Sonocatalytic oxidation of olefins catalyzed by heteropolyanion-montmorillonite nanocomposite

Article abstract of DOI:10.1016/j.ultsonch.2009.05.009

A Keggin-type heteropolyanion compound (HPO) was doped within the montmorillonite (MMT) structure by impregnation method. The synthesized catalyst was characterized by FT-IR, XRD, UV-vis, CV, SEM and elemental analysis. Based on chemical adsorption between HPO, and hydroxyl surface groups, HPOs nanoparticles were successfully located on the MMT. Moreover, the obtained nanocomposite was found as an efficient catalyst for oxidation of hydrocarbons under reflux and ultrasonic irradiation conditions.

Full text of DOI:10.1016/j.ultsonch.2009.05.009

Ultrasonics Sonochemistry 17 (2010) 145–152
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Sonocatalytic oxidation of olefins catalyzed by heteropolyanion–montmorillonite
nanocomposite
Hossein Salavati a, , Shahram Tangestaninejad , Majid Moghadam , Valiollah Mirkhani ,
b
b
b
*
b
Iraj Mohammadpoor-Baltork
a
Department of Chemistry, Payame Noor University (PNU), Isfahan 81395-671, Iran
Department of Chemistry, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A Keggin-type heteropolyanion compound (HPO) was doped within the montmorillonite (MMT) struc-
ture by impregnation method. The synthesized catalyst was characterized by FT-IR, XRD, UV–vis, CV,
SEM and elemental analysis. Based on chemical adsorption between HPO, and hydroxyl surface groups,
HPOs nanoparticles were successfully located on the MMT. Moreover, the obtained nanocomposite
was found as an efficient catalyst for oxidation of hydrocarbons under reflux and ultrasonic irradiation
conditions.
Received 7 February 2009
Received in revised form 6 May 2009
Accepted 12 May 2009
Available online 18 May 2009
Keywords:
Ó 2009 Elsevier B.V. All rights reserved.
Heteropolyanion
Supported catalyst
Montmorillonite
Nanocomposite
Epoxidation
1
. Introduction
Heteropolyanion (HPO), a class of molecularly defined inorganic
lead to the formation of large pores with dimensions of >2.0 nm,
depending up on the extent of pillaring and the pillared material
[13,14]. The presence of such large pores in montmorillonite-K10
is expected to facilitate the introduction and stabilization of the
heteropolyanions. Montmorillonite is an interesting support for
the immobilization of charged complexes [15].
metal–oxide clusters, has interesting structures and unexpected
properties, and attracts increasing attention worldwide [1–4]. Also,
HPOs have some properties related to developing functional mate-
rials, such as catalytic activity for chemical transformations [5],
molecule-based conductivity [6], and so on. Recently, HPOs-poly-
electrolyte multilayer films have been fabricated by the layer-by-
layer (LbL) self-assembly method [7,8]. Such multilayer films
shows electrochromism, photoluminescence and nonlinear optical
properties [9–11], which suggest that the functional properties of
HPOs can be successfully incorporated into the multilayer film
materials. However, the joining force between layers in the film
was only electrostatic interaction, so that film’s stability is insuffi-
cient. It was well-known that heteropolyanion is generally an an-
ionic cluster, in which its negative charges mostly distribute on
the surface oxygen atoms of HPO anion, thus photoinduced reac-
tions can occur by activation of these surface oxygen atoms [12].
Intercalation of clays is known to transform the unstable clay
structures into highly porous and stable structures. The robust
oxide particles, which form the pillars in between the clay layers,
prevent the collapse of the expanded layers and simultaneously
Supported heteropolyanions are important for many applica-
2
tions, because bulk HPO have low specific surface area (1À10 m /
g). In the case of unsupported heteropolyanions, when the reac-
tants have a polar character, the catalytic reactions occur not only
at the surface but also in the bulk of the solid heteropolyanions
[16]. This is the reason why despite their low surface areas they
demonstrate quite high catalytic activity. When non-polar reac-
tants are used, it is important to increase the surface area or even
better to increase the number of accessible acid sites of the HPO.
This can be achieved by dispersing the heteropolyanions on solid
supports with high surface areas [15]. It is known, that HPO
strongly interact with supports at low loading levels, while the
bulk properties of heteropolyanions prevail at higher loadings.
2
Acidic or neutral substances like SiO , active carbons or aluminosil-
icates are suitable as supports. Due to the interaction of HPO and
the OH surface groups protons are transferred to the silica surface
which results in a decrease of acidity and the appearance of redox
properties of HPOs. Interacting with the surface occurs very readily
for samples with loadings lower than 20 wt.% [16].
*
Corresponding author. Tel.: +98 311 7380005; fax: +98 311 7381002.
The main effect of ultrasonic in liquids is cavitation phenome-
non, which involves numerous tiny gas bubbles called cavitation
E-mail addresses: hosseinsalavati@yahoo.com, majidmoghadamz@yahoo.com
(
H. Salavati).
1
350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.05.009

