A facile method for synthesis of Keggin-type cesium salt of iron substituted lacunary phosphotungstate supported on MCM-41 and study of its extraordinary catalytic activity

Article abstract of DOI:10.1016/j.cattod.2012.04.067

A novel catalyst, Keggin-type Cs salt of Fe substituted mono lacunary phosphotungstate supported MCM-41 was synthesized from H₃PW ₁₂O₄₀, FeCl₂ and Cs₂CO₃ and characterized by different techniques like X-ray diffraction, UV-vis diffused reflectance spectroscopy (UV-vis DRS), nitrogen adsorption-desorption, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectra and Fourier-Transform Infrared Spectroscopy (FT-IR). Diffused reflectance spectra and FT-IR studies confirmed the undegraded Keggin structure of the lacunary salt after supporting it on MCM-41. The material showed its amazing catalytic activity toward two different types of reactions. Among various samples prepared, 50 wt% Fe substituted lacunary salt supported MCM-41 showed remarkable catalytic performances. In acid catalyzed bromination of phenol, it showed 95% conversion obtaining p-bromophenol with 99% selectivity and in case of oxidation of trans-stilbene using hydrogen peroxide oxidant, the same catalyst gave 52% conversion with 99% trans-stilbene oxide selectivity.

Full text of DOI:10.1016/j.cattod.2012.04.067

Catalysis Today 198 (2012) 52–58
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Catalysis Today
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A facile method for synthesis of Keggin-type cesium salt of iron substituted
lacunary phosphotungstate supported on MCM-41 and study of its extraordinary
catalytic activity
∗
Surjyakanta Rana, Sujata Mallick, Lagnamayee Mohapatra, G. Bishwa Bidita Varadwaj, K.M. Parida
Colloids and Materials Chemistry Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751013, Odisha, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel catalyst, Keggin-type Cs salt of Fe substituted mono lacunary phosphotungstate sup-
Received 14 December 2011
Received in revised form 23 February 2012
Accepted 18 April 2012
ported MCM-41 was synthesized from H3PW12O40, FeCl2 and Cs2CO3 and characterized by different
techniques like X-ray diffraction, UV–vis diffused reflectance spectroscopy (UV–vis DRS), nitrogen
adsorption–desorption, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS),
Raman spectra and Fourier-Transform Infrared Spectroscopy (FT-IR). Diffused reflectance spectra and
FT-IR studies confirmed the undegraded Keggin structure of the lacunary salt after supporting it on MCM-
Available online 27 June 2012
Keywords:
Lacunary
Keggin
Oxidation
Bromination
4
1. The material showed its amazing catalytic activity toward two different types of reactions. Among
various samples prepared, 50 wt% Fe substituted lacunary salt supported MCM-41 showed remarkable
catalytic performances. In acid catalyzed bromination of phenol, it showed 95% conversion obtaining
p-bromophenol with 99% selectivity and in case of oxidation of trans-stilbene using hydrogen peroxide
oxidant, the same catalyst gave 52% conversion with 99% trans-stilbene oxide selectivity.
©
2012 Elsevier B.V. All rights reserved.
[XW11O39](
n+4)−
is substituted by other transition metal cations, it
1
. Introduction
gives rise to transition metal-modified lacunary heteropoly com-
pounds having the general formula [XW11O39M] (where M = first
n−
Polyoxometalates are polyionic metal oxide clusters that have
attracted the interest of scientists for many decades. Their interest-
ing properties include high thermal stability, rich redox chemistry
and photochemical efficiency etc. These properties help them to
be catalytically active towards several industrially and biologi-
cally significant reactions [1–5]. So, polyoxometalates (POMs) have
applications in fields such as materials science [6,7], analytical
chemistry [8] and medicine [9]. An efficient approach to the syn-
thesis of new polyoxometalate architectures is mainly based on
lacunary POMs, i.e. species with defect structures.
row transition metal). These species have recently attracted con-
siderable attention [12], because of their thermal and chemical
stability and the range of possibilities for their modification of
electro catalytic property without affecting the primary Keggin
structure [13].
