Synthesis of diethyl carbonate from ethyl carbamate and ethanol over ZnO-PbO catalyst

Article abstract of DOI:10.1016/j.apcata.2012.05.023

The synthesis of diethyl carbonate (DEC) from ethyl carbamate and ethanol was investigated over a series of double metal oxides. Among the catalysts, ZnO-PbO showed the best catalytic activity and the highest DEC yield was 20.6%. Furthermore, ZnO-PbO had an excellent reusability. According to the results of XRD measurement, IR and element analysis, ZnO and PbO in ZnO-PbO catalyst were separately converted to Zn(NCO)₂(NH₃)₂ and metal Pb during the reaction, indicating that the mixture of Zn(NCO) ₂(NH₃)₂ and metal Pb may be the real active composition for DEC synthesis and ZnO-PbO is the precursor. In addition, a possible reaction mechanism for DEC synthesis was proposed.

Full text of DOI:10.1016/j.apcata.2012.05.023

Applied Catalysis A: General 433–434 (2012) 229–235
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis of diethyl carbonate from ethyl carbamate and ethanol over ZnO-PbO
catalyst
Hualiang An, Xinqiang Zhao∗, Lian Guo, Chunyao Jia, Baoguo Yuan, Yanji Wang
Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, Hebei University of Technology, Tianjin 300130, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of diethyl carbonate (DEC) from ethyl carbamate and ethanol was investigated over a series
of double metal oxides. Among the catalysts, ZnO-PbO showed the best catalytic activity and the highest
DEC yield was 20.6%. Furthermore, ZnO-PbO had an excellent reusability. According to the results of XRD
measurement, IR and element analysis, ZnO and PbO in ZnO-PbO catalyst were separately converted to
Zn(NCO)2(NH3)2 and metal Pb during the reaction, indicating that the mixture of Zn(NCO)2(NH3)2 and
metal Pb may be the real active composition for DEC synthesis and ZnO-PbO is the precursor. In addition,
a possible reaction mechanism for DEC synthesis was proposed.
Received 21 December 2011
Received in revised form 11 May 2012
Accepted 17 May 2012
Available online 24 May 2012
Keywords:
Diethyl carbonate
Ethyl carbamate
Ethanol
© 2012 Elsevier B.V. All rights reserved.
Urea
ZnO-PbO
1. Introduction
containing lead oxide were prepared and the activity evaluation
results revealed that ZnO-PbO presented the highest catalytic activ-
Diethyl carbonate (DEC) is not only an important organic inter-
mediate but also an attractive alternative oxygen-containing fuel
additive. The processes for the synthesis of DEC mainly include
phosgenation of ethanol [1], transesterification of organic carbon-
ates [2], oxidative carbonylation of ethanol [3,4], ethanolysis of CO2
[5] and ethanolysis of urea [6]. Among them, the ethanolysis of
urea exhibits great advantages such as available raw materials and
simple product separation. In such an approach, the intermediate
ethyl carbamate (EC) is first produced from urea and ethanol and
then further converted to DEC by consecutive reaction with ethanol.
The reaction of EC with ethanol to DEC is the rate-control step. Sev-
eral kinds of catalysts such as organometallic compounds, metal
salts and metal oxides have been tested toward the reaction of urea
and alcohols [7–9]. Among them, metal oxide catalysts have drawn
much more attention because they can overcome the drawbacks
in the separation of product and the recovery of the homogeneous
catalysts [10,11].
ity. The effect of preparation conditions and DEC synthesis reaction
parameters on the catalytic performance of ZnO-PbO was studied,
and the catalysis and reusability of ZnO-PbO were discussed, and a
possible reaction mechanism for DEC synthesis was proposed.
