Cycloaddition of styrene oxide and CO2 mediated by pyrolysis of urea

Article abstract of DOI:10.1039/c3ra42803f

This paper reports the effectiveness of carrying out the cycloaddition reaction of styrene oxide (SO) and CO₂ without the use of any catalysts. A naturally abundant and cheap material, urea, was utilized in promoting the reaction by controlling the reaction conditions. The influence of reaction temperature, carbon dioxide (CO₂) pressure, amount of urea, and the reaction time on the selectivity of styrene carbonate (SC) was investigated. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra42803f

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COMMUNICATION
Cycloaddition of styrene oxide and CO2 mediated by
pyrolysis of urea3
Cite this: RSC Advances, 2013, 3, 14290
Jose Tharun, Dong Woo Kim, Roshith Roshan, Amal Cherian Kathalikkattil,
Manickam Selvaraj and Dae-Won Park*
Received 7th June 2013,
Accepted 25th June 2013
DOI: 10.1039/c3ra42803f
www.rsc.org/advances
This paper reports the effectiveness of carrying out the cycloaddi-
tion reaction of styrene oxide (SO) and CO2 without the use of any
catalysts. A naturally abundant and cheap material, urea, was
utilized in promoting the reaction by controlling the reaction
conditions. The influence of reaction temperature, carbon dioxide
(CO2) pressure, amount of urea, and the reaction time on the
selectivity of styrene carbonate (SC) was investigated.
The synthesis of cyclic carbonates from urea and diols have been
reported by Li et al.5a and Zhou et al.5b using metal oxides and
metal carbonates, respectively, as the catalysts. On another aspect,
the synthesis of cyclic carbonates from epoxides and CO2 has been
carried out by Han et al. using choline chloride/urea as a
heterogeneous catalyst system.6 In this study, styrene carbonate
(SC) has been synthesized from CO2 and styrene oxide (SO) in the
presence of urea at high temperatures under solvent free
conditions (Scheme 1). The reactivity of urea could be controlled
by varying the reaction conditions for the selective production of
SC.
Introduction
A lot of research has been done on carbon dioxide (CO2)
transformations, where CO2 is used as a raw material in many
reactions, and these methods offer one of the most efficient ways
to reduce the atmospheric CO2 concentrations.1 Cyclic carbonates
are attractive and important compounds in a variety of chemical
research fields, which could be obtained by the coupling reaction
of CO2 and epoxides. Because of their high solubilizing power,
high boiling and flash points, low odor levels and evaporation
rates, low toxicities, and biodegradability; cyclic carbonates are
utilized as aprotic polar solvents in degreasing, paint stripping,
and cleaning processes. Similar to linear dialkyl carbonates, cyclic
carbonates are useful as intermediates for the synthesis of
polycarbonates, electrolytes in lithium ion batteries, or green
solvents.2 A wide variety of catalysts have been employed for the
synthesis of cyclic carbonates in which the catalysts possessing
strong nucleophilic groups showed better activity,1–3 where in
anionic group helps in the ring opening of epoxides for an
effective CO2 cycloaddition.1,2
Result and discussion
Due to the inert nature of CO2, its activation and incorporation
into organic substances is a daunting task. We investigated the
crucial role of temperature and CO2 pressure in the synthesis of SC
from SO and CO2 mediated by the pyrolysis of urea. Fig. 1 shows
the influence of temperature on the yield and selectivity of SC. It
can be seen clearly that the catalytic activity is sensitive to reaction
temperature. At 80 uC, no reaction took place; upon raising the
Urea is the major metabolic end product of nitrogen
substances in the human body. Being a naturally available, cheap,
and abundant material, it would be an excellent choice to utilize
urea as a starting material for the synthesis of various important
industrial products. In this regard, synthesis of glycerol carbonate
from glycerol and urea is one of the promising areas of research.4
Division of Chemical and Biomolecular Engineering, Pusan National University,
Busan 609-735, Korea. E-mail: dwpark@pusan.ac.kr; Fax: +82 51 512 8563; Tel: +82
51 510 2399
Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra42803f
Scheme 1 Cycloaddition of styrene oxide and CO2.
3
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Fig. 1 Effect of reaction temperature. Reaction conditions: SO = 18.6 mmol, urea
Fig. 3 Effect of urea amount. Reaction conditions: SO = 18.6 mmol, PCO = 2.0 MPa,
2
amount = 0.1 g, PCO = 2.0 MPa, time = 12 h.
T = 135 uC, time = 12 h.
2
temperature from 100 uC to 120 uC, there was a slight increase in
percentage conversion and reached a maximum conversion of
86% with 84% selectivity of SC at 135 uC, beyond which no further
increase in activity was observed. This could be explained on the
basis of thermal decomposition of urea (or pyrolysis of urea). Urea
is thermally stable up to 133 uC, after which it decomposes to an
intermediate called ammonium cyanate. The main requirement
for cycloaddition of epoxides and CO2 is the availability of an
anionic species that helps in the ring opening of epoxides.1,2 Urea
decomposing to ionic ammonium cyanate salt at 135 uC is the
main reason for the reactivity in the cycloaddition reaction of SO
and CO2.7 The other byproducts obtained during cycloaddition of
SO and CO2 were 2-oxazolidinone (POx) and styrene diol (SDiol)
with 10% and 6% selectivity respectively at the best reaction
condition (2.0 MPa, 12 h, 135 uC).
seen that the initial CO2 pressure has a great influence on the
percentage yield of SC. As more and more CO2 was applied to the
