Heteropolytungstate nanoparticles: Microemulsion-mediated preparation and investigation of their catalytic activity in the epoxidation of olefins

Article abstract of DOI:10.1016/j.materresbull.2015.12.036

Keggin type Q₃PW₁₂O₄₀ nanoparticles (Q = cetyltrimethylammonium cation) were synthesized in water-in-oil (w/o) microemulsion consisted of water/cetyltrimethylammonium bromide/n-butanol/isooctane. Reaction of Na₂WO₄, Na₂HPO₄ and hydrochloric acid within water containing nanoreactors of reverse micelles resulted in the preparation of Q₃PW₁₂O₄₀ nanoparticles. The resultant nanoparticles were analyzed by physicochemical methods such as FT-IR spectroscopy, X-ray diffraction, energy-dispersive X-ray analysis, thermogravimetric analyses (TGA-DTA), scanning and transmission electron microscopy and atomic force microscopy which show nearly uniform spherical nanoparticles with size of about 15 nm. Finally, catalytic activity of the Q₃PW₁₂O₄₀ nanoparticles was examined in the epoxidation of olefins with H₂O₂. The prepared nanoparticles acted as recoverable and reusable catalyst in the epoxidation of olefins with H₂O₂.

Full text of DOI:10.1016/j.materresbull.2015.12.036

Materials Research Bulletin 76 (2016) 332–337
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Heteropolytungstate nanoparticles: Microemulsion-mediated
preparation and investigation of their catalytic activity in the
epoxidation of olefins
M. Masteri-Farahani*, M. Ghorbani
Faculty of Chemistry, Kharazmi University, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 26 August 2015
Received in revised form 25 November 2015
Accepted 24 December 2015
Available online 29 December 2015
Keggin type Q PW12O40 nanoparticles (Q = cetyltrimethylammonium cation) were synthesized in water-
in-oil (w/o) microemulsion consisted of water/cetyltrimethylammonium bromide/n-butanol/isooctane.
3
Reaction of Na
micelles resulted in the preparation of Q
2
WO
4
, Na
2
HPO
4
and hydrochloric acid within water containing nanoreactors of reverse
PW12 40 nanoparticles. The resultant nanoparticles were
3
O
analyzed by physicochemical methods such as FT-IR spectroscopy, X-ray diffraction, energy-dispersive X-
ray analysis, thermogravimetric analyses (TGA-DTA), scanning and transmission electron microscopy and
atomic force microscopy which show nearly uniform spherical nanoparticles with size of about 15 nm.
Keywords:
A. Nanostructures
B. Chemical synthesis
C. Atomic force microscopy
D. Catalytic properties
3
Finally, catalytic activity of the Q PW12O40 nanoparticles was examined in the epoxidation of olefins with
2 2
H O . The prepared nanoparticles acted as recoverable and reusable catalyst in the epoxidation of olefins
with H O .
2 2
ã 2015 Elsevier Ltd. All rights reserved.
1
. Introduction
which are affected mainly by the synthetic process. Various
methods have been used for the preparation of different types of
inorganic nanoparticles. Among them, the microemulsion method
has been shown to be useful in the preparation of monodispersed
inorganic nanoparticles [19–24]. Water-in-oil microemulsion
systems are transparent, isotropic liquid media with nanosized
water droplets dispersed in a oil phase and stabilized by surfactant
molecules at the water/oil interface. These water droplets can act
as nanoreactors for the preparation and growth of inorganic
nanoparticles. Also, the surfactants inhibit the agglomeration of
particles and thus the obtained particles are stabilized and
generally are nanosized and monodispersed.
Polyoxometalates constitute an important class of inorganic
compounds with wide applications e.g. in catalysis, electronics and
photoelectronic [1–6]. They are polymeric oxoanions that are
composed of oxides of addenda atoms (e.g. molybdenum,
tungsten, vanadium) and heteroatom (e.g. phosphorous, silicon).
The formation mechanism of this type of compounds is based on a
self assembly process in an acidic aqueous solution. Among the
polyoxometalates, Keggin types with general formula XM12O
nÀ
40
are the main subject of great researches and several reports have
been devoted to investigation of their preparation, modification
and applications [7–10]. The Keggin polyoxometalates are com-
posed of a central tetrahedral XO
octahedral MO units.
Despite some reports published on the preparation of poly-
oxometalate nanostructures [25–27] there is no any report about
the preparation and characterization of phosphotungstate based
nanostructures in microemulsion system. On the other hand,
4
unit surrounded by 12
6
On the other hand, the development of uniform nanosized
+
+
particles has attracted many scientific researches in recent years
contrary to conventional polyoxometalates (POMs) based Na or K
[11–18]. These nanoparticles have a characteristic high surface to
salts, recent findings have shown that replacement of Na+ or K+
volume ratio as a large fraction of the metal atoms is at the surface
and thus show novel electronic, optical and chemical properties
significantly different from those of bulk materials. In general,
these properties depend on the size and structure of the particles,
salts with organic cations such as ionic liquids or quaternary
ammonium cations (Q ) leads to hybrid materials with advanced
catalytic and electrocatalytic properties [28–30]. So, herein we
wish to report for the first time the microemulsion-mediated
+
3
preparation and characterization of Q PW12O40 (Q = cetyltrime-
thylammonium cation) nanoparticles and investigation of their
catalytic activity in the epoxidation of olefins.
*
Corresponding author. Fax: +98 263 4551023.
E-mail address: mfarahany@yahoo.com (M. Masteri-Farahani).
http://dx.doi.org/10.1016/j.materresbull.2015.12.036
025-5408/ã 2015 Elsevier Ltd. All rights reserved.
0

