[PZnMo2W9O39]5- immobilized on ionic liquid-modified silica as a heterogeneous catalyst for epoxidation of olefins with hydrogen peroxide

Article abstract of DOI:10.1016/j.crci.2012.06.002

The epoxidation of alkenes with hydrogen peroxide catalyzed by [PZnMo ₂W₉O₃₉]^5-, ZnPOM, supported on ionic liquid-modified silica, Im-SiO₂, is reported. The immobilized catalyst, [ZnPOM@Im-SiO₂] was characterized by elemental analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), FT-IR and UV-Vis spectroscopic methods. This new synthesized hybrid catalyst was applied for efficient epoxidation of various olefins with aqueous H₂O₂ in acetonitrile under reflux conditions. This solid catalyst can be easily recovered by simple filtration and reused several times without significant loss of its catalytic activity.

Full text of DOI:10.1016/j.crci.2012.06.002

C. R. Chimie 15 (2012) 975–979
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´
Full paper/Memoire
[PZnMo2W9O39]5À immobilized on ionic liquid-modified silica as a
heterogeneous catalyst for epoxidation of olefins with hydrogen peroxide
*
*
*
Robabeh Hajian, Shahram Tangestaninejad , Majid Moghadam , Valiollah Mirkhani ,
Ahmad Reza Khosropour
Department of Chemistry, Catalysis Division, University of Isfahan, 81746 73441 Isfahan, Iran
A B S T R A C T
A R T I C L E I N F O
5-
Article history:
The epoxidation of alkenes with hydrogen peroxide catalyzed by [PZnMo2W9O39
] ,
Received 20 November 2011
Accepted after revision 6 June 2012
Available online 2 August 2012
ZnPOM, supported on ionic liquid-modified silica, Im-SiO2, is reported. The immobilized
catalyst, [ZnPOM@Im-SiO2] was characterized by elemental analysis, X-ray diffraction
(XRD), scanning electron microscopy (SEM), FT-IR and UV–Vis spectroscopic methods. This
new synthesized hybrid catalyst was applied for efficient epoxidation of various olefins
with aqueous H2O2 in acetonitrile under reflux conditions. This solid catalyst can be easily
recovered by simple filtration and reused several times without significant loss of its
catalytic activity.
Keywords:
Immobilized ionic liquid
Hydrogen peroxide
Epoxidation
ß 2012 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
Heterogeneous catalyst
5-
[PZnMo2W9O39
]
1. Introduction
polyoxometalates can act as effective catalysts for H2O2-
5À
based epoxidation [20–23]. The [PZnMo2W9O39
]
Epoxidation of olefins is an important class of industrial
reactions since epoxides are used as chemical intermedi-
ates [1,2]. Because of the unique properties of polyox-
ometalates (POMs) such as tunable size, shape, charge
density, acidity, redox potential, stability and solubility,
these compounds have attracted much attention for many
years [3–5]. These compounds have many applications in
electrochemistry [6], catalysis [7–9], medicine [10], and
material science [11]. An important subclasses of POM
clusters include lacunary POMs in which one or more
metal atoms are substituted by low valent-transition-
metals [12]. These compounds so called transition-metal-
substituted polyoxometalates (TMSPs), which have been
used as oxidation catalysts [13–19]. Keggin-type polyox-
ometalates such as Zn, Ti, Fe, and Mn-substituted
(ZnPOM) has been reported to be an active catalyst for
epoxidation although its stability has not been clear
[20,24]. Although most current uses of TMSPs are in
homogeneous catalysis [12–19], but heterogeneous cata-
lysts incorporated the properties of both the polyoxome-
talate (oxidation catalysis, ion-exchange) and a high
surface area support. In addition, they can be easily
separated from the reaction mixture and are of high
stability and reusability [25–29]. Ionic liquids (ILs) have
many applications; such as high thermal and chemical
stability, high polarity, non-flammability and tunable
acidity make them suitable reaction media for a broad
range of catalytic applications [30]. Supported ionic liquid
catalysis is a concept that combines the advantages of ionic
liquids with those of heterogeneous catalysts [31]. First,
Davis recognized that functionalized ionic liquids can
serve, not only as reaction media, but as catalyst as well
[32,33]. Ionic liquid-modified supported catalysts provide
the hydrophobic environment for organic reactions
[34–36].
* Corresponding authors.
E-mail addresses: stanges@sci.ui.ac.ir (S. Tangestaninejad), mogha-
damm@sci.ui.ac.ir (M. Moghadam), mirkhani@sci.ui.ac.ir (V. Mirkhani).
1631-0748/$ – see front matter ß 2012 Published by Elsevier Masson SAS on behalf of Acade´mie des sciences.
http://dx.doi.org/10.1016/j.crci.2012.06.002

