Phosphotungstic acid supported on magnetic nanoparticles as an efficient reusable catalyst for epoxidation of alkenes

Article abstract of DOI:10.1016/j.materresbull.2012.07.001

A new magnetically separable catalyst consisting of phosphotungstic acid 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). This immobilized phosphotungstic acid was shown to be an efficient heterogeneous catalyst for the epoxidation of various alkenes using tert-butylhydroperoxide (t-BuOOH) as oxidant. 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.materresbull.2012.07.001

Materials Research Bulletin 47 (2012) 3473–3478
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Phosphotungstic acid supported on magnetic nanoparticles as an efficient
reusable catalyst for epoxidation of alkenes
*
M. Kooti , M. Afshari
Department of Chemistry, College of Science, Shahid Chamran University, Ahvaz 61357- 43169, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A new magnetically separable catalyst consisting of phosphotungstic acid 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). This immobilized phosphotungstic
acid was shown to be an efficient heterogeneous catalyst for the epoxidation of various alkenes using
tert-butylhydroperoxide (t-BuOOH) as oxidant. The catalyst is readily recovered by simple magnetic
decantation and can be recycled several times with no significant loss of catalytic activity.
ß 2012 Elsevier Ltd. All rights reserved.
Received 13 March 2012
Received in revised form 26 June 2012
Accepted 4 July 2012
Available online 13 July 2012
Keywords:
A. Nanostructures
A. Magnetic materials
A. Composites
D. Catalytic properties
1. Introduction
The main advantage of a catalytic system based on magnetic
nanoparticles is that the nanoparticles can be efficiently isolated
Epoxidation of alkenes is an important reaction due to the wide
applications of epoxides in the production of various valuable
materials [1]. So far, many transition metal compounds have been
used as catalyst in the transformation of alkenes to epoxides.
Among them polyoxometalates (POMs), an immense class of
oxygen bridged metal cluster anions of mainly tungsten(VI) and
molybdenum(VI), are known as highly efficient green catalysts for
epoxidation reactions [2–4]. Despite of some advantages, there
exist a number of drawbacks about catalysis with POMs based
systems, such as high solubility in polar solvents and low surface
area. Therefore, in a homogeneous reaction the isolation of the
products and reuse of the catalyst become difficult. One of the most
promising solutions to these problems seems to be immobilization
of POMs onto solid support such as silica [5], zeolite [6], and
alumina [7]. While supporting of POMs on solid materials can
greatly enhance the catalytic activity, this approach still could not
avoid the separation difficulty of catalyst from solution media.
Therefore, further development of the catalytic system will require
advanced materials that can efficiently catalyze chemical reactions
and be recycled through simple separation processes.
from the product solution through a simple magnetic separation
process after completing the reactions. Therefore, catalytic
systems developed on MNPs supports have been successfully
used in catalyzing a wide range of organic reactions including C–C
coupling [11], hydrogenation [12], oxidation [13] and polymeriza-
tion [14]. The catalytic systems devised by employing super-
paramagnetic nanoparticles as a support and their performance in
catalyzing various organic reactions have been recently discussed
in two excellent reviews [15,16].
Among various MNPs, CoFe2O4 has gained extensive attention
due to its prominent chemical stability and other interesting
properties [17–19]. However, cobalt ferrite and other MNPs tend to
aggregate in liquid media due to their magnetic dipole–dipole
interaction. This will lead to the reduction of their surface area and
partial loss of magnetism. Therefore, some strategies have been
used to chemically stabilize the naked MNPs and prevent their
agglomeration in solutions, such as encapsulation them with
polymers [20,21] or hydroxyapatite [22] and coating with a thin
layer of silica. Silica has been considered to be one of the most ideal
materials for coating of MNPs due to its characteristic including
high chemical stability, biocompatibility, and easy functionaliza-
tion [23–25].
Magnetic nanoparticles (MNPs) have recently emerged as
attractive materials either as catalysts or support for immobiliza-
tion of homogeneous catalysts [8–10].
There are many reports on the preparation and applications of
silica coated magnetic nanoparticles [26–28]. However, to the best
of our knowledge, only a few reports were devoted to the grafting
of POMs on MNPs supports [29–31]. In the present work, a novel
method for immobilization of phosphotungstic acid (PTA) on
imidazole-functionalized silica coated cobalt ferrite nanoparticles
* Corresponding author. Tel.: +98 916 1115451; fax: +98 611 3331042.
E-mail address: m_kooti@scu.ac.ir (M. Kooti).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.07.001

