Synthesis and characterization of supported heteropolymolybdate nanoparticles between silicate layers of Bentonite with enhanced catalytic activity for epoxidation of alkenes

Article abstract of DOI:10.1016/j.materresbull.2011.07.037

A new heterogeneous catalyst (PVMo/Bentonite) consisting of vanadium substituted heteropolymolybdate with Keggin-type structure Na ₅[PV₂Mo₁₀O₄₀]·14H ₂O (PVMo) supported between silicate layers of bentonite has been synthesized by impregnation method and characterized using X-ray diffraction, Fourier-transformed infrared spectroscopy, scanning electron microscopy, UV-vis diffuse reflectance spectroscopy, transmission electron microscopy and elemental analysis. X-ray diffraction and scanning electron microscopy analysis indicated that PVMo was finely dispersed into layers of bentonite as support. The PVMo/Bentonite used as an efficient heterogeneous catalyst for epoxidation of alkenes. Various cyclic and linear alkenes were oxidized into the corresponding epoxides in high yields and selectivity with 30% aqueous H₂O ₂. The catalyst was reused several times, without observable loss of activity and selectivity. The obtained results showed that the catalytic activity of the PVMo/Bentonite was higher than that of pure heteropolyanion (PVMo).

Full text of DOI:10.1016/j.materresbull.2011.07.037

Materials Research Bulletin 46 (2011) 1853–1859
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Synthesis and characterization of supported heteropolymolybdate
nanoparticles between silicate layers of Bentonite with enhanced
catalytic activity for epoxidation of alkenes
*
Hossein Salavati , Nahid Rasouli
Department of Chemistry, Payame Noor University (PNU), 19395-4697, Tehran, Islamic Republic of Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A new heterogeneous catalyst (PVMo/Bentonite) consisting of vanadium substituted heteropolymo-
lybdate with Keggin-type structure Na5[PV2Mo10O40]ꢁ14H2O (PVMo) supported between silicate layers
of bentonite has been synthesized by impregnation method and characterized using X-ray diffraction,
Fourier-transformed infrared spectroscopy, scanning electron microscopy, UV–vis diffuse reflectance
spectroscopy, transmission electron microscopy and elemental analysis. X-ray diffraction and scanning
electron microscopy analysis indicated that PVMo was finely dispersed into layers of bentonite as
support. The PVMo/Bentonite used as an efficient heterogeneous catalyst for epoxidation of alkenes.
Various cyclic and linear alkenes were oxidized into the corresponding epoxides in high yields and
selectivity with 30% aqueous H2O2. The catalyst was reused several times, without observable loss of
activity and selectivity. The obtained results showed that the catalytic activity of the PVMo/Bentonite
was higher than that of pure heteropolyanion (PVMo).
Received 16 February 2011
Received in revised form 14 July 2011
Accepted 27 July 2011
Available online 5 August 2011
Keywords:
A. Composite
A. Layered compounds
A. Oxides
C. X- ray diffraction
ß 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Oxidation of organic compounds through ultrasonic irradiation
of liquids is due to acoustic cavitation. In the presence of dissolved
Polyoxometalates (POMs) have been widely studied in relation
to their expressive conductive, catalytic, antiviral, photochromic
and many other properties [1–3]. Unlike metal oxides and zeolites,
polyoxometalates possess a discrete and mobile ionic structure.
Moreover, POMs display strong Brønsted acidity and appropriate
redox properties, which can be tuned by varying the chemical
composition of the catalyst [4–6]. Among various structural
classes, Keggin-type polyoxometalates have received considerable
attention due to their acid–base and redox properties, and ease of
preparation [7]. POMs are soluble in water and polar solvents and
have low surface area. These disadvantages can be overcome by
incorporation of the polyoxometalate into various stable porous
supports by formation of different composites [8–18]. Among
these materials, clay is an excellent support due to its thermal
stability, relative high surface area, large pore volume and uniform
pore size distribution. Bentonite is a clay mineral, which is mainly
composed of montmorillonite with chemical composition of SiO2,
Al2O3, CaO, MgO, Fe2O3, Na2O and K2O. It is a 2:1 aluminosilicate,
the unit layer structure of which consists of one Al3+ octahedral
sheet between two Si4+ tetrahedral sheets [19].
gases and tiny particles with dissolved gases in their crevices,
ultrasound wave consisting of cycles of compression and expan-
sion produce cavitation bubbles. Cavitation bubbles are produced
when applied sound pressure overcomes surface tensile forces.