M. Masteri-Farahani, M. Ghorbani / Materials Research Bulletin 76 (2016) 332–337
333
3
Fig. 1. Schematic illustration for preparation of the Q PW12O40 nanoparticles.
2. Experimental
3
2.2. Preparation of Q PW12O40 nanoparticles
2
.1. Materials and instrumentations
The quaternary microemulsion system consisted of water/
cetyltrimethylamonium bromide (CTAB)/n-butanol/isooctane and
was prepared as followed: n-butanol (9 g) was added to isooctane
(22 g) and then a solution of CTAB (7.5 g) in water (2 ml) was added
to this solution. The resulted mixture was stirred for 10 min until a
All chemicals purchased from Merck chemical company and
used without further purification.
FT-IR spectra were recorded on PerkinElmer sspectrum RXI FT-
IR spectrometer using pellets of the samples diluted with KBr.
Elemental analyses were performed by a scanning electron
microscope with EDX detector INCA Penta FETx3 and VARIAN
VISTA-MPX ICP-AES atomic emission spectrometer. Carbon and
nitrogen content of the samples was analyzed on Thermo Finnigan
clear solution was obtained. Then,
2 4
a solution of Na WO
(
Flash 1112 series EA) CHN analyzer. X-ray powder diffraction
measurements (XRD) were performed on a SIEFERT XRD 3003 PTS
diffractometer using Cu K radiation (wavelength, = 0.154 nm).
a
l
Thermogravimetric measurements were made on a PerkinElmer
diamond thermogravimeter. The temperature was increased to
ꢀ
ꢀ
7
00 C using a rate of 10 C/min in static air. Transmission electron
microscopy (TEM) analyses were performed using a Philips EM
08 S instrument with an accelerating voltage of 100 kV. Samples
2
were prepared for TEM by placing droplets of a suspension of the
sample in acetone on a polymer microgrid supported on a Cu grid.
Scanning electron microscope images were taken with a ZEISS-
DSM 960A microscope with attached camera operating at 30 kV.
Atomic force microscopy (AFM) was performed using Dual Scope
C-26 scanning probe and optical microscope. Diffuse reflectance
UV–vis spectra (DR UV–vis) were recorded under ambient
conditions in the wavelength range from 200 to 900 nm using a
Shimadzu UV–vis scanning spectrometer (Model 2101PC) with
4
BaSO as standard.
3 3 40
Fig. 2. FT-IR spectra of (a) Q PW12O40 nanoparticles and (b) bulk H PW12O .