Recently, Patel et al. reported the detailed synthesis and
characterization of Keggin-type manganese (II)-substituted phos-
photungstate and its activity toward liquid phase oxidation of
styrene [12]. However, iron is a low-cost and less hazardous metal
compared with other transition metals [14]. Therefore a great deal
of works has already been carried out on Fe metal substituted
heteropoly acids. Mizuno et al. [15] reported synthesis of Fe, Ni sub-
stituted Keggin-type heteropoly anion, and evaluated its catalytic
activity toward oxidation reaction. Nagai et al. reported iron in the
Keggin anion of heteropoly acid catalysts for selective oxidation
of isobutene [16]. Dai et al. reported that out of various transition
metals in the supported system, Fe is the best metal for oxidation
reactions [17]. The excellent activity of single-site iron catalysts for
epoxidation as well as oxidation reaction was reported by Thomas
and Raja [18]. But so far there is no literature available on the
catalytic aspects of supported Cs salt of iron substituted lacunary
anions.
The most investigated Keggin type heteropoly acids are rep-
n+
(8−n)−,
n+
resented by the general formula [X M12O40
]
where X is
4+
5+
a central hetero atom (Si , P etc.) and M is an addenda atom
W , Mo , V , etc.) [10]. Polyoxometalates containing replaced
6+
6+
5+
(
one or more addenda atoms are called lacunary polyoxometa-
lates. Removal of one or two MO units from the fully occupied
n−
polyoxometalates [XM12O40
]
, gives rise to mono-lacunary
VI
(n+4)−
VI
(n+5)−
[
XM11 O39
]
and di-lacunary [XM10 O36] polyoxomet-
alates respectively. Lacunary and di-lacunary polyoxometalates are
gaining more importance because of their unique structural prop-
erties [11]. It is well known that when the lacunary of Keggin anions
∗ Corresponding author. Tel.: +91 674 2581637 425; fax: +91 74 2581637.
E-mail address: paridakulamani@yahoo.com (K.M. Parida).
According to literature, among various heteropoly acids, phos-
photungstic acid is more acidic and generally used for acid
0
920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.04.067

S. Rana et al. / Catalysis Today 198 (2012) 52–58
53
catalyzed reactions [19]. Its acidity can be enhanced more by
2.4. Synthesis of cesium salt of iron substituted lacunary
phosphotungstate (LFeW) and Cs salt of lacunary
phosphotungstate (LW)
converting it into its corresponding acidic Cs salt. Therefore, cre-
ation of lacuna in the skeleton of the salt as well as its modification
with a transition metal will make it a suitable catalyst for both acid
catalyzed as well as oxidation reactions. With these concepts in
mind, for the first time we prepared a Cs salt of Fe metal modified
lacunary phosphotungstate supported on MCM-41 and studied its
catalytic activity toward acid catalyzed bromination of phenol as
well as oxidation of trans-stilbene. Our rationale in choosing MCM-
The preparation procedure for LFeW was same as described in
the section for the preparation of Na5FePW11O39 up to the addition
of FeCl . After which a saturated solution of the Cs CO was added
2
2
3
to the hot filtrate and the resulting mixture was allowed to stand
overnight at room temperature. The mixture was filtered and the
◦
4
1 support lies in the fact that it is endowed with a high surface area,
residue was dried at 110 C for 12 h, which resulted in LFeW. The
uniform and controllable pore sizes and the periodic orders of their
pore packing which provides a tremendous dispersion of the active
species over it. What we found from the results is that, the catalyst
is incredibly efficient toward both the reactions.
filtrate was used for the estimation of W and Fe, in order to see the
loss during synthesis.
The preparation procedure of LW is almost same with that of
LFeW. The only difference is that no FeCl2 was added to it. The rest
parts of the preparatory steps are completely same.
