A catalyst-free novel synthesis of diethyl carbonate from ethyl carbamate in supercritical ethanol

Article abstract of DOI:10.1016/j.cclet.2014.05.012

Diethyl carbonate has been synthesized via the alcoholysis of ethyl carbamate in supercritical ethanol under catalyst-free conditions. The influences of various parameters such as reaction temperature, reaction time, reaction pressure, ethanol/ethyl molar ratios and reaction loading volume on the yield of DEC were studied systematically. The experimental results indicated that the alcoholysis of ethyl carbamate was greatly improved in supercritical ethanol. The optimal reaction conditions were as follows: a reaction temperature of 573 K, a reaction time of 30 min, a reaction pressure of 13.2 MPa, an ethanol/ethyl carbamate molar ratio of 10 and a reactor loading volume of 285 μL respectively. The optimal yield of DEC was 22.9%.

Full text of DOI:10.1016/j.cclet.2014.05.012

G Model
CCLET 2990 1–4
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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A catalyst-free novel synthesis of diethyl carbonate from ethyl
carbamate in supercritical ethanol
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Q1 Li-Cai Zhao, Zhi-Qiang Hou, Chun-Ze Liu, Yuan-Yuan Wang *, Li-Yi Dai *
Department of Chemistry, East China Normal University, Shanghai 200241, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Diethyl carbonate has been synthesized via the alcoholysis of ethyl carbamate in supercritical ethanol
under catalyst-free conditions. The influences of various parameters such as reaction temperature,
reaction time, reaction pressure, ethanol/ethyl molar ratios and reaction loading volume on the yield of
DEC were studied systematically. The experimental results indicated that the alcoholysis of ethyl
carbamate was greatly improved in supercritical ethanol. The optimal reaction conditions were as
follows: a reaction temperature of 573 K, a reaction time of 30 min, a reaction pressure of 13.2 MPa, an
Received 21 January 2014
Received in revised form 29 April 2014
Accepted 4 May 2014
Available online xxx
Keywords:
ethanol/ethyl carbamate molar ratio of 10 and a reactor loading volume of 285
optimal yield of DEC was 22.9%.
mL respectively. The
Supercritical ethanol
Diethyl carbonate
Ethyl carbamate
Alcoholysis
ß 2014 Yuan-Yuan Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
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1. Introduction
DEC was 0%. Zhao et al. [12] reported that with PbO as catalyst, the 32
highest yield of DEC was 16.2% with 44.0% selectivity at 463 K in 33
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As a biodegradable compound, diethyl carbonate (DEC) has
attracted much attention in recent years and has been widely used
in organic synthesis, electrical chemistry, and fuel additives [1–3].
Currently, the existing methods for preparing DEC mainly include
phosgenation of ethanol, oxidative carbonylation of ethanol,
7 h. Compared to the single metal oxide catalysts, An et al. [13] 34
provided insights into double metal oxide for the reaction of ethyl 35
carbamate and ethanol, a higher yield of DEC of 20.6% was achieved 36
over ZnO–PbO but DEC selectivity was only 41.0% at 463 K in 7 h. 37
So, in previous studies, expensive catalysts were indispensable and 38
the reaction equilibrating time was very long. Accordingly, it is 39
vital to identify new methods to promote the reaction in a more 40
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transesterification of carbonate and ethanolysis of CO
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[4–6].
However, these methods suffer from drawbacks such as high
toxicity, inconvenient operations, complicated processes and poor
yields, which were undesirable for industrial productions [7].
Recently, a new process for preparing DEC from urea and
ethanol described in Scheme 1 becomes more attractive. The raw
materials are low in price and the byproduct ammonia can be used
as a recycled material for urea production [8]. This reaction
proceeds in two steps: ethyl carbamate (EC) is formed as an
intermediate initially and then it can be further converted to DEC.
The first step is fast and EC can be easily obtained even without a
catalyst. However, the subsequent EC ethanolysis in the second
step is difficult to perform [9]. And until now, this process was
usually conducted in the presence of catalysts such as organotin
compounds, alkali metal compounds and metal oxides [10]. Wang
et al. [11] conducted the urea ethanolysis over a series of metal
oxides at 463 K in 5 h. The highest yield of DEC 14.2% was achieved
using ZnO as a catalyst, while in the absence of catalysts the yield of
environmentally benign and commercially viable way.
41
Supercritical fluids (SCFs) technology has attracted increasing 42
attention in recent years. SCFs have been widely used in organic 43
syntheses [14], biodiesel production [15] and chemical degradation 44
[16]. The superiorities of SCFs are summarized as the follows: (1) 45
they are green and catalyst-free; (2) high yield and selectivity are 46
always achieved by manipulatingreactiontemperatureand pressure 47
of SCFs [17]. Supercritical ethanol is one of the most promising SCFs 48
because of its milder critical conditions (243.1 8C, 6.38MPa), non- 49
toxicity and high activity. To the best of our knowledge, there have 50
been no report on the synthesis of DEC from EC in supercritical 51
ethanol. Based on the unique properties of supercritical ethanol, we 52
expected its good performance in the ethanolysis reaction.
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2. Experimental
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Ethyl carbamate (A.R.), ethanol (A.R.), diethyl carbonate (A.R.), 55
acetone (A.R.), dodecane (A.R.) were commercially available from 56
*
Corresponding authors.
E-mail address: lydai@chem.ecnu.edu.cn (L.-Y. Dai).
Sinopharm Chemical Reagent and were used as received.
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http://dx.doi.org/10.1016/j.cclet.2014.05.012
001-8417/ß 2014 Yuan-Yuan Wang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
1
Please cite this article in press as: L.-C. Zhao, et al., A catalyst-free novel synthesis of diethyl carbonate from ethyl carbamate in
supercritical ethanol, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.012

