SrFe2O4 complex oxide an effective and environmentally benign catalyst for selective oxidation of styrene

Article abstract of DOI:10.1016/j.molcata.2010.10.020

The SrFe₂O₄ spinel type catalyst is synthesized by citrate gel method. The precursor and oxide are well characterized by various techniques such as TG-DTG, FT-IR, X-ray diffraction, SEM, and X-ray fluorescence. The crystallization temperature of the spinel particle prepared by citrate gel method is 600 °C, which is lower than that of ferrite prepared by other methods. SrFe₂O₄ catalysts prepared by citrate gel method show high activity for styrene oxidation in the presence of H ₂O₂ (30%) as an oxidizing agent and aqueous medium. GCMS analysis revealed that, during the course of reaction, the oxidative cleavage of CC double bond of styrene takes place selectively to give benzaldehyde selectivity up to 64 mol%, whereas phenyl acetaldehyde selectivity 28 mol% and others' selectivity 8 mol% as minor product. Solvents have marked influence on the product distribution in the selective oxidation of styrene; water seems to be the best solvent. The optimization and effect of various reaction conditions on the conversion of styrene and product distribution were also studied. The highest benzaldehyde yield was obtained at 70 °C in water solvent over 0.1 g of catalyst with 18 h of reaction.

Full text of DOI:10.1016/j.molcata.2010.10.020

Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
SrFe2O4 complex oxide an effective and environmentally benign catalyst for
selective oxidation of styrene
Satish K. Pardeshi∗, Ravindra Y. Pawar
Department of Chemistry, University of Pune, Ganeshkhind, Pune, Maharashtra-411007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2010
Received in revised form 19 October 2010
Accepted 25 October 2010
Available online 30 October 2010
The SrFe2O4 spinel type catalyst is synthesized by citrate gel method. The precursor and oxide are well
characterized by various techniques such as TG–DTG, FT-IR, X-ray diffraction, SEM, and X-ray fluores-
cence. The crystallization temperature of the spinel particle prepared by citrate gel method is 600 ◦C,
which is lower than that of ferrite prepared by other methods. SrFe2O4 catalysts prepared by citrate gel
method show high activity for styrene oxidation in the presence of H2O2 (30%) as an oxidizing agent
and aqueous medium. GCMS analysis revealed that, during the course of reaction, the oxidative cleavage
of C C double bond of styrene takes place selectively to give benzaldehyde selectivity up to 64 mol%,
whereas phenyl acetaldehyde selectivity 28 mol% and others’ selectivity 8 mol% as minor product. Sol-
vents have marked influence on the product distribution in the selective oxidation of styrene; water
seems to be the best solvent. The optimization and effect of various reaction conditions on the conver-
sion of styrene and product distribution were also studied. The highest benzaldehyde yield was obtained
at 70 ◦C in water solvent over 0.1 g of catalyst with 18 h of reaction.
Keywords:
Spinel SrFe2O4
Styrene oxidation
Aqueous medium
Hydrogen peroxide
Benzaldehyde selectivity
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
perature and coagulation area by a heat transfer simulation that
takes the heat generation of ferrite powder, the metabolism heat
Spinel ferrites are of great fundamental and technological
important due to their structural, electronic, magnetic and cat-
alytic properties [1–3]. It has been recognized that they were
used as permanent magnets, recording media, telecommunication,
components in microwave, higher-frequency, and magneto opti-
cal devices [4–10]. The physico-chemical properties of the ferrites
are strongly dependent on the sites and the nature of the catalyst
[11,12], which are closely related to the method of preparation.
There are two kinds of lattices for cation occupancy. A and B sites
have tetrahedral and octahedral coordination respectively. In the
normal spinel structure divalent atom, occupying tetrahedral A
sites, while trivalent atom, are sitting on the octahedral B sites.
When ‘A’ sites being trivalent ions, while ‘B’ sites equally popu-
lated by divalent and trivalent ions, the spinel structure is referred
to as the inverse kind [13,14].
and the cooling effect of blood flow into account. The strontium
ferrite is widely applied in permanent magnetic material due to
various advantages such as abundant raw material, low manufac-
turing cost, stable properties and moreover the sample cannot be
oxygenated [15].
Several techniques have been used to prepare strontium ferrites
such as ball milling [16], salt melting [17], chemical precipitation
[18], glass crystallization [19] and self propagating high temper-
ature synthesis [20]. However, most of these methods cannot
be economically applied on a large scale because they require
expensive and often toxic reagents, complicated synthetic steps,
high reaction temperatures and long reaction times. This not only
results in waste of energy but also harms our environment. These
routes require prolonged thermal treatment to improve the crys-
tallinity, purity, and morphology of the powders. Hence a chemical
route can be excellent method for the synthesis of highly pure
multi-component oxide due to its simplicity, good-control grain
size, better homogeneity, better compositional control and lower
processing temperatures which are few potential advantages of
this wet chemical route over the conventional solid state reac-
tion method [21]. Spinel ferrites are found to be highly active
towards many aromatic alkylation reactions such as methylation
of phenol, aniline, pyridine, phenol tert-butylation, etc. [22–24].
