Selective epoxidation of styrene to styrene oxide over TS-1 using urea-hydrogen peroxide as oxidizing agent

Article abstract of DOI:10.1006/jcat.2001.3352

The use of anhydrousurea-hydrogen peroxide adduct as an oxidizing agent in the epoxidation of styrene catalyzed by a titanium-silicate (TS-1) molecular sieve resulted in very high selectivity (～85%) for styrene oxide. When aqueous hydrogen peroxide (H

Full text of DOI:10.1006/jcat.2001.3352

Journal of Catalysis 204, 64–70 (2001)
doi:10.1006/jcat.2001.3352, available online at http://www.idealibrary.com on
Selective Epoxidation of Styrene to Styrene Oxide over TS-1
Using Urea–Hydrogen Peroxide as Oxidizing Agent
S. C. Laha and R. Kumar1
Catalysis Division, National Chemical Laboratory, Pune-411 008, India
Received February 14, 2001; revised June 11, 2001; accepted June 11, 2001
using TS-1 as a catalyst and aqueous hydrogen perox-
The use of anhydrousurea–hydrogen peroxide adduct as an oxi- ide as the oxidizing agent (5, 6) but the selectivity of de-
dizing agent in the epoxidation of styrene catalyzed by a titanium– sired styrene oxide was poor (ca. 5–15% ). Hence, it was
silicate (TS-1) molecular sieve resulted in very high selectivity
thought that instead of aqueous hydrogen peroxide, anhy-
(
� 85%)for styreneoxide. When aqueoushydrogen peroxide(H O )
2
2
drous urea–hydrogen peroxide (UHP) can be used as the
oxidizing agent for the epoxidation of styrene in the pres-
ence of TS-1 as catalyst for increased selectivity for epox-
ide. The use of UHP in the chemo- and diastereoselective
epoxidation of allyic alcohols catalyzed by TS-1 (7) pro-
duced good epoxide yields. A major advantage of UHP lies
in its potential for releasing anhydrous H2O2 into solution
was used for styrene epoxidation, the styrene oxide selectivity was
very poor (5–10%) mainly due to its isomerization into phenylac-
etaldehyde. The formation of different types of Ti–superoxo com-
plexes was also observed by the solid–solid interaction with an-
hydrous urea–hydrogen peroxide and TS-1. It was confirmed by
the characteristic continuous absorption band in the UV-Vis region
(300–500 nm) and also by an intense and anisotropic EPR spectrum
for the superoxide radical ion stabilized on Ti (IV) centers of TS-1 in a controlled manner (8).
samples. �c 2001 Academic Press
It is also important to understand and establish the differ-
Key Words: epoxidation; styrene; styrene oxide; urea–hydrogen ent structural properties of TS-1 for its remarkable activity
peroxide; TS-1; UV-Vis; EPR.
toward oxidation reactions (9). The formation of different
Ti–superoxo compounds was observed in the UV-Vis spec-
tra of TS-1 mixed with aqueous H2O2 (10). However, in
most of the cases, these different Ti–superoxo compounds
INTRODUCTION
are indistinguishable from one to another mainly due to
the presence of water coming from aqueous H2O2. Ear-
Epoxidation of olefins or substituted olefins is a very im-
portant and sometimes a necessary step in a number of
important organic transformation reactions. Epoxides are
lier EPR results of TS-1 and aqueous H2O2 also show a
broad spectrum but the existence of different Ti4 species
in TS-1 is not clear (11). This motivated us to use anhydrous
solid urea–hydrogen peroxide adduct as the source of H2O2
in which the absence of water molecules will not increase
and/or equalize the coordination number of different Ti
species during the formation of Ti–superoxo compounds.
In this work, we describe and discuss direct spectroscopic
evidence to establish the existence of different Ti species in
TS-1. We also report detailed studies of the epoxidation of
styrene usingTS-1asa solid efficient catalyst and anhydrous
urea–hydrogen peroxide as an oxidizing agent. The results
obtained under the most favorable conditions using UHP
as oxidizing agent are compared with those obtained with
aqueous H2O2 under the same reaction conditions.
+
industrially important bulk chemicals. These materials are
largely used for the synthesis of several perfume materials,
anthelmintic preparations, epoxy resins, plasticizers, drugs,
sweeteners, etc. Therefore the synthesis of an epoxide by
an easier method and a low cost route is of great interest to
researchers working in this field.