2.5. Physico–chemical characterization
2
. Experimental
Micromeritics ASAP 2020 instrument determined the specific
2
.1. Catalyst preparation
surface area, pore size distribution and pore volume. The sam-
◦
ples were degassed at 250 C for 4 h. Powder X-ray diffraction
Synthesis of the catalyst was carried out in different steps. Since
(
1
PXRD) patterns of the samples were taken in the 2Â range of
direct impregnation of Cs salt of Fe metal modified lacunary phos-
photungstate to MCM-41 is not possible because of the insolubility
of the Cs salts in any solvent, in the first step sodium salt of iron
substituted lacunary phosphotungstate was prepared. The second
◦ ◦ ◦
to 10 at a rate of 2 /min in steps of 0.01 (Rigaku Miniflex
set at 30 kV and 15 mA) using Cu K␣ radiation. The high angle
XRD patterns of powdered samples were taken in the 2Â range of
◦ ◦ ◦
1
0 –80 at a rate of 1.2 /min (Philips analytical 3710) using Cu K␣
+
step involved the impregnation of Cs ion on MCM-41, followed
radiation. The FT-IR spectra of the samples were recorded using
by the desired lacunary anion produced from sodium salt of iron
substituted lacunary phosphotungstate. The detailed procedure is
given below.
For comparison purpose during various characterizations, a
plain Cs salt of lacunary phosphotungstate without Fe substitution
and a Cs salt of Fe substituted lacunary phosphotungstate have also
been prepared.
−1
Varian FTIR-800 in KBr matrix in the range of 4000–400 cm . Dif-
fuse reflectance UV–vis (DRUV-vis) spectra of the photo catalyst
samples were recorded with a Varian Cary 100 spectrophotome-
ter equipped with a diffuse reflectance accessory in the region
00–800 nm. Raman measurements were made using a Jobin-Yvon
T64000 Raman spectrometer, configured in a single spectrograph
mode. The spectra were recorded against the boric acid back-
2
ground. The acid character of the samples was studied by NH -TPD
3
AutoChem-II, (micromeritics) Chemisorption analyzer equipped
with a thermal conductivity detector (TCD). About 1 g of powdered
sample contained in a quartz “U” tube was degassed at 250 C for 1 h
2
.2. Synthesis of sodium salt of iron substituted lacunary
◦
phosphotungstate (Na5FePW11O39
)
with ultra-pure nitrogen gas. After cooling the sample to room tem-
perature, NH (20% NH balanced with helium) gas was passed over
The sodium salt of lacunary heteropoly compound modified
with iron ion was prepared by the alkalization of a solution of dode-
catungstophosphoric acid with an aqueous solution of NaHCO3.
First H PW12O40·nH O (2.88 g) was dissolved in water (10 mL) and
3
3
◦
−1
the sample while it was heated at a rate of 10 C min and the pro-
file was recorded. The scanning electron microscopic figures of the
sample were recorded using a Hitachi S3400 N. Elemental analysis
was performed by AAS using PerkinElmer Analysis 300 with acety-
lene flame and by ICP-OES using PerkinElmer Optima 2100 DV. The
X-ray photoelectron spectra of Fe, W, Si and O were recorded using
KRATOS apparatus with Mg, Al and Cu K␣ as X-ray sources.
3
2
the pH of the solution was adjusted to 4.8 using NaHCO3 solu-
tion. This resulted in the formation of lacunary heteropoly anion
7
−
◦
[
PW11O39
constant stirring. A solution of FeCl2 (0.197 g, 1 mmol) in water
10 mL) was added to this hot solution. The Na5FePW11O39 was
]
. The solution having pH 4.8 was heated to 90 C with
(
2.6. Catalytic activity toward bromination of phenol and
obtained by solvent evaporation, and recrystallization from water,
◦
oxidation of trans-stilbene
followed by subsequent drying at 110 C for 12 h.
Bromination of phenol was carried out in a 50 ml two-necked
round bottom flask, charged with 0.2 g catalyst, phenol (2 mmol) in
2
.3. Synthesis of Cs salt of iron substituted lacunary
acetic acid (4 ml) and KBr (2.2 mmol). Then 30% H O2 (2.2 mmol)
2
phosphotungstate supported onto MCM-41 (xLFeW/MCM-41)
was added drop wise to the reaction mixture and the contents in
the flask was stirred continuously at room temperature for 5 h [22].
After 5 h of the reaction, the catalyst was filtered and the solid was
washed with ether. The combined filtrates were washed with sat-
urated sodium bicarbonate solution and then shaken with ether
in a separating funnel. The organic extract was dried over anhy-
drous sodium sulfate. The products were analyzed by GC, through
capillary column.