The alkaline earth metal ferrites like MgxFe3−xO4 and CaFe2O4 are
used as catalyst for the selective oxidation of styrene in the pres-
The alkaline earth metal ferrites (MgFe2O4, CaxMg1−xFe2O4)
powder with a particle diameter of 7–15 nm are used for the
necrosis of the tumor; hyperthermia is an alternative to the con-
ventional surgical, chemical therapy and radiation therapies for
cancer. Mg0.5Ca0.5Fe2O4 powder is used to predict the tumor tem-
∗
Corresponding author. Tel.: +91 020 25601394x514; fax: +91 020 25691728.
E-mail addresses: skpar@chem.unipune.ac.in, skpar@chem.unipune.ernet.in
(S.K. Pardeshi).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.020

36
S.K. Pardeshi, R.Y. Pawar / Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
ence of 30% H2O2 as an oxidizing agent and acetone as a solvent
[25,26].
a - TGA curve
b - DTA curve
c - DTG curve
The catalytic effectiveness of this system is due to the abil-
ity of metallic ions to migrate between the sub lattices without,
altering the structure, which makes the catalyst efficient for many
organic transformation reactions. The selective liquid-phase oxida-
tion reaction is of significant importance in the fine chemicals and
pharmaceutical industries [27,28]. Some strong oxidants such as
KMnO4, CrO3, and HNO3 [29,30] were applied traditionally for the
oxidation of various substrates, which might result in much serious
pollution and some potential risks in the process of operation. In
terms of atom efficiency and environment friendly, after oxygen,
aqueous hydrogen peroxide (30% H2O2) is a very attractive oxidant
for industrial applications since water is the only by-product, and it
is easy to be dealt with after reactions. The latest studies have there-
fore focused on the catalytic oxidation of styrene with H2O2 as the
terminal oxidant [31–36]. Almost all these oxidation reactions have
been carried out in organic solvents.
In the present paper, first time we are reporting the results for
styrene (one of the most important pro-chiral alkenes) oxidation
reaction in water as solvent using environmentally benign oxidant
H2O2 over SrFe2O4. SrFe2O4 (s), of Sr–Fe–O system was synthesized
by citrate–nitrate gel combustion route and characterized by vari-
ous techniques. The conditions for maximum conversion of styrene
as well as selectivity for desired product have been optimized by
varying different parameters such as temperature, molar ratio of
styrene to H2O2, amount of catalyst, reaction time, calcination tem-
perature and various oxidizing agents.
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.0005
c
0.0000
2
196ºC
9.62%
309ºC
-0.0005
-0.0010
b
0
53.63%
-0.0015
29.56%
72ºC
601ºC
-0.0020
a
-0.0025
-2
14.45%
-0.0030
-0.0035
-40.0040
192ºC
200
100
300
400
500
600
700
800
900
Temperature (ºC)
Fig. 1. TG–DTG–DTA study of strontium iron citrate precursor prepared by citrate
gel method.
are examined by scanning electron microscopy (SEM) using a JEOL-
2011 microscope. Chemical analysis of strontium ferrite carried out
by X-ray fluorescence method.
2.3. Catalytic activity
2. Experimental
The selective oxidation of styrene was carried out in a round
bottom flask (25 ml) equipped with X-crossed Teflon coated mag-
netic stirrer and a reflux condenser. 10 milimole styrene [99+%,
Aldrich], 10 ml water, 0.1 g catalyst, 1.0 ml of hydrogen peroxide
(30% aq. Merck), (styrene/H2O2)molar = 1 are added successively
into the flask. The flask is then immersed in oil bath at a fixed
temperature and time. The reaction was carried out for the differ-
ent temperatures and time. The reaction products were analyzed
with a gas chromatograph equipped with a XE-60 capillary col-
umn (30 m × 0.25 × 0.3 ␮m) and a flame ionization detector. The
injector and column temperature were 280 and 140 ◦C respectively.
3-Nitrotoluene (99+%, Aldrich) is used as an internal standard. The
residual H2O2 in solution is determined by ceric sulphate titration
method. The styrene conversion and the selectivity of the reaction
products are determined by the formulae [26,37].
2.1. Synthesis of catalyst
Analytical grade Fe(NO3)3·9H2O, Sr(NO3)2 and anhydrous cit-
ric acid obtained from MERCK (specialties Pvt. Ltd., Mumbai, India)
were used without further purification as raw materials to prepare
SrFe2O4. The initial solution is prepared by dissolving ferric nitrate,
strontium nitrate and citric acid into double distilled water. Con-
centrations of ferric nitrate, strontium nitrate are 0.5 M and 0.25 M
respectively. The molar amount of citric acid is equal to that of metal
nitrates in the solution. Citric acid helps in the homogeneous dis-
tribution of the metal ions to get segregate from the solution. The
solution is continuously stirred for 1 h and kept at a temperature of
60 ◦C until it turns to sol. Then the stabilized nitrate–citrate sol was
rapidly heated to 80 ◦C and stirred constantly. Viscosity and color
changed as the sol turned into a transparent stick gel. The gel was
heated in evaporating dish where an auto combustion process takes
place. Finally, the brown, floppy precursor is calcined at 700 ◦C for
2 h in muffle furnace [26].
