Green and Sustainable Chemistry, 2012, 2, 1-7 Published Online February 2012 (
Styrene Epoxidation in Aqueous over Triazine-Based
Microporous Polymeric Netwo rk as a Me ta l- F ree Catalyst
Mohd Bismillah Ansari, Eun-Young Jeong, Sang-Eon Park*
Laboratory of Nano-Green Catalysis and Nano Center for Fine Chemicals Fusion Technology,
Department of Chemistry, Inha University, Incheon, Korea
Email: *
Received November 11, 2011; revised December 18, 2011; accepted December 27, 2011
Tirazine based microporous polymeric (TMP) network was found to be an efficient metal-free catalyst for the epoxida-
tion of styrene. The reactions were performed in water as an environmentally benign medium using H2O2 as a green
oxidant at ambient temperature. The reaction afforded higher yield with 90% conversion of styrene and 98% selectivity
to styrene oxide in 6 h. The triazine based microporous polymeric network can be readily recovered and reused up to 4
cycles without significant loss in catalytic activity and selectivity.
Keywords: Triazine Microporous Polydendritic Network; Metal-Free Catalyst; Aqueous Medium; Epoxidation; Styrene
1. Introduction
Design and development of nitrogenous materials and
their utilization in catalysis is of great interest. Depend-
ing upon the type of N-moieties these materials have been
enormously utilized in catalyzed organic transformations
such as Knoevenagel condensation [1], aldol condensa-
tion [2], C-H bond activation [3], isomerization [4], oxi-
dation [5] and epoxidation [6]. Recently nitrogen based
molecules such as guanidine [7], cinchonidine [8], urea
[9], aminoacids [10], 1, 4-diazabicyclo [2.2.2] octane (DA-
BCO) [11] and proline derivatives have drawn substan-
tial attention in catalysis due to their green nature. How-
ever the use of these nitrogenous material as an organo-
catalysts have a number of formidable problems such as
corrosion, deposition on reactor walls and moreover te-
dious product/catalyst separation procedure due to their
homogenous modus operandi. To overcome these prob-
lems efforts have been made towards immobilization of
nitrogenous materials onto organic, polymeric, or inorga-
nic nanoporous supports [12]. Although immobilization of
the homogenous catalyst reduces several drawback but are
prone to problems like less number of active sites, high
loading and leaching of anchored organocatalyst during
In this context it is highly desirable to design a nano-
porous catalyst which possess in built catalytic function-
alities as a part of framework. One such promising de-
sign of catalyst is porous polymers due to their metal free
nature, high surface properties and flexible framework,
compared to inorganic siliceous materials. Moreover the
monomeric units in these polymeric networks can be tuned
according to the desired catalytic reaction. It has been re-
ported by Chehardoli et al. that melamine have potential
to catalyze the chemo- and homoselective oxidation of
thiols and suldes [13] by hydrogen peroxide. With re-
spect to these viewpoints we have synthesized high sur-
face area polymeric network containing terephthalalde-
hyde and melamine as monomeric unit and were intri-
gued by possibility to utilize this triazine based micropo-
rous polymeric (TMP) network as a heterogeneous cata-
lyst in epoxidation of styrene to styrene oxide.
Conventionally styrene oxide is prepared via two ho-
mogenous routes, namely dehydrochlorination of styrene
chlorohydrin with a base or oxidation of styrene using or-
ganic peracids with transition metals. Both these methods
are hazardous and show poor selectivity for styrene ep-
oxide and produce copious amount of undesirable wastes,
leading to disposal problem of toxic solid and liquid
wastes. Since last few decades Payne epoxidation system
[14] have been enormously used [15], this system oper-
ates via Radziszewski reaction in which hydrolysis of an
organic nitrile by alkaline hydrogen peroxide occurs via
peroxycarboximidic acid intermediate leading to end
products amide, water and oxygen. The presence of ole-
fin in this reaction mixture acts as a better reducing agent
and potentially eliminates the hydrolysis of the intermedi-
ate with hydrogen peroxide thereby accomplishing ep-
oxidation of the olefin.