3
34
M. Masteri-Farahani, M. Ghorbani / Materials Research Bulletin 76 (2016) 332–337
3 3 40
Fig. 5. XRD patterns of: (a) Q PW12O40 nanoparticles (a), and (b) H PW12O .
2
.3. Catalytic epoxidation of olefins
Epoxidation of olefins was carried out in a 25 ml round
bottomed flask equipped with a condenser and a magnetic stirrer.
In a typical method, to a mixture of cyclooctene (8 mmol) and
catalyst (100 mg) in acetonitrile (10 ml) was added H O (1.6 ml)
2 2
under nitrogen atmosphere and the mixture was refluxed for
appropriate time. In order to investigate the progress of reaction,
samples were withdrawn in given times and were analyzed using a
gas chromatograph (HP, Agilent 6890N) equipped with a capillary
column (HP-5) and a FID detector. Products were analyzed using
isooctane (1 g, 8.75 mmol) as internal standard. GC–MS of products
were recorded using a Shimadzu-14A fitted with a capillary
column (CBP5-M25). After completing, the reaction mixture was
filtered and the solid catalyst was washed with methanol and dried
in vacuum oven overnight to reuse in next run.
Fig. 3. UV–vis spectra of: (a) Q
3
PW12
O
40 nanoparticles, and (b) H
3
PW12
O
40
.
(
(
0.33 g, 0.001 mol) and Na
2
HPO
4
(0.043 g, 0.0003 mol) in water
3 ml) was added to the above mixture and stirred for 5 min to give
a transparent mixture. While stirring, 2.5 ml concentrated HCl was
added and the reaction was continued for another 2 h. The
resulting white product was collected by centrifugation and
washed several times with water and ethanol to remove the
3. Results and discussion
3.1. Preparation of Q PW12O40 nanoparticles
3
ꢀ
remained organic residue and dried at 110 C for 12 h.
Fig. 1 shows the schematic representation of the steps involved
in the preparation of Q
cation) nanoparticles. Addition of hydrochloric acid to the micro-
emulsion system containing Na WO –Na HPO aqueous solution
3
PW12O40 (Q = cetyltrimethylammonium
2
4
2
4
results in the preparation of phosphotungstate nanoparticles
whose negative charges are compensated with quaternary
cetyltrimethylammonium cations. The obtained nanoparticles
are insoluble in water and organic solvents and thus precipitate
immediately. On the other hand, the cetyltrimethylammonium
species act as protecting groups against the agglomeration and
growth of the as-prepared nanoparticles.
3
3
.2. Characterization of the prepared Q PW12O40 nanoparticles
FT-IR spectroscopy provides a useful evidence for confirming
the preparation of Q 40 nanoparticles. In the FT-IR spectrum
3
PW12O
of the product (Fig. 2a), the four main features of the Keggin type
À1
phosphotungstate anions [31] are observed at 1079 cm
(n P ÀÀ O),
3
Fig. 4. TGA and DTA curves of the obtained Q PW12O40 nanoparticles.