1
46
H. Salavati et al. / Ultrasonics Sonochemistry 17 (2010) 145–152
Fig. 1. XRD patterns of: (A) MMT and (B) PVMo–MMT nanocomposite.
Fig. 2. FT-IR spectrum of: (A) PVMo–MMT and (B) recovered PVMo–MMT.

H. Salavati et al. / Ultrasonics Sonochemistry 17 (2010) 145–152
147
bubbles. The collapse of the bubbles generates high temperature
and pressures at the centre of the bubbles. These local effects pro-
duce a variety of radicals and highly active intermediates, which
initiate other secondary chemical reactions in the bulk liquid.
These radicals can react with contaminants to form by products.
The most successful applications of ultrasound have been found
in the field of heterogeneous chemistry involving solids and met-
als. This is due to the mechanical impact of ultrasound on solid sur-
faces. In conventional chemistry there are several problems
associated with reactions involving solids or metals: small surface
area of the solid/metal may lead to excessive reactivity, penetra-
tion of reactants into deeper areas is not possible, oxide layers or
impurities can cover the surface, reactants/products have to diffuse
onto and from the surface and reaction products can act as deposit
on the surface and prevent further reactions.
The mechanical effects of ultrasound offer an opportunity to
overcome the following types of problem associated with conven-
tional solid/metal reactions: break-up of the surface structure al-
lows penetration of reactants and/or release of materials from
surface, degradation of large solid particles due to shear forces in-
duced by shock waves and microstreaming leads to reduction of
particle size and increase of surface area and accelerated motion
of suspended particles leads to better mass transfer [17].
obtained as potassium bromide pellets in the range 400–
À1
4000 cm
with a Nicollet-Impact 400D instrument. Scanning
electron micrographs of the catalyst and support were taken on
SEM Philips XL 30. Powder X-ray diffraction data were obtained
on a D Advanced Bruker using Cu Ka radiation (2h = 5–70°). Gas
8
chromatography experiments (GC) were performed on a Shimadzu
GC-16A instrument using a 2 m column packed with silicon DC-
200 or Carbowax 20 m using n-decane as internal standard. 1
H
NMR spectra were recorded on a Bruker–Arance AQS 300 MHz.
Conversions and yields were obtained by GC experiments and
the products were identified after isolation and purification. A
UP 400S ultrasonic processor equipped with a 3 mm wide and
140 mm long probe, which was immersed directly into the reac-
tion 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. The tempera-
ture of the solution was kept at 40 °C by a thermostat bath during
sonication.
The Na
erature [20].
5
[PV
2
Mo10O
2
40]Á14H O was prepared according to the lit-
2.1. Preparation and characterization of HPO–MMT nanocomposite
Recently, we reported the sonocatalytic oxidation of organic
compounds catalyzed by vanadium containing polyphosphomo-
The typical preparation of PVMo–MMT nanoparticles is as fol-
lowing: PVMo (1 g) and MMT (5 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 centrifugalized. The washing and centrifuging
processes were repeated 5 times. The wet nanoparticles of HPO–
MMT were obtained and dried in vacuum for 5 h (60–80 °C).
lybdate supported on MCM-41 and TiO
this paper, we wish to report the sonocatalytic oxidation of alkenes
with H catalyzed by Na PV 40 (PVMo) deposited on
2
nanoparticles [18,19]. In
2
O
2
5
2
Mo10O
montmorillonite. The effect of ultrasonic irradiation on the cata-
lytic activity of this catalyst was also investigated.
2
. Experimental
All reagents were purchased commercially and used without
further purification. Elemental analysis was performed on a Per-
kin–Elmer 2400 instrument. The amount of Mo was measured by
a Leaman inductively coupled plasma (ICP) spectrometer. Diffuse
reflectance spectra were recorded on a Shimadzu UV-265 instru-
4
ment using optical grade BaSO as reference. FT-IR spectra were
Fig. 3. UV–vis spectrum of: (A) PVMo and (B) PVMo–MMT.
Fig. 4. SEM images of: (A) MMT and (B) PVMo–MMT nanocomposite.