In last decade Richardson et al. have introduced bicar-
bonate-activated peroxide (BAP) system which operates
*Corresponding author.
opyright © 2012 SciRes. GSC
in water for oxidation of sulfides [16] and also for wa-
ter-soluble olefins [17], however this system have limita-
tion for hydrophobic olefins. Later Burgess et al. [18]
made further progress in BAP system for epoxidation of
hydrophobic olefins using various transition metal salts,
additives and also co-solvents. Tong et al. have also de-
veloped an epoxidation protocol for lipophilic olefins ba-
sed on manganese sulphate and BAP system in which ionic
liquid served as solvent media as well as phase transfer
agent [19]. These systems are associated with drawbacks
like use of buffer, additive, co-solvent, phase transfer agent
and moreover tedious product/catalyst separation procedure.
In this context here we report a designed eco-friendly
protocol using BAP system for epoxidation of styrene in
water at room temperature using triazine based polymeric
network as metal-free heterogenous catalyst.
2. Experimental
2.1. Chemicals
All chemicals were of analytical grade and used without
further purification. Styrene was purchased from Sigma
Aldrich. NaHCO3 and 28% H2O2 were purchased from
Duksan chemicals South Korea. The water was deionized
by aqua MaxTM basic water purification system, Young
Lin, Korea.
2.2. Catalyst Preparation
Synthesis of TMP-Network: The 1,3,5-triazine based mi-
croporous network was synthesized by slightly modi-
fied procedure as reported earlier [20]. A flame dried two
necked round bottom flask fitted with a condenser and a
magnetic stirring bar, was charged with terephthalaldehy-
de dissolved in dimethyl sulfoxide and heated up to 100˚C
and temperature was maintained till a yellow colour so-
lution was observed. Warmed melamine solution in di-
methyl sulfoxide was added to hot terephtahaldehyde so-
lution under nitrogen atmosphere. The hot solution is
maintained at 100˚C with sonication for 3 h. The result-
ing mixture was heated at 180˚C up to 48 h under an in-
ert atmosphere. After cooling to room temperature the
precipitated TMP network was isolated by filtration over
a Buchner funnel and washed with excess acetone fol-
lowed by tetrahydrofuran and dichloromethane. The iso-
lated TMP network was dried under vacuum for over-
night at 60˚C and gave 75% yield.
2.3. Characterization
Fourier transform infrared (FT-IR) spectroscopy (Nicolet
6700) and Raman spectroscopy (HR 800, Horiba/Jobin-
Yvon) were employed to analyze the chemical structure
of the catalyst. The NMR (Nuclear Magnetic Resonance)
spectra were recorded on Varian UnityNOVA solid state
600 MHZ spectrometer. The Brunauer-Emmett-Teller
(BET) nitrogen adsorption and desorption were measured
at –196˚C using a Micromeritics porosimeter (model AS-
AP-2020). Prior to the measurement, the samples were
degassed at 160˚C for 5 h. Scanning electron microscopic
(SEM) image was collected with a JEOL 630-F micro-
scope. The aliquot of reaction mixture were analyzed by
GC and GC-MS (Agilent technologies 5975).
2.4. Catalytic Activity
Styrene 1 mmol along with 2 ml of H2O and 20 mg of
catalyst were taken in a vial with magnetic bar. Appro-
priate amount of NaHCO3 (0.5 - 2.0 mol) in one ml water
was added to this reaction mixture. The reaction mixture
was stirred for 30 min followed by drop wise addition of
H2O2 at room temperature. After completion of reaction
the reaction mixture was separated by the addition of die-
thyl ether and separated using separating funnel. The
ether layer was collected dried over anhydrous sodium
sulphate, filtered, concentrated by rota-evaporator and sub-
jected to GC analysis. The obtained products were cha-
racterized by GC-MS.
