Catalysis by lead oxide for diethyl carbonate synthesis from ethyl carbamate and ethanol

Article abstract of DOI:10.1016/S1872-2067(11)60373-2

The catalysis by lead oxide in the reaction of ethyl carbamate (EC) with ethanol to form diethyl carbonate (DEC) was studied. The lead oxide catalyst exhibited an excellent stability, which could be reused five times without a significant loss in catalytic activity. X-Ray powder diffraction analysis showed that the recovered catalyst was a mixture of cubic metal Pb and orthorhombic PbO₂, with the latter shown to be the real active component for the synthesis of DEC. Verification experiments showed that the reaction between DEC and PbO was the main reason for the reduction of PbO to metal Pb.

Full text of DOI:10.1016/S1872-2067(11)60373-2

CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 4, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 595–600.
COMMUNICATION
Catalysis by Lead Oxide for Diethyl Carbonate Synthesis
from Ethyl Carbamate and Ethanol
GUO Lian, ZHAO Xinqiang*, AN Hualiang, WANG Yanji
Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, Hebei University of Technology, Tianjin
3
00130, China
Abstract: The catalysis by lead oxide in the reaction of ethyl carbamate (EC) with ethanol to form diethyl carbonate (DEC) was studied.
The lead oxide catalyst exhibited an excellent stability, which could be reused five times without a significant loss in catalytic activity.
X-Ray powder diffraction analysis showed that the recovered catalyst was a mixture of cubic metal Pb and orthorhombic PbO , with the
2
latter shown to be the real active component for the synthesis of DEC. Verification experiments showed that the reaction between DEC and
PbO was the main reason for the reduction of PbO to metal Pb.
Key words: diethyl carbonate; ethyl carbamate; lead oxide; ethanol; urea
Diethyl carbonate (DEC) is not only an important organic
more difficult [10]. Hence, it is important and necessary to
study the second reaction. Zhao et al. [11] investigated the
synthesis of dimethyl carbonate (DMC) from methyl car-
bamate (MC) and methanol over zinc compound catalysts.
intermediate but also an attractive alternative oxy-
gen-containing fuel additive. The DEC synthesis routes mainly
include phosgenation of ethanol, transesterification of organic
carbonates, oxidative carbonylation of ethanol, ethanolysis of
ZnCl showed the highest catalytic activity and led to a DMC
2
CO , and ethanolysis of urea. The phosgenation of ethanol is
gradually being phased out because phosgene is highly toxic
and the byproduct hydrogen chloride causes serious corrosion
yield of 33.6% under the optimal conditions. The homogene-
ous nature of the metal salts lead to separation and recovery
problems. Since heterogeneous catalysis can overcome these
drawbacks, this type of catalyst has attracted much more at-
tention. Wu et al. [12] have prepared Zn-based catalysts, and
the DMC yield obtained was 12.7% over zinc powder. Wang
et al. [13,14] investigated the catalytic performance of mixed
oxide catalysts for the reaction for DMC synthesis, and
zinc/iron mixed oxide possessed the best catalytic ability giv-
ing a DMC yield of 31.5%. Compared with many studies on
DMC synthesis, there were a few reports on DEC synthesis.
Zhao et al. [15] found that CaO had good catalytic activity for
the reaction of ethyl carbamate (EC) and ethanol to DEC, and
the yield of DEC was 10.8%.
2
[
1]. The transesterification of organic carbonates is
thermodynamically governed. Moreover, the starting material
such as propylene oxide) is sourced from depleting petroleum
(
reserves [2,3]. The oxidative carbonylation of ethanol suffers
from low single-pass conversion, high catalyst costs, and poor
stability [4–8]. The ethanolysis of CO is thermodynamically
2
unfavorable and still in the stage of fundamental studies [9].
The ethanolysis of urea, however, is a potential route because it
exhibits great advantages such as availability of raw materials
and simple product separation [10].