1
48
H. Salavati et al. / Ultrasonics Sonochemistry 17 (2010) 145–152
1
2
.2. General procedure for oxidation reactions with H
O
2 2
under reflux
or a silica gel column. IR and H NMR spectral data confirmed the
identities of the products.
conditions and under ultrasonic irradiation
In a typical experiment, the reaction vessel was loaded with the
supported catalyst (30 mg, containing 4.76 mmol of PVMo), alkene
3. Results and discussion
(
2 2
0.8 mmol) in acetonitrile (5 ml). H O (1 ml) was added and the
mixture was refluxed or exposed to ultrasonic waves. The progress
of the reaction was monitored by GC. After the reaction was com-
3.1. Catalyst characterization
pleted, the reaction mixture was diluted with Et
tered. The catalyst was thoroughly washed with Et
combined washings and filtrates were purified on a silica gel plates
2
O (20 mL) and fil-
and
Since heteropolyanions of the Keggin structure have molecular
diameters of ꢀ12 A°, it is feasible to support the polyoxometalates
such as PVMo on MMT. Fig. 1 shows the XRD patterns of MMT and
2
O
Table 1
Epoxidation olefins with H
catalyzed by PVMo–MMT under reflux conditions.a
Products
2
O
2
Entry
1
Substrate
Time (h)
10
Conversion (%)b,c
58
Epoxide selectivity (%)
65
TOF (h
16.3
À1
)
OH
O
OH
OH
O
2
3
4
5
6
10
10
10
10
10
58
61
72
45
42
72
16.6
17.5
20.6
12.9
12
O
O
O
58d
78e
46f
40g
H
O
O
CH3
O
OH
O
O
1-Dodecene
OH
O
7
1-Octene
10
18
19h
5.2
OH
a
Reaction conditions: olefin (0.8 mmol); catalyst (2.86
Based on the starting olefin.
GC yield.
2 2 3
lmol); H O (1 ml); CH CN (5 ml).
b
c
d
e
f
2
1
1
2
4
6% Benzaldehyde was produced.
6% Acetophenone was detected as by product.
5% Verbenone and 10% verbenol were produced.
5% Alcohol was produced.
g
h
% Alcohol was detected.

H. Salavati et al. / Ultrasonics Sonochemistry 17 (2010) 145–152
149
PVMo–MMT. These patterns indicated that PVMo has been intro-
duced in the substitutional position of MMT. PVMo has a striking
effect on the width and intensity of the main reflection at the high
cated that PVMo has been supported on MMTs (Fig. 2). These peaks
could be attributed to (P = Oa), (Mo = Ot), (Mo–O –Mo) and
(Mo–O –Mo), respectively (Ot = terminal oxygen, O
m
m
m
b
m
c
b
= bridged
d
100 spacing and this line becomes broader and weaker as the cat-
oxygen of two octahedral sharing a corner and O
c
= bridged oxygen
alyst loading increases. At higher loadings, interactions between
PVMo and the MMT surface may have a negative effect on the
activity of these materials, as they appear to be responsible for se-
vere distortion of the structural wall [21,22].
sharing an edge) [23].
The UV–vis spectra of PVMo in CH
3
CN displayed absorption
bands at 266 and 308 nm, which are associated with octahedrally
6
+
coordinated Mo and arise due to the Mo–O and V–O charge–
transfer absorptions in the heteropoly cage [23–25]. These two
absorption bands were appeared in the solid-state UV–vis spectra
of catalyst and since pure MMT show no UV absorption; therefore,
FT-IR spectrum of the prepared catalyst showed absorption
À1
bands at 1056, 957, 867 and 798 cm , corresponding to the four
typical skeletal vibrations of the Keggin polyoxoanions, which indi-
Table 2
Epoxidation of olefins with H
2
O
2
catalyzed by PVMo–MMT under ultrasonic irradiation.a
Conversion (%)b,c
71
Epoxide selectivity (%)
77
TOF (h
394
À1
)
Entry
1
Substrate
Time (min)
30
OH
O
OH
OH
O
2
3
4
35
35
35
88
83
92
96
419
395
438
O
O
O
83d
H
O
O
95e
CH3
V
VV
V + H O
H O
2 2
2
2
.
O
VV
H O
2 2
VIV + HOO + H
+
5
30
82
81f
390
O
OH
O
6
1-Dodecene
35
55
53g
262
OH
O
7
1-Octene
35
28
30h
133
OH
a
Reaction conditions: olefin (0.8 mmol); catalyst (2.86
Based on the starting olefin.
GC yield.
2 2 3
lmol); H O (1 ml); CH CN (5 ml).
b
c
d
e
f
1
5
1
2
2
4% Benzaldehyde was produced.
% Acetophenone was detected as by product.
0% Verbenone heteropolyanion and 5% verbenol were produced.
6% Alcohol was produced.
0% Alcohol was detected.
g
h