The conversions were calculated on the basis of mole
percent of styrene, the initial mole percent of styrene was
divided by initial area percent (CYA peak area from GC)
to get the response factor. The unreacted moles of sty-
rene remained in the reaction mixture were calculated by
multiplying response factor with the area percentage of
the GC peak for CYA obtained after the reaction. The
conversion was calculated as Styrene Conversion (mol%)
= [(Initial mol% Final mol%)/initial mol%] × 100 and
selectivity was calculated as Styrene oxide Selectivity %
= (GC peak area of styrene oxide/ΣGCpeak area of all
products) × 100.
3. Results and Discussion
3.1. Catalyst Preparation and Characterization
The TMP network was synthesized by catalyst free po-
lymerization via condensation of terephthalaldehyde and
melamine with modified procedure as reported earlier
(Scheme 1). The modified procedure contributed to im-
proved yield (75%) and as well as reduced the reaction
time from 72 h to 48 h [20]. Fourier transform infrared
(FTIR) spectroscopy (Figure 1) of TMP network depicted
the absence of the bands at 3470 cm–1 and 3420 cm–1
(NH2 stretching) which corresponds to the primary amine
group of melamine [21]. The absence of band at 1690
cm–1 (C=O stretching) corresponding to carbonyl func-
tion of the aldehydes confirmed the formation of TMP
network. The well distinct quadrant vibrations at (1550
cm–1) and semicircle stretching vibrations (1480 cm–1) of
heteroaromatic ring systems are observed in FTIR spec-
tra. No band corresponding to imine linkages such as the
Copyright © 2012 SciRes. GSC
Scheme 1. Synthesis of TMP network.
Figure 1. Fourier transform infrared (FTIR) spectra of Me-
lamine and TMP network.
C=N stretching vibration around 1600 cm–1 are obser-
ved. The UV spectrum (Figure 2) depicted K and B bands
which arise from π to π* transitions as a result of a group
containing multiple bond being attached to the aromatic
ring. The incorporation of melamine showed bathochro-
mic shift in K band from 220 nm to 250 nm and in B
band it shifted from 300 nm to 350 nm [22]. The above
all characteristics depicted the incorporation of melamine
and formation of TMP framework.
The 13C cross-polarization magic angle spinning (CP-
MAS) NMR (Figure 3) spectrum shows three resonances
which appears at 167, 114, and 54 ppm. The peak at 167
ppm is assignable to the carbon atoms present in the tri-
azine ring of the melamine, while the signal at 114 ppm
corresponds to C-H aromatic carbons of the benzene and
resonance at 54 ppm originates from the tertiary carbon
atoms formed upon the addition of the primary amine
groups of melamine. The 15N CP-MAS spectrum (Figure
4) of TMP network showed two major resonances at –215
and –287 ppm, respectively. The peak at –215 ppm is as-
signed to the nitrogen atoms in the triazine ring, whereas
the peak at –287 ppm may be attributed to the secondary
amine present in the aminal motif. All the characteriza-
tion results were in consonance with those reported ear-
lier [20].
300 450 600 750 900
Wavelength nm.
B Ban d
K Band
Figure 2. U.V. spectra of TMP network.
Figure 3. Cross-polarization (CP) 13C MAS natural abun-
dance NMR spectrum of TMP network.
Figure 4. Cross-polarization (CP) 15N MAS natural abun-
dance NMR spectrum of TMP network.
The surface properties of TMP networks were analyzed
by nitrogen sorption analysis. The adsorption isotherm (Fi-
gure 5) showed a steep gas uptake at low relative pres-
sures and also flat course in the intermediate section, which
reflects the microporous nature of the polymeric net-
works [23]. The Brunauer-Emmet-Teller (BET) surface
Copyright © 2012 SciRes. GSC
Figure 5. Nitrogen adsorption and desorption isotherm of
TMP network.
area found to be 738 m2/g indicated high degree of cross-
linking in TMP network which is also evidenced by SEM
image showing globular aggregates of TMP network (Fi-
gure 6). The micropore volume calculated by nonlocal
density functional theory (NLDFT) found to be 0.22 cm3/g
indicated conformational flexibility of TMP network.
3.2. Catalytic Results
The TMP network was used as catalyst for the epoxida-
tion of styrene with various oxidants in the presence and
absence of promoter. Initially the optimization of molar
ratio of oxidant to substrate was investigated in presence
of NaHCO3 as promoter using acetonitrile as solvent sys-
tem and the results are listed in Table 1.