The alcoholysis of urea can be divided into two steps: the
synthesis of an intermediate alkyl carbamate from urea and an
alcohol followed by reaction of the alkyl carbamate with the
alcohol to form a dialkylcarbonate. The first step is fast and
highly selective even without a catalyst, but the second step is
In our previous work, lead oxide was found to be the most
active catalyst for the reaction of EC and ethanol to DEC
among a series of metal oxides studied, and the DEC yield was
16.2% [16]. Hence, a further study on the catalysis and reus-
Received 6 November 2011. Accepted 20 December 2011.
*
Corresponding author. Tel: +86-22-60202427; Fax: +86-22-60204294; E-mail: zhaoxq@hebut.edu.cn
The work was supported by the Special Program for Basic Research Program of the Ministry of Science and Technology of China (2010CB234602), the National
Natural Science Foundation of China (20976035, 21076059), and the Natural Science Foundation of Hebei province (B2010000019).
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60373-2

GUO Lian et al. / Chinese Journal of Catalysis, 2012, 33: 595–600
ability of lead oxide was carried out in this work. The reaction
ƻ
conditions in this work were identical to those described in our
previous work [16].
Ƹ
Ƹ
ƻ
ƻ
ƻ
(4)
3)
Lead oxide was prepared by thermal decomposition of lead
carbonate at 300 °C for 4 h in air. The DEC synthesis reaction
was carried out in a 500 ml stainless steel autoclave with a
packed column and a back-pressure valve for ammonia gas
removal. In a typical process, 30.5 g EC, 158 g ethanol, and 1.9
g catalyst were introduced into the autoclave which was firstly
purged with nitrogen and pressurized to 2 MPa. After that, the
reaction mixture was heated to 180 °C with stirring. After the
completion of reaction, the autoclave was cooled to room
temperature and the product mixture was weighed, filtered, and
analyzed.
(
(2)
ƽ
ƹ
ƽ
Tetragonal PbO
Cubic Pb
Ƹ
Orthorhombic PbO
ƻ
ƽ
2
ƹ
PbO4xH O
2
ƽ
ƽ
ƽ
ƽ
Ƹ
ƽ
(1)
10
20
30
40
50
60
70
80
o
2ꢀꢁꢂ ꢃ
The gaseous products were analyzed on a SP 3400 gas
chromatograph with a PLOT-Q packed column. The analysis
Fig. 1. XRD patterns of the fresh and recovered lead oxide catalysts. (1)
Fresh; (2) First recovered; (3) Third recovered; (4) Fifth recovered.
conditions were as follows: N as carrier, TCD temperature 120
2
°
C, the injection port temperature 120 °C, and column tem-
perature 50 °C. The liquid products were analyzed on an SP
100 gas chromatograph with a PEG-20M capillary column.
metal Pb and orthorhombic PbO catalyze the reaction for DEC
synthesis from EC and ethanol because the catalytic composi-
2
2
tion consisting of cubic metal Pb and orthorhombic PbO had
2
The analysis conditions were as follows: N as carrier, FID
2
not changed since the second run.
temperature 220 °C, the injection port temperature 200 °C, and
the program-controlled column temperature. The reaction
products were also qualitatively analyzed on a 6890/5973
GC-MS equipped with a HP-INNOWax column and mass
spectra were obtained in a range of 10–500 amu. X-ray dif-
fraction (XRD) analysis of the catalyst was performed on a
D/MAX-2500 diffractometer with Cu K radiation, 40 kV tube
voltage, 100 mA tube electric current, and scanning range of 5°
to 90°.
First of all, the reusability of lead oxide catalyst was inves-
tigated. The used lead oxide catalyst was recovered by filtra-
tion, washed with ethanol, and dried at 80 °C for 12 h in vac-
uum. The activity evaluation shown in Table 1 reveals that both
DEC yield and selectivity remained almost constant, indicating
that lead oxide had high catalytic stability.
In order to verify the correctness of this deduction, a metal
lead sample was prepared firstly by reducing lead oxide with a
mixture of H and N (volume ratio 1:1) at 300 °C for 6 h.