2. Experimental
2.1. Preparation of double metal oxides
All of double metal oxide catalyst samples were prepared by
calcinating their corresponding metal compounds. The prepara-
tion process was as follows: the metal compounds were mixed and
ground first and then calcined at a certain temperature for a period
of time in air. For example, for the preparation of ZnO-PbO with PbO
weight percentage of 10%, Zn2(OH)2CO3 (41.3 g) and PbCO3 (4.0 g)
were mixed and then ground in a mortar by hand for about 3 min
and finally calcined at 500 ◦C for 4 h in air.
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 single metal oxides but DEC yield was still unsatisfactory [12].
Furthermore, it was found that the addition of a little lead oxide
into some metal oxides could promote their catalytic performance.
Therefore, in the present work, a series of double metal oxides
2.2. DEC synthesis reaction
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 first, and then
the mixture was rapidly heated to 180 ◦C under stirring and kept
for 7 h. After the completion of reaction, the autoclave was cooled
to room temperature and the product mixture was weighed and
∗
Corresponding author. Tel.: +86 22 6020 2427; fax: +86 22 6020 4294.
E-mail address: zhaoxq@hebut.edu.cn (X. Zhao).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.05.023

230
H. An et al. / Applied Catalysis A: General 433–434 (2012) 229–235
Table 1
Catalytic performance of the double metal oxides.
Catalyst
Precursor
Calcination temperature/◦C
Conversion of EC (%)
Yield of DEC (%)
Selectivity of DEC (%)
MgO-PbO
CaO-PbO
SrO-PbO
BaO-PbO
ZnO-PbO
ZrO2-PbO
Al2O3-PbO
Fe2O3-PbO
CuO-PbO
NiO-PbO
TiO2-PbO
La2O3 -PbO
None
Mg(NO3)2·6H2O
CaCO3
Sr(NO3)2
500
900
1100
700
500
500
500
500
300
300
500
800
18.8
37.4
18.9
23.1
33.9
26.5
20.1
26.1
42.6
26.2
14.0
20.8
1.7
4.7
9.4
1.6
3.7
13.8
5.4
1.4
5.8
5.9
4.5
0.28
5.0
0.5
25.0
25.1
8.5
16.0
40.7
20.4
7.0
22.2
13.8
17.2
2.0
24.0
29.4
Ba(NO3)2
Zn2(OH)2CO3
Zr(NO3)4·5H2O
Al(NO3)3·9H2O
Fe (NO3)3·9H2O
CuCO3·Cu(OH)2·2H2O
NiCO3·2Ni(OH)2·4H2O
TiO(OH)2
La(NO3)3·6H2O
The weight percentage of lead oxide in double metal oxides was 10% and the calcination time was 4 h.
Catalytic activity evaluation conditions: catalyst weight percentage 1%, molar ratio of ethanol: EC = 10:1, 180 ◦C, and 7 h.
filtrated. The filtrate was analyzed by a SP 2100 gas chromatograph
with a FID and a PEG-20 M capillary column using cyclohexanol as
the internal standard. The yield and selectivity of DEC were calcu-
lated based on EC:
reaction parameters on the catalytic performance of ZnO-PbO cat-
alyst.
3.2. Effect of preparation conditions
moles of DEC formed
moles of EC charged
DEC yield(%) =
× 100
Reaction conditions for evaluating the catalytic performance of
ZnO-PbO sample were as follows: molar ratio of ethanol/EC of 10:1,
catalyst weight percentage of 1%, 180 ◦C, and 7 h.
moles of DEC formed
moles of EC converted
DEC selectivity (%) =
× 100
3.2.1. Effect of precursor
The catalytic performance of ZnO-PbO samples prepared from
different precursor is listed in Table 2. As a result, ZnO-PbO sample
prepared with Zn2(OH)2CO3 and PbCO3 as the precursors shows
the highest activity; the yield and selectivity of DEC are 13.8% and
40.7%, respectively. Hence, Zn2(OH)2CO3 and PbCO3 are selected as
the precursors of ZnO-PbO catalyst.
2.3. Catalyst characterization
XRD analysis of the catalyst was performed on a D/MAX-2500
diffractometer with Cu K␣ radiation operated at 40 kV and 100 mA.