Low conversion of styrene was observed for unimolar
ratio of substrate to oxidant (Table 1, Entry 1) further
increase in molar ratio was very effective to increase the
conversion of styrene to styrene oxide (Table 1, Entries
2-4). The highest conversion (50%) and selectivity (90%)
was observed at molar ratio 1:5 (Table 1, Entry 5), how-
ever further increase in amount of oxidant increases the
conversion but dropped selectivity (Table 1, Entry 6).
After optimization of oxidant to substrate molar ratio reac-
tions were carried out in aqueous and organic (acetone-
trile, DMF, DMSO) solvent systems (Table 2). Interest-
ingly, conversion of styrene to styrene oxide was found
60% with good selectivity (98%) in aqueous conditions
(Table 2, Entry 4). Surprisingly acetonitrile (Table 2,
Entry 1) gave better conversion (50%) and high selectiv-
ity (90%) among the non-aqueous solvents, whereas mo-
derate conversion and selectivity was observed in DMF
and DMSO (Table 2, Entries 2 and 3).
The selectivity and conversion in aqueous media were
higher; therefore further studies were performed in aque-
ous using various oxidants namely H2O2, tert-Butyl hy-
droperoxide (TBHP) and iodosylbenzene. However the
conversion and selectivity was poor with TBHP and io-
dosylbenzene compared with H2O2, which may be due to
lesser degree of solvation of oxidant in reaction media.
To further optimize the amount of promoter variation in
NaHCO3 concentration from the range of 0.0 - 2.0 mole
% was used. In absence of NaHCO3 the conversion was
low (5%). The maximum conversion (90.8%) and selec-
tivity (97.8%) was achieved at 1 mole% of NaHCO3 (Ta-
ble 3, Entry 3), further increase in concentration of
Figure 6. Scanning electron microscopy image of TMP net-
Table 1. Optimization substrate to H2O2 molar ratio.
Entry Styrene:H2O2
Conversion of
Styrene %
Selectivity for
Styrene Oxide %
1 1:1 10 96
2 1:2 22 96
3 1:3 30 94
4 1:4 36 93
5 1:5 50 90
6 1:6 54 85
Reaction condition: 20 mg catalyst, Styrene 1 mmol, 1 - 6 mmol 30% H2O2,
0.5 mmol NaHCO3, acetonitrile 2 ml, 25˚C, 6 h.
Table 2. Influence of solvent and oxidant on conversion.
Entry SolventOxidantConversion
of Styrene %
Selectivity for
Styrene Oxide %
1 ACN H2O2 50.0 90.0
2 DMF H2O2 45.5 75.0
3 DMSOH2O2 38.9 65.0
4 H2O H2O2 60.3 98.7
5 H2O TBHP 30.1 45.0
6 H2O PhIO 15.3 ND
Reaction condition: 20 mg catalyst, Styrene 1 mmol, 5 mmol oxidant, 0.5
mmol NaHCO3, solvent 2 ml, 25˚C, 6 h.
Copyright © 2012 SciRes. GSC
Table 3. Effect of base and temperature on conversion.
Entry NaHCO3
(mmol) Time Conversion of
Styrene %
Selectivity for
Styrene Oxide %
1 0 12 5.0 ND
2 0.5 6 60.3 98.7
3 1 6 90.0 97.8
4a 1 6 30.3 99.0
5b 1 6 >99 59.2
6 1.5 6 92.2 70.2
7 2 6 97.1 65.2
8c 1 12 10.0 ND
Reaction condition: 20 mg catalyst, Styrene 1 mmol, 5 mmol H2O2, 0.5
mmol NaHCO3, 25˚C, H2O 2 ml, 6 - 12 h, a15˚C, b40˚C, cabsence of catalyst.