3. Results and discussion
3.1. Characterization
2.2. Physico-chemical characterization
3.1.1. Thermal decomposition of strontium iron citrate precursor
Fig. 1 shows TG–DTG–DTA curve for the thermal decomposition
of strontium iron citrate precursor. The first weight losing peak was
observed in the range of 50–180 ◦C and the corresponding weight
loss is 9.6% corresponding to partial dehydration of the five crys-
tallizing water molecule. This dehydration step is clearly seen as a
peak in DTG at 72 ◦C. The second weight loss is 29.6% in the range of
175–300 ◦C corresponding to the loss of remaining water molecules
and partial decomposition of citrate ligand, it matches well with the
calculated value of 28.9%, this is also seen in the form of well defined
peak at 196 ◦C in DTG curve and one small shoulder at 309 ◦C. The
further weight loss in TG curve is of 14.5% (calculated 14.3%) in the
range of 500–600 ◦C corresponding to the complete decarboxyla-
tion of the precursor into mixed oxide. A peak at 601 ◦C in DTG
corresponds to the decarboxylation step. After 601 ◦C no notable
The thermal decomposition of the precursor is studied by
thermo gravimetric analysis (TG/DTG/DTA) is performed on a
Rigaku Thermo flex system with ramp rate of 10 ◦C min−1 in flowing
air. The stretching vibration frequencies of the precursor and the
catalyst are studied by FTIR (Fourier transform infrared spectra)
spectroscopy on a Nicollet NEXUS 7000C spectrophotometer using
the KBr pellet method. The phases of the synthesized catalysts were
characterized by X-ray diffraction (XRD) using an ARL SCINTAG
X’TRA X-ray powder diffractometer equipped with a position-
sensitive detector, allowing all angles between 20◦ and 80◦ to be
read simultaneously, at a scan rate of 4◦ min−1. The mean crystallite
size was estimated by high intensity broadening technique using
the Scherrer equation [24]. The morphology and size of the particles

S.K. Pardeshi, R.Y. Pawar / Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
37
weight loss was found which indicate that SrFe2O4 was formed
a - Precursor
above 600 ◦C along with the trace quantity of strontium carbonate.
The DTA curve indicates a broad endothermic peak at 80 ◦C,
corresponding to dehydration step. Two exothermic peaks in DTA
curve at 196 and 309 ◦C are attributed to the partial dehydration
and decomposition of citrate ligand (29.6% weight loss). The whole
thermal process of decarboxylation is accompanied by the evo-
lution of large amount of gas, due to vigorous oxidation reaction
[38]. The exothermic temperature in the DTA pattern is consistent
with the major weight loss temperature in the TG pattern. Finally
an endothermic peak at 601 ◦C in DTA indicates the transition of
precursor to strontium ferrite.
b - calcined at 600ºC
c - calcined at 700ºC
d - calcined at 800ºC
e
d
c
*
e - calcined at 900ºC
*
*
# - SrFe O4
* - SrCO23
*
*
#
#
#
#
b
a
25 30 35 40 45 50 55 60 65 70 75 80
3.1.2. XRD studies of the as-prepared precursor and the calcined
samples
Two Theta
Fig. 2. X-ray diffraction pattern of strontium iron citrate precursor (a) strontium
ferrite prepared by citrate gel method; (b) calcined sample at 600 ◦C, (c) 700 ◦C, (d)
800 ◦C and (e) 900 ◦C.
The crystallinities and phase purity of the as-prepared precur-
sor and the calcined products were examined by powder X-ray
diffraction (XRD). Fig. 2 shows the XRD patterns of the precursor
as well as the samples after calcination at different temperatures.
All the peaks of the pattern for the calcined samples can be easily
indexed to cubic SrFe2O4 with spinel structure, where the diffrac-
tion peaks at 2Â values of 32.7, 37.8, 46.8 and 58.3◦ of pattern (c)
can be ascribed to the reflection of (0 0 2), (3 3 1), (4 2 0) and (5 2 0)
planes of the spinel SrFe2O4 respectively [39]. While the peak at
68.5 and 78 (shown by * in Fig. 3) 2Â values indicate the presence of
trace quantity of strontium carbonate associated with the SrFe2O4
phase (JCPDS card nos. 74-1491 and 84-1778). Using the Scherrer
equation [40], the average size of the crystallites can be estimated
from the full-width at half maximum (FWHM) of the highest inten-
sity peak. As the calcination temperature increases from 700 ◦C to
900 ◦C the average crystallite size also increases from 50 nm to
82 nm which indicates the aggregation of the crystals. From the
XRD studies it was confirmed that the single phase SrFe2O4 with
trace of SrCO3 is formed at 700 ◦C temperature so this compound
was selected for the further characterization.
Table 1
Elemental analysis of strontium ferrite associated with strontium carbonate by X-ray
fluorescence method in percentage by weight.