The oxidation of trans-stilbene was carried out in a 50 ml two-
necked round bottom flask, provided with a mercury thermometer
for measuring the reaction temperature and a reflux condenser. The
reaction mixture containing 0.015 g catalyst, 1.82 g trans-stilbene
The parent MCM-41 was synthesized by sol–gel method [20].
A series of catalysts having different loading of Cs salt of Fe modi-
fied lacunary phosphotungstate (30–60 wt%) were synthesized by
incipient weight impregnation method by adopting the following
procedure.
MCM-41 was first impregnated with aqueous solution of the
Cs+ precursor (Cs CO ), dried at 110 C for 12 h [21]. Following
this, an aqueous solution of Na5FePW11O39 was impregnated, dried
at 110 C for 12 h and calcined at 200 C for 3 h. The catalysts are
designated as xLFeW/MCM-41 (x = 30–60 wt%).
◦
2
3
◦
◦

5
4
S. Rana et al. / Catalysis Today 198 (2012) 52–58
1
.0
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
a
0.8
0.6
(
a) MCM-41
(
b) 50LFeW/MCM-41
(a) MCM-41
b) 50LFeW/MCM-41
(
0
.4
0.2
.0
a
b
b
0
0
2
4
6
8
10
0.0
0.2
0.4
0.6
0.8
1.0
Pore Diameter (nm)
Relative pressure p/po
Fig. 2. Pore size distribution curve of MCM-41 (a) and 50 LFeW/MCM-41 (b).
Fig. 1. N2 adsorption–desorption isotherm of MCM-41 (a) and 50 LFeW/MCM-41
b).
(
The PXRD patterns of MCM-41 and 50 LFeW/MCM-41 are shown
in Fig. 3(A). The XRD patterns indicate that the samples exhib-
ited hexagonal structures with a high degree of structural ordering,
(
10 mmol) and 30% H O (20 mmol), 20 ml of acetonitrile was
2 2
◦
heated at 60 C for 4 h in an oil bath with stirring. The reaction prod-
ucts were analyzed by gas chromatograph using capillary column
(
◦
since the materials like MCM-41 exhibit a strong peak at 2Â = 2.2
due to (1 0 0) plane and small peaks due to (1 1 0), (2 0 0) and (2 1 0)
ZB MAX).
◦
plane reflections within 5 indicate the formation of well ordered
mesoporous materials [25]. The XRD patterns of both the samples
are nearly same indicating that the mesoporosity remain intact
after introduction of the LFeW on the silica network. So it is con-
sidered that LFeW salt was highly dispersed on MCM-41. There is
a little bit reduction in intensity, shifting and broadening of the
3
. Results and discussion
Elemental analysis for W and Fe was carried out on the filtrate
by gravimetric and volumetric methods [23], the other elements by
ICP-OES and AAS methods respectively. The observed values for the
elemental analysis of the isolated complex were in good agreement
with the theoretical values. Found: Cs, 19.21; W, 58.33; P, 0.91; Fe,
(
1 0 0) peak in case of 50 LFeW/MCM-41 sample, indicating a slight
disturbance in hexagonal symmetry after modification with LFeW
26].
The wide angle XRD of LFeW and LFeW/MCM-41 is shown in
[
1
.64; Calc: Cs, 19.35; W, 59.52; P, 0.91; Fe, 1.64.
The nitrogen adsorption–desorption isotherms for MCM-41 and
0 LFeW/MCM-41 are shown in Fig. 1. N2 adsorption–desorption
Fig. 3(B). The XRD pattern of LFeW shows that it is crystalline
in nature. But in case of 50 LFeW/MCM-41 sample, the XRD pat-
tern shows a broad peak showing no characteristic peaks of LFeW,
5
resulted in typical type IV isotherm which is defined by Brunauer
et al. [24]. It is observed that there are three different well-defined
stages in the isotherm of MCM-41. The initial increase in nitrogen
uptake at low p/p0 may be due to monolayer adsorption on the
pore walls, a sharp steep increase at intermediate p/p indicates the
capillary condensation in the mesopores and a plateau portion at
(
A) 20000
0
(100)
b
a
higher p/p is associated with multilayer adsorption on the external
surface of the materials [24].
