Optimization of reaction conditions in selective oxidation of styrene over fine crystallite spinel-type CaFe2O4 complex oxide catalyst

Article abstract of DOI:10.1016/j.materresbull.2010.01.011

The CaFe₂O₄ spinel-type catalyst was synthesized by citrate gel method and well characterized by thermogravimetric analysis, atomic absorption spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction and transmission

Full text of DOI:10.1016/j.materresbull.2010.01.011

Materials Research Bulletin 45 (2010) 609–615
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Optimization of reaction conditions in selective oxidation of styrene over fine
crystallite spinel-type CaFe2O4 complex oxide catalyst
*
Satish K. Pardeshi , Ravindra Y. Pawar
Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India
A R T I C L E I N F O
A B S T R A C T
Article history:
The CaFe2O4 spinel-type catalyst was synthesized by citrate gel method and well characterized by
thermogravimetric analysis, atomic absorption spectroscopy, Fourier-transform infrared spectroscopy,
X-ray diffraction and transmission electron microscopy. The crystallization temperature of the spinel
particle prepared by citrate gel method was 600 8C which was lower than that of ferrite prepared by
other methods. CaFe2O4 catalysts prepared by citrate gel method show better activity for styrene
oxidation in the presence of dilute H2O2 (30%) as an oxidizing agent. In this reaction the oxidative
cleavage of carbon–carbon double bond of styrene takes place selectively with 38 ꢀ 2 mol% conversion.
The major product of the reaction is benzaldehyde up to 91 ꢀ 2 mol% and minor product phenyl
acetaldehyde up to 9 ꢀ 2 mol%, respectively. The products obtained in the styrene oxidation reaction were
analyzed by gas chromatography and mass spectroscopy. The influence of the catalyst, reaction time,
temperature, amount of catalyst, styrene/H2O2 molar ratio and solvents on the conversion and product
distribution were studied.
Received 21 August 2009
Received in revised form 30 September 2009
Accepted 11 January 2010
Available online 18 January 2010
Keywords:
A. Oxides
B. Chemical synthesis
C. X-ray diffraction
D. Catalytic properties
ß 2010 Elsevier Ltd. All rights reserved.
1. Introduction
divalent and trivalent (Fe3+) ions, the spinel structure is referred to
as inverse kind [9].
Spinel ferrites with a general formula AB2O4 are a class of
chemically and thermally stable materials, which attract the
interest of researchers, because of their versatile applications [1].
The spinel ferrites are used in magnetic recording media and
magnetic fluids for the storage and retrieval of information [2],
magnetic resonance imaging (MRI) enhancement [3], magnetically
guided drug delivery [4], sensors [5], pigments [6], etc. The fine
particles of ferrites possess great potential for application in high
quality ceramics and supramagnetic material [7]. Mixed metal
oxide materials are good alternative to both zeolites, such as TS-1,
TS-2, Ti-MCM-41, V-ZSM-11 and Cu-ZSM-5 and aluminium
phenolate for many alkylation reactions [8]. Binary and ternary
oxides possessing a spinel structure have attracted much attention
due to their remarkable transport, magnetic and catalytic
properties. On the basis of distribution of cations there are two
kinds of lattice for cation occupancy; ‘A’ and ‘B’ sites have
tetrahedral and octahedral coordination, respectively. In the
normal spinel structure divalent atoms, occupying tetrahedral
‘A’ sites, while trivalent atoms, sitting on the octahedral ‘B’ sites.
When ‘A’ site being Fe3+ ions, while ‘B’ site equally populated by
The catalytic effectiveness of this system is due to the ability of
the metallic ions to migrate between the sublattices without
altering the crystal structure. This property makes the catalyst
efficient for many organic transformation reactions and number of
industrial processes such as oxidative dehydrogenation of hydro-
carbons [10], decomposition of alcohols and hydrogen peroxide
[11], treatment of automobile exhaust gases [12], oxidation of
various organic compounds such as chlorobenzene and carbon
monoxide [13,14]. The spinel ferrites were also applied in phenol
hydroxylation [15], hydrodesulphurization of crude petroleum
[16], catalytic combustion of methane [17]. Catalytic oxidation is
widely employed in the manufacture of bulk chemicals from
aromatics and more recently an environmentally attractive
method for the production of fine chemicals. Both in academic
and in industry the use of hydrogen peroxides in the oxidation of
organic molecules such as styrene, phenol, cyclohexene and trans-
stilbene is preferred, since it can give good oxidation conversion,
generate only water as byproduct and has high content of active
oxygen, this reaction can be taken as a kind of green technology
[18].