Strontium
Calculated
42.64
Iron
Observed
41.70
Calculated
27.18
Observed
28.32
3.1.3. X-ray fluorescence analysis
The elemental composition of the SrFe2O4 prepared from
strontium iron citrate calcined at 700 ◦C was analyzed by X-ray flu-
orescence method. The percentage composition of the strontium
and iron matches with the theoretical value as shown in Table 1.
The result also indicates that the catalyst is associated with the trace
quantity of strontium carbonate which gives the shoulder peak at
1435 cm−1 in IR spectrum.
Fig. 3. Scanning electron microscopy image of strontium ferrite prepared by citrate gel method calcined at 700 ◦C.

38
S.K. Pardeshi, R.Y. Pawar / Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
and 1516 cm−1 [41] for the samples calcined at 600 ◦C, 700 ◦C and
90
80
70
60
50
40
30
20
10
0
d
c
800 ◦C.
a - 600ºC
b - 700ºC
c - 800ºC
d - 900ºC
437
3.2. Catalytic properties of SrFe2O4 in selective oxidation of
styrene
1516
1516
1518
1435
869
The oxidation of hydrocarbon to oxygenic compounds is a piv-
otal reaction in organic chemistry, both the fundamental research
and industrial manufacturing [42,43]. Now, from both economic
and environmental point of view, much attention has recently
been focused on the aerobic catalytic oxidation of hydrocarbon to
oxygenic compounds using metal catalysis. The metal ferrites are
known to be highly efficient oxidation catalysts which have been
successfully used to heterogeneously catalyze the transfer of an
oxygen atom from oxidizing agent into hydrocarbon molecules in
inorganic solvent [25].
SrFe2O4 synthesized by citrate gel method, characterized for
the structure, morphology and confirmed to be fine materials with
spinel structure was then used as catalyst to evaluate its catalytic
efficiency. The oxidation of styrene to benzaldehyde was used as a
model reaction.
567
580
442
453
b
a
1435
1433
867
867
2359
2500
2000
1500
1000
500
Wavenumber(Cm-1)
Fig. 4. FTIR spectra of strontium ferrite prepared by citrate gel method calcined at
different temperatures (a) 600 ◦C, (b) 700 ◦C, (c) 800 ◦C and (d) 900 ◦C.
3.1.4. Scanning electron microscope
The scanning electron microscope (SEM) measurement was car-
ried out to study the morphology of the sample. Fig. 3 shows
the microstructure and morphology of strontium ferrite pow-
der calcined at 700 ◦C for 2 h. From the photograph we can find
that the morphology of the material is like fine flakes and well
defined monodispersed particles at different magnifications are
observed.
3.3. The role of catalyst
The selective oxidation of styrene was carried out in presence
of 0.1 g of SrFe2O4 catalyst, 30% H2O2, water (10 ml) as reaction
solvent, and at 70 ◦C temperature for 18 h, 50.8 mol% styrene con-
version take place along with 63.7 mol% selectivity and 32.4 mol%
yield of benzaldehyde. A free radical mechanism may be involved
in selective oxidation of styrene over SrFe2O4 catalysts. In case of
styrene oxidation in presence of H2O2 (30%) as oxidant, free radicals
can be generated on the surface of catalyst, where catalyst acceler-
ates the decomposition of hydrogen peroxide into free radicals.
In order to check the role of strontium ferrite towards selective
oxidation of styrene, a blank reaction was carried out using 30%
H2O2, water (10 ml) and temperature 70 ◦C, and the reaction was
stirred under the similar reaction conditions for 18 h. In absence of
SrFe2O4 catalyst no reaction takes place as seen from GCMS analy-
sis. This observation ruled out the possibility of the reaction-taking
place due to the thermal decomposition of H2O2.
3.1.5. FTIR studies
Fig. 4(a)–(d) shows FTIR spectrum of SrFe2O4 powder calcined
at different temperatures such as 600 ◦C, 700 ◦C, 800 ◦C and 900 ◦C
respectively. No significant peaks are found below 1000 cm−1 in
spectrum (a), while in spectrum (b), two assigned absorption bands
appeared around 580 cm−1 (ꢀ1), attributed to stretching vibration
of tetrahedral groups Sr2+–O2− and that around 453 cm−1 (ꢀ2),
attributed to the octahedral group complex Fe3+–O2−. These two
weak absorption bands are typical of spinel ferrites and the posi-
tions of these infrared bands are in the range which corresponds
to strontium ferrite. Band localized at 867 cm−1 is assigned for
deformation vibration of the C–H group in Fig. 4(a) and (b) which
is not found in spectrum (c) and (d). The spectrum also reveals
that the carboxylates of the precursor transform into metal car-
bonate with the characteristic stretching location at 1435 cm−1
In Scheme 1, the formation of metal-oxy radical (Fe4+–O•) on the
catalyst surface is the initiation step of the reaction and the prop-
agation of the reaction chain occurs in solution. A highest yield
of benzaldehyde is possibly due to further oxidation of styrene
18 h/ 40 oC
Acetone
OH
-H2O
Fe3+
Catalyst
surface
Oct. site
.
Fe4+
O
Fe3+
+
H2O2(30%)
O H
Iron peroxo species
C
CH2
Fe4+
CH CH2
+
Fe4+
.