The epoxidation of alkenesusingtitanium–silicate (TS-1)
as a solid catalyst and aqueous hydrogen peroxide as oxi-
dant has been studied extensively (1–4). However, in some
cases the use of aqueous hydrogen peroxide decreases the
selectivity of the desired epoxide due to isomerization and
hydrolysis of the epoxide and also the formation of other
cleaved products. The presence of highly polar water in the
system usually makes a difference by facilitating isomeriza-
tion and hydrolysis of the desired product.
EXPERIMENTAL
One of the most interesting cases is the epoxidation of
styrene. The epoxidation of styrene was studied in detail
Synthesis
1
The catalyst TS-1 was synthesized employing the concept
of promoter-induced synthesis of zeolite materials (12, 13).
To whom correspondence should be addressed. Fax: +91-20-5893761/
5
893355. E-mail: rajiv@cata.ncl.res.in.
64
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

EPOXIDATION OF STYRENE TO STYRENE OXIDE
RESULTS AND DISCUSSION
Catalyst Characterization
65
In a typical preparation, 20.8 g tetraethyl orthosilicate
(
TEOS) wasadded to 50.8gtetrapropylammonium hydrox-
ide (TPAOH, 20% aqueous solution) under vigorous stir-
ring for 2 h. Then 1.13 g tetrabutyl orthotitanate (TBOT)
dissolved in 5.7 g dry isopropanol (IPA) was added slowly
to the above clear solution of TPA–silicate under vigorous
stirring. Stirring was continued for another 30 min and then
a solution of 0.77 g H3PO4 in 4.4 ml water was added very
slowly under vigorous stirring, which was continued for an-
other hour. The initial molar gel composition of the reac-
tion mixture was 1 TEOS : 0.5 TPAOH : 0.033 TBOT : 0.067
H3PO4 :25 H2O. The crystallization of the sample was car-
ried out in a stainless-steel autoclave at 433 K for 6 h un-
der agitation (60–65 rpm). After crystallization the solid
product wascollected bycentrifugation, washed thoroughly
first with deionized water and then with dilute acid solution
The XRD pattern (characteristic of MFI topology), dif-
fuse reflectance UV-Vis spectrum (sharp absorption at
2
9
10 nm), framework region IR spectrum (band at
60 cm ), etc., characteristic of a titanium silicate molec-
� 1
ular sieve, clearly indicate the incorporation of Ti in the
framework and the absence of any extra framework TiO2
species. The scanning electron micrograph of TS-1 sample
synthesized using H3PO4 as a promoter exhibits small crys-
tallites (100–200 nm). The Si/Ti molar ratio in the solid was
3
2 as determined by EDX analysis.
Figure 1 depicts the UV-Vis spectra of TS-1 samples.
Curve A (TS-1) exhibits a sharp absorption at ca. 210 nm.
When TS-1 sample was mixed with UHP (TS-1/UHP), two
absorption bands (curve B) were obtained. One is a sharp
absorption at 210 nm as observed in pure TS-1 and another
is a continuous absorption band in the UV-Vis region from
(
containing 2 wt% H2SO4 and 5 wt% H2O2), washed again
with deionized water, dried at 393 K for 4 h, and calcined
at 813 K in air for 12 h.
3
00 to 500 nm, which is due to the formation of different
Characterization Techniques
Ti–superoxo complexes formed by the solid–solid interac-
The TS-1 sample was mainly characterized by X-ray tion between TS-1 and UHP. A similar type of curve C was
diffraction (XRD), scanning electron microscopy (SEM), also obtained when TS-1 was mixed with aqueous hydrogen
diffuse reflectance UV-Vis spectroscopy, FTIR spec- peroxide (HP), except some absorption in the 250–300 nm
troscopy, energy dispersive X-ray analysis (EDX), and elec- range, which is due to the coordinated water molecules on
tron paramagnetic resonance (EPR) spectroscopy.
The X-ray diffractogram of the sample was recorded
on a Rigaku D MAX III VC, Ni-filtered Cu K� radia-
Ti centers coming from aqueous hydrogen peroxide (10).
