Magnetic nanoparticle supported polyoxometalate: An efficient and reusable catalyst for solvent-free synthesis of α-aminophosphonates

Article abstract of DOI:10.1016/j.molcata.2013.02.018

A new magnetically separable catalyst consisting of phosphotungstic acid (PTA) supported on imidazole functionalized silica coated cobalt ferrite nanoparticles was prepared. The synthesized catalyst was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometry (VSM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The immobilized phosphotungstic acid was shown to be an efficient heterogeneous catalyst for the synthesis of α-aminophosphonates under solvent-free conditions at room temperature. The catalyst is readily recovered by simple magnetic decantation and can be recycled several times with no significant loss of catalytic activity.

Full text of DOI:10.1016/j.molcata.2013.02.018

Journal of Molecular Catalysis A: Chemical 373 (2013) 25–29
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Magnetic nanoparticle supported polyoxometalate: An efficient and reusable
catalyst for solvent-free synthesis of ␣-aminophosphonates
H. Hamadia, M. Kootia,∗, M. Afsharia, Z. Ghiasifarb, N. Adibpourc
a Department of Chemistry, College of Science, Shahid Chamran University, Ahvaz 61357-43169, Iran
b Department of Chemistry, Science and Research Khouzestan Branch, Islamic Azad University, Ahvaz, Iran
c Department of Pharmacology and Toxicology, School of Pharmacy, Ahvaz Jundishpur University of Medical Sciences, Ahvaz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 June 2012
Received in revised form 7 January 2013
Accepted 17 February 2013
Available online 28 February 2013
A new magnetically separable catalyst consisting of phosphotungstic acid (PTA) supported on imidaz-
ole functionalized silica coated cobalt ferrite nanoparticles was prepared. The synthesized catalyst was
characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), vibrating
sample magnetometry (VSM), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR), and
inductively coupled plasma atomic emission spectroscopy (ICP-AES). The immobilized phosphotungstic
acid was shown to be an efficient heterogeneous catalyst for the synthesis of ␣-aminophosphonates
under solvent-free conditions at room temperature. The catalyst is readily recovered by simple magnetic
decantation and can be recycled several times with no significant loss of catalytic activity.
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Cobalt ferrite
␣-Aminophosphonates
Magnetic nanoparticles
Surface functionalization
Phosphotungstic acid
1. Introduction
Therefore, cobalt ferrite can show better performance than mag-
netite as support for homogeneous catalysts. In most cases, the
The choice of an efficient support could significantly improve
the activity, selectivity, recycling, and reproducibility of catalyst
systems. In recent years, magnetic nanoparticles (MNPs) have
emerged as attractive solid supports for immobilization of homoge-
neouscatalysts[1,2]. Thisis becausethemagneticnanoparticlescan
be well dispersed in the reaction mixtures in the absence of mag-
netic field providing large surface for readily access of substrate
molecules. More importantly, after completing the reactions, the
MNPs supported catalysts can be isolated efficiently from the prod-
uct solution through a simple magnetic separation process thereby
eliminating the requirement of catalyst filtration and centrifuga-
tion [3,4].
Among the various magnetic nanoparticles, magnetite (Fe3O4)
is the most widely used magnetic substance for catalysts sup-
port [5,6]. However, Fe3O4 is fairly reactive to acidic environment
and since it contains Fe2+, which is readily oxidized, the MNPs of
Fe3O4 are vulnerable to lose magnetism. On the contrary, cobalt
ferrite (CoFe2O4) which is a typical ferromagnetic oxide with
spinel structure has high thermal stability, moderate magnetiza-
tion, remarkable chemical stability and mechanical hardness [7,8].
catalytic species cannot be introduced directly on the surface of
MNPs supports and modification or functionalization of the MNPs
surface is required prior to catalyst immobilization. In a recent
review, Basset and co-workers have reported myriad catalytic sys-
tems devised by employing some superparamagnetic nanoparticles
as a support, and their performance in catalyzing various organic
reactions [9].
Polyoxometalates (POMs) and their derivatives have been
widely used in organic synthesis and catalytic reactions as acid and
oxidation catalysts because of their strong acidity and favourable
redox properties [10,11]. Although many procedures have been
performed to immobilize polyoxometalates on solid materials, only
few reports have recently appeared in the literature about poly-
oxometalates supported-MNPs. The first example of non-covalent
immobilization of POMs on MNPs was reported by Luo and co-
workers [12]. These researchers have used Fe3O4 MNPs as core and
after coating this core with silica, the surface was functionalized
with imidazulium group. In the final step, PTA was immobilized on
the surface of silica coated MNPs in THF by sonication. The prepared
composite was used as efficient acid catalyst for the Friedel–Crafts
reaction of indole and chalcones, In another report, oxidation of
dibenzothiophene with H2O2 was studied by Ding and co-workers
using two types nanosized catalysts consisting of phosphotungstic
acid (H3PW12O40) supported on surface-modified Fe3O4. In this
∗
Corresponding author. Tel.: +98 916 1115451; fax: +98 6113331042.
E-mail address: m kooti@scu.ac.ir (M. Kooti).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.02.018