O
.
O
Styrene
Pi-bnded transient species
OH
O
O
C
CH
+
+
OH
O
Fe4+
Metalloepoxy species
Benzaldehyde
1-Phenyl-1,2-ethanediol
Phenyl acetaldehyde
Scheme 1. Reaction mechanism for the selective mechanism of styrene to benzaldehyde over SrFe2O4 catalyst.

S.K. Pardeshi, R.Y. Pawar / Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
39
80
70
100
a
b
c
d
e
a
90
b
80
c
d
e
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Time (h)
Water
Acetonitrile
Methanol
Ethanol
Acetone
Fig. 6. Effect of reaction time on styrene conversion and product selectivity over
SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of benzaldehyde, (c) phenyl
acetaldehyde, (d) others and (e) yield of benzaldehyde.
Solvents
Fig. 5. Effect of reaction medium (solvent) on styrene conversion and product selec-
tivity over SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of benzaldehyde,
(c) phenyl acetaldehyde, (d) others and (e) yield of benzaldehyde.
hyde is a maximum (63.7 mol%) in water with 32.4% yield. This
effect of solvents strongly supports that the selective oxidation of
styrene to benzaldehyde takes place through radical mechanism.
The fact was confirmed by carrying the same reaction in presence
of tertiary butyl alcohol as a scavenger and by keeping all the reac-
tion conditions constant. It is observed that only 5 mol% styrene get
converted to benzaldehyde. This observation confirms a free radical
mechanism is involved in styrene oxidation over a heterogeneous
catalyst (see Scheme 1).
oxide formed in the first step by a nucleophilic attack of Fe4+–O• on
styrene followed by cleavage of the intermediate carbon–carbon
double bond (see Scheme 1). As the reaction is carried out in water
as solvent and high amount of water is present in 30% H2O2 may
partly be responsible for the possible hydrolysis of styrene oxide to
1-phenylethane-1,2-diol in reaction.
3.4.2. Influence of reaction time
3.4. Optimization of reaction condition
The styrene conversion and product selectivity is plotted as a
function of reaction time, at 70 ◦C, styrene/H2O2 molar ratio 1:1,
in water as reaction medium over 0.1 g of SrFe2O4 catalyst. Under
these reaction conditions, the reaction was carried out by vary-
ing the time from 6 h to 30 h. The results are shown in Fig. 6. It
was found that at 6 h the conversion of styrene is 13.6 mol% with
13.3 mol% yield of benzaldehyde. As the reaction time increases
to 12 h, styrene conversion increased to 33.5 mol% with 71.1 mol%
selectivity of benzaldehyde and 28.9 mol% of phenyl acetalde-
hyde. At 18 h styrene conversion increases up to 50.8 mol%, while
the selectivity and yield of benzaldehyde is 63.7 mol% and 32.4%
respectively, along with the formation of byproducts like phenyl
acetaldehyde and 1-phenyl-1,2-ethanediol up to 28.1 and 8 mol%
respectively. If the same reaction was run for 24 and 30 h, marginal
increment is observed in styrene conversion i.e. 54.2 and 55.8 mol%,
but the selectivity of benzaldehyde suddenly decreases to 53.5 and
51.6 mol% with % yield 29 and 28.8 respectively. The result indicates
that as the reaction is run for long time the selectivity of byproducts
increases up to 49 mol%. This may be due to the complete exhaus-
tion of H2O2 in the reaction mixture as the reaction was run for
long duration [45].
3.4.1. Influence of the solvents
The influence of solvent was studied by carrying the reaction
in different solvents like ethanol, methanol, acetonitrile, acetone
and water on the styrene conversion and selectivity of products
over SrFe2O4 catalyst. The results are shown in Fig. 5. The same
reaction was also carried out in absence of solvent in which very
less (4 mol%) amount of styrene conversion is observed. Since H2O2
(30%) is added to the solution as a 70% aqueous solution, and
polar solvent such as water (H2O), ethanol (C2H5OH), methanol
(CH3OH), acetonitrile (CH3CN) and acetone are used to avoid
separation of the solvent during the reaction. The selectivity of
benzaldehyde with respect to percentage yield is in the order of
H2O > CH3OH > C2H5OH > CH3CN > CH3COCH3. In the present study
the catalyst shows better efficiency in water than the other sol-
vents such as ethanol, methanol, acetonitrile and acetone. The
styrene conversion in methanol, ethanol, acetonitrile, and acetone
is 12.8 mol%, 26.8 mol%, 21.3 mol% and 12.6 mol% with, 8.8 mol%,
6.6 mol%, 6 mol% and 3 mol% yield of benzaldehyde respectively.