976
R. Hajian et al. / C. R. Chimie 15 (2012) 975–979
O
H2O2, CH3CN
[PZnMoW@SiO2-Im], reflux
Scheme 1. Epoxidation of alkenes with hydrogen peroxide catalyzed by
[PZnMo2W9O39]5- immobilized on ionic liquid-modified silica.
In this work, the catalytic activity of [(n-Bu4N)5PZn-
Mo2W9O39] supported on ionic liquid-modified silica, Im-
SiO2, in the alkene epoxidation with aqueous H2O2 is
reported (Scheme 1).
Fig. 1. FT-IR spectra of: (a) SiO2-Im and (b) [ZnPOM@Im-SiO2].
2. Experimental
2.1. Materials
2.2. Preparation of ZnPOM immobilized on ionic liquid-
modified silica [ZnPOM@Im-SiO2-Im]
All chemicals were of commercial reagent grade and
were used without further purification. Hydrogen
peroxide (30%) was titrated by
a standard KMnO4
To a solution of [(n-Bu4N)5PZnMo2W9O39] (1 g) in
acetonitrile, SiO2-Im (4 g) was added and refluxed for
24 hours. Finally, the catalyst was filtered and dried at
room temperature.
`
solution. Polyoxometalate and SıO2-Im were prepared
according to the literature [24,35]. The prepared catalyst
was characterized by elemental analysis, X-ray diffrac-
tion (XRD), scanning electron microscopy (SEM), FT-IR
and UV–Vis spectroscopic methods. FT-IR spectra were
obtained as potassium bromide pellets in the range 400–
4000 cmÀ1 with a Jasco Impact 6300D spectrometer.
Scanning electron micrographs of the catalyst and
support were taken on a SEM Philips XL 20 instrument.
UV–Vis spectra were obtained in the range 200-
800 cmÀ1 with a Jasco V-670 spectrometer. Powder X-
ray diffraction patterns were obtained on a D8 Advanced
2.3. General procedure for epoxidation of alkenes with H2O2
catalyzed by [PZnMoW@SiO2-Im] under reflux conditions
In a 25 mL round-bottom flask equipped with
a
magnetic stirring bar, to a mixture of alkene (0.5 mmol),
and the [ZnPOM@Im-SiO2] (200 mg, 0.01 mmol) in
acetonitrile (10 mL) was added H2O2 (30%, 0.5 ml) and
refluxed. The progress of the reactions was monitored by
GC. At the end of the reaction, the catalyst was filtered
and washed with Et2O (20 ml). The organic layer was
separated, concentrated and after chromatography on a
short column of silica gel, the pure product was
obtained. IR and 1H NMR spectral data confirmed the
identities of the products.
Bruker using Cu K radiation (2
u
= 5–60ꢀ). Gas chroma-
tography experiments (GC) were performed with
Shimadzu GC-16A instrument using 2 m column
packed with silicon DC-200 or Carbowax 20M. In GC
experiments, n-decane was used as internal standard. 1
a
a
a
H
NMR spectra were recorded on a Bruker-Avance AQS
300 MHz.
Scheme 2. The preparation route for catalyst.

R. Hajian et al. / C. R. Chimie 15 (2012) 975–979
977
`
Fig. 2. XRD patterns of: (A) SıO2-Im; (B) ZnPOM and (C) [ZnPOM@Im-
SiO2] composite.
3. Results and discussion
Fig. 3. DR UV–Vis spectra of: (A) SiO2-Im and (B) [ZnPOM@Im-SiO2].
3.1. Preparation and characterization of catalyst,
ZnPOM@Im-SiO2
ZnPOM and [ZnPOM@Im-SiO2] are shown in Fig. 2. From
these patterns, it is clear that PVMo has been introduced in
The preparation route for the catalyst is shown in
Scheme 2. First 3-chloropropyl functionalized silica, SiO2,
was reacted with N-methylimidazole at 80 8C to prepare
the supported ionic liquid. The IR spectrum of SiO2-Im
(Fig. 1a) showed the characteristic band of the parent ionic
`
the substitutional position of SıO2-Im. Diffuse reflectance
(DR) UV–Vis spectra of the ZnPOM showed a broad
absorption peak at 258 nm which can be attributed to a
ligand metal charge transfer (LMCT). This absorption band
presents in the UV–Vis spectra of [ZnPOM@Im-SiO2]
liquid (
vibration of the polyoxometalates appear at 1057 (
953 ( M=O), 889 ( M–Ob–M) and 807 ( M–Oc–M) (Ob: corner-
n
C=N) at 1638 cmÀ1. The four typical skeletal
P-O),
n
(Fig. 3B) and since pure Sı
`O -Im shows no UV absorption
2
n
n
n
peak in this area (Fig. 3A), it is clear that ZnPOM has been
supported on the Im-SiO2. The SEM images of SiO2-Im and
[ZnPOM@Im-SiO2] are shown in Fig. 4. The clear change in
the morphology of catalysts indicated that the PZnMoW
has been supported on the SiO2-Im.
sharing; Oc: edge-sharing), respectively [24]. After an ion-
exchange with polyoxometalate anion, owing to the
overlap of P–O band with those of Si-O-Si stretching
vibrations of silica gel, only the bands corresponding to
Mo-Ob-Mo (887 cmÀ1) and Mo-Oc-Mo (807 cmÀ1) and
M = O (946 cmÀ1) are observed in the supported catalyst
(Fig 1b). The FT-IR spectra indicated that ZnPOM had been
successfully supported on ionic liquid-modified SiO2. The
support and the catalyst were characterized by elemental
analysis. The nitrogen content of the SiO2-Im was about
0.86%. According to this value, the amount of the ionic
liquid fragment, which was attached to SiO2, was
0.31 mmol/g of support. The amounts of Mo and W were
measured by ICP. Both values showed that the catalyst
loading is about 0.05 mmol/g. The XRD patterns of Im-SiO2,
3.2. Catalytic activity
The synthesized catalyst [ZnPOM@Im-SiO2] was used in
the oxidation of various alkenes with aqueous H2O2. First,
the effect of different solvents such as dichloromethane,
chloroform, acetonitrile, acetone and methanol were
checked on the epoxidation of cis-cyclooctene. Amongst
them, acetonitrile was chosen as reaction media because
the highest epoxide yield was observed (Table 1). The
catalytic activity of this catalyst was investigated in the
Fig. 4. Scanning electron micrographs of: (A) SiO2-Im and (B) [ZnPOM@Im-SiO2].

