Catalytic alcoholysis of urea to diethyl carbonate over calcined Mg-Zn-Al hydrotalcite

Article abstract of DOI:10.1039/c4ra15897k

The synthesis of diethyl carbonate (DEC) from urea and ethanol was carried out over Mg-Zn-Al composite oxide catalysts derived from hydrotalcites (HTs). The catalytic results showed that the ternary hydrotalcites calcined at 450 °C with Mg:Zn:Al = 1:1.7:1 exhibited superior catalytic activity, and the highest DEC yield was 67.8%. Similar to ethanol, other alcohols such as methanol and butanol can also be transformed to corresponding dialkyl carbonates. Catalysts were characterized by XRD, BET, SEM and TPD with the aim of establishing a relationship between performance and structure. The results indicated that MgZn_1.7Al-450 with nanoplate morphology and more accessible active medium basic sites were favourable for obtaining much superior catalytic activity. Recycling experiments demonstrated that the catalyst could be successfully reused. This journal is

Full text of DOI:10.1039/c4ra15897k

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PAPER
Catalytic alcoholysis of urea to diethyl carbonate
over calcined Mg–Zn–Al hydrotalcite†
Cite this: RSC Adv., 2015, 5, 19534
ab
a
ab
ab
c
Peixue Wang, Shimin Liu, Feng Zhou, Benqun Yang, Ahmad S. Alshammari
and Youquan Deng*a
The synthesis of diethyl carbonate (DEC) from urea and ethanol was carried out over Mg–Zn–Al composite
oxide catalysts derived from hydrotalcites (HTs). The catalytic results showed that the ternary hydrotalcites
ꢀ
calcined at 450 C with Mg : Zn : Al ¼ 1 : 1.7 : 1 exhibited superior catalytic activity, and the highest DEC
yield was 67.8%. Similar to ethanol, other alcohols such as methanol and butanol can also be
transformed to corresponding dialkyl carbonates. Catalysts were characterized by XRD, BET, SEM and
TPD with the aim of establishing a relationship between performance and structure. The results indicated
that MgZn1.7Al-450 with nanoplate morphology and more accessible active medium basic sites were
favourable for obtaining much superior catalytic activity. Recycling experiments demonstrated that the
catalyst could be successfully reused.
Received 6th December 2014
Accepted 9th February 2015
DOI: 10.1039/c4ra15897k
www.rsc.org/advances
DEC is an important organic intermediate, which can be
used as an effective ethylization, a carbonylation agent and raw
1
. Introduction
As an abundant, renewable and economical carbon resource, material for manufacturing polycarbonates. Also, DEC has
transformation of CO
attracted signicant interest. However, CO
2
to useful carbon compounds has signicant applications in the eld of lithium batteries
1
5
is the most electrolyte. More importantly, DEC has been mentioned as a
2
oxidized state of carbon and acts as a relatively inert molecule, potential additive to fuel oil due to its high oxygen content and
therefore, the activation of CO normally needs the usage of a octane number, good gasoline/water distribution coefficient,
2
6
highly active starting material or to shi the equilibrium to the low toxicity, and rapid biodegradability. The high oxygen
product side by removing a particular compound. New method content of DEC (40.6% by wt) signicantly reduces the amount
2
such as CO
CH , etc., has been extensively studied. Nevertheless, this
technique is severely restricted by current hydrogen supply and CO
only applicable in small scale. In this context, exploration of mainly include phosgenation of ethanol,
2
hydrogenation to produce HCOOH, CH
3
OH, of smoke and particulates in engine exhaust.
According to the different carbonyl sources (COCl
, DMC and urea), the processes for the synthesis of DEC
3
, CO,
2
4
2
7
oxidative
alternative ways for CO activation and utilization in large scale carbonylation of ethanol, esterication of carbon dioxide with
8
2
9
10
are highly desired. Until now, urea production is almost the ethanol, transesterication of organic carbonates and alco-
unique way for large scale chemical xation of CO2.