This may be partially due to easy reaction of H2O2 with these sol-
vents and partially due to the adsorption of solvent molecule on the
surface of catalyst which blocks the active acidic sites of the cata-
lyst and avoid the further adsorption of styrene and H2O2 molecule
[44]. When the same reaction was carried out in water as solvent
the styrene conversion is increased up to 50.8 mol% with selectivity
and percentage yield of benzaldehyde as 63.7 mol% and 32.4 mol%
respectively. Water is found to be the good solvent, because of its
polar and dissolving nature for a wide range of oxidant without
complication due to the presence of acidic protons. Apart from this
the strong interaction between H2O and H2O2 inhibit the coordi-
nation of substrate and enhance desorption of the products from
the active sites, which results in the prevention of deep oxidation
of benzaldehyde to benzoic acid. Hence the selectivity of benzalde-
Moreover, the impurities were also found to be formed when
the reaction was allowed to run for long time. From the results it
can be concluded that at 18 h the selectivity of benzaldehyde is
better with respect to styrene conversion. The results for styrene
conversion and selectivity of all the products were determined with
the accuracy of 5% error.
3.4.3. Influence of reaction temperature
The response of the reaction towards raise in temperature has
been studied at different temperatures from 40 ◦C to 80 ◦C as shown
in Fig. 7. The reaction was carried out by using 10 mmol (1.04 g),
styrene, 10 mmol 30% H2O2 (1.16 g), over 0.1 g of SrFe2O4 cata-

40
S.K. Pardeshi, R.Y. Pawar / Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
100
90
80
70
60
50
40
30
20
10
0
90
e
b
a
b
c
d
e
80
70
60
50
40
30
20
10
0
c
d
e
40
50
60
70
80
1:0.5
1:1
1:2
1:3
Temperature (ºC)
Styrene: H2O2 Molar ratio
Fig. 7. Effect of reaction temperature on styrene conversion and product selectivity
over SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of benzaldehyde, (c)
phenyl acetaldehyde, (d) others and (e) yield of benzaldehyde.
Fig. 8. Effect of styrene/H2O2 molar ratio on styrene conversion and product selec-
tivity over SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of benzaldehyde,
(c) phenyl acetaldehyde, (d) others and (e) yield of benzaldehyde.
lyst in 10 ml of H2O as solvent for 18 h. At 40 ◦C very less styrene
conversionisobserved(6.6 mol%), whiletheselectivityofbenzalde-
hyde is up to 86.2 mol% with 13 mol% of phenyl acetaldehyde as a
byproduct. As temperature increases to 50 ◦C the styrene conver-
sion increases by approximately three fold i.e. 18.0 mol%, while the
selectivity of benzaldehyde decrease to 78.2 mol% with increase in
the percentage selectivity of phenyl acetaldehyde to 21.8 mol%. Fur-
ther increase in temperature is favorable for the styrene oxidation.
At 60 ◦C conversion of styrene increases to 24.1 mol%, while the
selectivity of benzaldehyde decrease to 71.7 mol% with increase
in percentage yield up to 17.3. When the same reaction was
carried out at 70 ◦C the styrene conversion sharply increases to
50.8 mol% with the better selectivity of benzaldehyde i.e. 63.7 mol%
with 32.4% yield along with the formation of 28.1 mol% of styrene
oxide, which get isomerizes to phenyl acetaldehyde, 1-phenyl-1,2-
ethanediol up to 8.1 mol% as byproduct. Further rise in temperature
(80 ◦C) the marginal increment in styrene conversion was observed
(52.0 mol%) with very poor selectivity of benzaldehyde (31 mol%).
These results indicate that the cleavage of C C bond is favorable
with increase in temperature up to 70 ◦C, but however at high tem-
perature instead of C C bond cleavage, C–O bond cleavage is done
preferably, which results into increase of byproduct like phenyl
acetaldehyde and 1-phenyl-1,2-ethanediol up to 53.0 mol%. The
observation is in agreement with previous reports [25,46]. The
results for styrene conversion and selectivity of all the products
were determined with the accuracy of 5% error.
in Fig. 8. From the results it is observed that the conversion of
styrene is comparatively less when the styrene/H2O2 molar ratio
is 2:1 (about 23.8 mol%). This may be due to significantly reduced
•OH radicals as less amount of H2O2 compete with excess styrene
for coordination. As the concentration of H2O2 increases (1:1), the
styrene conversion as well as the selectivity of benzaldehyde also
increases. The percentage conversion of styrene is 50.8% and the
product selectivity and yield for benzaldehyde is 63.7% and 32.4%
respectively. At this molar ratio of 1:1, H2O2 could freely coordi-
nate to metal and decompose to form •OH radical. When the same
reaction was carried out at styrene/H2O2 molar ratio 1:2 and 1:3
in presence of 0.1 g catalyst the styrene conversion was suddenly
decreased to 25.0 mol% and 16.5 mol% with decreasing the percent-
age yield of benzaldehyde to 19.2 and 13.6 respectively which may
be due to the presence of excess water supplied by H2O2 itself [47].
