Dimethyl carbonate synthesis over ZnO-CaO bi-functional catalysts

Article abstract of DOI:10.1016/j.catcom.2013.10.040

ZnO-CaO catalysts were prepared and tested for the synthesis of dimethyl carbonate (DMC) from urea and methanol. Meaningfully, the bi-functional catalysts ZnO-CaO exhibited noticeable DMC yield (41.2%) with the Zn/Ca molar ratio of 4:1, which might ascribe to the synergistic effect between ZnO and CaO. The plausible mechanism for the DMC synthesis was as follows: urea was activated to generate metal isocyanato group on the acid sites and subsequently nucleophilic attacked by the activated methanol on the basic sites.

Full text of DOI:10.1016/j.catcom.2013.10.040

Dimethyl carbonate synthesis over ZnO-CaO bi-functional catalysts
Xiaomin Wu, Min Kang, Ning Zhao, Wei Wei, Yuhan Sun
PII:
DOI:
Reference:
S1566-7367(13)00411-1
doi: 10.1016/j.catcom.2013.10.040
CATCOM 3700
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Catalysis Communications
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Revised date:
Accepted date:
17 September 2013
29 October 2013
30 October 2013
Please cite this article as: Xiaomin Wu, Min Kang, Ning Zhao, Wei Wei, Yuhan Sun,
Dimethyl carbonate synthesis over ZnO-CaO bi-functional catalysts, Catalysis Communi-
cations (2013), doi: 10.1016/j.catcom.2013.10.040
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ACCEPTED MANUSCRIPT
Dimethyl carbonate synthesis over ZnO-CaO bi-functional catalysts
Xiaomin Wu a,b, Min Kang a,b, Ning Zhao a*, Wei Wei c*, Yuhan Sun a,d
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, PR China
b University of Chinese Academy of Sciences, Beijing 100049, PR China
c Center for Greenhouse Gas and Environmental Engineering, Shanghai Advanced
Research Institute, Chinese Academy of Sciences, Shanghai 201210, PR China
d CAS Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai
Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, PR
China
* Corresponding authors. Address: State Key Laboratory of Coal Conversion, Institute
of Coal Chemistry, Chinese Academy of Science, Taiyuan 030001, PR China.
Tel.: +86 351 4049612; fax: +86 351 4041153.
E-mail addresses: zhaoning@sxicc.ac.cn, weiwei@sxicc.ac.cn
Abstract
ZnO-CaO catalysts were prepared and tested for the synthesis of dimethyl
carbonate (DMC) from urea and methanol. Meaningfully, the bi-functional catalysts
ZnO-CaO exhibited noticeable DMC yield (41.2 %) with the Zn/Ca molar ratio of 4:1,
which might ascribe to the synergistic effect between ZnO and CaO. The plausible
mechanism for the DMC synthesis was as follows: urea was activated to generate
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metal isocyanato group on the acid sites and subsequently nucleophilic attacked by
the activated methanol on the basic sites.
Keywords: Dimethyl carbonate; Urea alcoholysis; ZnO; CaO; Heterogeneous catalyst
1. Introduction
Dimethyl carbonate (DMC) is achieving importance in the chemical industry
mainly for its non-toxicity and versatility. Typically, DMC can be applied in
methylation and carbonylation to substitute toxic methyl halides, dimethyl sulfate and
phosgene[1]. Two processes for DMC synthesis have been commercialized. The first
is based on the oxidative carbonylation of methanol over CuCl or nitric oxide while
the other one is derived from the transesterification of cyclic carbonate using baisic
catalysts[2]. However, there are drawbacks of high pressure and catalysts deactivation
caused by the formation of water. The transesterification also suffers from the
disadvantage of the availability of raw material propylene oxide. Therefore, the
synthesis of DMC from urea and methanol, as a novel and attractive route, has been
proposed recently[3].
Typically, in the process of DMC synthesis from urea and methanol, methyl
carbamate (MC) can be easily produced, whereas the DMC synthesis from MC and
methanol is the rate-control step (Scheme 1). Many catalysts such as bases, organic
tin, metal oxides have been employed[3-5]. Among the catalysts, ZnO is highly active
toward DMC synthesis due to its acid-base property[6].
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Scheme 1 Synthesis of dimethyl carbonate by urea alcoholysis