IR spectroscopic spectrum of the catalyst was recorded on a Bruker
Vector 22 FT-IR spectrophotometer in the 4000–400 cm−1 range.
TG of the precursors (Zn2(OH)2CO3 and PbCO3) was carried out on
a SDT Q600 simultaneous DSC-TGA instrument at a heating rate of
10 ◦C/min under a flow of air. Element analysis of the recovered
catalyst was determined by a Flash EA1112 elemental analyzer.
3.2.2. Effect of calcination temperature
The effect of calcination temperature on the catalytic perfor-
mance of ZnO-PbO is illustrated in Table 3. It can be seen that
DEC selectivity reaches the maximum 40.7% at the calcination tem-
perature of 500 ◦C (DEC yield 13.8%) while DEC yield attains the
maximum 13.9% at the calcination temperature of 300 ◦C (DEC
selectivity 38.2%). From the viewpoint of full use of reactants, the
suitable calcination temperature is determined as 500 ◦C.
3. Results and discussion
3.1. Screening for catalysts
ZnO-PbO samples calcined between 300 ◦C and 600 ◦C were ana-
lyzed by XRD and the result is shown in Fig. 1. The diffraction peaks
of ZnO in different samples can be indexed to hexagonal structured
ZnO but lead oxides with different valence states and crystal forms
can be observed at different calcination temperature. Tetragonal
PbO is obtained from the samples prepared at 300 ◦C and 400 ◦C
As shown in Table 1, ZnO-PbO exhibits the highest activity
among the prepared double metal oxide catalysts and the yield and
selectivity of DEC are 13.8% and 40.7%, respectively. So it is neces-
sary to study the effect of preparation conditions and DEC synthesis
Table 2
Catalytic performance of ZnO-PbO prepared from different precursor.
Precursor of zinc oxide
Precursor of lead oxide
Calcination temperature/◦C
Conversion of EC (%)
Yield of DEC (%)
Selectivity of DEC (%)
Zn2(OH)2CO3
Zn2(OH)2CO3
Zn2(OH)2CO3
Zn(NO3)2
Zn(NO3)2
Zn(NO3)2
Zn(CH3CO2)2
Zn(CH3CO2)2
Zn(CH3CO2)2
Zn(OH)2
Zn(OH)2
Zn(OH)2
PbCO3
500
500
500
400
400
400
400
400
400
500
500
500
33.9
21.6
27.1
19.8
19.4
17.8
29.7
23.4
24.9
27.8
19.6
26.3
13.8
9.5
9.7
7.1
4.6
6.9
11.0
7.0
6.8
10.9
5.6
40.7
44.0
35.8
35.9
23.7
38.8
37.0
29.9
27.3
39.2
28.6
37.3
Pb(NO3)2
Pb(CH3COO)2
PbCO3
Pb(NO3)2
Pb(CH3COO)2
PbCO3
Pb(NO3)2
Pb(CH3COO)2
PbCO3
Pb(NO3)2
Pb(CH3COO)2
9.8
The weight percentage of lead oxide in double metal oxides was 10% and the calcination time was 4 h.
Catalytic activity evaluation conditions: catalyst weight percentage 1%, molar ratio of ethanol: EC = 10:1, 180 ◦C, and 7 h.

H. An et al. / Applied Catalysis A: General 433–434 (2012) 229–235
231
Table 3
Effect of calcination temperature on the catalytic performance of ZnO-PbO.
Table 4
Effect of lead oxide content on the catalytic performance of ZnO-PbO.