15000
0
Parent MCM-41 sample exhibits N2 uptake at a relative pres-
sure of 0.32 which corresponds to the pre-condensation loop. The
isotherm shows a H4 type hysteresis loop (according to IUPAC
nomenclature) with well-developed step in the relative pressure
range ≈0.9. The incorporation of LFeW in the MCM-41 framework is
(a) MCM-41
1
0000
(b) 50 LFeW/MCM-41
5000
(110)
4
(
200)
5
found to lower the p/p value for capillary condensation step, indi-
0
cating the shift in pore size to lower value due to incorporation of
lacunary acid. The pore diameter is found to decrease with increase
in loading of LFeW content over the MCM-41 surface (Fig. 2).
The textural properties such as BET surface area, pore diame-
ter and pore volume derived from the N2 adsorption–desorption
measurements are included in Table 1. The parent MCM-41 has a
0
2
3
6
7
8
9
10
2
θ (Degree)
(
B)
(
a) 50 LFeW/MCM- 41
b) LFeW
(
2
surface area of 1250 m /g. But there is a gradual decrease in the
b
value with increasing LFeW content in MCM-41.
As can be seen from Fig. 1, there are considerable number
of micropores present in MCM-41. Therefore we suppose, during
impregnation, LFeW goes to the micropores present in MCM-41,
thereby blocking them. After filling the micropores, it occupies the
mesopore walls of MCM-41. Therefore, though there is a decrease in
surface area with increase in loading of LFeW on MCM-41, the trend
of reduction in surface area and pore volume is not proportional to
the decrease in pore size.
a
2
0
30
40
50
60
70
80
2
θ (Degree)
Total acidity of the samples was measured from NH -TPD and
3
is presented in Table 1. It shows that the total acidity of MCM-41
increases with the increase in loading of LFeW.
◦
Fig. 3. (A) Low angle (0–10 ) XRD patterns of MCM-41 (a) and 50 LFeW/MCM-41
◦
(b). (B) Wide angle (10–80 ) XRD patterns of (a) 50 LFeW/MCM-41 and (b) LFeW.

S. Rana et al. / Catalysis Today 198 (2012) 52–58
55
Table 1
Textural properties of different samples.
−1
3
˚
−1
Acidity (mmol g )
Catalyst
MCM-41
Surface area (m2
g
)
Pore volume (cm /g)
Pore diameter (A)
1250
850
715
610
575
0.85
0.72
0.68
0.59
0.53
22.3
21.9
21.4
21.0
20.6
0.238
45.8
48.2
52.7
49.4
30 LFeW/MCM-41
40 LFeW/MCM-41
50 LFeW/MCM-41
60 LFeW/MCM-41
indicating an undegraded and very high dispersion of LFeW in a
non-crystalline form on the surface of MCM-41.
which suggests that the LFeW structures remained intact regard-
less of their functionality. The shifting in the positions of the IR
absorption peaks are due to both hydrogen bonding and chemi-
cal interactions that exist between the surface of LFeW and the
MCM-41.
The UV–vis DRS gives information about the non-reduced het-
eropoly anion due to charge transfer from oxygen to metal. The
UV–vis DRS spectra of the LW, 50 LFeW/MCM-41 and LFeW are
shown in Fig. 5. The LFeW sample exhibits two absorption max-
ima at 200–206 nm and 260–320 nm (ꢁmax), which are attributed to
The FT-IR spectra of various samples are shown in Fig. 4. In
−
1
case of LFeW/MCM-41 the broad band around 3500 cm
may
be attributed to surface silanols and adsorbed water molecules,
while deformational vibrations of adsorbed molecules cause the
−
1
absorption bands at 1623–1640 cm
[20]. The spectrum of the
, shows prominent bands at 1080, 985,
90 and 800 cm which are characteristic of Keggin structure and
are assigned to ꢀ(P O), ꢀ(W O), corner-sharing ꢀ(W O W), and edge-
3−
Keggin anion [PW12O40
8
]
−1
−
1
sharing ꢀ(W O W), respectively [27]. In case of LW the 1080 cm
oxygen-to-tungsten charge-transfer (OMCT) at W O and W
O W
band has been splitted into two components (1084–1044 cm−1),
due to the symmetry decrease of the PO4 tetrahedron. The other
bands found are 953 (ꢀas(W Od)), 860 (ꢀas(W Ob W)), 809 and
bonds, respectively. A broad peak at 680 nm was obtained in LFeW
4
6
which arises due to T ← A transition and a similar weak absorp-
2
1
tion peak was observed for LFeW in the range of around 540 nm
42 cm−1(ꢀas(W Oc W)), and differ from those of [PW12O40
]
3−
[27].