M. Masteri-Farahani, M. Ghorbani / Materials Research Bulletin 76 (2016) 332–337
335
Fig. 6. (a) Scanning electron micrograph, and (b) EDX spectrum of the prepared
Q
3
PW12O
40 nanoparticles.
À1
À1
as, cornersharingW ÀÀ O ÀÀ W) and 818 cm (n
as,
9
65 cm
(n
W = O), 897 (
that verify the Keggin structure of the
3
PW12O40 nanoparticles. Compared with FT-IR spec-
n
edgesharingMo ÀÀ O ÀÀ Mo
prepared Q
)
trum of the bulk phosphotungstic acid (Fig. 2b), the
n
as,
cornersharingW-O-W and
nanoparticles shifted from 890 to 897 cm and 803 to 818 cm
n
as, edgesharingW ÀÀ O ÀÀ W peaks in Q
3
PW12O
40
À1
À1
,
respectively. It has been shown that the protons in the heteropoly
acids are linked to the bridging oxygens rather than terminal ones
[
32]. So, the blue shift of these peaks indicates the substitution of
the H by CTA+ cations with stronger interactions. On the other
+
À1
hand, the observation of the bands at 2849 and 2918 cm
is
+
attributed to the stretching vibrations of C ÀÀ H groups of CTA
cations associated with phosphotungstate nanoparticles. Further-
more, the intensity of the O ÀÀ H stretching vibration at about 3400–
Fig. 7. (a) Transmission electron micrograph, (b) atomic force microscopy image,
and (c) size distribution histogram of the prepared Q PW12O40 nanoparticles.
3
À1
3
600 cm is reduced in comparison with bulk phosphotungstic
acid. This observation can be explained with hydrophobic nature
and water insolubility of the Q 40 nanoparticles.
Another evidence for the formation of Q PW12
particles was obtained by recording the UV–vis spectrum of the
product (Fig. 3a). In the UV–vis spectra of Keggin type poly-
oxometalates, there are two main absorptions corresponding to
the ligand to metal charge transfer (from mainly oxygen 2p orbitals
to mainly metal d orbitals) [33]. In the UV–vis spectrum of the
obtained nanoparticles, the strong peak at 267 nm is assigned to
3
PW12O
shoulder at 321 nm is attributed to the charge transfer in
corner-shared WO octahedra which further confirm the presence
of Keggin type heteropolytungstate nanoparticles. In comparison
with the corresponding peaks of bulk solid H 40 (Fig. 3b),
the observed blue shift is due to the decrease in the particle size of
heteropolytungstate species which results in the increasing the
energy difference between the electronic energy levels.
3
O
40 nano-
6
3
PW12O
The thermal behavior of the prepared Q
3
PW12O40 nanoparticles
the charge transfer in edge-shared WO
6
octahedra and the
was investigated by TGA-DTA analyses and the results are shown in

3
36
M. Masteri-Farahani, M. Ghorbani / Materials Research Bulletin 76 (2016) 332–337
Table 1
The resuls of catalytic epoxidation of olefins with H
Table 2
2
O
2
in the presence of
Selectivitya (%)
Results of EDX and ICP-AES analyses of the heteropolytungstate nanoparticles
before and after catalytic tests.
Q
3
PW12
O
40 nanoparticles.
Entry
Olefin
Time (h)
Conversion (%)
Material
P:W
EDX analysis)
P:W
(
(ICP-AES analysis)
1
2
3
4
5
6
7
Cyclooctene
Cyclooctene
Cyclooctene
2
2
2
2
2
6
6
89
85
80
10
81
84
59
>99
>99
>99
b
c
Q
3
Q
3
Q
3
PW12
PW12
PW12
O
O
O
40 before catalytic test
40 after 1st recycling
40 after 2nd recycling
1:11.6
1:11.0
1:11.5
1:11.9
1:11.5
1:11.6
Cyclooctened
Cyclohexene
1-Hexene
74
e
91f
>99
>99
1-Octene
amorphous [35]. The long chain cetyl groups in the prepared
nanoparticles inhibit the good ordering of the individual Keggin
units to form any crystalline phase. Moreover, this may be a result
of small size of the prepared nanoparticles and the absence of
various crystal planes.
The morphology and chemical composition of the prepared
nanoparticles were investigated using scanning electron micros-
copy (SEM) and energy dispersive X-ray analysis (EDX). A typical
Reaction conditions: catalyst (50 mg), cyclooctene (8 mmol),
refluxing CH CN (10 ml).
2 2
H O (1.6 ml),
3
a
Selectivity toward the epoxide.
b
c
d
e
f
Catalytic test with the 1st recycled catalyst.
Catalytic test with the 2nd recycled catalyst.
Reaction was carried out without the catalyst.
The side product is cyclooctyl hydroperoxide.
The side product is 2-cyclohexen-1-one.
3
scanning electron micrograph of the prepared Q PW12O40 nano-
Fig. 4. The TGA curve indicates three different stages of mass
particles is shown in Fig. 6a. The SEM image indicates that the
obtained nanoparticles are composed of nanospheres with some
agglomeration of the prepared nanoparticles. EDX analysis of the
prepared nanoparticles in Fig. 6b shows the molar ratio P:W near
the stoichiometric composition (1:12) and further confirms the
formation of Keggin type nanoparticles.
reduction as a function of temperature. The first stage, about 200–
ꢀ
3
00 C, is attributed to the loss of physisorbed molecules involved
in the preparation process. The corresponding DTA curve in this
region is endothermic because of the evaporation of adsorbed
ꢀ
molecules. The second stage (13.5% weight loss) about 300–450 C
can be assigned to the combustion of approximately 2 equivalent
CTA+ cations, which is in good agreement with exothermic DTA
peak in this region. The third stage of mass reduction (7.7% weight
The morphology and microstructure of the obtained product
was further investigated by transmission electron microscopy
(
TEM) and atomic force microscopy (AFM). Fig. 7a–b shows the
ꢀ
loss) about 450–650 C is due to the decomposition of the
typical TEM and AFM images of the Q 40 nanoparticles.
3
PW12O
+
remaining 1 equivalent CTA cation which its corresponding
These images illustrate that the obtained nanoparticles have
spherical morphology. The histogram of size distribution in Fig. 7c
shows that the Q PW12O40 nanoparticles are relatively mono-
3
DTA curve is exothermic. Our earlier study [25] has been shown
that in this stage, the Keggin structure of the nanoparticles
collapses and transforms into the corresponding metal oxide and
phosphorous oxide.
dispersed with average diameter of about 15 nm.
In the X-ray diffraction pattern of the Q
3
PW12
O
40 nanoparticles
ꢀ
(
Fig. 5a) a sharp peak at about 2
u
= 8 is observed which is a typical
feature of the Keggin structure [34]. In comparison with XRD
pattern of bulk H 40 (Fig. 5b), the other peaks are weak and
3
PW12O
very broad. According to the work of Koliadima et al., these
observations indicate that the prepared polyoxometalate nano-
particles have not good crystalline nature and are mainly
Fig. 8. The proposed schematic mechanism for the epoxidation of olefins in the
Fig. 9. (a) X-ray diffraction pattern, and (b) transmission electron microscopy
presence of the prepared Q
3
PW12
O
40 nanoparticles.
image of the 2nd recycled catalyst.