978
R. Hajian et al. / C. R. Chimie 15 (2012) 975–979
Table 1
The effect of solvent on the epoxidation of cis-cyclooctene catalyzed by
[ZnPOM@Im-SiO2] under reflux conditions.a
Entry
Solvent
Epoxide yield (%)b
Time (h)
1
2
3
4
5
CH2Cl2
CHCl3
8
7
2
2
2
2
2
(CH3)2CO
CH3OH
CH3CN
50
42
98
a
Reaction conditions: cis-cyclooctene (0.5 mmol), catalyst (200 mg,
0.01 mmol), solvent(10 mL), H2O2 (0.5 ml).
b
GC yield based on the starting cyclooctene.
epoxidation of different olefins (Table 2). The reactions
were continued until no further progress was observed. In
these reactions, cis-cyclooctene, cyclohexene, 1-heptene
and 1-octene were completely converted to their corre-
sponding epoxides with 100% selectivity. In the case of
indene, the major product was epoxide in 90% yield, and
indene-1-one was produced as by-product. In the oxida-
Fig. 5. The results obtained from catalyst reuse and stability in the
oxidation of cyclooctene with H2O2 by [ZnPOM@Im-SiO2] under reflux
conditions.
from the reaction mixture after each experiment, washed
thoroughly with acetonitrile and n-hexane successively and
dried before being used in the subsequent run. The results of
the recycling experiments indicated that the yields reduced
in three first runs and after then no significant loss of its
catalytic activity was observed (Fig. 5). The filtrates were
used for determination of the catalyst leaching by atomic
absorption spectroscopy (AAS). The results showed that in
two first runs some catalyst is leached from support (4.45%
in the first run and 0.74% in the second run), but in the next
runs no leaching was observed. The FT-IR spectroscopy of
the recovered catalyst after using for 10 times showed no
changeinits IRspectra(Fig. 6), whichconfirmedthefactthat
the catalytic activity of the catalyst did not change
significantly compared to fresh catalyst.
tion of
a-pinene, the major product was a-pinene oxide
(60%), while verbenone and verbenol were produced as
minor products. Control experiments in the absence of the
catalyst were also investigated and it was observed that
the amount of product is negligible.
3.3. Catalyst reuse and stability
The stability of the [ZnPOM@Im-SiO2] catalyst was
monitored using multiple sequential oxidation of cyclo-
octene with hydrogen peroxide under reflux conditions. To
assess stability and reusability, the catalyst was separated
Table 2
Oxidation of alkenes with H2O2 catalyzed by [ZnPOM@Im-SiO2] under reflux conditions.a
Entry Substrate
Product
Time (h) Conversion Epoxide
TOF(hÀ1
24.5
)
(%)b
selectivity
(%)
1
2
98
100
O
2
3
5
90
100
90
9.0
O
1.5
100
33.3
O
O
4
5
2.5
100
60
20
O
O
OH
5
5
55
50
100
100
5.5
5.0
O
6
O
a
Reaction conditions: alkene (0.5 mmol), H2O2 (0.5 ml), catalyst (200 mg, 0.01 mmol), acetonitrile (10 ml).
b
GC yield based on the starting alkene.

R. Hajian et al. / C. R. Chimie 15 (2012) 975–979
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