Calcination
Conversion
of EC (%)
Yield of
DEC (%)
Selectivity
of DEC (%)
Lead oxide
content (%)
Conversion
of EC (%)
Yield of
DEC (%)
Selectivity
of DEC (%)
temperature/◦C
300
400
500
600
36.4
35.6
33.9
35.3
13.9
13.6
13.8
10.6
38.2
38.2
40.7
30.0
5
10
15
20
25
33.6
33.9
43.2
41.4
18.1
9.4
13.8
14.3
13.2
3.4
28.0
40.7
33.1
31.9
18.8
The double metal oxides were prepared using Zn2(OH)2CO3 and PbCO3 as the pre-
cursors which were calcined for 4 h, and the weight percentage of lead oxide in
double metal oxides was 10%.
The double metal oxides were prepared using Zn2(OH)2CO3 and PbCO3 as the pre-
cursors which were calcined at 500 ◦C for 4 h.
Catalytic activity evaluation conditions: catalyst weight percentage 1%, molar ratio
Catalytic activity evaluation conditions: catalyst weight percentage 1%, molar ratio
of ethanol: EC = 10:1, 180 ◦C, and 7 h.
of ethanol: EC = 10:1, 180 ◦C, and 7 h.
by calcining at 300 ◦C. The difference between TG and XRD may be
caused by the calcination time, i.e. ZnO-PbO catalysts were calcined
at 300 ◦C for 4 h, and TG analyses is only a process of temperature-
programmed decomposition. After complete decomposition of the
precursors, there does not exist any changes with increasing tem-
perature in the TG curve, which may be responsible for the similar
catalytic behavior for the samples prepared at different calcination
temperatures.
3.2.3. Effect of lead oxide content
The effect of lead oxide content on the catalytic activity of ZnO-
PbO is demonstrated in Table 4. When lead oxide weight percentage
in ZnO-PbO sample reaches 10%, DEC selectivity attains the maxi-
mum 40.7% (DEC yield 13.8%) while the maximal DEC yield of 14.3%
(DEC selectivity 33.1%) is obtained over the catalyst with lead oxide
weight percentage of 15%. By comprehensive consideration of both
DEC yield and selectivity, lead oxide weight percentage of 10% is
selected.
20
30
40
50
60
70
80
2θ/(degree)
Fig. 1. XRD patterns of ZnO-PbO catalysts prepared at different calcination temper-
ature. (1) 300 ◦C, (2) 400 ◦C, (3) 500 ◦C, (4) 600 ◦C. The double metal oxides were
prepared using Zn2(OH)2CO3 and PbCO3 as the precursors which were calcined for
4 h, and the weight percentage of lead oxide in double metal oxides was 10%.
3.3. Effect of reaction conditions
3.3.1. Effect of catalyst concentration
The effect of catalyst concentration on DEC synthesis reaction is
shown in Fig. 3(a). The conversion of EC rises monotonically with an
increase of catalyst concentration while DEC yield and selectivity
reach their maximum 13.8% and 40.7% at the catalyst weight per-
centage of 1%. The main side-reaction in this reaction system is the
N-ethylation of EC to N-ethyl ethyl carbamate (N-EEC). Higher cat-
alyst concentration will promote this side-reaction and cause the
consumption of DEC [6] so as to decrease the yield and selectivity
of DEC. Therefore the suitable catalyst concentration is 1 wt%.
while orthorhombic PbO2 is present in the sample prepared at
400 ◦C and 500 ◦C. Orthorhombic PbO appears and tetragonal PbO
disappears when the calcination temperature is above 500 ◦C.
To investigate further the reason for similar catalytic behaviour
of the samples, the thermal decomposition of precursors
(Zn2(OH)2CO3 and PbCO3) was studied by thermal analyses and the
result is shown in Fig. 2. TG curve indicates that the major weight
loss is at around 190–330◦C and there is no other weight loss when
the temperature is higher than 330◦C. XRD determination results
(see Fig. 1) show that the precursors are decomposed completely
3.3.2. Effect of ethanol/EC molar ratio
Fig. 3(b) shows the influence of ethanol/EC molar ratio on the
DEC synthesis reaction. The DEC yield and selectivity rise with an
increase of ethanol/EC molar ratio first and then decrease. The
DEC yield reaches the maximum 16.8% (DEC selectivity 36.6%) at
ethanol/EC molar ratio of 7:1 while DEC selectivity attains the max-
imum 40.7% (DEC yield 13.8%) at ethanol/EC molar ratio of 10:1.