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M. Kooti, M. Afshari / Materials Research Bulletin 47 (2012) 3473–3478
is reported. The prepared composite was successfully used as
catalyst toward epoxidation of some alkenes in the presence of t-
BuOOH.
this solution and the whole mixture was refluxed overnight. After
that, the expected final product, PTA/Si–imid@ Si–MNPs, was
separated by magnetic decantation and washed with acetonitrile
and dichloromethane and left to dry in a desiccator.
2. Experimental
2.3. Catalytic studies
2.1. General
In a typical run, a 25 mL round-bottom flask equipped with a
condenser and magnetic stirrer, was charged with: PTA/Si–imid@
Si–MNPs catalyst (0.1 g), 1,2-dichloroethane (5 mL), alkene
(1 mmol), and t-BuOOH (2 mmol). This mixture was heated in
an oil bath at 70 8C and the progress of the reaction was monitored
by GC. At the end of the reaction, CH2Cl2 was added to dilute the
reaction mixture and the organic layer was simply decanted by
means of an external magnet. The decanted solution was purified
on a silica-gel plate or a silica-gel column to obtain the pure
product. The identities of the products were confirmed by FT-IR
and 1H NMR spectral data.
All chemicals were purchased from Sigma–Aldrich or Merck
and used as received. Phosphotungstic acid (PTA) was synthesized
according to the previously reported method [32]. 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 the
studied MNPs were measured with a vibrating sample magnetom-
eter (VSM), Meghnatis Daghigh Kavir Company at room tempera-
ture. GC experiments were performed with a Shimadzu GC-16A
instrument using a 2 m column packed with silicon DC-200 or
Carbowax 20 m. 1H NMR spectra of the epoxidation products were
recorded in CDCl3 on a Bruker Advanced DPX 400 MHz spectrom-
eter.
3. Results and discussion
3.1. Characterization of phosphotungstic acid supported on imidazole
silica-coated cobalt ferrite
2.2. Synthesis of phosphotungstic acid immobilized on imidazole
functionalized Si–MNPs
Immobilization of PTA on imidazole functionalized MNPs
combines the advantages of ionic liquids with those of heteroge-
neous catalysts [35]. The as-prepared ionic liquid-modified
Cobalt ferrite MNPs were prepared using the procedure
reported by Maaz et al. [33]. Silica coated CoFe2O4 nanoparticles
(Si–MNPs) were made by using sol–gel method [34]. A schematic
representation for the synthesis of PTA anchored on imidazole
functionalized silica coated cobalt ferrite nanoparticles is shown in
Scheme 1. In the first step, a mixture of 3-chloromethoxypro-
pylsilane (4.4 mL, 24 mmol) and imidazole (1.63 g, 24 mmol) was
heated at 110 8C for 24 h with continuous stirring under N2
atmosphere. The product (Si–imid) of this step was dissolved in
25 mL of ethanol. After addition of Si–MNPs (0.75 g) to the
ethanolic solution, the mixture was refluxed for 24 h. The produced
imidazole functionalized silica coated magnetic nanoparticles (Si–
imid@ Si–MNPs) were magnetically separated and washed twice
with ethanol and ether. Finally, Si–imid@ Si–MNPs were dispersed
into 30 mL of acetonitrile, PTA (1.56 g, 0.54 mmol) was added to
supported catalyst provides
a hydrophobic environment for
organic reactions, as reported by other research groups [36–38].
The PTA moiety is bonded strongly to Si–imid@ Si–MNPs by means
of ionic interaction which prevents/reduces leaching of the catalyst
in polar solvents [39]. The PTA/Si–imid@ Si–MNPs catalyst was
synthesized by a multi-step procedure, as shown in Scheme 1, and
characterized by various techniques.
The FT-IR spectrum of MNPs (Fig. 1A) shows the expected Fe–O
stretching absorption at 577 cmÀ1 which shifts to high wave
numbers of 588.9 cmÀ1 after coating with silica [40]. Two new
bands at 1088 cmÀ1 and 802 cmÀ1 were observed which are
ascribed to the symmetrical and asymmetrical vibrations of the Si–
O–Si bonds [41]. The FT-IR spectrum of PTA/Si–imid@ Si–MNPs
Scheme 1. Schematic representation of the formation of PTA/Si–imid@ Si–MNPs catalyst.