26
H. Hamadi et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 25–29
Scheme 1. PTA/Si-imid@ Si-MNPs catalyzed synthesis of ␣-aminophosphonates.
study high conversions were obtained and the catalysts could be
easily separated from the reaction solution by applying an exter-
nal magnetic field and recycled several times [13]. The oxidation
of dibenzothiophene with hydrogen peroxide was also studied by
another researcher group using a recyclable amphiphilic catalyst in
which phosphotungstic acid was immobilized on surface modified
MNPs [14].
of 12 L/min. NMR spectra were recorded in CDCl3 on a Bruker
Advanced DPX 400 MHz spectrometer.
2.2. Synthesis of PTA supported on imidazole functionalized
Si-MNPs
In the present work, we designed a methodology for con-
structing of a magnetically recoverable nanocatalyst in which
phosphotungstic acid (PTA) was immobilized on imidazole func-
tionalized silica coated cobalt ferrite nanoparticles. In view
of the immense importance of ␣-aminophosphonates [15–18],
the as-synthesized catalyst was utilized for the preparation of
these organophosphorous compounds at room temperature under
solvent-free conditions (see Scheme 1).
Cobalt ferrite MNPs were synthesized using the procedure
reported by Maaz et al. [20]. Silica coated CoFe2O4 nanoparti-
cles (Si-MNPs) were prepared by using a reported sol–gel method
[21]. The synthesis of PTA supported on imidazole functional-
ized Si-MNPs were achieved by using the previously reported
method with some modification [22]. In our modified procedure 3-
chloromethoxypropylsilane and imidazole in 1:1 molar ratio were
reacted first to obtain product 1 (see Scheme 2). This product was
then dissolved in ethanol followed by addition of Si-MNPs and the
mixture was refluxed for 24 h to give product 3. Finally, PTA/Si-
imid@ Si-MNPs composite was obtained by treating of product 3
with PTA in refluxing acetonitrile.
2. Experimental
2.1. General
All chemicals were purchased from Sigma–Aldrich or Merck
and used as received. Phosphotungstic acid (PTA) was synthe-
sized according to the literature [19]. X-ray diffraction (XRD)
patterns were recorded with a Philips X-ray diffractometer (Model
PW1840). FT-IR spectra were obtained using BOMEM MB-Series
1998 FT-IR spectrometer. Magnetic properties of all nanoparti-
cles were measured with a vibrating sample magnetometer (VSM,
Meghnatis Daghigh Kavir Company, Iran) at room temperature.
The content of tungsten of the PTA/Si-imid@ Si-MNPs catalyst
was determined using an ICP-AES instrument (HORIBA Jobin Yvon,
Longjumeau Cedex, France). The catalyst sample was first dissolved
in concentrated hydrochloric acid and after filtration, the aqueous
solution is converted to aerosols via a nebulizer. The aerosols are
transported to the inductively coupled plasma with argon flowrate
2.3. General procedure of catalytic ˛-aminophosphonates
synthesis
A mixture of an aldehyde (1 mmol), aniline (1 mmol), triethyl
phosphate or trimethyl phosphate (1.2 mmol) and PTA/Si-imid@ Si-
MNPs catalyst (0.05 g) was stirred for an appropriate time at room
temperature. The progress of the reaction was monitored by TLC.
At the end of the reaction, CH3Cl was added to dilute the reaction
mixture and the organic layer was simply decanted by means of an
external magnet. The isolated solution was purified on a silica-gel
plate to obtain pure product. The identities of the products were
confirmed by FT-IR and 1H NMR spectral data.
Fig. 1. FT-IR spectrum of PTA/Si-imid@ Si-MNPs.