The physico-chemical properties of the ferrites are strongly
dependent on the sites and the nature of the catalyst [19,20], which
are closely related to the method of preparation. There are many
known methods of producing ferrites which can be divided into
two main groups: dry and wet methods. The most popular
* Corresponding author. Tel.: +91 020 25601394x514; fax: +91 020 25691728.
E-mail address: skpar@chem.unipune.ernet.in (S.K. Pardeshi).
0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2010.01.011

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methods, which have been recently reported, are combustion
method [21], sol–gel method [22], co-precipitation of hydrous
oxides from salts solution followed by calcination at high
temperatures [23–25] and hydrothermal routes [26,27]. Other
than these self-propagating process (SHS method) [28] and reverse
micelle synthesis technique [29,30] were also used to synthesize
the ferrites. 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 time. This not only
results in waste of energy but also harms our environment. These
routes require prolonged thermal treatment to improve the
crystallinity, purity, and morphology of the powders. Hence a
chemical route can be an 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 reaction method [31].
The catalytic efficiency of various catalysts was checked in
various organic transformations such as the oxidation of styrene
over nickel ferrite and zinc ferrite has been studied by Debanjan et
al. [1]. They have observed 31.4 mol% and 26.1 mol% styrene
conversion with selectivity of benzaldehyde as 55.6 mol% and
50.4 mol%, respectively. 20 mol% styrene conversions and
94 mol% selectivity of benzaldehyde were investigated by Gao
and Gao [18] over Cobalt VSB-5 at 70 8C temperature. Maurya and
Kumar [32] reported 76 mol% conversion of styrene with 65 mol%
selectivity of benzaldehyde over oxovanadium based coordina-
tion polymer at 75 8C temperature. The styrene oxidation was also
carried out over MgFe2O4 [33] prepared by co-precipitation and
citrate gel method, it was found that 7.43 mol% and 34.0 mol%
styrene conversion, while the benzaldehyde selectivity is
94.0 mol% and 63.2 mol%, respectively.
2.2. Catalyst characterization
The decomposition of the precursor was studied by FT-IR
spectra on a Nicollet NEXUS 7000C spectrophotometer using the
KBr pellet method. X-ray powder diffraction (XRD) patterns were
obtained on a Rigaku D/MAX-2A diffractometer with Cu K
a
radiation and a Ni-filter. The morphology and size of the particles
were examined by transmission electron microscopy (TEM) using a
JEOL-2011 microscope. Thermogravimetric analysis (TG/DTG) was
performed on a Rigaku Thermoflex system with ramp rate of
10 8C minꢂ1 in flowing air. Chemical analysis of calcium ferrite was
carried out by atomic absorption spectroscopy and chemical
method.
2.3. Reaction testing
Styrene oxidation was carried out in a round bottom flask
(25 ml) equipped with X-crossed Teflon coated magnetic stirrer
and a reflux condenser. 10 mmol styrene [99+% extrapure, Acros
organics], 10 ml solvent, 0.1 g catalyst, 1.0 ml of hydrogen peroxide
(30%, MERCK), (styrene/H2O2 molar ratio = 1) were added succes-
sively into the flask. The reaction mixture was refluxed in
temperature-controlled oil bath and the reaction was carried
out at different temperatures and time periods. The reaction
products were analyzed by gas chromatography–mass spectros-
copy (GC–MS). The gas chromatograph equipped with a XE-60
capillary column (30 m ꢃ 0.25 ꢃ 0.3
mm) flame ionization detec-
tor was used. The injector and column temperatures were 280 8C
and 140 8C, respectively. 3-Nitrotolune (99+%, MERCK) was used as
an internal standard. The residual H2O2 in solution was determined
by ceric sulphate titration method [34]. The styrene conversion
and the selectivity of the reaction products were determined by the
following formulae [35].