M. Kooti, M. Afshari / Materials Research Bulletin 47 (2012) 3473–3478
3475
Fig. 4. Hysteresis loops of (A) MNPs, (B) Si–MNPs and (C) PTA/Si–imid@ Si–MNPs.
Fig. 1. FT-IR spectra of (A) MNPs, (B) Si–MNPs, (C) neat PTA and (D) PTA/Si–imid@
Si–MNPs.
Fig. 5. TGA of PTA/Si–imid@ Si–MNPs.
(Fig. 1D) shows weak peaks in the range between 1400 and
1630 cmÀ1 assigned to the imidazole ring [42]. Two other bands at
890 cmÀ1 and 981 cmÀ1 are also observed in this spectrum, which
are respectively attributed to W–O–W and W55O stretching modes
of PTA [39,42].
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 retention
of cubic reverse spinel structure of CoFe2O4 after coating and
functionalization. A weak broad hump appeared in the spectra of
Si–MNPs and PTA/Si–imid@ Si–MNPs (Fig. 2B and C), at 2u = 18–
288, could be assigned to an amorphous silica phase in the shell of
CoFe2O4 nanoparticles. Moreover, there are no characteristic peaks
Fig. 2. XRD pattern of (A) MNPs, (B) Si–MNPs and (C) PTA/Si–imid@ Si–MNPs.
Fig. 3. TEM images of (A) MNPs, (B) Si–MNPs and (C) PTA/Si–imid@ Si–MNPs.

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M. Kooti, M. Afshari / Materials Research Bulletin 47 (2012) 3473–3478
Table 1
Oxidation of alkenes by t-BuOOH using PTA/Si–imid@ Si–MNPs catalyst.a
Entry
1c
Alkenes
Product
Conversion (%)b
95
Selectivity (%)b
90
O
2c
80
95
90
92
O
3c
O
O
O
4c
5c
90
95
95
95
O
O
6c
7c
70
65
95
90
O
O
8d
45
–
80
–
CHO
O
9e
10f
42
–
65
–
O
11g
O
O
12h
91
90
a
Reaction conditions: alkene (1 mmol); t-BuOOH (2 mmol); catalyst (0.1 g); 1,2-dichloroethane (5 mL); T: 70 8C; time: 6 h.
b
c
d
e
f
GC yields based on starting alkenes.
Catalyst: PTA/Si–imid@ Si–MNPs.
Catalyst: MNPs.
Catalyst: Si–MNPs.
Catalyst: PTA.
g
h
Catalyst: Si–imid@ Si–MNPs.
Fifth recycled catalyst.
of PTA in the XRD spectrum of PTA/Si–imid@ Si–MNPs. This
the TEM images of Si–MNPs and PTA/Si–imid@ Si–MNPs samples
(see Fig. 3B and C).
indicates that PTA species are well-dispersed in the surface of the
functionalized MNPs and probably there is no crystalline phase of
this heteropoly acid to be detected by XRD analysis [43,44].
The TEM images of MNPs, Si–MNPs and PTA/Si–imid@ Si–MNPs
are presented in Fig. 3, which reveal that most of the particles have
quasi-spherical shape. The average size of these nanoparticles is in
the range of 20–30 nm which show a close agreement with the
values calculated by XRD analysis. Interestingly, the magnetic core
is visible as a dark spot inside the bright spherical SiO2 thin shell in
Magnetic measurements for MNPs, Si–MNPs and PTA/Si–imid@
Si–MNPs samples were performed using a vibrating sample
magnetometer (VSM) with a peak field of 8 kOe and their
hysteresis curves are shown 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, respectively. The decrease in mass saturation magnetization in
the last two composites is ascribed to the contribution of the