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CCLET 2990 1–4
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L.-C. Zhao et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Scheme 1. The reaction equation between urea and ethanol (1), ethyl carbamate
and ethanol (2) and the side reaction of NEEC and diethyl carbonate.
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An assembled stainless-steel reactor with a volume of 500
served as the batch-type reactor. Before it use, the reactor should
be cleaned at 573 K for 8 h with 300 L of deionized water.
The reaction solution was prepared by dissolving EC in ethanol
with the aid of ultrasound at room temperature. An exact amount
of reactant was fed to the stainless-steel reactor by a microliter
syringe. Reactant volumes at different temperature were as
m
L
Fig. 1. The effects of reaction temperature and time on the yield of DEC. The molar
ratio of ethanol/EC: 10.
m
yield of DEC were both enhanced as the temperature increased
from 543 K to 573 K and the yield of DEC reached its maximum of
22.9% with an EC conversion of 48.3% at 573 K. This revealed that
synthesis of DEC in supercritical ethanol was greatly affected by
temperature. The effect of temperature lied in two aspects: on one
hand, as reported in the literature, the reaction was endothermic,
so elevated temperatures are favorable according to the laws of
thermodynamics [12,13]; on the other hand, the temperature
played a decisive role on the properties of supercritical ethanol
[18–20]. As we know, ethanol under supercritical conditions
mainly exists in the form of small oligomers instead of H-bonded
aggregates in the liquid. Monomers dominated in these small
oligomers and its percentage (ranging from 35.2% to 92.2%) rises as
temperature increases. As a result, at higher temperatures, more
monomers of ethanol were directly exposed to EC, which increased
the chance of collisions of molecules and promoted the reaction.
This hypothesis was consistent with our experimental results.
However, when the temperature exceeded 573 K, the conversion of
EC increased but the yield of DEC decreased. Meanwhile, we also
found that the major by-product N-EEC increased significantly,
which suggested that at higher temperatures the side reaction
described in Eq. (3) by consuming DEC and EC became more
significant, which leads to higher conversion of EC and lower yield
of DEC. Therefore, the optimum reaction temperature was 573 K
for the synthesis of DEC. The effect of temperature on the yield of
DEC agreed with the findings in the literature [13].
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follows: 315
285 L at 573 K and 275
sealed and immersed into a molten-salt bath (consisted of 40 wt%
NaNO , 7 wt% NaNO and 53 wt% KNO ) that has been preheated to
m
L at 543 K, 305
mL at 553 K, 295 mL at 563 K,
m
m
L at 583 K. After loading, the reactor was
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the required temperature. The temperature was controlled at ꢀ2 K
by an Omega temperature controller. After a desired reaction time,
the reactor was placed into ice water for about 10 min to quench the
reaction. The liquid reactant mixture was taken out by microliter
syringe. Then the reactor was treated with acetone for three times to
ensure that the reaction mixture was completely transferred.
The liquid reaction contents were analyzed quantitatively by
gas chromatograph (SHIMADZU GC-2014) equipped with an
RT-10223 capillary column (30 m ꢁ 0.25 mm ꢁ 0.25
mm). The
analytical conditions were: injector temperature 553 K, column
temperature program-raised from 313 K to 523 K at a rate of 15 K/
min, nitrogen at a flow rate of 3 mL/min served as the carrier gas.
N-dodecane was selected as an internal standard.
The products were identified by GC–MS (Varian Saturn
3900/2100) equipped with
a
VF-5MS non-polar column
(30 m ꢁ 0.25 mm ꢁ 0.25 m). The analytical conditions were:
m
injector temperature 523 K, column temperature was settled at
333 K for 3 min, then program-raised to 523 K at a rate of 15 K/min,
helium gas at a flow rate of 1 mL/min served as the carrier gas. The
product DEC was verified by the fragment ion peaks of m/z 18, 29,
45, 63, 75, 91, 118. The fragment peaks of N-EEC were m/z18, 28 44,
58, 72, 88, 102, 117. These mass data matched exactly with the
standard chromatograms in mass spectral library.
The influence of reaction time on the DEC synthesis was also
described in Figs. 1 and 2. The yield of DEC and conversion of EC
The yield and selectivity in this work were defined as the
follows:
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moles of DEC
YieldðDEC; %Þ ¼
ꢁ 100%
moles of EC added
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Yield of DEC
SelectivityðDEC; %Þ ¼
ꢁ 100%
Conversion of EC
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3. Results and discussion
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3.1. Effect of reaction temperature and reaction time on the yield of
DEC
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Figs. 1 and 2 depict the effects of reaction temperature
(543–583 K) and time (20–75 min) on the conversion of EC and
the yield of DEC. It is observed that the conversion of EC and the
Fig. 2. The effects of reaction temperature and time on the conversion of EC. The
molar ratio of ethanol/EC: 10.
Please cite this article in press as: L.-C. Zhao, et al., A catalyst-free novel synthesis of diethyl carbonate from ethyl carbamate in
supercritical ethanol, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.012