The styrene conversion is defined as
In the present work, we report the synthesis, characterization
and catalytic activity of calcium ferrite complex oxide with a fixed
Ca:Fe ratio. The calcium ferrite catalyst synthesized by citrate gel
method shows good catalytic performance. It gives the highest
selectivity of benzaldehyde with respect to conversion of styrene
in presence of 30% H2O2, an important chemical intermediate for
the fine chemical industry. Moreover, the method of synthesis of
catalyst is simple and economical. The influence of the catalyst,
reaction time, temperature, amount of catalyst, styrene/H2O2
molar ratio and solvents on the conversion and product distribu-
tion were studied.
styrene ðinÞ ꢂ styrene ðoutÞ
conversion ðmol%Þ ¼ 100 ꢃ
(1)
styrene ðinÞ
The selectivity was calculated based on the peak areas of the gas
chromatogram by considering the different sensitivity factors of
the flame ionization detector. The mol% of styrene as well as
analytes is calculated from chromatogram by the following
formula and using a common internal standard solution.
area of analyte from the reaction mixture
mol% ¼ 100 ꢃ
(2)
area of fix quantity of same analyte ðstandardÞ
The selectivity to product (i) is defined as
2. Experimental
corrected area ðiÞ
sum of all corrected areas
selectivity ðiÞ ðmol%Þ ¼ 100 ꢃ
(3)
2.1. Synthesis of catalyst
Analytical grade Fe(NO3)3ꢁ9H2O, Ca(NO3)2ꢁ4H2O and anhydrous
citric acid obtained from MERCK (specialties Pvt. Ltd., Mumbai,
India) were used without further purification as raw materials to
prepare CaFe2O4. The initial solution was prepared by dissolving
calcium nitrate, ferric nitrate and citric acid into double distilled
water. Concentrations of ferric nitrate, calcium 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 the
homogeneous distribution of the metal ions to get segregated from
the solution. The solution was continuously stirred at 30 8C for
30 min, and then was concentrated until a viscous liquid was
obtained. The liquid was dried in an oven at 120 8C for 12 h to get a
fluff mass, which was ground to powder. The powder was heated in
an evaporating dish where an autocombustion process takes place.
Finally, the brown, floppy precursor was calcined at different
temperatures for 1 h in muffle furnace [33].
3. Results and discussion
The citrate gel method was recommended for the preparation of
submicron-sized particle of calcium ferrite spinel. This method has
the advantage of lower calcination temperature and increase in
purity of powder by keeping the Ca2+:Fe3+ in the molar ratio of 1:2.
The synthesized calcium iron citrate is characterized by TG–DTG.
The following method was used to characterize the synthesized
calcium iron citrate complex.
3.1. TG–DTG analysis
Fig. 1 shows TG–DTG curve for thermal decomposition of
calcium iron citrate complex. There are four major weight losses in
TG curve. The first weight loss within the range of 50–110 8C is
14.16% (calculated 12.19) as shown in Table 1 corresponds to

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611
Fig. 1. TG/DTG curves of the calcium ferrite precursor prepared by citrate gel
method.
Fig. 2. XRD pattern of the calcium ferrite prepared by citrate gel method and
calcined at (a) 600 8C, (b) 700 8C, (c) 800 8C, and (d) 900 8C for 1 h.
dehydration step. This dehydration step is clearly seen as a peak in
DTG at 67 8C. The second weight loss is 33.06% in the range of 160–
210 8C corresponds to the decomposition of citrate ligand it
matches well with the calculated value of 31.27%. The loss of ligand
is also seen in the form of well-defined peak at 191 8C in DTG curve.
The further weight loss in TG curve is 12.06% (calculated 13.33%) in
the range of 300–350 8C corresponds to the decarboxylation of
precursor to mixed oxide. A peak at 333 8C in DTG corresponding to
the decarboxylation step is also seen. Finally a small weight loss of
5.54% (calculated 7.30%) in the range of 700–730 8C corresponds to
the loss of adsorbed carbon dioxide centered at 720 8C in DTG
curve. After 720 8C no notable weight loss was found which
indicate that CaFe2O4 was formed above 720 8C. Thus the calcium
ferrite oxide was calcined at different temperatures from 600 8C to
900 8C and the calcined samples were characterized by different
methods as follows.
3.4. Transmission electron microscopy studies
The TEM micrograph of the calcium ferrite fine particle
prepared by citrate gel method calcined at 800 8C is presented
in Fig. 3(a and b). It consists of rather uniform small spherical
particles with a mean size of 500–600 nm. This also indicates that
the synthesis procedure adopted result in particles which were
mostly fine.
3.5. Characterization by FT-IR
With the view to study the effect of dependence of normal
modes and their frequency on change of the substitution of ions in
calcium ferrite, FT-IR frequency data for the respective sites were
analyzed using the observed FT-IR spectra of calcium ferrite
calcined at 800 8C is shown in Fig. 4. The sample shows the two IR
bands, n1 and n2 around 690 cmꢂ1 and 501 cmꢂ1, respectively.