Once a cavitation bubble is formed in a liquid, it absorbs the
ultrasound energy until the bubble grows to a critical size where it
collapses violently during sound compression cycles. These
collapsing bubbles can have temperatures and pressures as high
as 5000 K and 1000 atm, respectively. Under such extreme
conditions during bubble collapse, organic compounds are
oxidized by direct pyrolysis and hydroxyl free radical oxidation
(8OH) reactions. With direct pyrolysis an organic compound
absorbed in a cavitation bubble is converted to by-products due to
the extremely high temperature and pressure inside the bubble.
Under these extreme conditions free radicals are also formed when
water molecules are split:
H2O ! Hꢀ
þ
ꢀOH
(1)
(2)
(3)
(4)
Hꢀ þ O2 ! HO2
ꢀ
ꢀ
ꢀ
HO2 þ HO2 ! H2O2 þ O2
* Corresponding author. Tel.: +98 311 7380003; fax: +98 311 7381002.
E-mail addresses: hosseinsalavati@yahoo.com, hosseinsalavati@pnu.ac.ir
(H. Salavati).
ꢀ
Hꢀ þ HO2 ! H2O2
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.07.037

1854
H. Salavati, N. Rasouli / Materials Research Bulletin 46 (2011) 1853–1859
2.4. General oxidation procedure catalyzed by PVMo/Bentonite
ꢀOH þ ꢀOH ! H2O2
(5)
(6)
(7)
nanocomposite under reflux
The catalytic reaction was performed in a 25 mL round-
bottomed flask equipped with a magnetic stirring bar and a reflux
condenser. In a typical experiment, hydrogen peroxide (30%)
(1 mL), acetonitrile (5 mL), alkene substrate (0.8 mmole) and
ꢀOH þ ꢀHO2 ! H2O þ O2
ꢀ
ꢀOH þ H2O2 ! HO2 þ H2O
catalyst (2.86
mmol of PVMo) were added in the flask. The reaction
These radicals can react with organic compounds to form
products. The main oxidation pathway is direct pyrolysis, but in
the presence of oxidants such as HO28 and H2O2, radicals produced
in implosive bubbles can also be an important reaction pathway
[20–22].
In the present study, vanadium substituted heteropolymolyb-
date supported into bentonite (PVMo/Bentonite nanocomposite)
was synthesized. To the best of our knowledge, this is the first
report about synthesis of heteropolymolybdate with layered
structure such as bentonite. The PVMo/Bentonite exhibited an
excellent catalytic activity for epoxidation of alkenes. Also, the
catalytic activity of the PVMo/Bentonite nanocomposite for
epoxidation of alkenes was improved as compared with the
unsupported heteropolymolybdate (PVMo).
was performed at 78 8C and was monitored by GC analysis. After
10 h, conversion of the substrate was measured by GC with the
internal standard method. At the end of reaction, the mixture was
diluted with Et2O (20 mL) and filtered. The catalyst was thoroughly
washed with Et2O and combined washings and filtrates were
purified on a silica gel plate or silica gel column. The catalyst alone
did not show any activity in the reactions studied.
2.5. General oxidation procedure catalyzed by PVMo/Bentonite
nanocomposite under ultrasonic irradiation
A UP 400S ultrasonic processor equipped with a 3 mm wide and
140 mm long probe, which was immersed directly into the
reaction mixture was used for sonication. The operating frequency
was 24 kHz and the output power was 0–400 W through manual
adjustment. The final volume of solution was 6 ml and the
temperature of the solution reached 60 8C during sonication.
