzyl alcohol to benzaldehyde
over various conditions
a
b
Entry Catalyst
Amount
Solvent T (°C) Time (min) Conv. (%) Yield (%) Selectivity
of catalyst
(%)
(
mg)
A
B
–
1
2
3
4
5
6
7
8
9
1
1
1
1
1
–
–
CH CN 40
60
60
30
30
30
30
120
30
30
60
60
60
60
120
–
–
–
3
MNZ@PVImW 1
MNZ@PVImW 2
MNZ@PVImW 4
MNZ@PVImW 2
MNZ@PVImW 2
MNZ@PVImW 2
MNZ@PVImW 2
MNZ@PVImW 2
MNZ@PVImW 2
MNZ@PVImW 2
MNZ@PVImW 2
MNZ@PVImW 2
CH CN 40
78
96
45
64
95
39
75
85
25
58
75
85
68
75
95
43
60
92
45
78
80
28
65
80
87
65
85.7 14.3
95.4 4.6
97.3 2.7
3
CH CN 40
3
CH CN 40
3
CH CN r.t
78.2 21.8
1.3 98.7
88.5 11.5
80.9 19.1
30.7 69.3
89.3 10.7
21.5 78.5
92.4 7.8
3
CH CN 80
3
c
CH CN 40
3
Water
Water
40
80
0
1
2
3
4
CH Cl2 r.t
2
CH Cl2 40
2
–
–
40
80
34.8 65.2
64.6 35.4
HPW
50
CH CN 40
3
Reaction conditions: benzyl alcohol (1 mmol), H O (4 mmol), 3 mL solvent
2
2
a
b
c
Conversions were calculated based on initial mmol of benzyl alcohol
Yields were determined by GC
2
mmol H O
2 2
Catalyst activity study of MNZ@PVImW
the amount of H O , the oxidation of benzyl alcohol with
2 2
2
mmol H O under same reaction conditions was investi-
2 2
Initially, in order to investigate the catalytic activity of
MNZ@PVImW, the oxidation of benzyl alcohol to benzal-
dehyde was examined under diꢀerent conditions as a model
reaction, and the results were listed in Table 1. For the initial
study, we used CH CN (3 mL) as solvent and H O (4 mL)
gated that demonstrated only 39% conversion is achieved in
2 h (Table 1, entry 7). Consequently, 4 mmol H O is the
2
2
optimum amount under these conditions. In order to opti-
mize the reaction conditions, the model reaction was exam-
ined in various solvents such as H O and CH Cl instead of
3
2
2
2
2
2
as oxidant for the oxidation of benzyl alcohol in the presence
of MNZ@PVImW as catalyst at 40 °C. In the absence of
catalyst, no desired product was formed (Table 1, entry 1).
Oxidation of benzyl alcohol has been carried out over 1, 2,
and 4 mg of catalyst under model reaction (Table 1, entries
CH CN, and results showed that CH CN is the best solvent
3
3
for this reaction (Table 1, entries 8–11). The oxidation of
benzyl alcohol was also investigated in solvent-free condi-
tion (Table 1, entries 12–14), the selectivity and yield of
benzaldehyde were 92.4% and 80%, respectively. All these
results demonstrate that the optimum conditions for oxi-
dation reaction of benzyl alcohol to benzaldehyde is 2 mg
MNZ@PVImW as catalyst and 4 mmol H O in CH CN at
2
–4). The results revealed that both the conversion of benzyl
alcohol and the selectivity of benzaldehyde increased. No
obvious improvement was observed in the catalytic perfor-
mance of MNZ@PVImW when catalyst loading increased
to 4 g (Table 1, entry 4), but the selectivity increased from
2
2
3
40 °C (Table 1, entry 3).
The MNZ@PVImW was evaluated in the catalytic oxi-
dation of benzylic alcohols under optimized reaction con-
ditions. The oxidations of various benzylic alcohols with
electron-donating and electron-withdrawing substituents
to corresponding aldehydes have been studied. Under this
method, oxidation of substituted benzyl alcohols with elec-
tron-donor groups (CH , OCH ) take place faster than ben-
9
2
3
5.4 to 97.3%. From the viewpoint of yield of benzaldehyde,
mg MNZ@PVImW gives the best results. (Table 1, entry
). Oxidation of benzyl alcohol to benzaldehyde with H O
2
2
on MNZ@PVImW was explored in various temperatures
(
Table 1, entries 5 and 6). When the reaction was carried
out at a higher temperature, the selectivity for benzaldehyde
A) was reduced with the formation of benzoic acid (B) from
5.4 to 1.3% at 80 °C (Table 1, entry 6). Also, the selec-
3
3
(
zylic alcohols with electron-withdrawing groups (Cl, NO ).
2
9
The reaction of cyclic alcohols such as cyclohexanol and
aliphatic alcohols were also investigated, and results dem-
onstrated that the desired products were obtained in lower
yields and longer reaction times than benzylic alcohols.
tivity was decreased from 95.4 to 78.2% with reducing the
temperature to 25 °C (Table 1, entry 5). According to the
results, 40 °C was chosen to be the best reaction temperature
for the selective oxidation of benzyl alcohol. To optimize
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Journal of the Iranian Chemical Society
Table 2 Oxidation of various alcohols catalyzed by MNZ@PVImW
Time
min)
Conv.a
(%)
Yield
(%)
Entry
Substrates
Product
(
A
B
1
30
25
96
95
2
92
92
3
4
25
25
91
96
90
95
5
20
60
91
82
90
80
6
7
8
90
45
79
92
75
91
9
45
92
90
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Journal of the Iranian Chemical Society
Table 2 (continued)
Entry
Time
min)
Conv.a
(%)
Yield
(%)
Substrates
Product
(
A
B
1
0
40
92
91
1
1
1
2
40
30
90
90
89
88
–
–
1
1
3
4
45
40
92
91
90
90
–
1
5
–
–
40
60
88
90
85
89
1
6
1
1
7
8
–
–
60
60
70
30
69
86
Reaction conditions: substrate (1 mmol), H O (4 mmol), MNZ@PVImW (2 mg), CH CN (3 mL) and 40 °C
2
2
3
a
Conversions were calculated based on initial mmol of benzyl alcohol
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Journal of the Iranian Chemical Society
Acknowledgements This research is supported by the Islamic Azad
University, Ayatollah Amoli Branch, I. R. Iran.
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1
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1
0 mmol. In this regard, after each run, the catalyst was
1
separated by an external magnet and reused as such for the
next experiment. As seen in Fig. 7, the eꢂciency of this
catalyst remained unchanged even after eight runs. Although
the catalyst can be easily separated by magnets from the
reaction mixture, such studies are not enough to prove
tungsten leaching from the polymer matrix. Therefore, in
other methods, tungsten leaching from the magnetic sup-
port was investigated in the oxidation of benzyl alcohol. The
model reaction was stopped after 15 min and the catalyst
was removed from the solution and the yield was calculated
by GC. Then, the reaction mixture was allowed to stir for
another 15 min without catalyst, and the results show that no
benzaldehyde is generated in the absence of catalyst. After
separation of the catalyst, tungsten leaching into the reaction
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