1
50
H. Salavati et al. / Ultrasonics Sonochemistry 17 (2010) 145–152
these results indicated that primary Keggin structure has been
introduced into the nanostructure framework (Fig. 3).
in 58% and 78% epoxide selectivity. In the oxidation of
a-pinene,
the major product was -pinene oxide, while verbenone and ver-
a
Fig. 4 shows the SEM images of nanoparticles. The morpholo-
gies and microstructure of the nanoparticles are approximately
spherical. Some nanoparticles are importantly aggregated. Assum-
ing that a nanoparticle is spherical, the average diameter of nano-
particles is estimated to be between 50 and 250 nm by averaging
its diameters measured in several directions in the SEM images.
The larger size may be due to the aggregation of particles.
benol were produced as minor products. Oxidation of linear
alkenes such as 1-octene and 1-dodecene was accompanied
by allylic oxidation. Blank experiments in the absence of the
2 2
catalyst showed that H O has low ability in the epoxidation of
alkenes.
3.2.2. Alkene epoxidation with H
2 2
O catalyzed by PVMo–MMT under
ultrasonic irradiation
3
.2. Catalytic activity
As mentioned in the previous works, ultrasonic irradiation can
be used as an efficient tool to influence the product yield and selec-
tivity [18,19]. Therefore, we decided to investigate the effect of
ultrasonic waves on the epoxidation of different alkenes with
Since heterogeneous catalysts are recoverable, hence, heteroge-
nization of homogenous catalysts is of great interest. Therefore, we
decided to immobilize PVMo on MMT nanoparticles, and investi-
2
H O
2
catalyzed by PVMo–MMT.
2 2
gate its catalytic activity in the epoxidation of alkenes with H O
As shown in Table 2, application of ultrasonic waves in this cat-
under agitation with magnetic stirring and under ultrasonic
irradiation.
alytic system has reduced the reaction times and improved the
yields and product selectivities. The system under ultrasonic irra-
diation showed a good catalytic activity in the oxidation of linear
alkenes such as 1-octene and 1-dodecene.
It is reported that peroxo radicals are generated at the PVMo
sites (Scheme 1). Subsequent oxidation of the hydrocarbon can oc-
cur both within the pores and in the bulk liquid solution [3,26].
3
.2.1. Alkene epoxidation with H
reflux conditions
First, the ability of the prepared catalyst, PVMo–MMT, was
investigated in the epoxidation of cyclooctene with H in ace-
2 2
O catalyzed by PVMo–MMT under
2 2
O
tonitrile. The reactions were continued until no further progress
was observed. The results showed that the conversion was 58%
with 65% epoxide selectivity. Epoxidation of cyclohexene
showed 58% conversion and 72% epoxide selectivity, and allylic
oxidation products (cyclohexene-1-one and cyclohexene-1-ol)
were detected in the reaction mixture. In the case of styrene
1
00
Epoxide Selectivity (%)
Conversion (%)
9
8
7
6
5
4
0
0
0
0
0
0
and
a-methyl styrene, the corresponding epoxides were obtained
V
VV
V + H O
H O
2 2
2
2
.
30
VV
H O
VIV + HOO + H
+
2
2
2
1
0
0
0
Scheme 1. Mechanism for generation of peroxo radicals at PVMo sites.
2
0
40
60
80
100
(
A) Particle distribution of PVMo/MMT before sonication
Ultrasound intensity
25
20
15
10
5
0
Fig. 6. The effect of ultrasonic irradiation intensity on the oxidation of
styrene with H catalyzed by PVMo–MMT.
a-methyl
2 2
O
0
-50
50-100
100-150 150-200 200-250
>250
particle size (nm)
(
B) Particle distribution of PVMo/MMT after sonication
50
40
30
20
10
0
0
-50
50-100
100-150 150-200 200-250
particle size (nm)
>250
Fig. 7. Comparison of ultrasonic irradiation and magnetic agitation in the oxidation
of -methyl styrene with H catalyzed by PVMo–MMT (the reactions were
carried out as described in the Section 2).
a
2 2
O
Fig. 5. The particle size distribution PVMo–MMT before and after sonication.