may be the presence of 4T ← 6A transition of iron [28]. The ꢁmax
1
1
7
The spectra for LFeW showed characteristic splitting for the P
O
of 50 wt% loading of LFeW on MCM-41 is same as that for LFeW
(Fig. 5). This confirms the presence of the undegraded LFeW on the
surface of MCM-41 which is online with the XRD results.
−1
bond frequency at 1074 and 1052 cm , which faced a slight shift-
ing toward lower frequency compared to bulk lacunary unit. This
clearly indicated that Fe was introduced into the octahedral lacuna.
The slight shifting of bands in FT-IR spectra of LFeW sample com-
pared to bulk LW may be due to formation of pseudo-symmetric
environment that resulted from the replacement of a W atom with
a Fe atom. It can be observed that 50 LFeW/MCM-41 sample has
similar vibration bands to those of the corresponding pure LFeW,
To study the surface morphology and to assess the dispersion of
the active components over the surface of MCM-41, SEM investiga-
tion was performed for 50 LFeW/MCM-41 (Fig. 6). The sample does
not have a well-defined hexagonal structure like MCM-41. Further,
aggregates without regular shapes are observed, which is in agree-
ment with the reported literature for metal incorporated materials
[
4
29], indicating a slight reduction in hexagonal symmetry of MCM-
1 due to metal incorporated lacunary salt modification, which is
in line with the low angle XRD results.
The XPS investigation of binding energies and intensities of the
surface elements provides information on the chemical states and
relative quantities of the outermost surface compounds. Fig. 7a dis-
plays the XPS survey spectrum obtained for 50 LFeW/MCM-41. The
main peaks indicate the presence of W,O, P, Cs, Si and Fe in the sam-
ple. One distinct peak can be observed at binding energy 724 eV for
the 50 LFeW/MCM-41 samples. The binding energy of about 724 eV
+
for the Cs 3d5/2 peak is characteristic of Cs . The W 4f XPS spectrum
of 50 LFeW/MCM-41 is shown in Fig. 7b. The catalyst shows two
peaks at 35.62 and 37.15 eV. The shifting of W 4f peaks toward a
higher BE indicates the flow of electron density toward silica from
LFeW through the Si
O W linkages [30].
Fig. 4. FT-IR spectra of LW (a), LFeW (b) and 50 LFeW/MCM-41(c).
Fig. 5. UV–vis DRS of LW (a), 50 LFeW/MCM-41 (b) and LFeW (c).

5
6
S. Rana et al. / Catalysis Today 198 (2012) 52–58
Fig. 6. SEM image of 50 LFeW/MCM-41.
The Fe 2p XPS spectrum of 50 LFeW/MCM-41 samples is shown
number values due to strong interactions between the MCM-41
support and lacunary Keggin unit [35]. These results confirm the
incorporation of undegraded LFeW on MCM-41 surface.
in Fig. 7c. Two distinct iron peaks can be observed at binding ener-
gies 709.76 and 723.59 eV for the 50 LFeW/MCM-41 samples. The
binding energy of about 709.5 eV for the Fe 2p3/2 main peak is in
agreement with the typical values for the iron oxides reported in
the literature [31,32]. The peaks are shifted toward a higher bind-
ing energy, which indicated the strong interaction of iron within
the lacunary structure and with the support.
3.1. Catalytic activity
Many studies were already carried out on oxidation of trans-
stilbene as well as bromination of phenol using various catalysts.
Maurya and Kumar [36] reported oxidation of trans-stilbene.