To a mixture of alkene (0.8 mol) and the catalyst (containing
2. Experimental
2.1. Chemicals
2.86
mmol of PVMo) in acetonitrile (5 mL) was added H2O2 (1 mL,
All chemicals were of analytical grade and used without further
purification. The Na5[PV2Mo10O40]ꢁ14H2O (PVMo) was prepared as
described in the literature [23]. Also, The PVMo/Bentonite with 20%
loading of PVMo was prepared. The UV–vis spectrum of solution
before and after impregnation showed that more than 99% of PVMo
was supported on the bentonite.
30%) and the mixture was exposed to ultrasonic irradiation. The
reaction was monitored by GC. After the reaction was completed,
the reaction mixture was diluted with Et2O (20 mL) and filtered.
The catalyst was thoroughly washed with Et2O and the combined
washings and filtrates were purified on a silica gel plates or silica
gel column.
2.2. Preparation of the PVMo/Bentonite nanocomposite
3. Results and discussion
The typical preparation method of PVMo/Bentonite is as
follows: PVMo (1.0 g) and bentonite (4.0 g) were mixed and put
into an agate mortar. The mixture was thoroughly ground for
20 min, washed in a supersonic washing machine using absolute
alcohol as dispersant and centrifuged. The washing and centrifug-
ing processes were repeated five times. The wet PVMo/Bentonite
were obtained and dried at 60 8C.
3.1. Characterization of the PVMo/Bentonite nanocomposite
The size of the PVMo nanoparticles were calculated from the
data of broadened XRD peaks by the Scherrer equation: D = 0.89
l/
b
cos . The bentonite has pores, whose diameters range from 10
u
to 50 nm and the heteropolyanions of the Keggin type structure
have molecular diameter of ꢃ1–2 nm. Therefore, it is feasible to
insert polyoxometalates such as Na5[PV2Mo10O40] between the
silicate layers of bentonite [24,25]. Also, the presence of the
heteropolymolybdate (PVMo) was confirmed by X-ray diffraction.
As shows in Fig. 1, the insertion of heteropolymolybdate
nanoparticles into the silicate layers of bentonite would make
the later expand, which results in the shift of the according
diffraction peak to smaller angle in the XRD patterns (Bragg’s
equation), but due to the strong interactions between PVMo and
the bentonite surface, the XRD patterns showed little angle shift,
which the heteropolymolybdate (PVMo) exhibited typical peaks at
2.3. Catalyst characterization
Elemental analysis was performed on a Perkin–Elmer 2400
instrument. Atomic absorption analyses were carried out on a
Shimadzu 120 spectrophotometer. Compositional and elemental
investigations were performed by backscattered electrons and EDX
analysis, respectively. UV–vis diffuse reflectance spectra were
recorded on a Shimadzu UV-265 instrument using optical grade
BaSO4 as reference. FT-IR spectra were obtained as potassium
bromide pellets in the range 400–4000 cmꢂ1 with Nicollet-Impact
400D instrument. Scanning electron micrographs of the heteroge-
neous catalyst and support were taken on SEM Philips XL 30.
Powder X-ray diffraction data were obtained on a D8 Advanced
2u
: 8.898, 9.268, 10.158, 27.168 and 29.448 [15], while the as-
prepared PVMo/Bentonite composite showed peaks at 2 : 8.788,
u
26.728 and 28.888 that showed the PVMo was introduced in the
substitutional position of bentonite.
Bruker using Cu K radiation (2
experiments (GC) were performed on Shimadzu GC-16A
u
= 5–708). Gas chromatography
FT-IR spectrum of the PVMo/Bentonite catalyst in the range
700–1100 cmꢂ1 showed absorption bands at 1052, 952, 873 and
787 cmꢂ1, corresponding to the four typical skeletal vibrations of
the Keggin polyoxometalate (PVMo), which indicated that PVMo
has been supported into layeres of bentonite (Fig. 2). These peaks
a
a
instrument using a 2 m column packed with silicon DC-200 or
Carbowax 20 m. In all experiments the n-decane was used as the
internal standard. 1H NMR spectra were recorded on a Bruker–
Arance AQS 300 MHz using CDCl3 as solvent and tetramethylsilane
(TMS) as the internal reference. Conversions and yields were
obtained by GC experiments and the products were identified after
isolation and purification.