When the molar ratio of ethanol/EC is lower, EC concentration is
higher and the side reaction of N-ethylation of EC will take place
easily so as to lower DEC yield and selectivity. However, too much
higher molar ratio of ethanol/EC will result in the reduction of EC
concentration. As a result, the reaction rate decreases and a lower
DEC yield will be obtained. Based on the consideration of decreas-
ing ethanol separation costs, the suitable molar ratio of ethanol/EC
is 7.
110
100
90
80
70
60
50
3.3.3. Effect of reaction temperature
0
100
200
300
400
500
600
The effect of reaction temperature on the DEC synthesis reac-
tion is presented in Fig. 3(c). The conversion of EC rises with an
increase of reaction temperature but DEC yield and selectivity get
Temperature (ºC)
Fig. 2. TG curve of the mixture of Zn2(OH)2CO3 and PbCO3.

232
H. An et al. / Applied Catalysis A: General 433–434 (2012) 229–235
(a)
(b)
Conversion of EC
Selectivity of DEC
Yield of DEC
50
40
30
20
10
0
Conversion of EC
Selectivity of DEC
40
Yield of DEC
30
20
10
2
4
6
8
10
12
14
16
1
2
3
4
Molar ratio of ethanol/EC
Catalyst weight percentage (%)
(c)
(d)
60
50
40
30
20
10
50
Conversion of EC
Selectivity of DEC
Yield of DEC
Conversion of EC
Selectivity of DEC
Yield of DEC
40
30
20
5
6
7
8
9
170
180
190
200
210
Reaction time (h)
Reaction temperature (ºC)
Fig. 3. Effect of reaction conditions on the reaction for DEC synthesis. (a) Effect of catalyst weight percentage. Reaction conditions: molar ratio of ethanol: EC = 10:1, 180 ◦C,
and 7 h. (b) Effect of ethanol/EC molar ratio. Reaction conditions: catalyst weight percentage 1%, 180 ◦C, and 7 h. (c) Effect of reaction temperature. Reaction conditions:
catalyst weight percentage 1%, molar ratio of ethanol: EC = 7:1, and 7 h. (d) Effect of reaction time. Reaction conditions: catalyst weight percentage 1%, molar ratio of ethanol:
EC = 7:1, and 190 ◦C.
their maximum of 20.6% and 41.0% at 190 ◦C and then decrease.
Since the synthesis of DEC is an endothermic reaction, it is beneficial
at higher reaction temperature [6]. However, higher reaction tem-
perature will accelerate the side-reaction of N-ethylization either
and cause the decrease of DEC yield and selectivity beyond 190 ◦C.
Hence, the reaction temperature of 190 ◦C is selected.
drying at 80 ◦C for 12 h in vacuum. Their catalytic performance was
evaluated and the results are listed in Table 5. With the reuse of
ZnO-PbO catalyst, the conversion of EC decreases at the second run
and then keeps almost unchanged, DEC selectivity rises firstly and
then keeps even while DEC yield remains almost constant. It could
be inferred that ZnO-PbO has an excellent reusability.
3.3.4. Effect of reaction time
3.5. Analysis for catalysis of ZnO-PbO
The effect of reaction time on the DEC synthesis reaction is
demonstrated in Fig. 3(d). As the reaction proceeds, the conver-
sion of EC increases. However, DEC yield and selectivity reach their
maximal value 20.6% and 41.0% at 7 h and then drop due to the
side-reactions such as N-ethylization of EC. So the suitable reaction
time is chosen as 7 h.
It could be concluded that the suitable reaction conditions for
DEC synthesis over ZnO-PbO catalyst are as follows: catalyst con-
centration of 1 wt%, ethanol/EC molar ratio of 7:1, 190 ◦C, and 7 h.