But the inherent disadvantages associated were higher tempera-
ture, and longer reaction time. Our group (Parida and co-workers
[37]) reported bromination of phenol over heteropoly acid (HPA)-
impregnated zirconium phosphate (ZrP), with 86% conversion. But
the most enchanting part of the present study is that, this single
catalyst is showing its superlative catalytic activity toward both
the reactions.
Raman scattering spectroscopy is considered to be an effective
technique for studying the structure of LFeW and its incorpora-
tion on MCM-41. The Raman scattering spectra of LFeW and 50
LFeW/MCM-41 are shown in Fig. 8. The bulk LFeW gives peaks at
−1
9
66, 921, 885 and 824 cm which are attributed to the stretch-
ing vibrations of P O, W Ob W, W Oc W and W Ot bonds
of metal modified mono lacunary Keggin unit, respectively. The
−1
band at 716 cm
may be due to WO3 [33]. The band at about
20 cm may be attributed to symmetric stretching vibration of
Oa band [34]. The 50 LFeW/MCM-41 sample shows all the above
−1
2
W
We have investigated the use of various LFeW/MCM-41 as cat-
alysts in the acid catalyzed bromination of phenol as well as in the
oxidation of trans-stilbene. The bromination of phenol was carried
out in acetic acid medium with KBr and hydrogen peroxide at room
described peaks of LFeW, but the intensity of the peaks are low,
compared to the bulk LFeW and slightly shifted toward higher wave
Table 2
Conversion and selectivity of various catalysts toward bromination of phenol.
Catalyst
Conversion (%)
Selectivity (%)
Yield (%) of p-bromophenol
p-Bromophenol
o-Bromophenol
MCM-41
LFeW
32
59
76
85
95
89
35
40
38
29
1
65
60
62
71
99
81
21
35
47
60
94
72
30 LFeW/MCM-41
40 LFeW/MCM-41
50 LFeW/MCM-41
60 LFeW/MCM-41
19
Condition: Phenol (2 mmol), KBr (2.2 mmol), catalyst (0.02 g), time (5 h).
Table 3
Conversion and selectivity of various catalysts toward oxidation of trans-stilbene.
Catalyst
Conversion (%)
Selectivity (%)
Yield (%) 0f Trans-stilbene oxide
Trans-stilbene oxide
Benzaldehyde
MCM-41
LFeW
5
14
16
36
52
46
43
63
75
87
99
90
57
37
25
13
1
2
9
12
31
51
41
3
4
5
6
0LFeW/MCM-41
0LFeW/MCM-41
0LFeW/MCM-41
0LFeW/MCM-41
10
Condition: Trans-stilbene (10 mmol), H2O2 (20 mmol), catalyst (0.015 g), time (4 h).

S. Rana et al. / Catalysis Today 198 (2012) 52–58
57
temperature. The results show that all the catalysts were efficient
for catalyzing the reaction. The selectivities of para- and ortho-
bromo phenol obtained are shown in Table 2. Among the catalysts
with different LFeW loading, 50 LFeW/MCM-41 showed highest
conversion 95% with 99% selectivity toward p-bromophenol. Fur-
ther, increase in the LFeW loading decreased the phenol conversion.
The acid sites of the catalysts are given in Table 1. It is evident from
the data that there is an initial increase in the acidity up to 50 wt%
loading, and thereafter the acidity decreases. The highest acidity
for 50 wt% loading may be due to the formation of monolayer cov-
erage of LFeW on MCM-41. The decrease in the surface acidity at
high LFeW concentration is probably due to the formation of poly
layer coverage of LFeW on MCM-41.
a
1
1
2000
0000
Fe
Cs
O
8
6
4
2
000
000
000
000
P
W
Si
0
200
400
600
800
1000
When the reaction was carried out without catalyst the conver-
sion was only 25% with nearly equal ortho and para selectivity.
From the selectivity data, it is supposed that phenol may inter-
act with the Lewis acid sites of the catalyst, as a result, the electron
density at ortho positions of the aromatic ring decreases, which hin-
Binding Energy (eV)
9
9
9
9
9
8
8
8000
6000
4000
2000
0000
8000
6000
b
W4f
7
/2
W4f
5/2
+
ders the attack of incoming electrophile (Br ) at two ortho position
and preferred to attack on para position [38,39].