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L.-C. Zhao et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Fig. 5. The optimized geometries for reactant (ethanol and EC), intermediate (M),
transition state (TS) and product.
Fig. 3. The effects of ethanol/EC molar ratio on the yield of DEC and the conversion of
EC. Reaction temperature: 573 K, reaction time: 30 min.
yield of DEC was lower. In contrast, when ethanol/EC molar ratios 159
were too high, the concentration of EC became lower and the 160
reaction was restricted. In conclusion, the optimal ethanol/EC 161
molar ratio was 10:1. The effect of ethanol/EC molar ratio on the 162
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both increased with increased reaction time at 543 K. At lower
temperature, the side reaction can hardly proceed due to the low
concentration of DEC, therefore the DEC continually accumulated
and the formation of EEC was negligible. Furthermore, at the
temperatures ranging from 553 K to 583 K, the conversion of EC
increased continuously but the yield of DEC increased initially and
then decreased at each temperature. In addition, we found that at
longer reaction time, the amount of by-product N-EEC increased
significantly, which suggested that the side reaction was acceler-
ated. As a result, the yield and selectivity of DEC were reduced with
prolonged reaction time. Therefore, the optimal reaction time was
30 min. The effect of time on the yield of DEC was also in good
accordance with the literature [13].
yield of DEC was similar to the findings in the literature [13].
163
3.3. Effect of reactant loading volume on the yield of DEC
164
We studied the effect of the reactant loading volume on the 165
yield of DEC by varying the volume from 225 L to 325 L in a 166
500 L stainless-steel batch type reactor. Reaction conditions are: 167
m
m
m
a temperature of 573 K, a reaction time of 30 min, a molar ratio of 168
1:10. The reaction pressures at a range of 11.5–14.1 MPa were 169
applied in a 18 mL reaction vessel with an accurate piezometer. As 170
shown in Fig. 4, the yield of DEC increased as the reactant volume 171
increased from 225 to 285
mL and decreased sharply when the 172
volume exceeded 285 L. The observation can be explained that in 173
m
1
47
3.2. Effect of ethanol/EC molar ratio on the yield of DEC
the case of lower volume, the number of molecules per unit volume 174
of the reaction vessel was fewer and the reaction rate was lower, so 175
the yield of DEC increased with an increased reactant volume. 176
However, as the reactant volume passed the optimum, the 177
increased pressure resulted in the re-formation of the H-bonded 178
aggregates at the expense of ethanol monomers. Therefore, the 179
optimal reaction conditions were as follows: reactant volume 180
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Effect of ethanol/EC molar ratio on the yield of DEC was shown
in Fig. 3, as the molar ratio of ethanol/EC varied from 5:1 to 19:1,
the yields increased gradually and then declined and the peak was
at the ratio of 10:1. As reported in the literature [11], in this
ꢂ
reaction, it is difficult for the weaker nucleophile CH
3
CH
2
O
to
ꢂ
displace the stronger nucleophile NH
2
. Therefore, at lower