H. Hamadi et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 25–29
27
Scheme 2. Schematic representation of the formation of PTA/Si-imid@ Si-MNPs.
3. Results and discussion
2ꢀ = 18–28◦, could be assigned to an amorphous silica phase in the
shell of CoFe2O4. There are, however, no characteristic peaks of
PTA in the XRD pattern of PTA/Si-imid@ Si-MNPs. This indicates
that PTA species are well-dispersed on the surface of Si-imid@ Si-
MNPs and probably there is no crystalline phase of this heteropoly
acid to be detected by XRD analysis [26,27]. The crystallite sizes
of cobalt ferrite MNPs, Si-MNPs and PTA/Si-imid@ Si-MNPs esti-
mated using the well-known Scherer formula [28] were found to
be 25.5 nm, 34.7 nm and 34.9 nm respectively. The growth of mag-
netic nanoparticles on coating with silica was also occurred in the
previously reported composites [29–31]. No explanation was given
for this observation in any of the reported works; however, this
increase might be due to the strong interaction of Si O groups with
the metal cations on the surface of the MNPs which in turn brings
the crystallites closer to each other and fuse them to somewhat
larger particles.
3.1. Characterization of PTA/Si-imid@ Si-MNPs
Immobilization of PTA on imidazole functionalized MNPs com-
bines the advantages of ionic liquids with those of heterogeneous
catalysts [23]. The PTA moiety in the synthesized catalyst is bonded
strongly to Si-imid@ Si-MNPs by means of ionic interaction. The as-
synthesized PTA/Si-imid@ Si-MNPs catalyst was characterized by
various techniques. The FT-IR spectrum of PTA/Si-imid@ Si-MNPs,
shows weak peaks in the range between 1400 and 1630 cm−1
assigned to the imidazole ring and two more bands at 890 cm−1
and 981 cm−1 attributed to the immobilized PTA [24]. In addition,
the characteristic peaks of Fe O at 588 cm−1 [25] and Si
O Si at
1079 cm−1 and 808 cm−1 [24] were also observed (Fig. 1).
The X-ray diffraction patterns of MNPs, Si-MNPs and PTA/Si-
imid@ Si-MNPs are shown in Fig. 2. The peaks are compatible with
pure CoFe2O4 phase (JCPDS PDF #221086), indicating the reten-
tion of cubic reverse spinel structure of CoFe2O4 during coating
and functionalization. A weak broad bump appeared in the pat-
terns of Si-MNPs and PTA/Si-imid@ Si-MNPs (Fig. 2b and c), at
The TEM image of PTA/Si-imid@ Si-MNPs is presented in Fig. 3,
showing that most of the particles have quasi-spherical shape. The
Fig. 2. XRD patterns of (a) MNPs, (b) Si-MNPs and (c) PTA/Si-imid@ Si-MNPs.
Fig. 3. TEM image of PTA/Si-imid@ Si-MNPs.