4
holysis of urea (Scheme 1). The phosgenation of ethanol has
Fortunately, using urea as feedstock to produce higher valued been phased out because phosgene is highly toxic and the
chemicals opened a new window for CO utilization and
alternative carbonyl sources to substitute phosgene, e.g.
manufacturing of isocyanates since direct CO activation is very
2
2
difficult but manufacturing urea is very mature and low cost.
The alcoholysis of urea to dialkyl carbonates, e.g. diethyl
carbonate (DEC), may provide another effective way for
CO utilization.
2
a
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou, 730000, China. E-mail: ydeng@licp.cas.cn;
Fax: +86-931-4968116; Tel: +86-931-4968116
b
Graduate School of the Chinese Academy of Sciences, Beijing, 100039, China
c
National Nanotechnology Research Center, King Abdulaziz City for Science and
Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra15897k
Scheme 1 The processes for synthesis of DEC with different carbonyl
sources.
1
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by-product hydrogen chloride could result in serious corrosion. Table 1 The performance of various catalysts in the alcoholysis of
a
urea to DEC
The oxidative carbonylation of ethanol suffers from low
production rate, need for corrosion resistant reactors, toxicity
and potential explosion of carbon monoxide. Likewise, the
Yield (%)
direct synthesis of DEC by reaction of ethanol and CO
2
is also Entry
Catalyst
DEC
EC
NEEC
limited by the unfavourable equilibrium constant. For the
transesterication of DMC with ethanol, the separation of DEC
is very difficult owing to the formation of three binary
azeotropes. Consequently, synthesis of DEC based on alcohol-
ysis of urea becomes a more promising approach. In such
process, urea can be used as the carbonylation agent, which is a
nontoxic material and has abundant resources with lower price.
1
2
3
4
5
6
7
8
9
HTMg2.7Al
HTZn2.6Al
HTMg1.9Zn0.8Al
HTMgZn1.7Al
Mg2.7Al-400
8.0
31.5
42.2
49.3
49.1
41.9
65.0
41.2
67.8
55.6
38.3
44
77.5
32.8
30.8
28.3
25.3
28.3
20.2
34.7
17.8
20.1
43.2
36
1.6
1.3
0.6
1
1.2
1.2
0.7
0.7
0.8
0.9
0.4
0.8
Zn2.6Al-400
Mg1.9Zn0.8Al-450
MgZn1.7Al-350
MgZn1.7Al-450
MgZn1.7Al-550
ZnO + HTMg2.7Al
ZnO/Mg2.7Al-450
Meanwhile, NH
converted to urea, since the urea synthesis from CO
3
co-produced in the reaction can be easily
and NH
2
3
10
has been established. Hence, this route could be considered as 11
12
indirect utilization of CO
2
to produce the green chemicals of
a
DEC. Wang et al. used ZnO as a catalyst to obtain only a
Reaction conditions: 20 mmol urea, 200 mmol ethanol, 0.1 g catalyst,
ꢀ
ꢀ
11
200 C, 4 h. DEC: diethyl carbonate, EC: ethyl carbamate, NEEC : N-ethyl
ethyl carbamate.
1
4.2% yield of DEC at 190 C. Aerwards, Sun et al. observed
that the solid ZnO completely dissolved and formed homoge-
12
nous catalyst Zn(NH ) (NCO) during the reaction. Recently,
3
2
2
Xin et al. reported that La
2
O
3
as a basic catalyst has better
catalytic activity, but the yield of DEC (38.6%) is still not 67.8% of DEC yield for Mg2.7Al-400, Zn2.6Al-400, Mg1.9Zn0.8Al-450
13
satisfactory. Therefore, developing an effective and stable and MgZn1.7Al-450 as catalysts were achieved, respectively
heterogeneous catalyst is still highly desirable for this impor- (entries 5–7 and 9). From these results, it can be seen that the
tant process due to the signicance in industrial manufacture. composite oxides derived from hydrotalcites possessed a good
In this study, ternary Mg–Zn–Al hydrotalcites were prepared catalytic performance for this reaction, and the ternary composite
by co-precipitation method, which was found to catalyze the oxides as catalysts were also better than binary composite oxides.