2
2
Figure 2 shows the XRD pattern of the reduced lead oxide
sample, and both cubic metal Pb and orthorhombic PbO were
2
observed. In Fig.1, PbO showed diffraction peaks at around 2ꢁ
2
ꢀ
=
26° (110) and 2ꢁ = 28° (111) while the diffraction peak of
PbO appeared only at around 2ꢁ = 28° (111) in Fig. 2. This
2
result may be caused by different treatment processes. Fur-
thermore, the catalytic performance of PbO , pure metal Pb,
2
and the metal lead sample prepared by reducing lead oxide was
evaluated separately in the reaction of EC and ethanol. The
results are listed in Table 2. Compared with the fresh lead
oxide, the pure metal Pb exhibited a weak activity; PbO dis-
2
played a slightly lower selectivity for DEC synthesis; the pre-
XRD patterns of the fresh, first recovered, third recovered,
and fifth recovered lead oxide catalyst are shown in Fig. 1.
Tetragonal PbO and orthorhombic PbO were observed in the
2
ƻ
ͪ
Cubic Pb
Orthorhombic PbO2
ƻ
fresh catalyst while cubic metal Pb and orthorhombic PbO2
appeared in the recovered catalyst. This suggested that
tetragonal PbO had been transformed into cubic metal Pb
during the reaction. Furthermore, we assumed that both cubic
ƻ
Table 1 Reuse of lead oxide catalyst
ƻ
Run EC conversion (%)
DEC yield (%)
15.7
DEC selectivity (%)
ƻ
1
2
3
4
5
36.5
36.8
37.0
33.4
36.7
43.0
44.0
43.5
45.2
44.7
ƻ
ƻ
ͪ
ƻ
16.2
16.1
15.1
16.4
10
20
30
40
50
ꢀ/( )
60
70
80
90
o
2
Reaction conditions: n(ethanol):n(EC) = 10, catalyst concentration 1 wt%,
80 °C, 7 h, 2 MPa. EC—Ethyl carbamate; DEC—Diethyl carbonate.
Fig. 2. XRD pattern of the sample prepared by reduction of the fresh
lead oxide.
1

GUO Lian et al. / Chinese Journal of Catalysis, 2012, 33: 595–600
Table 2 Catalytic activity of different catalysts
ƻ
ƻ
Ƹ Cubic Pb
Orthorhombic PbO
EC conversion DEC yield
DEC selectivity
Catalyst
2
(
%)
(%)
1.9
11.3
13.4
13.9
15.7
(%)
15.7
32.7
49.8
41.9
43.0
Pure metal Pb
12.1
34.6
26.9
33.2
36.5
PbO
Metal lead sample
Pb + PbO
Fresh lead oxide
2
*
ƻ
2
ƻ
ƻ
ƻ
Ƹ
*
Prepared by reducing lead oxide.
Ƹ
ƻ
ƻ
Reaction conditions: n(ethanol):n(EC) = 10, catalyst concentration 1
wt% , 180 °C, 7 h, 2 MPa.
10
20
30
40
50
ꢀ/( )
60
70
80
90
o
2
pared metal lead sample showed an excellent activity and
selectivity. DEC yield decreased slightly while DEC selectivity
increased over the metal lead sample. Since the prepared metal
Fig. 3. XRD pattern of lead oxide reacted with DEC.
lead sample was composed of metal Pb and PbO , the catalytic
indicating that the presence of DEC was the main reason for the
reduction of PbO.
2
performance of Pb/PbO mixture (mass ratio 1:1) prepared by
2
physical mixing was also evaluated. DEC yield and selectivity
were 13.9% and 41.9%, respectively, indicating that the cata-
To investigate the reaction of DEC and lead oxide thor-
oughly, the gaseous products were collected and analyzed after
reaction and the results showed the presence of hydrogen,
ethylene, and carbon dioxide. Furthermore, the liquid products
were qualitatively analyzed by GC-MS. The result showed that
besides DEC, the main products in this reaction system were
ethanol, diethylether, ethylacetate, and water. Notario and
coworkers [18] found that DEC could decompose to ethanol,
ethylene and carbon dioxide at temperatures of 280–440 °C. In
our blank test, DEC did not decompose at 180 °C but did par-
tially decompose at 180 °C in the presence of lead oxide. So we
suppose that lead oxide can promote the decomposition of
DEC.
lytic activity of Pb/PbO mixture was close to that of the fresh
2
lead oxide catalyst. Hence, we are convinced that the mixture
of metal Pb and PbO is the catalytic agent and the synergistic
2
action between metal Pb and PbO is responsible for the high
2
catalytic activity.