˚
tion (� = 1.5404 A). The SEM micrographs of the calcined
samples were obtained in a Leica Stereoscan 440. The
diffuse reflectance UV-Vis spectrum in the 200–600 nm
range was recorded with a Shimadzu UV-2101 PC spec-
trometer equipped with a diffuse reflectance attachment
using BaSO4 as a reference and the FTIR spectrum in the
�
1
4
8
00–1300 cm range was recorded on a Shimadzu FTIR-
201 PC (in Nujol on KBr disc technique), respectively. The
metal contents of the calcined samples were determined by
EDX analysis with Kevex equipment attached to a Jeol
JSM-5200 scanning microscope. The EPR spectra of the
samples were recorded on a Bruker EMX spectrometer op-
erating at X-band frequency and 100 kHz field modulation.
Catalytic Tests
The epoxidation of styrene was carried out in a batch re-
action. In a typical reaction, 2.08 g styrene (20 mmol) and
2
.08 g acetone were added slowly to the mixture of cata-
lyst (0.416 g, 20 wt% of the styrene) and urea–hydrogen
peroxide (0.47 g, 5 mmol) and the reaction mixture was
heated to 313 K under stirring. After the completion of the
reaction, the organic layer was collected by centrifugation
and analyzed using a Shimadzu 17A series gas chromato-
graph (HP 101 methyl silicone fluid, 50 m long 0.2 mm i.d.
with 0.2 �m thickness of coated film), GCIR (Perkin Elmer,
FIG. 1. Diffuse reflectance UV-Vis spectra of TS-1 (A), TS-1 + UHP
GC-IR 2000), and GCMS (Shimadzu, GCMS-QP 2000A). (B), and TS-1 + aqueous H2O2 (C).

6
6
LAHA AND KUMAR
molecules cannot fill vacant coordination sites of Ti in the
case of curve A. The g values of the three curves are in
good agreement with those reported in the literature for
the superoxide radical ion stabilized on Ti (IV) centers of
both supported and bulk titanium dioxide (14).
Epoxidation of Styrene
The effect of different sources of the oxidizing agent
(
H O ) on the styrene conversion (mol% ) and product se-
2 2
lectivity (mol% ) is given in Table 1, for comparative pur-
pose. It is observed that the styrene conversion as well as the
selectivity for the desired epoxide increases when instead
of aqueous hydrogen peroxide urea–hydrogen peroxide is
used. In the case of U + HP system, where urea and aque-
ous hydrogen peroxide solution (45 wt% ) were added sepa-
rately, both the conversion and the selectivity of the desired
epoxide were found to be slightly lower compared to the
solid UHP system and significantly higher in comparison to
the aqueous hydrogen peroxide system. Probably, the urea
acts not only as a dehydrating agent but also as a buffer
for the system, which prevents further isomerization and
hydrolysis of the desired epoxide.
The graphic profile (Fig. 3A) of temperature depen-
dence on styrene epoxidation shows that in the case of
both U + HP and UHP systems, the styrene conversion
first increases (273–313 K), reaches a maximum level at
3
13 K, and then decreases with an increase in temper-
ature (313–353 K). In the case of the HP system, al-
though initially the reaction was slow, the maximum con-
version was higher compared to that in the case of U + HP
and UHP systems at higher temperatures (>313 K). Fur-
ther there was no decrease in the conversion. At higher
FIG. 2. EPR spectra of TS-1 + UHP (A), TS-1 + UHP + Acetone
TABLE 1
(
B), and TS-1 + aqueous H2O2 (C).
Effect of Different Oxidants on the Epoxidation
a
of Styrene over TS-1
Figure 2 (curve A) depicts the anisotropic EPR spectra
Product distribution, mol% e
Styrene
of TS-1/UHP sample due to the interaction between TS-1
and UHP (� = 9.7589 GHz, 298 K), which represents di-
rect spectroscopic evidence for the existence of differ- Oxidantb mol%
d
conversion, TOF, Styrene
c
� 1
oxide Ph–CH2–CHO Ph–CHO Others f
h
ent Ti–superoxo species. The presence of three different
HP
U + HP
56
65
71
1.1
1.3
1.4
5
81
87
44
8
5
29
7
7
22
4
1
Ti–superoxo species (a, b, and c) in curve A indicates
the existence of different Ti4 sites in the original TS-1 UHP
+
(
9) and/or generated during reactions with UHP whereas
a
Reaction conditions: Catalyst wt = 0.416 g (20 wt% of the styrene);
free superoxo radicals weakly attached to lattice silicon
represent species “d”. Curve B is obtained when acetone
(
T = 313 K; styrene : oxidant (mol/mol) = 4.0; solvent = acetone; styrene :
acetone (wt/wt) = 1.0; reaction time (h) = 12.