NaHCO3 increases the conversion but selectivity for ep-
oxide was dropped (Table 3, Entries 6 and 7). The Opti-
mized amount of base was found to be one mole % of
NaHCO3. To know the effect of temperature for the con-
version of styrene to styreneoxide, the reaction was also
conducted at different temperatures (Table 3, Entries
3-5). At lower temperature (15˚C) the selectivity was
higher (98%), however the conversion (30.3%) was low
(Table 3, Entry 4). At higher temperature (40˚C) the
conversion was 100% however, the selectivity of styrene
oxide was dropped down to 59.2% and the diol was other
product which was formed due to ring opening at high
temperature (Table 3, Entry 5).
The ideal temperature was found to be 25˚C (Table 3,
Entry 3) where high conversion (90.8%) and excellent
selectivity (97.8%) was obtained, further increase in tem-
perature causes poor selectivity (Table 3, Entry 5).
The catalyst was recycled up to 4 cycles without sig-
nificant loss of activity and selectivity (Table 4).
3.3. Role of H2O
The higher activity in aqueous condition may be attrib-
uted to stabilization of catalytic active species peroxymo-
nocarbonate ion, (4) [19] which is also in agree-
ment with earlier report. We have also able to demon-
strate by 13C NMR the formation of peroxymonocarbon-
ate species (Figure 7). The NMR spectra in absence of
hydrogen peroxide depicted peak corresponding to bicar-
bonate anion 3 at 160.5 ppm (Figure 7(a) ). When
hydrogen peroxide was added to the solution a peak at
157.7 ppm (Figure 7(b)) could be observed suggesting
the formation of peroxymonocarbonate species [17-19].
Richardson et al. have proposed the formation of per-
oxymonocarbonate species leads to two transition states
in BAP system as depicted in (Scheme 2) transition state
1 and transition state 2 [17]. The two transition states pro-
posed earlier are different compositionally; transition state
1 involves olefin, peroxymonocarbonate species whereas
Table 4. Recyclability of the catalyst.
Entry No. of CycleConversion of
Styrene %
Selectivity for
Styrene Oxide %
1 1 90.9 98.0
2 2 89.1 98.0
3 3 87.9 97.5
4 4 88.6 97.4
Reaction condition: 20 mg catalyst, Styrene 1 mmol, 1 - 6 mmol 30% H2O2,
0.5 mmol NaHCO3, acetonitrile 2 ml, 25˚C, 6 h.
Figure 7. 13C NMR spectra for a solution at 25˚C in D2O. (a)
NaHCO3; (b) NaHCO3:H2O2 (1:5).
Transition state 1
Transition state 2
Scheme 2. TMP network catalyzed epoxidation of styrene
and transition states.
the transition state 2 involves H2O in addition (Scheme
2). This suggests that the mechanism in water proceeds
via two different pathways increasing the conversion.
In addition to lower transition state energy the reaction
of organic molecules in water are prone to “Breslow ef-
fect” [24]. In line of this effect it is presumed that styrene
molecules repel water molecules and are forced to form
Copyright © 2012 SciRes. GSC
aggregates in order to decrease the organic surface area
exposed to water. Organic reactions arising from these
hydrophobic aggregates in water will have reduced acti-
vation energies and significant rate enhancements. In ad-
dition to this the water medium brings the styrene mole-
cule in close proximity of TMP network by π-π stacking
interactions which influences the contact times of sub-
strate with catalyst [25].
4. Conclusion
TMP-network has been synthesized and its role as an ef-
ficient catalyst towards styrene epoxidation has been de-
monstrated with effective performance in aqueous me-
dium. The better performance in water is accredited to
dual pathway and “Breslow effect”. This eco-friendly epo-
xidation protocol afforded 90.9% conversion of styrene
with 97.8% selectivity to styrene oxide under aqueous
conditions using hydrogen peroxide as an oxidant at am-
bient temperature. The catalyst can be easily recycled and
used several times without significant loss in conversion
and selectivity.
5. Acknowledgements
Authors thank for financial support from National Re-
search Foundation of Korea (NRF) grant funded by the
Korea government (MEST) (No. 2012-0000911) and
MKE (Ministry of Knowledge Economy) for Nano Cen-
ter for Fine Chemicals Fusion Technology.
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