DEC synthesis from urea and ethanol with high efficiency Given the above, the ternary composite oxide with the molar ratio
2
(67.8% yield of DEC). XRD, BET, SEM and CO -TPD studies were of Mg and Zn was 1 : 1.7 displayed the best activity. In the
conducted to explore the relationship between structure and subsequent investigations, the effect of preparation conditions
performance. Additionally, the inuence of the preparation on the catalytic performance of MgZn1.7Al catalyst was studied in
parameters (such as Mg–Zn–Al molar ratio, catalyst activation detail. Among the catalysts calcined at different temperatures
temperature), the effect of reaction conditions and the structure (entries 8–10), the DEC yields were 41.2%, 67.8% and 55.6% over
of alcohols were also investigated in this work.
MgZn1.7Al-350, -450 and -550 catalyst, respectively, which
indicated that 450 C was the proper activation temperature for
ꢀ
better catalytic activity. It should be mentioned that the yield of
2
. Results and discussion
DEC (67.8%) obtained is much better than that previous
2.1 Syntheses of DEC from urea and ethanol
11–13
reports.
Additionally, the catalytic activity of the physical
A series of Mg, Zn, Al catalysts were employed based on previ- mixture of ZnO + HTMg2.7Al (50 mg ZnO and 50 mg HTMg2.7Al)
ously extensive catalysts screening (the ratios of Mg, Zn, Al in and ZnO/Mg2.7Al (20 wt% ZnO loaded on Mg–Al) were further
HTs were estimated by EDS analysis). Table 1 lists the reaction tested (entries 11–12). The results showed that the physical
results obtained over prepared catalysts. When the HTs without mixture and ZnO/Mg2.7Al-400 gave lower catalytic activity than
calcination were used as the catalyst for the direct synthesis of ternary composite oxides, demonstrating that the integration of
DEC (entries 1–4), the obtained results showed that the ternary Zn and Mg–Al components could remarkably improve the
HTs exhibited superior catalytic activity than the binary HTs, catalytic activity and the synergistic effect probably occurred.
and the yield of DEC could be up to 49.3% over HTMgZn1.7Al.
Over the above catalysts, a large quantity of ethyl carbamate
(EC) regarded as the intermediate of DEC formation was
detected. The results indicated that the EC could be easily
formed, but it is difficult to convert into DEC without proper
catalyst in this step (Scheme 2). When the DEC concentration in
the reaction mixture was relatively higher, the side reaction
between DEC and EC would occur and N-ethyl ethyl carbamate
(NEEC) was also found in the products.
As prepared HTs are thus oen inactive in a certain number of
14
base-catalysed reactions, so the HTs were activated by thermal
Scheme 2 Possible routes for the synthesis of DEC by alcoholysis of
decomposition. Aer calcination, 49.1%, 41.9%, 65.0% and urea.
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.2 Effect of reaction conditions
Paper
2
remarkable increase of reaction volume when excessive ethanol
was added.
In order to optimize reaction conditions, the inuence of
various reaction parameters was investigated. The impact of the
amount of catalyst charged on the DEC synthesis was rstly
investigated. The amount of catalyst charged was varied
between 4 and 12 (wt% respect to the mass of urea). It can be
seen in Fig. 1a, that the DEC yield increases with the increase of
the amount of catalyst charged initially. When the amount of
catalyst charged was 8%, the DEC yield reached the maximum
value of 67.8%, and the DEC yield declined with the further
increase of the amount of catalyst. Thus, the optimum amount
of catalyst charged was 8% in this batch reaction.
Then, the impact of the ethanol/urea molar ratio on DEC
synthesis was investigated, as shown in Fig. 1b. A signicant
increase in DEC yield was observed when the ethanol/urea
molar ratio increased from 4 to 8, and reached the maximum
when the ratio was 10. When the molar ratio of ethanol/urea is
lower, urea concentration is higher in the reactor and the
The impact of temperatures on DEC synthesis was also
investigated, Fig. 1c. The DEC yield sharply increases in the
ꢀ
temperature range of 180–200 C and decreases slightly when
ꢀ
the reaction temperature exceeds 200 C, probably due to urea
and DEC decompositions at higher temperatures.