According to Waldron [36] and White and De Angelis [37], the high
frequency band around 700 cmꢂ1 is due to the stretching vibration
of the tetrahedral M–O bond and the low frequency band at around
500 cmꢂ1 is due to the vibration of the octahedral M–O bond
present in the spinel structure.
3.2. X-ray diffraction studies
The XRD pattern of precursor calcined from 600 8C to 900 8C is
shown in Fig. 2. It is observed that, with increase in calcination
temperature XRD peaks become sharper and also increase in
intensity. The h k l values of calcium ferrite calcined at 800 8C and
900 8C matches exactly with the JCPDS data (JCPDS card no. ID 08-
0100), which are characteristic of spinel structure of CaFe2O4
(a = 9.18, b = 10.62 and c = 3.02). From the XRD studies it was
confirmed that the single phase CaFe2O4 formed at 800 8C and so
this compound was selected for further characterization.
3.6. Optimization of reaction condition for selective oxidation of
styrene over CaFe2O4 catalyst
The calcium ferrite calcined at 800 8C thus characterized for its
structure, morphology and confirmed to be fine particles with the
spinel structure was then used as catalyst to evaluate their
catalytic efficiency. The selective oxidation of styrene to benzal-
dehyde was used as the model reaction. The effect of different
solvents, temperature variation, and styrene/hydrogen peroxide
molar ratio, on the styrene conversion and product selectivity over
catalyst has been successively carried out and is discussed in
following subsections.
3.3. Elemental analysis of CaFe2O4
The chemical composition of calcined (fresh) catalyst was
determined by atomic absorption spectroscopy. The percentage of
calcium and iron matches well with the calculated values (Table 2).
Table 1
Thermogravimetric analysis of calcium iron citrate precursor.
Table 2
Steps
Temperature
Observed %
mass loss
Calculated %
mass loss
Percentage distribution of Ca and Fe in CaFe2O4 calcined at 800 8C.
range (8C)
Catalyst
CaFe2O4
% weight of calcium
% weight of iron
Calculated
51.79
I
50–110
160–210
300–350
700–730
14.16
33.06
12.06
05.54
12.19
31.27
13.33
07.30
II
Calculated
18.58
Observed
19.21
Observed
53.09
III
IV

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S.K. Pardeshi, R.Y. Pawar / Materials Research Bulletin 45 (2010) 609–615
Fig. 3. TEM micrograph of CaFe2O4 powder calcined at 800 8C.
3.6.1. Effect of temperature
from 6 h to 30 h (Fig. 6). Running the reaction fora long time is found
to be favorable for the oxidation of styrene; styrene conversion is
increased up to 38 ꢀ 2 mol% along with selectivity of benzaldehyde up
to 91 ꢀ 2 mol% at 18 h. After 18 h styrene conversion remains constant
up to 30 h, while the selectivity of benzaldehyde decreased to
83 ꢀ 2 mol%. This may be due to the complete exhaustion of H2O2 in
the reaction mixture as the reaction was run for long duration [38].
Moreover, the impurities were also found to be formed when the
reaction was allowed to run for long time.
The effect of reaction temperature on styrene conversion and
selectivity of products was studied. The reaction was carried out by
styrene/H2O2 molar ratio as 1:1 in acetone as a solvent with 0.1 g of
CaFe2O4 catalyst at various temperatures for 18 h. The response of
the reaction towards rise in temperature has been studied and the
results are shown in Fig. 5. At 30 8C styrene conversion is
28 ꢀ 2 mol% and it is found to increase with increase in temperature
and reaches to 47 ꢀ 2 mol% at 70 8C (Fig. 5). This is due to the fact that
H2O2 decomposition increases from about 54 to 96 ꢀ 2 mol% with the
increase in temperature. The selectivity of benzaldehyde increases up
to 40 8C (91 ꢀ 2 mol%) but as the temperature increases selectivity for
phenyl acetaldehyde increases at the cost of benzaldehyde. This may
be due to the fact that the cleavage of C55C bond is higher at lower
(40 8C) temperature which leads to the formation of benzaldehyde.
On the other hand at higher (70 8C) temperature the epoxidation
seems to compete more favorably against C55C bond cleavage (see
Scheme 1a) which helps to form phenyl acetaldehyde.