M. Kooti, M. Afshari / Materials Research Bulletin 47 (2012) 3473–3478
3477
Scheme 2. Oxidation of alkenes using PTA/Si–imid@ Si–MNPs catalyst.
non-magnetic silica shell and functionalized groups. Although the
3.3. Catalyst recycling
MS values of the silica coated and functionalized MNPs samples
have evidently decreased, they still could be efficiently separated
from solution media with a permanent magnet.
Catalyst reusability is of major importance in heterogeneous
catalysis. The recovery and reusability of the catalyst was studied
using styrene as model substrate. Catalyst recycling experiments
were achieved by fixing the catalyst magnetically at the bottom of
the flask and the solution was decanted after each run. The left
solid was washed with 1,2-dichloroethane twice, and fresh
substrate dissolved in 1,2-dichloroethane is introduced into the
flask, allowing the system to proceed for next run. The catalyst was
consecutively reused five times without any noticeable loss of its
catalytic activity. ICP-AES analysis has shown that leaching of the
catalyst from support (1.2% W) occurred only in the first run and no
leaching was observed in the next runs. Furthermore, the FT-IR
spectrum of the recovered catalyst showed no change after using
the catalyst for five successive times. This catalyst has some
remarkable features including ease of separation by an external
magnet and in contrast to other reported similar catalysts; no
regeneration process is required after each reaction [29].
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 8C) may be due to removal of surface adsorbed water of the
catalyst. The loss of weight at temperatures higher than 218 8C can
be ascribed to the decomposition of Si–imid groups. The TGA
analysis of the as-prepared catalyst showed an immidazolium
moiety loading of approximately 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 nanopaticles.
3.2. Oxidation of alkenes
Styrene was examined as a substrate model to optimize
reaction conditions using t-BuOOH as oxidant in the presence of
catalytic amount of PTA/Si–imid@ Si–MNPs. The obtained opti-
mum conditions are: catalyst 0.1 g, oxidant and substrate in a
molar ratio of 2:1, respectively. The reaction temperature was set
at 70 8C, since a very low conversion of styrene is observed at room
temperature. Various solvents, including acetone, chloroform,
acetonitrile, and 1,2-dichloroethane were examined to find out the
appropriate solvent and 1,2-dichloroethane gave the best results.
Besides t-BuOOH other oxidants such as H2O2 and H2O2/urea
(UHP) were also screened for the epoxidation of styrene, but t-
BuOOH was found to be the best source of oxygen. It was also found
that no epoxidation reaction of alkenes occurred in the absence of
catalyst or oxidant. Moreover, we have also examined bare
CoFe2O4, silica coated CoFe2O4, neat PTA and Si–imid@ Si–MNPs
for the epoxidation of styrene. As it seen in Table 1, bare CoFe2O4
can oxidize styrene to benzaldehyde with relatively good
conversion. However, CoFe2O4 loses its catalytic activity and
becoming inert surface after coating with silica. Also, according to
the results of Table 1, neat H3PW12O40 (PTA) shows much lower
catalytic activity (42% conversion and 65% selectivity) in the
epoxidation reaction of styrene compared with PTA/Si–imid@ Si–
MNPs catalyst. Therefore, the high catalytic activity of PTA
immobilized on Si–imid@ Si–MNPs catalyst can be attributed to
the nanosized character of the support which increases surface
area for more interaction with substrates.
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 as-
synthesized catalyst was confirmed by XRD, FT-IR, TGA, TEM, ICP-
AES, and VSM techniques. This catalyst was found to epoxidize
some alkenes efficiently in the presence of t-BuOOH as oxidant.
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.
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