Finally, the impact of reaction time on DEC synthesis was
investigated, Fig. 1d. It can be seen that aer 4 h reaction the
yield of DEC can reach 67.8%. Simultaneously, the yield of EC
gradually decreased with the increasing reaction time, which
provided the substantial support for the DEC formation via an
EC intermediate. Further prolonging the reaction time the
yield of DEC decreased, this may caused by the side reaction.
So, 4 h should be the proper reaction time for obtaining higher
yield of DEC.
decomposition of the urea could be observed. Since, we found 2.3 Recycling of the catalyst
that some white solid was formed on the pipelines at the top of
The reusability of solid catalysts is a key parameter in deter-
the autoclave. Theoretically, higher ethanol/urea molar ratio
favors an increase of the DEC yield. Nevertheless, further
increasing the ethanol/urea molar ratio to 14, the yield of DEC
was greatly reduced again. At this stage, although the detailed
reason is still not clear, it can be conjectured that insufficient
availability of catalyst species may be one of the reasons due to
mining their suitability in practical applications. Therefore,
attempts were made to reuse the MgZn1.7Al-450 catalyst 5 times
by ltration, washing with acetone, and drying aer each run.
The results obtained (Fig. 2) indicated that the catalyst can
retain its activity and selectivity performance during the
recycles.
Fig. 1 Effect of reaction conditions on the catalytic performance of MgZn1.7Al-450. Influence of (a) the amount of catalyst charged; (b) the
ꢀ
ethanol/urea molar ratio; (c) the temperature; (d) the reaction time. Reaction conditions: reaction temperatures 180–220 C, ethanol/urea molar
ratio 4–14, the amount of catalyst charged 4 and 12 (wt% of the mass of urea), reaction time 1–6 h.
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catalyst also has good catalytic performance for other dialkyl
carbonates synthesis by alcoholysis with urea.
2.5 Results of the catalysts characterization
Fig. 3 shows the XRD patterns of the different HTs samples. It
can be seen that the HTMg2.7Al and HTZn2.6Al showed the
characteristic peaks corresponding to well-crystallized
16
hydrotalcite structure. For HTMg1.9Zn0.8Al and HTMgZn1.7
-
Al, XRD patterns revealed that the position of the diffraction
peaks is still consisted with the peaks of HTs, indicating that
the prepared sample kept the layered structure. Aer
calcination, the layered structure disappeared completely in
HTMgZn1.7Al and oxide diffraction peaks are detected. As for
X-ray diffractions of the MgZn1.7Al-450, only ZnO
(
JCPDS 36-1451) crystal phase was present, no other phase could
Fig. 2 The reusability of MgZn1.7Al-450 catalyst for DEC synthesis.
be observed suggesting that the resulted Mg and Al oxides may
be amorphous aer calcination. The characteristic peaks of ZnO
became weak and a new phase of Zn(NH ) (NCO) was appeared
3
2
2
2
.4 Inuence of the structure of alcohols for the synthesis of
in the used catalyst, indicating that phase transformations
occurred during the reaction. This may be a reason for the
slightly decrease of DEC yield over spent catalysts.
dialkyl carbonates
The inuence of the structure of alcohol on the reaction was
also investigated. Methanol and butanol were used for the
alcoholysis with urea, and the results are presented in Table 2.
Methanol gives 43% selectivity towards dimethyl carbonate
ꢀ
(
DMC) at 200 C, 4 h. A very larger quantity of by-products of
methyl carbamate (MC) and N-methyl methyl carbamate
NMMC) was also formed. The yield of DMC decreased with the
(
increasing reaction time, which may caused by the serious side
reaction between DMC and MC. The reaction of butanol and
ꢀ
urea under 200 C produces dibutyl carbonate (DBC), butyl
carbamate (BC) and N-butyl butyl carbamate (NBBC), however,
only 40% yield of DBC and 1.0% NBBC were obtained aer 8 h
15
reaction. A previous report showed that the dialkyl carbonates
are an attractive alternative as alkylating reagents for
NH-containing compounds, and its alkylation ability at the
same conditions is as follows: DMC > DEC > DBC. Thus, a small
amounts of N-butylated product was observed over DBC
synthesis might be duo to its lower reactivity for the alkylation.