reaction, the conversion of SO increased, with an increase in SC
selectivity, while that for POx decreased. The conversion of SO
shows a steady increase with the increase in CO2 pressure from 0.8
MPa to 2.0 MPa, and reaches a maximum of 86% conversion.
Further increase in CO2 pressure beyond 2.0 MPa did not bring in
much of a change, while in fact slightly decreased the reactivity.
This observation can be explained by the effect of pressure on the
concentrations of CO2 and SO in the two phases (liquid and gas).
The upper phase in the reactor is the CO2-rich phase (gas), and the
bottom phase is the SO-rich phase (liquid). The reactions take
place mainly in the liquid phase because the urea is dispersed in
this phase. At first, the solubility of gaseous CO2 in liquid SO
increases with increasing pressure, which favors the reaction to
proceed. At higher pressures, however, more and more SO
becomes dispersed in the gaseous CO2-rich phase resulting in a
lower concentration in the liquid phase which reduces the rate of
reaction.8
Fig. 2 depicts the effect of CO2 pressure on the cycloaddition of
SO and CO2 in the presence of urea at 135 uC for 12 h. It can be
The influence of urea amount on the reaction was also
evaluated and illustrated in Fig. 3. When the amount of urea was
doubled (0.05 g to 0.1 g), the percentage conversion of SO also
increased from 60% to 86%. Further increase in the amount of
urea did not change the percentage conversion of SO significantly.
It can be explained by the fact that an excess of urea cannot be
well dispersed in the reaction mixture and so limits the mass
transfer between the active sites and the reactants.9 As a result, 0.1
g of urea was chosen as the optimal amount of urea in the present
cycloaddition reaction process. Moreover, as the amount of urea
increased, the selectivity for POx also increased in the reaction. In
a similar system, it was witnessed that at first the epoxide undergo
cycloaddition with CO2, forming a cyclic carbonate, followed by
reaction with amino alcohols to produce 2-oxazolidinones.10
The dependence of the product yield on the reaction time at
135 uC and 2 MPa CO2 pressure is shown in Fig. 4. The percentage
conversion of SO increased with the reaction time and reached a
maximum yield of 86% after 12 h. The percentage conversion of
Fig. 2 Effect of CO2 pressure. Reaction conditions: SO = 18.6 mmol, urea amount =
0.1 g, T = 135 uC, time = 12 h.
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Conclusions
SC has been successfully prepared by the cycloaddition of SO and
CO2 in the presence of urea at 135 uC. Urea pyrolysis occurring at
135 uC results in the formation of ionic ammonium cyanate salt
intermediate which helps in the epoxide ring opening of SO
followed by cycloaddition of CO2 to form SC. The influence of CO2
pressure, reaction temperature, urea amount and reaction time, in
selectively yielding SC product has been investigated. This work
shows the effectiveness of carrying out cycloaddition reaction of
SO with CO2 exploring the earlier thermal degradation of a
naturally abundant cheap material, urea instead of employing a
conventional catalyst.
Experimental
Urea (¢99%) and styrene oxide (SO) (97%) were purchased from
Aldrich and used without further purification. Carbon dioxide of
99.999% purity was used without further purification.
Dichloromethane was obtained from SK Chemicals, Korea, and
used as received.
Fig. 4 Effect of reaction time. Reaction conditions: SO = 18.6 mmol, urea amount =
0.1 g PCO = 2.0 MPa, T = 135 uC.
2
SO did not show any improvement by prolonging the reaction
time beyond 12 h, indicating that 12 h is the optimal reaction time
for the urea pyrolysis mediated cycloaddition reaction of SO and
CO2. Concomitant with the results, we propose a plausible
mechanism in which the intermediate ammonium cyanate
obtained by the urea pyrolysis at 135 uC attacks the b carbon
atom of the SO (less hindered), followed by cycloaddition of CO2,
resulting in the formation of cyclic carbonate (Scheme 2).
The formation of intermediate ammonium cyanate from urea
at 135 uC was confirmed by the DSC-TGA (11.47 mg of urea on an
SDT Q600 V20.9 Build 20 apparatus under a nitrogen flow of 100
mL min21 using a heating rate of 5 uC min21 from room
temperature to 400 uC) kept in supplementary data. The synthesis
of SC from SO and CO2 (Scheme 1) in the presence of urea was
performed in a 50 mL stainless steel autoclave equipped with a
magnetic stirrer. For each typical batch operation, urea (0.05–0.4 g)
and SO (18.6 mmol) were charged into the reactor without adding
any solvent, and purged by CO2 several times. The reactor was
then pressurized with CO2 to a preset pressure, at room
temperature. The reaction was carried out by heating the reactor
to a desired temperature while stirring the reaction mixture at 600
rpm. After a time period, the cycloaddition reaction was stopped
by cooling the reaction mixture to room temperature and venting
the remaining unreacted CO2. Product analysis was carried out by
gas chromatography/mass spectrometry (GC-MS, Micromass, UK)
1
analysis. H NMR (Varian 500 MHz, CDCl3) d (ppm): 7.36–7.43
(5H), 5.6 (1H), 4.8 (1H), 4.3 (1H). The percentage conversion of SO
was measured by gas chromatography (GC, HP 6890, Agilent
Technologies, Santa Clara, CA, USA) data.
Acknowledgements
This study was supported by the Korean Ministry of Education
through the National Research Foundation (NRF 2012-001507)
and the Global Frontier Program.
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Scheme 2 Proposed mechanism of urea mediated cycloaddition.
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