From Fig. 1 we know that tetragonal PbO was transformed
into cubic metal Pb during the DEC synthesis reaction. In order
to study the reason for the reduction of PbO in this reaction
system, the following experiments were designed and con-
ducted.
Experiment No.1: the reaction of ethanol and lead oxide
(
1
reaction conditions: mass ratio of ethanol to lead oxide = 60:1,
80 °C, 7 h). Acetaldehyde and ethylacetate were detected in
Experiment No. 4: to test whether PbO could be reduced to
metal Pb by the reaction products of experiment No. 3, the
reaction between lead oxide and a mixture of diethylether,
ethylacetate, acetaldehyde, and ethanol in the presence of
gaseous mixture of hydrogen, ethylene, carbon dioxide, and
nitrogen studied (reaction conditions: mass ratio of lead oxide
to diethylether to ethylacetate to acetaldehyde to ethanol =
1:1:1:1:60, 180 °C, 7 h) was carried out. The XRD analysis of
the solid sample after reaction revealed that PbO was not re-
duced to metal Pb, indicating that the above reaction system
does not effect this reduction.
the liquid phase after the reaction of ethanol and the lead oxide
sample. Idriss et al. [17] investigated the reaction of ethanol
with a series of metal oxides and found that the main reaction
products in all cases were acetaldehyde and ethylacetate. They
supposed that ethanol tended to dehydrogenate to acetaldehyde
which further reacted to form ethylacetate via the Tishchenko
reaction. The result of the reaction between ethanol and lead
oxide was in agreement with Idriss’s work [17]. Furthermore,
the XRD characterization of the solid sample after the reaction
revealed that no metal Pb was observed, indicating that the
presence of ethanol was not the reason for the reduction of PbO
to metal Pb.
Based on the above experiments, it can be inferred that the
reaction between DEC and PbO is the main reason for the
reduction of PbO to metal Pb.
Experiment No. 2: the reaction of EC and lead oxide (reac-
tion conditions: water as solvent, mass ratio of EC to PbO =
To further analyze the reason for the reduction of PbO to
metal Pb, experiment No. 5 was designed to test the effect of
reaction time on the reaction between DEC and PbO (reaction
conditions: mass ratio of DEC to lead oxide = 60:1, 180 °C, 3
h). GC analysis showed that acetaldehyde appeared at 3 h but
disappeared at 7 h (experiment No. 3) in the reaction solution,
and other products did not change at 3 and 7 h. This means that
the acetaldehyde formed must be further converted to other
products such as ethylacetate [17]. Additionally, the DEC
15:1, 180 °C, 7 h). The XRD analysis of the solid sample re-
vealed that no metal Pb was observed either, indicating that the
presence of EC was not the reason for the reduction of PbO.
Experiment No. 3: the reaction of DEC and lead oxide (re-
action conditions: mass ratio of DEC to PbO = 60:1, 180 °C, 7
h). The XRD analysis of the solid sample illustrated that
tetragonal PbO was transformed into cubic metal Pb (Fig. 3),

GUO Lian et al. / Chinese Journal of Catalysis, 2012, 33: 595–600
content decreased while ethanol content increased at 7 h in the
References
reaction system, illustrating that prolonging the reaction time
can promote the decomposition of DEC. Since acetaldehyde
was detected in the reaction solution of DEC and lead oxide at
the reaction time of 3 h, we suggest that the reaction between
DEC and PbO takes place as follows:
1
2
3
Muskat I E, Strain F. US 2 379 250. 1941
Frevel L K, Gilpin J A. US 3 642 858. 1972
Wang M, Zhao X Q, An H L, Wang Y J. J Chem Eng Chin Univ,
2010, 24: 663
4
5
Fenton D M. US 3 227 740. 1966
Zhang P B, Huang S Y, Yang Y, Meng Q S, Wang S P, Ma X B.