could be attributed to n(P55Oa), n(Mo55Ot), n(Mo–Ob–Mo) and n
(Mo–Oc–Mo), respectively (Ot = terminal oxygen, Ob = bridged
oxygen of two octahedral sharing a corner and Oc = bridged
oxygen sharing an edge) [25]. The FT-IR spectra indicated that the

H. Salavati, N. Rasouli / Materials Research Bulletin 46 (2011) 1853–1859
1855
Fig. 1. XRD patterns of: (A) Bentonite, (B) PVMo and (C) PVMo/Bentonite
nanocomposite.
Fig. 3. UV–vis (DRS) spectrum of: (A) PVMo and (B) PVMo/Bentonite
nanocomposite.
Fig. 2. FT-IR spectrum of: (A) PVMo/Bentonite composite and (B) recovered PVMo/
Bentonite nanocomposite.
structure of the polyoxometalate (PVMo) remains unchanged upon
impregnation method.
The UV–vis spectra of PVMo in CH3CN displayed absorption
bands at 246 and 308 nm, which are due to the Mo–O and V–O
charge-transfer absorptions in the heteropolycompound cage
[26,27]. These two absorption bands appeared in the diffuse
reflectance UV–vis spectrum of the heterogeneous catalyst and
since pure bentonite shows no UV absorption, then these results
indicated that PVMo was introduced into the layered framework of
bentonite (Fig. 3).
Fig. 4. SEM images of: (A) Bentonite and (B) PVMo/Bentonite nanocomposite.
PVMo/Bentonite nanocomposite indicated that the PVMo has been
supported into the layers of bentonite. TEM analysis was also
performed to determine the location and the particle size of the
PVMo/Bentonite which can be distinguished as dark dots in TEM
(Fig. 5). Also, the particle size obtained by this method was less
than 80 nm.
According to the SEM images, the particles are approximately
spherical and the average diameter of particles is estimated to be
about 50 nm (Fig. 4). A clear change in the morphology of the

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H. Salavati, N. Rasouli / Materials Research Bulletin 46 (2011) 1853–1859
clarify the catalytic active sites for epoxidation of alkenes, blank
reactions are carried out with bentonite support under identical
conditions. Also, a blank experiment in the absence of the catalyst
showed only small amounts of products in the epoxidation of
cyclooctene with H2O2. Epoxidation of cyclohexene with 80%
conversion produced cyclohexene oxide as major product (with
78% epoxide selectivity), and allylic oxidation products (cyclohex-
ene-1-one and cyclohexene-1-ol) were obtained as minor pro-
ducts. In the case of indene, styrene and
corresponding epoxides were obtained in 60%, 58% and 72%
epoxide selectivity (In the oxidation of -pinene the major product
was -pinene oxide, while verbenone and verbenol were also
a-methyl styrene, the
a
a
produced as minor products). Oxidation of linear alkenes such as 1-
octene and 1-dodecene was accompanied by allylic oxidation
(Table 1). As shown in Table 2, the behavior of the catalyst with
other oxidants reduced epoxide selectivity.
Fig. 5. TEM images of: PVMo/Bentonite nanocomposite.