H. Salavati et al. / Ultrasonics Sonochemistry 17 (2010) 145–152
151
Ultrasonic irradiation promotes the generation of radicals in the
reaction media and this can accelerate the reaction. On the other
hand, collapse of the produced bubbles results in generation of
high temperatures. As the reaction under mechanical stirring re-
quires a temperature of about 80 °C, therefore, the reaction can
be also accelerated under ultrasonic irradiation.
the main reasons for catalytic enhancement by ultrasonic
irradiation.
The power of ultrasound is a very important parameter and also
has a great influence on the phenomena of acoustic cavitation and
efficiency of ultrasound treatment. Fig. 6 shows the effect of irradi-
ation power on the epoxidation of a-methyl styrene, which indi-
In order to show the effect of ultrasonic irradiation on the cat-
alytic activity enhancement, the particle size distribution of the
catalysts was determined before and after sonication. The results
showed that the catalyst agglomerates break-up during the sonica-
tion process (Fig. 5). It seems that a part of ultrasonic irradiation
effect is due to this phenomenon. To stress this point, the catalytic
activity of a sonicated sample catalyst was studied in the epoxida-
tion of cyclooctene under conventional mechanical stirring. It was
found that the reaction time reduced from 10 to 6 h for epoxidation
of cyclooctene to its corresponding epoxide. These results show
that in addition to break-up of the agglomerates, other factors such
as thorough mixing of the reactants and producing of hot spots are
cates that increasing of ultrasound power will improve the
extent of oxidation and the highest conversion, was observed at
a power of 400 W.
Fig. 7 comprises the systems under ultrasonic irradiation and
under agitation with magnetic stirring, which indicating that the
catalytic activity of PVMo–MMT catalyst has been enhanced by
ultrasonic irradiation.
The blank experiments in the absence of catalyst showed that
ultrasonic irradiation has poor ability to epoxidize the alkenes with
hydrogen peroxide (about 10%).
In order to show the effect of supporting on the catalytic activ-
ity, all reactions were repeated in the presence of homogeneous
Table 3
Epoxidation of olefins with H
2
O
2
(30%) catalyzed by homogeneous PVMo.a
Products
Entry
1
Substrate
Time (h)
20
Conversion (%)b,c
55
Epoxide selectivity (%)
96
TOF (h
7.7
À1
)
OH
O
OH
O
OH
2
3
4
20
20
20
53
38
27
85
49
29
7.4
6.9
3.8
O
O
O
H
O
O
CH3
O
5
6
20
20
20
26
28
59
2.8
3.6
O
OH
O
OH
O
7
20
32
31
4.6
OH
a
Reaction conditions: olefin (0.8 mmol); catalyst (2.86
Based on the starting olefin.
GC yield.
l
mol); H
2
O
2
3
(1 ml); CH CN (5 ml).
b
c

1
52
H. Salavati et al. / Ultrasonics Sonochemistry 17 (2010) 145–152
Table 4
The results obtained from catalyst reuse and stability in the oxidation of
irradiation.a
2 2
a-methyl styrene with H O by PVMo–MMT under agitation with magnetic stirring and under ultrasonic
Run
Time
Conversion (%)b,c
MS
Epoxide selectivity (%)
MS
V leaching (%)d
MS (h)
US (min)
US
US
MS
US
1
2
3
4
10
10
10
10
35
35
35
35
72
71
70
69
92
91
90
89
78
77
76
76
95
94
94
93
1.0
0.8
0.4
0.3
1.0
0.7
0.5
0.4
a
b
c
Reaction conditions: olefin (0.8 mmol); catalyst (2.86
Based on the starting olefin.
GC yield.
2 2 3
lmol); H O (1 ml); CH CN (10 ml).
d
Determined by ICP.
PVMo catalyst and under the same reaction conditions. It is clear
from Tables 1 and 3 that the catalytic activity of PVMo–MMT
was much higher than that of unsupported heteropolyanion. The
conversions, selectivities and TOFs are higher for heterogeneous
PVMo–MMT in comparison with the homogeneous one. Therefore,
the catalytic activity increases by dispersing of the catalyst on the
montmorillonite. On the other hand, the ability of MMT, as cata-
tween hydroxyl groups of MMTs and PVMOs nanoparticles. This
method can be readily extended to label defects on MMT by using
PVMOs nanoparticles as a marker.
Acknowledgements
The support of this work by Payame Noor University (PNU) and
Catalysis Division of University of Isfahan (CDUI) are acknowledged.
2 2
lyst, was checked in the oxidation of cyclooctene with H O .
Recently, we have reported the use of polymer supported
molybdenum hexacarbonyl in the alkene epoxidation with tert-
BuOOH [26–28]. No doubt, these methods are good in terms of
reactivity and reusability; but the present method is superior in
terms of thermal stability, ease of preparation and lower cost of
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