Under the above reaction conditions, the DEC yield was 20.6%. The
catalytic performance of ZnO-PbO for dimethyl carbonate (DMC)
synthesis from methyl carbamate and methanol was also evalu-
ated and the yield and selectivity of DMC was 25.7% and 38.9%,
respectively. The results indicate that ZnO-PbO is effective for the
similar reactions.
3.5.1. XRD characterization
XRD patterns of the recovered ZnO-PbO catalyst are shown in
Fig. 4(a). Hexagonal ZnO, orthorhombic PbO and orthorhombic
PbO2 are observed in the fresh catalyst but only cubic metal Pb and
orthorhombic PbO2 appear in the recovered catalyst. This suggests
that ZnO has been transformed into an amorphous or microcrystal
phase while orthorhombic PbO has been reduced to cubic metal Pb.
Table 5
Reuse of ZnO-PbO Catalyst.
Runs
Conversion
of EC (%)
Yield of
DEC (%)
Selectivity
of DEC (%)
1
2
3
4
5
50.2
43.9
44.6
42.4
48.7
20.6
19.4
20.2
19.1
21.6
41.0
44.2
45.3
45.0
44.4
3.4. Reusability of ZnO-PbO catalyst
To investigate the reusability of ZnO-PbO catalyst, the used ZnO-
PbO was recovered by filtrating, washing with ethanol, and then
Reaction conditions: catalyst weight percentage 1%, molar ratio of ethanol: EC = 7:1,
190 ◦C, and 7 h.

H. An et al. / Applied Catalysis A: General 433–434 (2012) 229–235
233
(2)
(1)
4000
3500
3000
2500
2000
1500
1000
500
10
20
30
40
50
60
70
80
Wavenumber(cm-1)
2θ /(degree)
Fig. 5. IR spectra of the recovered catalysts. (1) First recovered, (2) third recovered.
(b)
From Table 5 and Fig. 4(a), it can be seen that orthorhombic
PbO has been reduced to cubic metal Pb in the recovered ZnO-
PbO catalyst and DEC selectivity is higher than that over the fresh
catalyst. In order to find the correlation between them, ZnO-Pb
sample was prepared by reducing ZnO-PbO sample with a mix-
ture of H2 and N2 (volume ratio 1:1) at 450 ◦C for 4 h and then the
catalytic performance of ZnO-Pb sample was evaluated. Under the
reaction conditions of catalyst weight percentage of 1%, ethanol/EC
molar ratio of 7:1, reaction temperature of 190 ◦C and reaction
time of 7 h, the yield and selectivity of DEC are 19.4% and 48.0%,
respectively, indicating that ZnO-Pb sample possesses high cat-
alytic performance. Fig. 4(c) shows the XRD patterns of the fresh
and recovered ZnO-Pb catalyst. Hexagonal ZnO, orthorhombic PbO2
and cubic metal Pb are observed in the fresh catalyst sample but
the recovered catalyst sample does not show any ZnO charac-
teristic peaks, illustrating that ZnO has been transformed into an
amorphous or microcrystal phase either, in accordance with the
result of ZnO-PbO catalyst. Compared with the catalytic perfor-
mance of ZnO-PbO catalyst, DEC yield is roughly the same while
DEC selectivity increases over ZnO-Pb catalyst, in agreement with
the experimental results of the reuse of ZnO-PbO catalyst. This
result illustrates that metal lead has better selectivity in DEC syn-
thesis reaction from ethanol and EC, which may be the reason for
DEC selectivity increase in the experiment of the reuse of ZnO-PbO
catalyst.