3.4.5. Influence of catalyst amount on the oxidation reaction
The reaction was carried out by varying the catalytic amount
from 0.05 g to 0.2 g while keeping the molar ratio of styrene/H2O2 at
1:1 and reaction temperature at 70 ◦C for 18 h. The percentage con-
version of styrene and percentage selectivity of products is shown
in Fig. 9. For 0.05 g of catalyst 28.8 mol% of styrene conversion takes
place with 72.7 mol% and 27.3 mol% selectivity for benzaldehyde
and phenyl acetaldehyde respectively. The yield of benzaldehyde
is 21.0%. As the catalytic amount increases to 0.1 g the styrene
conversion also increases to 50.8 mol%, while the selectivity of
benzaldehyde, phenyl acetaldehyde and 1-phenyl-1,2-ethanediol
are 63.7 mol%, 28.1 mol% and 8.1 mol% respectively. The yield of
benzaldehyde increases to 32.4%; this is due to the availability
of large surface area and the acid sites, which favors the disper-
sion of more active species. Therefore, the accessibility of the large
number of molecules of the reactants to the catalyst is favored.
As the amount of catalyst increases from 0.1 g to 0.15 g and 0.2 g,
styrene conversion decreases to 34.7 mol% and 26.5 mol% with the
selectivity of benzaldehyde as 62.3 mol% and 60 mol% respectively.
The selectivity of byproducts like phenyl acetaldehyde increases to
27.6 mol% and 28.6 mol% respectively and for that of 1-phenyl-1,2-
ethanediol is 10.0 mol% and 11.4 mol% respectively. The reason for
lower activity at higher amount of catalyst does may possibly be
due to adsorption or chemisorptions of two reactants on separate
catalyst particles, thereby reducing the chance to interact. Similar
3.4.4. Influence of styrene/H2O2 molar ratio
The styrene conversion is observed up to 63.0 mol% in the pres-
ence of catalyst without using any oxidant. This clearly indicates
that the possibility of the reaction occurring due to the participa-
tion of lattice oxygen of catalyst. But the reaction does not give the
selective product. The selectivity of benzaldehyde is only 16.0 mol%
with 10.0% yield. In absence of catalyst and in presence of 30% H2O2
the reaction proceed up to 60.0 mol%, but here also the reaction
proceed randomly and give 12.0 mol% of benzaldehyde with 7.2%
yield. Hence there is a need to optimize the styrene/H2O2 molar
ratio to carry out the reaction selectively keeping all the remaining
parameter like temperature, time, etc. constant. The reaction was
subsequently carried out in the presence of different styrene/H2O2
molar ratios such as 2:1, 1:1, 1:2 and 1:3, for which 0.1 g of cata-
lyst were taken in 10 ml of water at 70 ◦C. The results are shown

S.K. Pardeshi, R.Y. Pawar / Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
41
100
a
a
b
90
70
60
50
40
30
20
10
0
b
c
c
d
e
80
70
60
50
40
30
20
10
0
d
e
t-BuOOH
Oxidants
UHP
H2O2
50
100
150
200
Fig. 11. Effect of oxidizing agents on styrene conversion and product selectivity over
SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of benzaldehyde, (c) phenyl
acetaldehyde, (d) others and (e) yield of benzaldehyde.
Amount of catalyst (mg)
Fig. 9. Effect of amount of catalyst on styrene conversion and product selectivity
over SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of benzaldehyde, (c)
phenyl acetaldehyde, (d) others and (e) yield of benzaldehyde.
zaldehyde also decreases to 55.9 mol% and 47.2 mol% respectively
against the cost of increasing the percentage of phenyl acetalde-
hyde and 1-phenyl-1,2-ethanediol up to 29.6 and 23.1.
This results show that the catalyst calcined at 700 ◦C gives the
better transformation of styrene with highest selectivity of ben-
zaldehyde. This may be due to the maximum availability of the
number of Fe3+ ions on the octahedral site of the catalyst surface,
which is responsible in making the catalyst more effective for cat-
alytic reaction.
observations were noted by Maurya et.al. [48]. The results show
that 0.1 g of catalyst is the optimum amount to obtain maximum
conversion of styrene with better selectivity of benzaldehyde.
3.4.6. Effect of calcination temperature on the oxidation reaction
The percentage conversion of styrene and the percentage selec-
tivity of different products were studied over the 0.1 g of SrFe2O4
catalyst calcined at different temperatures from 600 ◦C to 900 ◦C at
the optimized conditions; the results are shown in Fig. 10. When
the reaction is carried over the catalyst calcined at 600 ◦C, the
conversion of styrene is 58.1 mol%, while the selectivity of ben-
zaldehyde, phenyl acetaldehyde and 1-phenyl-1,2-ethanediol are
47.6 mol%, 25.4 mol% and 23.1 mol% respectively. As the calcination
temperature increases to 700 ◦C, though the styrene conversion
decreases (50.8 mol%) the benzaldehyde selectivity is increased
up to 63.7 mol% with 32.4% yield. For further increase in temper-
ature to 800 ◦C and 900 ◦C, the styrene conversion is decreased
to 44.8 mol% and 32.9 mol% respectively. The selectivity of ben-
3.4.7. Effect of oxidants on the styrene oxidation
The effect of different oxidants on the catalytic activity of
SrFe2O4 in styrene conversion and product distribution is also
studied. Different oxidants such as H2O2, urea–H2O2 (UHP) and
tert-BuOOH were used as the oxygen source in water as solvent.