Since we note that butanol is less reactive to the respect of the
ꢀ
other alcohols, the temperature was increased from 200 C to
ꢀ
2
30 C, and the yield of DBC could be reach to 85%. From the Fig. 3 XRD patterns of (a) HTZn2.6Al; (b) HTMgZn1.7Al; (c) HTMg2.7Al;
preliminary results, it can be conjectured that MgZn1.7Al-450 (d) HTMg1.9Zn0.8Al; (e) MgZn1.7Al-450 and (f) MgZn1.7Al-450-reused.
3 2 2
(-: HTs; ;: ZnO; +: Zn(NH ) (NCO) ).
a
Table 2 Effect of the structure of alcohols for the synthesis of dialkyl carbonates
Yield (%)
ꢀ
Alcohol
Temp. ( C)
Time (h)
Dialkyl carbonate
Alkyl carbamate
N-substituted carbamate
CH
CH
3
OH
OH
200
200
200
200
200
230
4
8
4
4
8
4
43
20
68
22
40
85
28
17
18
63
44
9
14
26
1
0.6
1.0
1.2
3
C
2
C
4
C
4
C
4
H
H
H
H
5
9
9
9
OH
OH
OH
OH
a
Reaction conditions: 20 mmol urea, 200 mmol alcohols, 0.1 g catalyst.
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The representative morphology of HTMgZn1.7Al and Table 3 Physical properties of the composite oxides derived from
a
HTs
morphology evolution of MgZn1.7Al-450 samples aer
calcination were studied by means of SEM, as shown in Fig. 4.
2
ꢁ1
a
b
3
ꢁ1
Catalyst
S
BET (m g
)
d
p
(nm)
p
v (cm g )
The distinct nanoplates can be found in HTMgZn1.7Al (Fig. 4A).
ꢀ
Aer calcination at 450 C (Fig. 4B), the discriminable Mg2.7Al-400
158.1
103.0
154.9
123.0
120.6
115.4
8.5
0.38
0.52
0.56
0.67
0.65
0.66
ꢀ
nanoplates-like features was still exist, indicating that 450 C Zn2.6Al-400
19.1
11.8
20.5
20.4
20.7
Mg1.9Zn0.8Al-450
was the suitable calcination conditions in order to thoroughly
decompose the interlayer anions without exceeding the critical
temperature at which phase segregation and sintering take
MgZn1.7Al-350
MgZn1.7Al-450
MgZn1.7Al-550
17
place. However, the average diameter of the thickness
50–80 nm) is smaller for the MgZn1.7Al-450 and considerably
a
b
Average pore size. Average pore volume.
(
more uniform in distribution than HTMgZn1.7Al. In addition,
the homogeneous distribution of these tree metals was
conrmed by EDS mapping (Fig. S1†).
introduced into the Mg–Al oxide, which might be due to the fact
that the BET surface area of the Zn–Al oxide was much lower
than that of Mg–Al oxide. Meanwhile, the BET surface area of
MgZn1.7Al oxides decreased monotonically from 123.0 to
Aer controlled thermal decomposition, HTs are converted
into composite oxides with high specic surface areas
2
ꢁ1
(
>100 m g ) arising from the formation of a signicant
2
ꢁ1
1
15.4 m g with the increasing of calcination temperatures
porosity, Table 3. All of the samples exhibited a mesoporous
material (the average pore size ranging from about 8 to 21 nm).
Obviously, the BET surface area of the Mg–Zn–Al composite
oxides slightly decreased when the zinc component was
ꢀ
from 350 to 550 C. It indicated that the calcination tempera-
tures had a strong inuence on the catalyst textures. Besides,
the average pore size and the average pore volume of Mg–Zn–Al
oxides were larger than Mg–Al oxide and Zn–Al oxide, and
slightly changed as increased the calcination temperatures
ꢀ
from 350 to 550 C.