10
20
30
40
50
60
70
80
90
2θ /(degree)
(c)
3.5.2. IR analysis and element analysis
IR spectra of the recovered ZnO-PbO catalysts are presented
in Fig. 5. For the recovered catalyst from the first run, stretch-
ing vibrations of NH appears at 3464 cm−1 and the asymmetry
deformation vibration of NH3 is observed at 1624 cm−1. The char-
acteristic bands at 2215, 1263 and 619 cm−1 are attributed to the
stretching vibrations of NC and CO as well as the asymmetry defor-
mation vibration of NCO, respectively [13]. The recovered catalyst
from the third run also possesses the characteristic bands of NCO
and asymmetry deformation vibration of NH3, suggesting that the
recovered catalysts contain NCO group and NH3. Zhao et al. [14]
found that ZnO was transformed into Zn(NCO)2(NH3)2 in the reac-
tion of urea and alcohols. Hence, based on the IR analysis results, we
presume that ZnO (in ZnO-PbO catalyst) is probably transformed
into Zn(NCO)2(NH3)2 either in DEC synthesis reaction from ethanol
and EC. This is significantly supported by element analysis of the
recovered catalysts, i.e., the measured molar ratio of Zn, N, C and H
is 1:4:1.97:5.98, which is evidently consistent with the molecular
formula of Zn(NCO)2(NH3)2.
10
20
30
40
50
60
70
80
90
2θ /(degree)
Fig. 4. XRD patterns of the catalysts. (a) The fresh and recovered ZnO-PbO: (1) fresh,
(2) first recovered, (3) third recovered, (4) fifth recovered. (b) The fresh and recov-
ered ZnO catalyst: (1) fresh, (2) recovered. (c) The fresh and recovered ZnO-Pb: (1)
fresh, (2) recovered
When ZnO alone was used in this reaction, hexagonal ZnO could be
observed in the fresh and recovered catalyst (see Fig. 4(b)). So we
suppose that the existence of lead oxide accelerates the transfor-
mation of ZnO into an amorphous or microcrystal phase in ZnO-PbO
catalyst.

234
H. An et al. / Applied Catalysis A: General 433–434 (2012) 229–235
O
Zn
O
O
O
Zn
O
O
NCO
NCO
H2N
H2N
NH2
NH2
OCN
OCN
2NH3
O
C2H5O
2
Pb
H
O
(B)
(A)
C H OH
2
2
5
O
O
2NH2COOC2H5
(EC)
2Pb
C
O
NCO
OCN
H2N
H
H
NH3
Zn
O
Zn
NH2
H3N
OCN
O
NCO
C
O
H5C2OCOOC2H5
(DEC)
2
(D)
(C)
Scheme 1. Possible reaction mechanism for DEC synthesis from EC and ethanol.
3.5.3. Possible formation route to Zn(NCO)2(NH3)2
Combining the results of the present study and Zhao’s work [14],
a plausible formation route to Zn(NCO)2(NH3)2 is put forward as
follows:
not convert to metal Pb during the reaction. This result proves again
that both Zn(NCO)2(NH3)2 and metal Pb are the active components.
Therefore we speculated the reaction mechanism for diethyl car-
bonate formation from ethyl carbamate and ethanol over metal Pb
and Zn(NCO)2(NH3)2 as shown in Scheme 1.
O
As a transition metal element, divalent Zn2+ ion with d10 elec-
trons can coordinate with nitrogen, sulfur or phosphorus atom to
reach the stable construction of 18 electrons. In this reaction sys-
tem, Zn(NCO)2(NH3)2 can convert to Zn(NCO)2 and vice versa. Zn2+
ion coordinates with the nitrogen atom of amino group in a EC
molecule so as to activate EC. Since there exists a resonance struc-
ture for EC molecule, the complex of EC activated by Zn(NCO)2
has two kind of structures named as Species A and Species B in
Scheme 1. The hybridized orbital of the nitrogen atom will change
and most of sp2 changes to sp3 once it coordinates with Zn2+ ion,
and then the ␲-delocalized bond of the amide group becomes weak
correspondingly. Firstly, 1 equiv. of Zn2+ ion reaches the stable
construction of 18 electrons (the two nitrogen atoms in NCO
would provide one pair of electrons each) by the coordination
of 2 equiv. of EC through the nitrogen atom of the amide group.