Various solvents including carbon tetrachloride, hexane,
dichloromethane, methanol, acetonitrile and acetic acid were used
for this reaction. The best results were obtained when acetic acid
was used as a solvent compared to others. In presence of H O ,
2
2
acetic acid gives peracetic acid, which is a stronger oxidant than
−
+
H O and efficiently oxidizes the Br to Br , which could not be
2
2
observed in case of other solvents. It is assumed that the catalyst
LFeW/MCM-41 reacts with hydrogen peroxide and forms peroxo
species in presence of acetic acid [40]. The formed peroxo metal
species then enhances the oxidation of Br (KBr) to Br+ (HOBr),
which reacts in presence of active centers of LFeW/MCM-41 with
the phenol to give, brominated compounds.
3
0
32
34
36
38
40
42
44
46
Binding Energy (ev)
c
74000
−
Fe 2p
3
/2
Fe 2p
1/2
7
7
6
6
6
6
6
2000
0000
8000
6000
4000
2000
0000
In order to establish the oxidative nature of the synthesized
catalyst, oxidation of trans-stilbene was carried out using hydro-
gen peroxide as oxidant and the results are shown in Table 3. It is
found that pure MCM-41 showed 5% conversion. The conversion
of trans-stilbene increases with increase in loading of LFeW up to
5
0 wt%. An optimum 52% conversion with 99% selectivity toward
trans-stilbene oxide was observed using 50 LFeW/MCM-41 which
decreased on further increase in LFeW loading.
7
00
710
720
3.2. Heterogeneity test
Binding Energy (eV)
Since LFeW is water tolerant at room temperature, we carried
Fig. 7. (a) XPS spectra of 50 LFeW/MCM-41. (b) XPS of W4f7/2 and W4f5/2 of 50
LFeW/MCM-41. (c) XPS of Fe2p3/2 and Fe2p1/2 of 50 LFeW/MCM-41.
out a heterogeneity test only for the oxidation reaction which was
carried out at a higher temperature in order to know whether the
catalyst was truly behaving heterogeneously or not toward this
reaction. During the catalytic oxidation of trans-stilbene, the solid
catalyst was separated from the reaction mixture by filtration after
2
h of the reaction and the filtrate obtained was continuously stirred
under same reaction conditions for further 3 h. GC analysis showed
no increment in the conversion. This fact suggests that there is no
loss of catalyst components during the course of the reaction which
confirmed the heterogeneity of the catalyst.
3.3. Recyclability of the catalyst
The catalyst with 50 wt% loading was used for recycling exper-
iments. In order to regenerate the catalyst after completion of
reaction, it was separated by filtration, washed several times with
◦
conductivity water and dried at 110 C. The material was used for
further run in the bromination of phenol with a fresh reaction mix-
ture. In the regenerated sample after two cycles, the yield decreased
by 3%. The activity loss observed with the regenerated catalyst
Fig. 8. Raman spectra of LFeW (a) and 50LFeW/MCM-41 (b).

5
8
S. Rana et al. / Catalysis Today 198 (2012) 52–58
could be due to partial loss of acid sites of the catalyst during
reaction/regeneration.
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[
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We have developed a Cs salt of Fe substituted mono lacunary
2329–2364.
phosphotungstate material and supported it on MCM-41 to obtain
a new and robust heterogeneous catalyst. The XRD and nitrogen
adsorption–desorption studies revealed that the modified samples
retained the mesoporosity. The pore diameter and pore volume
decreased with an increase in LFeW loading. FT-IR, UV–vis DRS and
XPS studies revealed the presence of Fe atoms in lacunary unit and
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1. The material proved to be an efficient catalyst for acid catalyzed
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[
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bromination of phenol as well as oxidation of trans-stilbene. Good
catalytic activity and recycling efficiency for acid catalyzed as well
as oxidation reaction suggests the potential application of this Cs
salt of Fe substituted mono lacunary phosphotungstate supported
MCM-41 material for the synthesis of additional organic molecules.
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