28
H. Hamadi et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 25–29
Table 1
Effect of various catalysts and solvents on the synthesis of ␣-aminophosphonates.
Entry
Catalyst
Solvent
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
PTA/Si-imid@SiMNPs
PTA/Si-imid@SiMNPs
PTA/Si-imid@SiMNPs
PTA/Si-imid@SiMNPs
SiMNPs
CH3CN
CH2Cl2
EtOH
(CH3)2CO
Neat
20
30
45
80
85
75
60
30
74
95
45
120
120
15
Si-imid@SiMNPs
PTA/Si-imid@SiMNPs
Neat
Neat
a
All reaction conditions: benzaldehyde (1 mmol), aniline (1 mmol), P(OEt)3
(1.2 mmol), catalyst (0.05 g) at room temperature.
b
Isolated yield.
3.2. ˛-Aminophosphonates synthesis
In order to optimize the reaction conditions of ␣-
aminophosphonates synthesis, various experiments were carried
out at room temperature using benzaldehyde, aniline and tri-
ethylphosphate substrates in the presence of PTA/Si-imid@
Si-MNPs as catalyst. Firstly, we screened different solvents such
as CH3CN, acetone, CH2Cl2, and EtOH to find out the effect of
solvent on the progress of reaction. The results are given in Table 1.
When this reaction was carried out under solvent-free conditions,
however, much better results were obtained (see Table 1).
Secondly, to see the effectiveness of PTA species on this catalysis
reaction, we have also examined Si-MNPs and Si-imid@ Si-MNPs in
addition to PTA/Si-imid@ Si-MNPs as catalysts in separate experi-
ments. As seen in Table 1, PTA/Si-imid@ Si-MNPs showed the best
catalytic activity for the ␣-aminophosphonates synthesis.
Fig. 4. Hysteresis loops of (a) MNPs, (b) Si-MNPs and (c) PTA/Si-imid@ Si-MNPs.
average size of these nanoparticles is in the range of 20–30 nm
which shows a close agreement with the values calculated by XRD
data. Interestingly, the magnetic core is visible as a dark spot inside
the bright spherical SiO2 thin shell in the TEM image of PTA/Si-
imid@ Si-MNPs sample.
Magnetic measurements for MNPs, Si-MNPs and PTA/Si-imid@
Si-MNPs were performed using a vibrating sample magnetome-
ter (VSM) with a peak field of 8 kOe and their hysteresis curves
are presented in Fig. 4. It could be seen from the loops in Fig. 4
that saturation magnetization (MS) of MNPs, Si-MNPs and PTA/Si-
imid@ Si-MNPs are 59.42, 38.93 and 21.84 emu g−1 respectively.
The decrease in mass saturation magnetization may be ascribed to
the contribution of the non-magnetic silica shell and functionalized
groups. Interestingly, although the MS values of the silica coated
and functionalized MNPs samples have remarkably decreased, they
still could be efficiently separated from solution with a permanent
magnet.
In order to explore the scope and the limitations of this novel cat-
alytic method of ␣-aminophosphonates synthesis, we investigated
various aromatic aldehydes and amines containing either electron
withdrawing or electron donating functional groups with P(OEt)3
and P(OMe)3 under solvent-free conditions. The given results in
Table 2
Synthesis of diversified ␣-aminophosphonates in the presence of PTA/Si-imid@ Si-
MNPs.a
There are two weight loss steps in TGA curve of PTA/Si-imid@
Si-MNP catalyst (Fig. 5). The first mass loss of 1.7% (between 60 and
218 ◦C) may be due to removal of surface adsorbed water of the cat-
alyst. The loss of weight at temperatures higher than 218 ◦C can be
ascribed to the decomposition of Si-imid groups. The TGA analysis
of the as-prepared catalyst showed that the loading of immida-
zolium moiety about 0.36 mmol/g. Moreover, the tungsten content
of the catalyst, as determined by ICP-AES, was 0.98 mmol/g. This
is another proof for the fact that PTA was immobilized onto the
imidazole functionalized silica coated CoFe2O4.
Entry
Aldehyde
R1 (amine)
R2
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
H
H
H
H
H
H
H
H
2-Br
4-F
4-OMe
4-NO2
H
H
4-Br
4-F
4-OMe
H
H
4-Br
4-F
4-OMe
H
H
4-Br
4-F
2-Br
H
H
2-Aminopyridine
H
2-Aminopyridine
Me
Et
Et
Et
Et
Me
Me
Et
Et
Et
Et
Me
Et
Et
Et
Et
Me
Et
Et
Et
15
15
20
35
30
30
20
20
30
25
20
20
30
30
20
20
35
20
25
20
20
15
15
25
30
40
94
95
97
86
88
91
88
91
90
91
92
94
91
89
94
97
92
94
90
87
92
97
96
78
81
68
3-OMe
3-OMe
3-OMe
3-OMe
3-OMe
2-Br
2-Br
2-Br
2-Br
2-Br
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-NO2
4-NO2
4-NO2
Et
Me
Et
Me
Me
Me
Furfural
Furfural
a
All reaction condition: aldehydes (1 mmol), amine (1 mmol), P(OMe)3/P(OEt)3
(1.2 mmol) and 0.05 g of PTA/Si-imid@ Si-MNPs at room temperature.
b
Isolated yields.
Fig. 5. TGA of PTA/Si-imid@ Si-MNPs.

H. Hamadi et al. / Journal of Molecular Catalysis A: Chemical 373 (2013) 25–29
29
4. Conclusions
In summary, we have successfully developed a novel type of
non-covalently immobilized H3PW12O40 catalyst using surface-
modified CoFe2O4 magnetic nanoparticles as support. The
synthesized catalyst was confirmed by XRD, FT-IR, TGA, TEM, ICP-
AES, and VSM techniques. The immobilized phosphotungstic acid
was shown to be an efficient heterogeneous catalyst for synthesis of
␣-aminophosphonates under solvent-free conditions at room tem-
perature. Moreover, the immobilized PTA catalyst could be easily
recovered by simple magnetic decantation and reused at least five
times without significant loss of activity.
Acknowledgment
The authors wish to acknowledge the support of this work by
the Research Council of Shahid Chamran University, Ahvaz, Iran.
Fig. 6. Catalyst recycling experiments for ␣-aminophosphonates synthesis.
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To evaluate the stability and level of reusability of the catalyst,
we conducted experiments of ␣-aminophosphonates synthesis
using the recycled PTA/Si-imid@ Si-MNPs catalyst. After the com-
pletion of the first reaction, the catalyst was washed with CH3Cl
and the solution was removed by magnetic decantation. The left
used catalyst was washed with methanol and CHCl3 and dried.
A new reaction was then conducted with fresh reactants under
similar conditions. It was found that the developed catalyst could
be used at least five times without any change in reactivity (see
Fig. 6). ICP-AES analysis has shown that slight leaching of the cat-
alyst from support (0.92 wt%) occurred only in the first run and
no leaching was observed in the next runs. Moreover, the FT-
IR spectrum of the recovered catalyst showed no change after
using it for five times. This indicates that no significant leaching
of the PTA species occurred from support on using and reusing the
catalyst.