3.1.2. Epoxidation of alkenes catalyzed by PVMo/Bentonite
nanocomposite under ultrasonic irradiation
3.1.1. Catalytic performance of the PVMo/Bentonite nanocomposite
At first, the ability of the prepared PVMo/Bentonite was
investigated in the epoxidation of cyclooctene as a model reaction
with H2O2 as oxidant in acetonitrile. The reactions were continued
until no further progress was observed. The results showed that
the conversion was 60% with 64% epoxide selectivity. In order to
Due to the effect of ultrasonic irradiation on the product yield
and selectivity, we decided to investigate the effect of ultrasonic
waves on the epoxidation of alkenes with H2O2 catalyzed by PVMo/
bentonite. As shown in Table 3, application of ultrasonic waves in
this catalytic system reduced the reaction times and improved the
Table 1
Epoxidation of alkenes with H2O2 catalyzed by PVMo/Bentonite nanocomposite under reflux conditions.a
Entry
1
Substrate
Products
Time (h)
Conversion (%)b,c
Epoxide selectivity (%)b
TOF (hꢂ1
)
OH
OH
10
60
64
16.8
O
OH
O
2
10
80
78
22.4
O
O
O
3
4
10
10
64
60
60
17.9
16.8
O
O
58d
H
O
O
5
10
10
65
35
72e
18.2
9.8
CH3
O
6
7
32f
O
OH
O
10
10
15
30
24g
28h
4.2
8.4
OH
O
8
OH
mol), H2O2 (1 mL), CH3CN (5 mL).
a
Reaction conditions: alkene (0.8 mmol), catalyst (2.86
Based on the starting alkene.
m
b
c
d
e
f
GC yield.
25% benzaldehyde was produced.
18% acetophenone was detected as by product.
15% verbenone and 9% verbenol were produced.
11% alcohol was produced.
g
h
21% alcohol was detected.

H. Salavati, N. Rasouli / Materials Research Bulletin 46 (2011) 1853–1859
1857
Table 2
Epoxidation of cyclooctene with oxygen donors catalyzed by PVMo/Bentonitea nanocomposite under reflux conditions.
Entry
Oxygen donor
Solvent
Epoxide selectivity (%)b after (10 h)
PVMo
PVMo/Bentonite (MS)
PVMo/Bentonite (US)
1
2
3
4
5
6
7
NaIO4
CH3CN/H2O
CH3CN/H2O
CH3CN
61
54
90
32
39
41
28
33
31
64
27
32
36
25
60
56
92
39
41
45
32
Oxone (KHSO5)
H2O2
NaOCl
CH3CN
Tert-BuOOH
Bu4NIO4
O2
CH3CN
CH3CN
CH3CN
a
Reaction conditions: cyclooctene (0.8 mmol), catalyst (2.86
GC yield based on the starting cyclooctene.
m
mol), H2O2 (1 mL), CH3CN (5 mL).
b
Table 3
Epoxidation of alkenes with H2O2 catalyzed by PVMo/Bentonite nanocomposite under ultrasonic irradiation.a
Entry
1
Substrate
Products
Time (min)
40
Conversion (%)b,c
85
Epoxide selectivity (%)b
92
TOF (hꢂ1
)
OH
OH
360
O
OH
O
2
40
40
100
82
98
79
424
347
O
O
O
3
4
O
O
H
40
40
85
87
82d
84e
360
369
O
O
5
CH3
O
6
7
40
40
68
18
65f
288
76
O
OH
26g
O
OH
O
8
40
75
28h
318
OH
mmol), H2O2 (1 mL), CH3CN (5 mL).
a
Reaction conditions: alkene (0.8 mmol), catalyst (2.86
Based on the starting alkene.
b
c
d
e
f
GC yield.
15% benzaldehyde was produced.
14% acetophenone was detected as by product.
16% verbenone and 8% verbenol were produced.
13% alcohol was produced.
g
h
54% alcohol was detected.
product yield and selectivity. The system under ultrasonic
irradiation showed a good catalytic activity in the epoxidation
of linear alkenes such as 1-octene and 1-dodecene (Table 4).
It is reported that peroxo radicals were generated at the PVMo
sites (Scheme 1). Subsequent epoxidation of the alkenes can occur
both within the pores and in the bulk liquid solution [15–17,28–
31].
The results indicated that the increase of ultrasound power
improved the extent of epoxidation of cyclooctene and the highest
conversion was observed at a power of 400 W (Fig. 6).
Fig. 7 shows systems under ultrasonic irradiation and under
agitation with magnetic stirring, which indicate that the catalytic
activity of PVMo/Bentonite nanocomposite has been enhanced by
ultrasonic irradiation.