To elucidate the difference between the catalysts, base
properties of these composite oxide catalysts were studied by
TPD of adsorbed CO . The CO -TPD results of different oxides
2 2
derived from HTs are shown in Fig. 5. Generally, a peak of CO
2
ꢀ
desorption appeared at about 75 C was observed over all the
tested samples; this was derived from the weak strength basic
ꢀ
sites. For the Mg–Al oxides, a shoulder peak centered at 120 C
was also observed, which was attributed to the desorbed CO2
that adsorbed onto the Lewis basic sites of MgO. The peak of
ꢀ
CO desorption at 120 C ascribed to medium strength basic
2
sites was much reduced and nally disappeared if increased
zinc content. Besides, a CO desorption peak appeared at about
2
2
Fig. 5 CO -TPD profiles of (a) Mg2.7Al-400; (b) Zn2.6Al-400; (c)
Mg1.9Zn0.8Al-450; (d) MgZn1.7Al-350; (e) MgZn1.7Al-450; (f) MgZn1.7Al-
550 and (g) MgZn1.7Al-450-reused.
Fig. 4 SEM images of (A) HTMgZn1.7Al and (B) MgZn1.7Al-450.
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ꢀ
4
00 C in Mg2.7Al-400 sample, which was attributed to strong reaction, the autoclave was cooled down to room temperature.
2
ꢁ
strength basic sites by isolated O in MgO. Moreover, there was The catalyst was recovered by centrifugation and then qualita-
almost no difference on the CO -TPD prole between the fresh tively identied by HP-6890 GC-MS equipped with a SE-54
2
and used catalysts.
capillary column. The quantitative analysis of the product was
According to carbon dioxide desorption temperature determined by Agilent 7890 GC equipped with a SE-54 capillary
ꢀ ꢀ ꢀ
weak < 100 C, 100 < medium < 300 C, strong > 300 C) basic column and a FID detector (biphenyl was used as internal
(
sites can be roughly classied (Table 4). From the quantitative standard). The gas chromatograph applied a temperature ramp
ꢀ
results of CO
1
2
-TPD, the total basic sites sharply decreased from for the analysis: the column temperature was started at 80 C,
ꢁ
1
ꢀ
27 to 15 mmol g for Mg–Al oxides and Zn–Al oxides, respec- and then the temperature was raised 15 C per minute until a
tively, suggesting that the basic sites mainly derived from the temperature of 260 C was reached. The temperature was
ꢀ
MgO phase in the composite oxides. According to the quanti- maintained for ve minutes before the column oven was cooled
ꢀ
tative results of CO -TPD and catalytic performance results of to 80 C for the next analysis (the GC chromatograms were
2
those catalysts, the yields of DEC were mainly impacted by the supplied in our ESI as Fig. S2–S4†). The yields of products were
amount of the medium strength basic sites. At this stage, much calculated by the following formula:
work is needed before clearly discerning the role of metals in
M
Y ¼
ꢂ 100%
Mg–Zn–Al hydrotalcite in catalysis, however, previous studies
showed that the reaction between urea and alcohols promoted
with basic catalysts proceeded via a nucleophilic substitution
mechanism. Based on this reaction mechanism, the lower
activity of the Zn2.6Al-400 might be due to its weak basicity,
which were unfavorable for the nucleophilic attack on the
carbonyl of the urea. For the catalyst Mg2.7Al-400 with large
amounts of medium basic sites base shown relatively lower
catalytic activity was considered to be related to the presence of
starting urea ðmolÞ
here, M represents the cumulative mole number of DEC or ethyl
carbamate (EC), and two times of the mole of N-ethyl ethyl
carbamate (NEEC), respectively.