The weak amino–carbonyl bond of Species B facilitates the nucle-
ophilic attack of ethoxy group in an ethanol molecule. On the other
hand, Pb atom inserts into an ethanol molecule between an ethoxy
group and a hydrogen atom so as to activate ethanol, facilitating
the dissociation of ethanol molecule into an ethoxy group and a
hydrogen atom. Then, a lone pair of electrons of oxygen atom in
the ethoxy group can form a bond with the electrophilic carbonyl
carbon of Species B while the hydrogen proton breaking away from
the ethanol molecule combines with the amino group leaving from
EC molecule. As a result, a new Species C is formed while Pb atom is
reduced back. The amino–carbonyl bond of Species C breaks to pro-
duce DEC and Species D. Because of the strong nucleophilic ability
of EC and the dissociation of NH3 in Species D, the NH3 in the Species
D can be substituted by EC to form Species A. Thus a catalytic circle
finishes.
Pb
NH2COC2H5
HNCO + C2H5OH
(1)
(2)
(3)
ZnO + 2HNCO → Zn(NCO)2 + H2O
Zn(NCO)2 + 2NH3 ꢀ Zn(NCO)2(NH3)2
The decomposition reaction of EC was studied at 190 ◦C for 7 h
in order to test whether metal lead could accelerate this reaction
or not. The result shows that the yield of ethanol is separately
41.1% and 84.4% without a catalyst and over metal lead catalyst.
This illustrates that metal lead indeed accelerates the decomposi-
tion of EC. Consequently, ZnO reacts with the resultant HNCO to
give rise to Zn(NCO)2 (Eq. (2)) and then Zn(NCO)2 coordinates with
NH3 in the reaction system to form Zn(NCO)2(NH3)2 (Eq. (3)). Since
the deamination reaction of an ammonia complex can proceed at a
high temperature, we presume that the reaction of Zn(NCO)2 with
NH3 toZn(NCO)2(NH3)2 maybereversiblein DECsynthesis process.
While the temperature drops to room temperature, the complex is
mainly in the form of Zn(NCO)2(NH3)2.
3.5.4. Possible reaction mechanism
Since hexagonal ZnO and orthorhombic PbO were converted
separately into Zn(NCO)2(NH3)2 and cubic metal Pb at the first run,
and they did not change at the following runs (see Fig. 4(a)), we
presumed Zn(NCO)2(NH3)2 and metal Pb were the real active com-
ponents. To investigate whether the precursors of ZnO-PbO have
the same catalytic performance as that of ZnO-PbO, the catalytic
performance of Zn2(OH)2CO3 + PbCO3 was evaluated and the yield
and selectivity of DEC was 15.5% and 34.4%, which was lower than
that over ZnO-PbO. In the recovered Zn2(OH)2CO3 + PbCO3 cata-
lyst, we found Zn(NCO)2(NH3)2 but no metal Pb, suggesting that
Zn2(OH)2CO3 could convert to Zn(NCO)2(NH3)2 while PbCO3 could
According to this catalytic mechanism, EC is activated by
Zn(NCO)2, which is in accordance with the activation of methyl

H. An et al. / Applied Catalysis A: General 433–434 (2012) 229–235
235
carbamate by Zn(NCO)2 reported by Zhao et al. [14]. The reaction
between PbO and EC could reduce PbO to metal Pb which was
shown to be one of the real active component for the synthesis
of DEC [15] and transition metal could catalyze dehydrogenation
of ethanol to ethyl acetate [16], which give support for activition
of ethanol by Pb. DEC yield was 2.3% over ZnO, 16.2% over PbO and
20.6% over ZnO-PbO. The higher catalytic activity of ZnO-PbO may
be caused by the separate activation of EC and ethanol by Zn(NCO)2
and metal Pb.
Science Foundation of Hebei province (Grant B2010000019). The
authors are gratefully appreciated their contributions.
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Acknowledgements
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