solvent) was added to the mixture of TS-1 and UHP
� = 9.4565 GHz, 77 K). Whereas, curve C resulted from xide mixture (1 : 1, mole ratio); UHP, urea hydrogen peroxide adduct.
b
HP, hydrogen peroxide (45 wt% ); U + HP, urea and hydrogen pero-
(
c
(
Styrene conversion/theoretically possible styrene conversion) � 100.
the mixture of TS-1 and aqueous H2O2 (� = 9.4439 GHz,
d
Turnover frequency (TOF) = moles of H2O2 converted for producing
7
7 K). In both curves B and C, the dominant Ti–superoxo
styrene oxide + secondary products per mole of Ti per hour.
species is “a” which represents that other coordination
sites of Ti are occupied by solvent molecules (acetone and
e
Ph–CH2–CHO, phenylacetaldehyde; Ph–CHO, benzaldehyde.
High boiling products including diols, benzoic acid, and some uniden-
f
water, respectively, for curves B and C). However, solvent tified compounds.

EPOXIDATION OF STYRENE TO STYRENE OXIDE
67
U + HP, and UHP, the reaction proceeds in a similar
fashion with time in the order UHP > U + HP > HP. The
reaction is fast at the beginning, since the total amount
of oxidant was added in one lot at the beginning of the
reaction. The conversion for the reaction reaches a max-
imum level at ca. 12 h. However, as the hydrogen perox-
ide added becomes more and more anhydrous in nature
(
HP < U + HP < UHP), the conversion level increases.
Figure 4B depicts the selectivities of styrene oxide and
benzaldehyde + phenylacetaldehyde plotted against reac-
tion time for the three different systems.
The effect of styrene/UHP mole ratio on the styrene
conversion and the product selectivity is shown in Fig. 5.
The styrene conversion increases with the increase in
styrene/UHP mole ratio mainly due to increased H O uti-
2
2
lization for styrene oxidation. However, the selectivity of
styrene oxide increases very slowly over the range. It is also
FIG. 3. Effect of reaction temperature on the epoxidation of styrene.
Reaction conditions: catalyst TS-1 (20 wt% with respect to styrene);
styrene : H2O2 (mol/mol) = 4.0; styrene : acetone (wt/wt) = 1.0; reaction
time = 12 h. (A) Comparison of styrene conversion with different H2O2
sources:(᭿) HP, (᭹) U + HP, and (᭡) UHP. (B) Comparison ofselectivities
for styrene oxide (solid symbol) and benzaldehyde + phenylacetaldehyde
(
(
open symbol) with different H2O2 sources: (᭿ ᮀ) HP, (᭹ ᭺) U + HP, and
᭡ ᭝) UHP.
temperatures(>313K), the stabilityofsolid urea–hydrogen
peroxide adduct decreases. Therefore, the decomposition
of hydrogen peroxide becomes more competitive than
the desired epoxidation. However, in the case of HP
system, in which aqueous hydrogen peroxide is stabi-
lized by hydrogen bonding, the decomposition of hydro-
gen peroxide is rather low particularly at higher tem-
peratures (>313 K). In that case, the styrene conver-
sion increases with increasing temperature. The selectivity
of styrene oxide and benzaldehyde + phenylacetaldehyde
is plotted in Fig. 3B, for all the three systems. In all
cases, upon increasing the temperature from 273 to 353 K,
the selectivity of undesired secondary products increases
FIG. 4. Effect ofreaction time on the epoxidation ofstyrene. Reaction
at the expense of styrene oxide. However, at all reaction conditions: temperature = 313 K; catalyst TS-1 (20 wt% with respect to
styrene); styrene : H2O2 (mol/mol) = 4.0; styrene : acetone (wt/wt) = 1.0.
temperatures, the styrene oxide selectivity was quite high
in the case of UHP and U +HP (70–90% ), while in the case
of the HP system, the same was very low (5–10% ).
(
A) Comparison of styrene conversion with different H2O2 sources: (᭿)
HP, (᭹) U + HP, and (᭡) UHP. (B) Comparison of selectivities for styrene
oxide (solid symbol) and benzaldehyde + phenylacetaldehyde (open sym-
bol) with different H2O2 sources: (᭿ ᮀ) HP, (᭹ ᭺) U + HP, and (᭡ ᭝)
In the Fig. 4A, styrene conversion is plotted as a func-
tion of reaction time at 313 K. In all three systems, HP, UHP.