M. Masteri-Farahani, M. Ghorbani / Materials Research Bulletin 76 (2016) 332–337
337
Some agglomeration of the prepared nanoparticles is observed
which is in good agreement with the SEM observation.
The advantage of the presented method is one pot preparation
of relatively monodispersed polyoxometalate nanoparticles. Other
reported methods [26,27] involve the preparation of polyoxome-
talate nanoparticles with use of previously prepared heteropoly
acid in aqueous solution as one of the reagents and thus are
multistep reactions which need several time consuming work-up
processes. However, the use of this method for the preparation of
3
3.3. Catalytic activity of the prepared Q PW12O40 nanoparticles
Catalytic activity of the prepared nanoparticles was evaluated
in the epoxidation of olefins with H and the results have been
2 2
O
+
heteropoly acid nanoparticles failed as CTA cations can substitute
given in Table 1. As can be seen, the prepared nanoparticles can act
as active and selective catalyst in the epoxidation reaction and the
reaction progress in the absence of the nanoparticles is very slow.
On the other hand, the reactivity of the olefins increase in the order
of cyclooctene > cyclohexene > 1-hexene > 1-octene which shows
that the internal double bond olefins with higher electron density
are more reactive than terminal olefins. It has been shown that the
catalytic active sites in the epoxidation of olefins with hydrogen
peroxide in the presence of tungsten compounds have electro-
philic character [36,37]. So, with increasing the nucleophilicity of
the double bond, the rate of olefin epoxidation increases. The
proposed mechanism for the oxygen-transfer from the tungsten-
peroxo complex to the olefin involves the attack of olefin on one of
the peroxygens of the complex. The proposed mechanism for the
epoxidation of olefins was shown in Fig. 8.
protons in heteropoly acid. The preparation of heteropoly acid in
nonionic microemulsion systems is under investigation.
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4. Conclusion
1
[
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[
2 2
O .