3.6.3. Effect of catalyst amount on the oxidation reaction
The amount of catalyst was varied from 0.05 g to 0.20 g while
keeping the molar ratio of styrene/H2O2 at 1:1 and reaction
temperature at 40 8C. The reaction was carried out for 18 h and the
products were analyzed. The results are represented in Fig. 7. With
increase in catalyst amount from 0.05 g to 0.10 g, the conversion of
styrene also increases from 14.0 mol% to 38 mol%, while the
selectivity of benzaldehyde slightly decreases from 100 mol% to
91 mol% along with the formation of styrene oxide up to 9 mol%
which isomerizes to phenyl acetaldehyde as a byproduct. As the
3.6.2. Effect of reaction time
The effect of reaction time on the styrene conversion and product
selectivity is studied at 40 8C keeping the other conditions constant
Fig. 5. Effect of temperature (8C) on styrene oxidation: (a) styrene conversion, (b)
H2O2 conversion, (c) selectivity of benzaldehyde, (d) selectivity of others in mol%
(others—phenyl acetaldehyde, 1-phenyl-1,2-ethanediol, benzoic acid, etc.). Error
bars represent the standard deviation of triplicate samples.
Fig. 4. FT-IR spectra of CaFe2O4 powder calcined at 800 8C.

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Scheme 1. Reaction mechanism for selective oxidation of styrene using CaFe2O4.
Fig. 6. Effect of time (h) on styrene oxidation: (a) styrene conversion, (b) H2O2
conversion, (c) selectivity of benzaldehyde, (d) selectivity of others in mol%
(others—phenyl acetaldehyde, 1-phenyl-1,2-ethanediol, benzoic acid, etc.). Error
bars represent the standard deviation of triplicate samples.
Fig. 7. Effect of the amount of catalyst (mg) on styrene oxidation: (a) styrene
conversion, (b) H2O2 conversion, (c) selectivity of benzaldehyde, (d) selectivity of
others in mol% (others—phenyl acetaldehyde, 1-phenyl-1,2-ethanediol, benzoic
acid, etc.).

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Fig. 8. Effect of styrene:H2O2 molar ratio on styrene oxidation: (a) styrene
conversion, (b) H2O2 conversion, (c) selectivity of benzaldehyde, (d) selectivity of
others in mol% (others—phenyl acetaldehyde, 1-phenyl-1,2-ethanediol, benzoic
acid, etc.).
Fig. 9. Effect of solvents on styrene oxidation: (a) styrene conversion, (b) H2O2
conversion, (c) selectivity of benzaldehyde, (d) selectivity of others in mol%
(others—phenyl acetaldehyde, 1-phenyl-1,2-ethanediol, benzoic acid, etc.).
amount of catalyst increases from 0.05 g to 0.10 g the H2O2
conversion was also increased from 33 mol% to 73 mol%, respec-
tively. This is due to the availability of relatively large surface area
and the acid sites, which favors the dispersion of more active
species [39]. As a result, the accessibility of the large number of
molecules of the reactants to the catalyst is favored. Further
increase in the amount of catalyst from 0.15 g to 0.20 g styrene
conversion is decreased to 19.5 mol% while the selectivity of
benzaldehyde was 100 mol%. The reason for the lower activity at
higher amount of catalyst up to 200 mg may be possibly due to
adsorption or chemisorptions of two reactants on separate catalyst
particles, thereby reducing the chance to interact [40]. As the
catalyst amount increase from 0.15 g to 0.20 g the H2O2 conversion
also decreases (41 mol%) with respect to the conversion of styrene.
Therefore, an amount of 0.1 g catalyst may be considered as
appropriate to obtain maximum conversion of styrene with better
selectivity of benzaldehyde.
studied the reaction using protic and aprotic solvent. According to
Kumar et al., aprotic solvents are more favorable for styrene
oxidation than protic solvents [38]. Acetone is found to be a good
solvent, which gave a maximum styrene conversion of 37.91 mol%,
while the selectivity of benzaldehyde is 91.14 mol% at optimum
conditions of time 18 h and temperature 40 8C. When methanol,
acetonitrile and ethanol are used as solvents for the oxidation
reaction, the catalytic activity of the catalyst is reduced, this is
probably due to the adsorption of these solvents on the active
species at the surface of catalyst. Ethanol and methanol are
favoring the formation of byproducts like styrene oxide, 1-phenyl-
1,2-ethanediol and others. The percentage of these byproducts is
44 mol% and 52 mol% in ethanol and methanol, respectively.