On the other hand, CaO has been widely investigated in the transesterification.
Recently, Gryglewicz et al. have demonstrated that basic compounds derived from
alkaline earth metals could effectively catalyze the transesterification[7]. Furthermore,
CaO can effectively promote the activation of methanol due to its strong basicity[8].
Therefore, CaO might exhibit high catalytic activity for DMC synthesis, attributing to
its methanol activation property.
The aim of our article is to study the catalytic performance of ZnO-CaO catalysts
for DMC synthesis in mild reaction conditions. Heterogeneous and stable ZnO-CaO
catalysts with adequate acid-base sites have been fabricated and tested for the
synthesis of DMC. The plausible mechanism is then proposed.
2. Experimental
2.1. Catalyst preparation
ZnO-CaO mixed oxides were produced by the urea precipitation method of the
mixed aqueous solution of Ca(NO3)2·4H2O and Zn(NO3)2·6H2O[9]. The un-calcined
samples and calcined products are denoted as HZC and ZCO, respectively. For
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example, ZCO-4(1073K) stands for Zn/Ca feed molar ratio of 4 and calcinations
temperature of 1073 K.
2.2. Catalytic test
The catalytic performance for DMC synthesis was carried out in a stainless steel
autoclave with a magnetic stirrer. Typically, 6.0 g urea, 64 g methanol and 0.4 g
catalyst were charged into the reactor, heated and kept at 453 K for 10 h. The reactor
was then cooled to room temperature. The product was analyzed by a gas
chromatograph (GC) with thermal conductivity detector (TCD) detector equipped
with Agilent HP-Plot u column.
2.3. Characterization
Powder X-ray diffraction (XRD) was performed on a Panalytical X’Pert Pro X-ray
diffractometer with Cu Kα radiation. The morphology of the powder was examined
using a JSM-7001F scanning electron microscope (SEM) with an accelerating voltage
of 10-500 kV. The elemental analysis of ZnO-CaO catalysts was determined by
inductively coupled plasma (ICP) emission spectrometer. Thermal decomposition of
the catalyst precursors was studied by Thermogravimetric analysis (TG) method using
a NETZSCH TG209F1 thermal analyzer. The Infrared Spectroscopy (IR) were
recorded with a Nicolet Nexus 470 Fourier Transform instrument with KBr or CsI at a
resolution of 4 cm−1. The IR spectra of samples were obtained according to the
method described by Zhao et al[6].
3. Results and discussion
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3.1. Characterization of ZCO catalysts
The elemental analysis of ZCO samples are summarized in Table 1. It can be seen
that the Zn/Ca molar ratio of mixed oxides was lower than that of the feed (except
ZCO-0.25(1073K)), owning to the greater affinity of Ca2+ ions to the precipitation
than Zn2+ ions. As a result, Zn2+ was partially retained in the mother solutions during
the precursors preparation[10, 11]. Thereby, the Zn/Ca molar ratio of obtained ZCO
catalysts was considerably less than that of the feed.
Table 1 Elemental composition and particle size of ZCO catalysts with different Zn/Ca ratio
Particle sizea (nm)
Entry
Catalyst
Zn/Ca molar ratio
ZnO
~100
~100
~100
~100
~100
CaO
1
2
3
4
5
ZCO-10(1073K)
ZCO-4(1073K)
4.80
2.72
0.64
0.19
0.37
-
-
ZCO-2(1073K)
-
ZCO-0.85(1073K)
ZCO-0.25(1073K)
400-800
400-1200
a Determined by SEM technique.
The XRD patterns of ZCO catalysts calcined at 1073 K are shown in Fig. 1. Only
diffraction peaks corresponding to ZnO and CaO were observed, which revealed the
ZCO catalysts did not contain any other specific structures but presented as separate
oxide clusters[12]. The formation of oxide clusters in the precipitate particles was also
confirmed by SEM (Fig. 2). Noticeably, two types of oxide particles were unveiled
for the samples with the Zn/Ca molar ratio from 10 to 0.25. For the samples with
Zn/Ca molar ratio from 10 to 2, only small round-shape particles (about 100 nm) were
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observed. However, the small particles merged and became embedded in the large
ones for the samples with Zn/Ca molar ratio of 0.85 and 0.25. Therefore, it could be
derived that the smaller particles were mainly composed of ZnO while CaO was the
major component of the larger particles[12].
: ZnO
: CaO