The results are shown in Fig. 11. In presence of 30% H2O2 the reac-
tion proceeds with 50.8 mol% styrene conversion and 63.7 mol%
selectivity of benzaldehyde. The yield of benzaldehyde under this
condition is 32.4%. When the same reaction was carried out in pres-
ence of ureated hydrogen peroxide (UHP) the styrene conversion
takes place up to 43.6 mol%, but the selectivity of benzaldehyde is
very less (7.3 mol%). In presence of tert-BuOOH the styrene con-
version proceed up to 60.7 mol% with 52.0 mol%, 26.2 mol% and
21.9 mol% selectivity of benzaldehyde, phenyl acetaldehyde and 1-
phenyl-1,2-ethanediol respectively. In terms of percentage yield
of benzaldehyde, the oxidizing agents shows the efficiency in the
order of H2O2 (30%) > tert-BuOOH > UHP with 32.4, 31.5 and 7.3
respectively. Among all these oxidants, H2O2 (30%) shows the best
performance since it can give good oxidation conversion; more-
over it generate only water as by-product; and it has high content
of active oxygen. The results for styrene conversion and selectivity
of all the products were determined with the accuracy of 5% error.
a
b
c
d
e
60
50
40
30
20
10
0
3.4.8. Effect of sonication time on styrene oxidation
The effect of ultrasonic radiation on SrFe2O4 catalyst is also stud-
ied by varying the sonication time. The sonication was carried out
by taking 10 ml of solvent and 0.1 g SrFe2O4 catalyst only; neither
H2O2 nor styrene was added in this process. The results are shown
in Fig. 12. The reaction which was carried out by using catalyst
without sonication gave the percentage styrene conversion and
percentage selectivity of benzaldehyde as 50.8 and 63.7 respec-
tively. When the same reaction was carried out by sonicating the
catalyst in solvent for the interval of 5, 10 and 15 min, the styrene
conversion is decreased to 32.4 mol%, 28.0 mol% and 17.1 mol%
600
700
800
900
Cacination temperature (ºC)
Fig. 10. Effect of calcination temperature of catalyst on styrene conversion and
product selectivity over SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of
benzaldehyde, (c) phenyl acetaldehyde, (d) others and (e) yield of benzaldehyde.

42
S.K. Pardeshi, R.Y. Pawar / Journal of Molecular Catalysis A: Chemical 334 (2011) 35–43
catalyst due to the loss during filtration. But if the catalyst is col-
a
b
c
lected from several batches and then reused it was found that the
same catalytic activity with a 6–8% decrease in the percentage con-
version. The distribution for the selectivity of the products remains
the same.
90
80
70
60
50
40
30
20
10
0
d
e
4. Conclusions
The phase analysis, metal oxygen stretching frequencies, mor-
phology and elemental analysis done by various techniques
confirmed the formation of single phase SrFe2O4 with trace quan-
tity of SrCO3.
SrFe2O4 is an efficient catalyst for selective oxidation of styrene
in water as a solvent, in the presence of 30% H2O2 as oxidizing
agent. The heterogeneously catalyzed aqueous phase oxidation of
styrene proceeds by a free radical mechanism which involves the
initiation of the reaction on the catalyst surface and heterogeneous
propagation in liquid.
The conversion of styrene up to 51.0 mol% along with the forma-
tion of benzaldehyde (63.7 mol%) as major product, while styrene
oxide (28.0 mol%) as minor, products was catalyzed by SrFe2O4 in
absence of organic solvent and optimum condition as 70 ◦C, 18 h,
styrene/H2O2 molar ratio as 1, protic and polar solvent like water
and catalyst amount as 0.1 g. Solvents have marked influence on
the product distribution in selective oxidation of styrene; water
seems to be the best solvent. The selectivity of benzaldehyde with
respect to percentage yield for various solvents is in the order of
H2O > CH3OH > C2H5OH > CH3CN > CH3COCH3. The ultrasonication
of catalyst shows the adverse effect, which may be due to damaged
active sites of catalyst.
0
5
10
15
Sonication time (min.)
Fig. 12. Effect of ultrasonication time on styrene conversion and product selectivity
over SrFe2O4 catalyst, (a) styrene conversion, (b) selectivity of benzaldehyde, (c)
phenyl acetaldehyde, (d) others and (e) yield of benzaldehyde.
respectively while the selectivity of benzaldehyde increases as
71.0 mol%, 80.5 mol% and 91.2 mol% respectively. These results
indicate that the ultrasonication of catalyst shows the adverse
effect which may be due to high intensity shock waves generated
by ultrasonic irradiation cause promotion of mass transport and/or
activation of solid surface. Ultrasonic dispersion method is effec-
tive for dispersing the material; but at the same time, the high
energy may damage the active sites of catalyst and results in the
degradation of the mechanical properties of the composite [49].
Acknowledgement
This work is supported by University of Pune under UPE grant.
3.5. Recycling and repeated use of catalyst
The efficiency of the SrFe2O4 was checked by carrying out the
reaction under optimized amount of catalyst i.e. 0.1 g and by keep-
ing all the conditions constant, the results are shown in Fig. 13. The
amount of catalyst is very small (0.1 g). Hence the recycling of cat-
alyst is not very efficient. There were difficulties in collection of the
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