3.7. The influence of catalyst and oxidizing agent
The role of catalyst and oxidizing agent is also observed, the
conversion of styrene is only up to 4–5% in the presence of catalyst
without using any oxidant. This clearly indicates that there is a
possibility of the reaction occurring due to the participation of
lattice oxygen alone. The oxygen vacancies favor the catalytic
activity in oxidation reaction because they increase the lattice
oxygen mobility [42].
3.6.4. Effect of styrene/H2O2 molar ratio
The influence of styrene/H2O2 molar ratio on the conversion of
styrene and product selectivity is also studied and the results are
shown in Fig. 8. It is found that when the styrene/H2O2 molar ratio is
increased from 1:2, 1:1 and 2:1 the styrene conversion decreases
from 41 mol% to 20 mol% with decrease in the H2O2 conversion from
80 mol% to 34 mol%. This is attributed to the increased percentage of
styrene in the reaction mixture. Interestingly, on the other hand the
selectivity for benzaldehyde increases from 41 mol% to 100 mol%.
Maximum styrene conversion is obtained at a molar ratio of styrene/
H2O2 as 1:2. The coordination of H2O2 on the metal, and its
subsequent decomposition to ꢄOH radical [41] are the necessary
steps involved in this reaction (see Scheme 1b). At the molar ratio of
1:1, H2O2 could freely coordinate to metal and decompose to form
ꢄOH radical. Hence, high styrene conversion is attained along with
the formation of styrene oxide as a byproduct up to9 mol%. While, at
the molar ratio of 2:1, low amount of H2O2 may be competing with
excess styrene for coordination, and hence formation of ꢄOH radicals
might be significantly reduced. Thus, the conversion is decreased.
Similarly, the same reaction when carried out in the absence of
catalyst but by using 10 mmol 30% H2O2 under the similar reaction
condition, 0% styrene conversion was done. It suggests that no
reaction takes place in the absence of catalyst. This observation
ruled out the possibility of reaction taking place due to the thermal
decomposition of hydrogen peroxide. The fact was confirmed by
carrying the same reaction in the presence of tertiary butyl alcohol
as a scavenger and by keeping all the reaction conditions constant,
only 4 mol% styrene got converted to benzaldehyde. This
observation confirms a free radical mechanism is involved in
styrene oxidation over a mixed catalyst (Scheme 1b). In this type of
reaction free radicals are generated on the surface of catalyst. The
catalyst accelerates the rate of decomposition reaction of H2O2 into
radicals. The Fe3+ ions from the octahedral sites of catalyst react
with hydrogen peroxide to form Cat–Fe4+–OH complex and
hydroxyl radicals as shown in Scheme 1(b). The results shows
that the redox cycle of spinel phase with high oxygen deficiencies
coming from the crystal structure can participate into the styrene
3.6.5. Effect of solvent
The effect of solvents on the styrene oxidation and product
selectivity was also observed and results are shown in Fig. 9. We

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615
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or oxygen vacancies can be generated to maintain the structure
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CaFe2O4 complex oxide catalyst is prepared by the citrate gel
method. This complex type ferrite catalyst is found to be highly
active than other reported metal ferrites for the oxidation of
styrene with H2O2 as an oxidant. The promoting effect of the
oxidation can be ascribed to the activation of oxygen ad species
(such as O2ꢂ), which was accompanied with the reduction of
abnormal valence iron (Fe4+) site in the surface layer of catalyst.
The active oxygen species reversibly appeared and disappeared on
the iron site due to sorption of atmospheric oxygen at the same
temperature range. The heterogeneously catalyzed liquid phase
oxidation of styrene proceeds by a free radical mechanism as
confirmed by using tertiary butyl alcohol as a scavenger. The free
radical involves initiation on the catalyst surface and homoge-
neous or heterogeneous propagation in the liquid. Styrene
undergoes a C55C bond cleavage preferentially over calcium ferrite
oxide catalyst to give benzaldehyde as a major product and
formation of byproduct such as styrene oxide (epoxide), benzoic
acid, phenyl acetaldehyde as minor products. Acetone as a reaction
medium, reaction temperature 40 8C, reaction time 18 h and
styrene:hydrogen peroxide ratio 1:1 was found to be favorable for
increasing the selectivity of benzaldehyde. The 0.1 g of catalyst
amount is optimized for the maximum conversion of styrene up to
38 ꢀ 2 mol% with the selectivity of benzaldehyde and phenyl
acetaldehyde is up to 91 ꢀ 2 mol% and 9 ꢀ 2 mol%, respectively,
due to the accessibility of the large number of molecules of the
reactants to the catalyst is favored.
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This work is supported by University of Pune under UPE grant.
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