Catal Today, 2010, 149: 202
(
C H O) CO + PbO ꢄꢄꢅ
2 5 2
C H + CH CHO + CO ꢆ H O ꢆ Pb
(1)
2
4
3
2
2
As mentioned above, ethanol, diethylether, ethylacetate,
and hydrogen were detected in the reaction between DEC and
lead oxide. We assume that ethanol can be formed by the de-
composition of DEC [18], and the intermolecular dehydration
of ethanol can generate diethylether [19]. Ethylacetate and
hydrogen can be formed by the dehydrogenation of ethanol
6
Zhu D J, Mei F M, Chen L J, Mo W L, Li T , Li G X. Fuel,
2011, 90: 2098
Zhang P B , Ma X B. Chem Eng J, 2010, 163: 93
Ma X B, Huang Sh Y, Wang Sh P, Zhang P B. Petrochem Tech-
nol, 2010, 39: 697
7
8
9
0
1
Gasc F, Thieloaud-Roux S, Moulougui Z. J Supercrit Fluids,
2009, 50: 46
[
20], and ethylacetate can be also produced via the Tishchenko
1
1
Wang D P, Yang B L, Zhai X W, Zhou L G. Fuel Process Tech-
nol, 2007, 88: 807
Zhao W B, Wang F, Peng W C, Zhao N, Li J P, Xiao F K, Wei
W, Sun Y H. Ind Eng Chem Res, 2008, 47: 5913
reaction of acetaldehyde [17], as shown in the following equa-
tions:
(
C H O) CO ꢄ ꢄꢅ C H + C H OH + CO
(2)
(3)
(4)
(5)
2
5
2
2
4
2
5
2
2
C H OH ꢄ ꢄꢅ C H OC H + H O
12 Wu C C, Zhao X Q, Wang Y J. Catal Commun, 2005, 6: 694
2
5
2
5
2
5
2
1
3
Wang D F, Zhang X L, Gao Y Y, Xiao F K, Wei W, Sun Y H.
Catal Commun, 2010, 11: 430
2
C H OH ꢄ ꢄꢅ CH COOC H + 2H
2
2
5
3
2
5
2
CH CHO ꢄ ꢄꢅ CH COOC H
3 3 2 5
14 Wang D F, Zhang X L, Zhao W B, Peng W C, Zhao N, Xiao F
K, Wei W, Sun Y H. J Phys Chem Solids, 2010, 71: 427
Zhao W B, Peng W C, Wang D F, Zhao N, Li J P, Xiao F K, Wei
W, Sun Y H. Catal Commun, 2009, 10: 655
In summary, the lead oxide catalyst displayed excellent sta-
bility. The recovered catalyst was found to be a mixture of
1
5
6
cubic metal Pb and orthorhombic PbO , indicating that the
2
1
Zhao H L, Zhao X Q, An H L, Wang Y J. Petrochem Technol,
synergistic action between them is responsible for the high
catalytic activity observed. Furthermore, tetragonal PbO pro-
moted the decomposition of DEC to ethanol, ethylene, and
carbon dioxide, and PbO itself was simultaneously reduced to
metal Pb. The catalytic mechanism of cubic metal Pb and
2009, 38: 139
1
1
7
8
Idriss H, Seebauer E G. J Mol Catal, 2000, 152: 201
Notario R, Quijano J, Sánchez C, Vélez E. J Phys Org Chem,
2005, 18: 134
19 Varisli D, Dogu T, Dogu G. Chem Eng Sci, 2007, 62: 5349
20 Inui K, Kurabayashi T, Sato S, Ichikawa N. J Mol Catal, 2004,
216: 147
orthorhombic PbO in the synthesis of DEC from EC and
2
ethanol is currently under investigation.