3
.2 Catalysts preparation and characterization
strong basic sites, which would promote the by-product All chemicals were purchased from local chemical agents and
13
formation. Therefore, incorporation of a zinc component were of reagent grade without further purication.
into Mg–Al oxides remarkably affected the basicity strength and
The Mg/Al hydrotalcite (HTMg2.7Al) was prepared by co-
16
the amount of basic sites. For the slightly decrease of DEC yield precipitation according to the literature procedures. The
ꢀ
over spent catalysts, it may be due to the decrease of medium hydrotalcite formed was calcined at 400 C for 4 h and denoted
ꢁ1
ꢁ1
basic sites from 28 mmol g to 21 mmol g . In this sense, the as Mg2.7Al-400.
specic kinds and amounts of the basic sites might be the The Zn/Al hydrotalcite (HTZn2.6Al) was prepared by
major reason for the high catalytic activity of MgZn1.7Al-450. co-precipitation according to the literature procedures. The
18
+
ꢀ
Moreover, the role of the Lewis acid site of M (M ¼ Mg, Zn, hydrotalcite formed was calcined at 400 C for 4 h and denoted
Al, synergistic effect between Mg, Zn and Al probably occurred), as Zn2.6Al-400.
was also considered to contribute the high catalytic activity.
The Mg–Zn–Al hydrotalcites (HTMg–Zn–Al) were prepared by
co-precipitation method. Solution 1 contained Al(NO $9H O,
Zn(NO $6H O and Mg(NO $6H O were dissolved in distilled
3
)
3
2
3
)
2
2
3
)
2
2
3. Experimental
water with the molar ratio of (Zn + Mg) : Al ¼ 3 : 1. Solution 2
3.1 DEC synthesis from urea and ethanol
contained 2 M NaOH and 0.125 M Na CO . Then solution 2 was
2
3
added to solution 1 under vigorous stirring, the pH was main-
All reactions were carried out in a 90 mL stainless steel
autoclave with a glass tube inside and magnetic stirring. In a
typical test, 20 mmol urea, 200 mmol ethanol and 0.1 g catalyst
were added into the reactor. The reaction mixture was heated up
ꢀ
tained at ꢃ12. The resulting slurry was aged at 60 C overnight,
ꢀ
followed by ltration, washing, and drying at 60 C for 12 h. The
composition of the Mg–Zn–Al hydrotalcite can be altered by
ꢀ
varying the ratio of Zn/Mg. Then precursors were calcined at
to 200 C and the reaction pressure was about 2 Mpa. Aer the
ꢀ
temperatures 350, 450 and 550 C for 4.5 h, respectively. The
resulted catalysts are denoted as Mg Zn Al -T, l, m, n are molar
l
m
n
Table 4 The catalyst basicity measured by CO
2
-TPD
ratios of Mg, Zn and Al, T is the calcination temperature
ꢀ
(T ¼ 350, 450 and 550 C).
ꢁ1
Basic sites (mmol g
)
X-ray diffraction (XRD) was measured on a Siemens
D/max-RB powder X-ray diffract meter. Diffraction patterns were
recorded with Cu Ka radiation (40 mA, 40 kV) over a 2q range of
Catalyst
Weak
Medium
Strong
Total
ꢀ
ꢀ
Mg2.7Al-400
Zn2.6Al-400
60
15
31
20
34
21
34
52
—
21
9
28
12
21
15
—
—
—
—
—
—
127
15
52
29
62
33
55
20 to 80 and a position-sensitive detector using a step size of
ꢀ
0
.01 and a step time of 0.15 s.
Mg1.9Zn0.8Al-450
MgZn1.7Al-350
MgZn1.7Al-450
MgZn1.7Al-550
MgZn1.7Al-450-reused
The morphological structures were examined by eld emis-
sion scanning electron microscopy (FE-SEM, JSM-6701F). The
EDS method was adopted to estimate the ratios of the Mg, Zn, Al
in HTs and oxides.
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The BET surface areas, pore volumes and average pore
diameters of catalysts were obtained with physisorption of N2
using a Micromeritics ASAP 2010.
The surface acid–base properties of the catalysts were
measured by temperature programmed desorption (TPD) of
R. P. A. Sneeden, in Comprehensive Organometallic
Chemistry, 1982; (d) Comprehensive Organometallic
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R. P. A. Sneeden, CHEMTECH, 1982, 12, 108.