6
8
LAHA AND KUMAR
FIG. 5. Effect of styrene to H2O2 molar ratios on the epoxida-
tion of styrene. Reaction conditions: temperature = 313 K; catalyst TS-1 tion of styrene. Reaction conditions: temperature = 313 K; catalyst TS-1
FIG. 6. Effect of acetone to styrene weight ratios on the epoxida-
(
20 wt% with respect to styrene); styrene : acetone (wt/wt) = 1.0; reaction
(20 wt% with respect to styrene); styrene : H O (mol/mol) = 4.0; reaction
2
2
time = 12 h. (᭿) Styrene conversion, (᭹) styrene oxide (SO) selectivity,
and (᭡) benzaldehyde + phenylacetaldehyde (B + P) selectivity.
time = 12 h. (᭿) Styrene conversion, (᭹) styrene oxide (SO) selectivity,
and (᭡) benzaldehyde + phenylacetaldehyde (B + P) selectivity.
observed that the concentrations of phenylacetaldehyde +
benzaldehyde remain unchanged with the increase of the
styrene/UHP molar ratio.
are facilitated. However, styrene conversion decreases with
increases in acetone/styrene wt ratios. Here, it is pertinent
to note that although the substrate to catalyst ratio was the
same in these experiments, catalyst concentrations with re-
spect to total reaction volume were changed considerably.
Figure 7 depicts the effect of catalyst concentration on
styrene epoxidation. It is observed that an increase in the
catalyst concentration (with respect to styrene) resulted
in an increase in the styrene conversion initially before
Table 2 displays the effect of solvent on styrene epoxida-
tion over the TS-1/UHP system. As expected, in aprotic sol-
vents like acetone and acetonitrile, the product selectivity is
higher than in proticsolvent like methanol. In methanol, the
concentrations of undesired secondary products (pheny-
lacetaldehyde, benzaldehyde, and styrene diol) are higher
compared to those of acetone and acetonitrile. Still, the se-
lectivity of the desired styrene oxide (� 72% ) is quite high
in methanol due to anhydrous reaction conditions.
Figure 6 illustrates the effect of acetone concentration
on styrene conversion and styrene oxide selectivity. Upon
increasing the acetone/styrene wt ratio, the selectivity of
styrene oxide increases, at the expense of secondary prod-
ucts, indicating that at lower dilutions secondary reactions
TABLE 2
Effect of Solvent on Epoxidation of Styrene
a, b
with UHP over TS-1
Product distribution, mol%
Styrene
conversion, TOF, Styrene
�
1
Solvent
mol%
h
oxide Ph–CH2–CHO Ph–CHO Others
Acetone
Acetonitrile
Methanol
71
51
54
1.4
1.0
1.1
87
82
72
5
7
11
7
6
9
1
5
8
FIG. 7. Effect of catalyst concentration on the epoxidation of styrene.
Reaction conditions: temperature = 313 K; styrene : H2O2 (mol/mol) =
4.0; styrene : acetone (wt/wt) = 1.0; reaction time = 12 h. (᭿) Styrene
conversion, (᭹) styrene oxide (SO) selectivity, and (᭡) benzaldehyde +
phenylacetaldehyde (B + P) selectivity.
a
Reaction conditions: Catalyst wt = 0.416 g; T = 313 K; styrene : UHP
mol/mol) = 4.0; styrene : solvent (wt/wt) = 1.0; reaction time = 12 h.
(
b
Also see Table 1.

EPOXIDATION OF STYRENE TO STYRENE OXIDE
69
leveling off at ca. 70 � 5 mol% . Styrene oxide selectiv-
ity also increases with increases in catalyst concentration
before reaching a limiting value of 85 � 5 mol% . As ex-
pected, at low catalyst concentrations, the concentration of
Ti–superoxo complex is low and therefore secondary reac-
tions are facilitated. With the increase in the catalyst con-
centration, the concentration of Ti–superoxo complex also
increases which facilitates the formation of desired styrene
oxide inside the zeolitic pore.
The addition of urea has a significant effect on the se-
SCHEME 1. Different cyclic Ti species active for epoxidation
lectivity of the epoxide. This motivated us to carry out the reactions.
epoxidation of styrene with aqueous H2O2 in the presence
of varying amounts of urea separately added to the reac-
The Ti–OOH species, formed by the interaction between
tion mixture. The results obtained are plotted as a func-
tion of the urea/hydrogen peroxide molar ratio in Fig. 8.