The blank experiments in the absence of the catalyst showed
that ultrasonic irradiation has a poor ability to epoxidize the
alkenes with hydrogen peroxide.
In order to show the effect of supporting on the catalytic
activity, all reactions were repeated in the presence of PVMo as
catalyst under the same reaction conditions. It is clear from Tables
1
and 3 that the catalytic activity of the PVMo/Bentonite
nanocomposite was much higher than that of unsupported
heteropolyanion (PVMo). The TOFs are higher for PVMo/Bentonite
nanocomposite in comparison with the parent PVMo. By disper-

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H. Salavati, N. Rasouli / Materials Research Bulletin 46 (2011) 1853–1859
Table 4
Epoxidation of alkenes with H2O2 (30%) catalyzed by PVMo under reflux conditions.a
Entry
1
Substrate
Products
Time (h)
20
Conversion (%)b,c
65
Epoxide selectivity (%)b
90
TOF (hꢂ1
)
OH
OH
9.1
O
OH
O
2
20
67
85
9.4
O
O
O
3
4
20
20
35
43
49
29
4.9
6.0
O
O
H
O
O
5
20
27
42
3.8
CH3
O
6
7
20
20
20
26
52
58
2.8
3.7
O
OH
O
OH
O
8
20
30
31
4.2
OH
mol), H2O2 (1 mL), CH3CN (5 mL).
a
Reaction conditions: alkene (0.8 mmol), catalyst (2.86
Based on the starting alkene.
GC yield.
m
b
c
sion of the vanadium-substituted heteropolymolybdate on ben-
tonite, the available vanadium active sites were isolated which in
turns increased the catalytic activity. On the other hand, the ability
of bentonite alone as support of catalyst was checked in the
epoxidation of cyclooctene with H2O2. The obtained results
(Table 5). Addition of fresh alkene and oxidant to the filtrates
showed that the amount of epoxide is comparable to the blank
experiments. These results are in accordance with the leaching
data. The nature of the recovered catalyst was followed by FT-IR.
After reusing the catalyst for several times, no change in the FT-IR
spectrum was observed (Fig. 2).
showed that bentonite has
a poor ability to catalyze the
epoxidation of cyclooctene and the amount of epoxide was less
than 4%.
3.1.3. The stability and reusability of PVMo/Bentonite nanocomposite
The reusability of the supported catalyst was investigated in the
epoxidation of cyclooctene with hydrogen peroxide under reflux
and ultrasonic irradiation. At the end of each consecutive run, the
heterogeneous catalyst was filtered, washed thoroughly with
acetonitrile and 1,2-dichloroethane successively, and dried before
being used with fresh cyclooctene and hydrogen peroxide. In both
cases, the catalysts were consecutively reused four times. The
amount of leaching of catalyst after each run was determined by
ICP analysis. In this manner the filtrates were collected after each
run and used for determining of the amounts of vanadium leached
Fig. 6. The effect of ultrasonic irradiation intensity on the epoxidation of
Scheme 1.
cyclooctene with H2O2 catalyzed by PVMo/Bentonite.

H. Salavati, N. Rasouli / Materials Research Bulletin 46 (2011) 1853–1859
1859
Table 5
The results obtained from the reusability and stability of catalyst under magnetic stirring (MS) and ultrasonic irradiation (US).a
Run
Time
Conversion (%)b
MS
Epoxide selectivity (%)
Vanadium leaching (%)c
MS (h)
US (min)
US
MS
US
MS
US
1
2
3
4
10
10
10
10
40
40
40
40
60
59
58
58
85
84
84
83
64
63
62
62
92
91
90
89
1.0
0.8
0.4
0.3
1.0
0.7
0.5
0.4
a
Reaction conditions: cyclooctene (0.8 mmol), catalyst (2.86
GC yield based on the starting cyclooctene.
Determined by ICP.
m
mol), H2O2 (1 mL), CH3CN (10 mL).
b
c
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Acknowledgements
The support of this work by Payame Noor University (PNU) and
Catalysis Division of Isfahan University is acknowledged.