CO
2
and carried out on TPD ow system equipped with a MS
4 (a) F. Shi, Y. Q. Deng, T. L. SiMa, J. J. Peng, Y. L. Gu and
B. T. Qiao, Angew. Chem., Int. Ed., 2003, 42, 3257; (b)
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detector (DM300, AMETEK, USA). In a typical experiment, the
solid sample (200 mg with particle size 160–200 mm) was pre-
treated at catalyst calcination temperature for 1 h under argon
gas ow (50 mL min ) and then cooled to room temperature.
The sample was subsequently exposed to CO2 stream
ꢁ
1
ꢁ
1
(50 mL min ) at room temperature for 1 h and ushed again
5 M. Herstedt, M. Stjerndahl, T. Gustafsson and K. Edstrom,
Electrochem. Commun., 2003, 5, 467.
with argon gas for 1 h to remove any physico-adsorbed CO
desorption prole was recorded at a heating rate of 10 C min
from room temperature to pretreated temperature and main-
tained at this temperature until the MS signal of CO
to the baseline. The quantitative analysis for CO desorption is
calculated based on the integration of the corresponding TPD
traces, preliminarily calibrated by the injection of pure CO2
pulses. CO pulses (0.1727 mL) were injected into the carrier gas
intermittently and the whole process was detected by a MS. The
2
. The
ꢀ
ꢁ1
6 (a) M. A. Pacheco and C. L. Marshall, Energy Fuels, 1997, 11,
2; (b) D. M. Dillon, U.S. Pat., 4,891,049, 2 January 1990.
7 I. E. Muskat and F. Strain, U.S. Pat., 2,379,250, 1941.
8 (a) U. Romano, R. Tessi, G. Ciprianni and L. Micucci, U.S.
Pat., 4,218,391, 1980; (b) B. C. Dunn, C. Guenneau,
S. A. Hilton, J. Pahnke, E. M. Eyring, J. Dworzanski,
H. L. C. Meuzelaar, J. Z. Hu, M. S. Solum and
R. J. Pugmire, Energy Fuels, 2002, 16, 177; (c) Z. Zhang,
X. B. Ma, J. Zhang, F. He and S. P. Wang, J. Mol. Catal. A:
Chem., 2005, 227, 141; (d) N. S. Roh, B. C. Dunn,
E. M. Eyring, R. J. Pugmire and H. L. C. Meuzelaar, Fuel
Process. Technol., 2003, 83, 27; (e) T. C. Liu and
C. S. Chang, Appl. Catal., A, 2006, 304, 72.
2
returned
2
2
basic amounts were expressed as the number of CO
per gram of catalyst (mmol CO per g).
2
molecules
2
4
. Conclusions
In summary, the dialkyl carbonates could be successfully
synthesized from alcoholysis of urea by the catalysts derived
from HTs. The MgZn1.7Al-450 catalyst could be reused for 5 runs
with only slightly deactivation. The characterization results of
9 (a) O. Arbel ´a ez, A. Orrego, F. Bustamante and A. L. Villa, Top.
Catal., 2012, 55, 668; (b) E. Leino, N. Kumar, P. M ¨a ki-Arvela,
A. Aho, K. Kord ´a s, A. Leino, A. Shchukarev, D. Y. Murzin and
J. Mikkola, Mater. Chem. Phys., 2013, 143, 65.
the catalysts suggested that the high specic surface areas with 10 C. Murugan and H. C. Bajaj, Fuel Process. Technol., 2011, 92,
nanoplates morphology, in combination with specic kinds
77.
and amounts of the basic sites may be responsible to the high 11 D. Wang, B. Yang, X. Zhai and L. Zhou, Fuel Process. Technol.,
catalytic activity of MgZn1.7Al-450. In conclusion, urea based
2007, 88, 807.
DEC synthesis will be one of the preferable alternatives in the 12 W. Zhao, W. Peng, D. Wang, N. Zhao, J. Li, F. Xiao, W. Wei
quest for an environmentally benign route.
and Y. Sun, Catal. Commun., 2009, 10, 655.
13 S. Xin, L. Wang, H. Li, K. Huang and F. Li, Fuel Process.
Technol., 2014, 126, 453.
Acknowledgements
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This work was supported by National Natural Science Founda-
tion of China (no. 21002107).
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19540 | RSC Adv., 2015, 5, 19534–19540
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