The styrene conversion first increases steadily, and then
it reaches a limiting value of 65 � 2 mol% , when the
urea/hydrogen peroxide molar ratio becomes� 1. The selec-
tivity of styrene oxide increases considerably from 5 mol%
framework Ti atom and H2O2 molecule, is able to form
a stable five-membered cyclic structure with a donor hy-
droxyl moiety coordinated on titanium (species I) as shown
in Scheme 1. The enhanced acid strength may be generated
in charge-separated species (species II) due to greater hy-
drogen bonding. The acid centers (species I and II) are able
to catalyze the epoxide ring opening. When aqueous H2O2
(
urea/H2O2 = 0.0) to 61 mol% (urea/H2O2 = 0.25) by the
addition of a small amount of urea which indicates the im-
portance of its presence in the reaction mixture. Further
increase in the urea/H2O2 molar ratio also increases the
styrene oxide selectivity before leveling off at ca. 82 �
(
HP) isused asoxidant for styrene epoxidation, the selectiv-
ity of the desired epoxide was very poor (� 5 mol% ) mainly
because of its isomerization to phenylacetaldehyde, cat-
alyzed by the acid centers generated on TS-1. However, in
the case of the U + HP system, the selectivity of styrene ox-
ide increases considerably (� 80 mol% ). This is mainly due
to the presence of urea, which acts as a dehydrating agent
as well as buffer for the system and thereby reduces the
acid-catalyzed isomerization of styrene oxide as was seen
in Fig. 8. Expectedly, maximum selectivity of styrene oxide
3
mol% . On the contrary, the selectivity of undesired
benzaldehyde + phenylacetaldehyde decreases drastically
by the addition of a small amount of urea and reaches a
limiting value of 15 � 3 mol% , when the urea/hydrogen
peroxide molar ratio becomes � 1.
It hasbeen reported that TS-1developsacid centersin the
presence of H2O2 in alcoholic or aqueous solutions (15, 16).
(
� 85 mol% ) is achieved in the case of the UHP system be-
cause of the absence of water in the reaction mixture. Still,
the formation of water molecules in the vicinity of active
Ti centers due to consumption of H2O2 during the epoxida-
tion reaction causes some epoxide ring opening leading to
isomerized product. In methanol, the selectivity of styrene
oxide is less compared to that in other solvents also because
of the same acid-catalyzed isomerization of the epoxide.
The difference between curve B and curve C in the
UV-Vis spectra (Fig. 1) is that curve C absorbs in the
2
50–300 nm region which is due to the coordination of wa-
ter (solvent) molecules with Ti centers coming from aque-
ous H2O2 or solvent (10). However, in the case of UHP no
such absorption was observed (curve B). Most interestingly,
curves B and C in the EPR spectra (Fig. 2) are very similar
and contain mostly solvent-coordinated Ti–superoxo com-
plexes (species ‘a’), which are also shown in Scheme 1
(
species “I”). But since acetone being an aprotic solvent
is not able to generate acid centers on Ti species, the se-
lectivity of styrene oxide is very high compared to that
in aqueous H2O2. In the case of methanol such species
will also lower the styrene oxide selectivity. The exis-
tence of different Ti–superoxo species as evidenced by the
diffuse reflectance UV-Vis spectra and EPR spectra and
FIG. 8. Effect of urea to H2O2 molar ratios on the epoxidation of
styrene. Reaction conditions: temperature = 313 K; catalyst TS-1 (20 wt%
with respect to styrene); styrene : H2O2 (mol/mol) = 4.0; styrene : acetone
(
wt/wt) = 1.0;reaction time = 12h. (᭿) Styrene conversion, (᭹) styrene ox-
ide (SO) selectivity, and (᭡) benzaldehyde + phenylacetaldehyde (B + P)
selectivity.

7
0
LAHA AND KUMAR
the variation of different reaction parameters explain why authors thank Dr. D. Srinivas for providing EPR data and for helpful
discussions.
anhydrous urea–hydrogen peroxide is highly selective for
the epoxidation of styrene.
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S.C.L. gratefully acknowledges the Council of Scientific and Industrial
Research (CSIR), India, for granting him a research fellowship. The
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