ZCO-0.25(1073K)
ZCO-0.85(1073K)
ZCO-2(1073K)
ZCO-4(1073K)












ZCO-10(1073K)

20
30
40
50
60
70
2 / degree
Fig. 1 XRD patterns of ZCO catalysts
Fig. 2 SEM images of catalysts with different zinc/calcium molar ratio:
(a) ZCO-4(1073K); (b) ZCO-2(1073K); (c) ZCO-0.85(1073K); (d) ZCO-0.25(1073K).
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FT-IR spectra tests were conducted for fresh ZCO catalysts in Table 2. The sharp
band at 3643 cm−1 was ascribed to OH- groups of Ca(OH)2 for all samples. The bands
of physically-sorbed water stretching vibrations were disclosed in the range of
3400-3100 cm−1 and 1640 cm−1[13]. Meanwhile, the presence of carbonates was also
confirmed by the broad peak located at 1465 cm−1, resulting from the stretching
vibrations of O-C-O. In view of IR results, the carbonation and hydration of CaO in
the surface of ZCO catalysts was rapid in the initial few hours, resulting in the
preservation of bulk ZnO and CaO[14]. The conclusions were also identified by the
XRD results.
Table 2 Infrared wavenumber and assignment of vibrational mode of ZCO catalysts
Wavenumber (cm-1)
Vibrational mode
3643
OH-
3400-3100
H2O
1640
H2O
1465
O-C-O
3.2. Catalytic activity
The catalytic performance of ZCO catalysts for DMC synthesis was summarized in
Table 3. CaO as basic catalyst exhibited lower DMC yield (15.8 %), while ZnO
possessing both acidic and basic sites had highly enhanced performance up to 30.8 %.
However, the dissolution of ZnO in methanol led to the problem for the separation of
the products and catalysts[15]. Therefore, heterogeneous and stable ZCO catalysts
were prepared and exhibited better DMC yield (41.2 %) with Zn/Ca molar ratio of 4:1,
far more than that over ZnO and CaO. Meaningfully, ZCO-4(1073K) was stable after
several catalytic runs.
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Table 3 Results of DMC over ZCO catalysts
Entry
Catalyst
MC yield (%) DMC yield (%) Total yield (%) Regeneration
1
2
3
4
5
6
7
8
9
None
CaOa
84.8
76.9
59.0
70.9
55.3
50.5
41.4
49.9
57.5
6.5
91.3
92.7
89.8
93.8
88.0
91.7
82.5
82.5
80.8
-
Partially
No
15.8
30.8
22.9
32.7
41.2
41.1
32.6
23.3
ZnOa
ZnO + CaOb
ZCO-10(1073K)
ZCO-4(1073K)
ZCO-2(1073K)
ZCO-0.85(1073K)
ZCO-0.25(1073K)
Partially
Yes
Yes
Yes
Yes
Yes
Reaction condition: Urea, 0.1 mol; Methanol, 64 g; Catalyst amount, 0.4 g;
Reaction time, 10 h; Reaction temp. 453K;
a Catalyst amount, 0.5 g;
b Physically mixed and the molar ratio of Zn/Ca was 2.72.
The influence of ZCO-4(1073K) catalyst amount was then investigated. As shown
in Fig. 3, the MC yield increased from 39.6 % to 58.5 % when catalyst amount varied
from 5 wt.% to 10 wt.%. Meanwhile, the DMC yield reached maximum (41.2 %) at
the catalyst amount of 6.67 wt.%, attributing to the appropriate amount of acidic/basic
sites in the reaction system[16]. However, the DMC yield decreased with the further
increase of catalyst amount, owning to the decomposition of DMC on CaO basic