285
mL, pressure 13.2 MPa, temperature 573 K, reaction time 181
ethanol/EC molar ratio, ethanol is insufficient to promote the
reaction thermodynamically. Besides, when the ethanol/EC molar
ratio was lower, the amount of by-product N-EEC was higher,
which implied that the side reaction of EC N-ethylation took place
more easily due to the higher concentration of EC. As a result, the
30 min, a maximum yield of DEC was 22.9% with a selectivity of 182
48.0%.
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3.4. Possible reaction mechanization
The possible reaction mechanization was obtained on the basis 185
of DFT study preliminarily. As shown in Fig. 5, by geometry 186
optimization of ethyl carbamate and methanol, a weak H-bonded 187
complex (R) formed, originated from which the transition state 188
(
TS) was appeared after further optimization. In TS, the carbonyl C 189
and ethoxy O became closer and the H atom in hydroxyl was 190
attached to the amino N atom which resulted in the bond of N–C 191
2
extended. At the same time, the carbonyl C changes from sp
192
3
hybridization to almost sp hybridization and the dihedral angle of 193
TS was 132.88. Then, the product diethyl carbonate and ammonia 194
formed. The results were accurately consistent with the literature 195
published [21]. The possible reaction mechanization was shown in 196
Scheme 2.
197
Fig. 4. The effects of reactant loading volume on the yield of DEC and the conversion
of EC. Reaction temperature: 573 K, reaction time: 30 min, the molar ratio of
ethanol/EC: 10, reaction pressure: 11.5 MPa (225
mL), 12.1 MPa (245
mL), 12.6 MPa
Scheme 2. Possible reaction mechanism for DEC synthesis from EC in supercritical
(
265 L), 13.2 MPa (285 L), 13.6 MPa (305 L), 14.1 MPa (325
m
m
m
m
L).
ethanol.
Please cite this article in press as: L.-C. Zhao, et al., A catalyst-free novel synthesis of diethyl carbonate from ethyl carbamate in
supercritical ethanol, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.012

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L.-C. Zhao et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
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4. Conclusion
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A novel process for the synthesis of DEC from EC was achieved
under catalyst-free conditions in supercritical ethanol, in which
ethanol act as both solvent and active reactant. The possible reaction
mechanism was also proposed preliminarily on the basis of DFT
study. Compared with traditional methods, the method presented
herein showed many advantages, such as satisfactory yield of 22.9%,
high selectivityof 48.0%, remarkablyshort reaction time and absence
of catalysts. In addition, our work brought the synthesis of DEC from
EC and ethanol much closer to green industrial development.
Accordingly, we expect eagerly that supercritical ethanol can play a
more important role in chemical industry in the near future.
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Please cite this article in press as: L.-C. Zhao, et al., A catalyst-free novel synthesis of diethyl carbonate from ethyl carbamate in
supercritical ethanol, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.cclet.2014.05.012