sites[17]. Therefore, suitable acidic/basic sites of ZnO-CaO catalysts were beneficial
for the DMC synthesis.
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70
DMC
MC
60
50
40
30
20
5
6
7
8
9
10
Catalyst amount / wt.%
Fig. 3 Dependence of DMC and MC yield on catalysts amount
The regeneration of catalysts was performed by washing the used ZCO catalysts
with methanol, followed by calcination at 1073 K for 2 h. It was clearly observed that
the catalytic activity of ZCO-4(1073K) sample could be recovered despite a little
decrease of MC yield (Fig. 4). The reusability of ZCO-4(1073K) could be ascribed to
the removal of organic deposit by air oxidation at 1073 K.
80
DMC
MC
60
40
20
0
1
2
3
4
Repetition Time
Fig. 4 Reusability of ZCO-4(1073K) for DMC synthesis (Reaction conditions: see Table 3).
3.3. Mechanism for DMC synthesis
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3.3.1 Synergistic effect of ZnO and CaO
TG and DTG curves of the HZC-0.85 and HZC-2 were studied. As shown in Fig. 5,
the decomposition of zinc carbonate was different from pure Zn precipitate (~220 oC),
which could be caused by the generation of various zinc carbonate precipitate during
the precipitation[18]. Moreover, the peak corresponding to CaCO3 decomposition
o
o
o
(pure CaCO3 at 785 C) shifted to lower temperature (from 753 C to 742 C) with a
decrease of total weight loss (weight loss of HZC-0.85 was 37.5 wt.% compared with
HZC-2 was 29.35 wt.%) when increased the Zn content of ZCO catalysts[19]. The
result indicated that the incorporation of Zn decreased the temperature decomposition
of CaCO3. Therefore, there was a synergetic effect between ZnO and CaO in the ZCO
catalysts comparable to physically mixture of pure ZnO and CaO. Meanwhile, the fact
of the synergistic effect of ZnO and CaO can also be demonstrated by CO2-TPD[20,
21].
1
(A)
100
0
-1
262oC
-2
80
-3
-4
-5
-6
60
753oC
700
0
100
200
300
400
500
600
800
900
0
(B)
100
80
-1
-2
-3
-4
-5
205oC
200
60
742oC
0
100
300
400
500
600
700
800
900
Temperature / oC
Fig. 5 TG and DTG curves of the precursors: (A) HZC-0.85; (B)HZC-2.
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FT-IR spectra of the synergistic effect between ZnO and CaO in ZCO catalysts are
illustrated in Fig. 6. The bands at 478, 438 and 326 cm-1 corresponding to the Zn-O
and Ca-O stretching vibrations were observed, respectively. When raised the Ca
content of ZCO catalysts, the intensity of Zn-O stretching vibrations decreased while
the strength of Ca-O stretching vibration increased. Moreover, the blue shift of CaO
band significantly revealed the existence of synergistic effect between ZnO and CaO
of ZCO catalysts, compared with physically mixture of pure ZnO and CaO.
CaO
ZnO+CaO
ZCO-0.85(1073K)
ZCO-2(1073K)
ZCO-4(1073K)
ZnO
438
478
326
700
600
500
400
300
Wavenumber (cm-1)
Fig. 6 FT-IR spectra of ZCO catalysts
3.3.2 Plausible mechanism
The synergistic effect of ZnO and CaO in ZCO catalysts could promote DMC
synthesis, which was ascribed to its sufficient acidic/basic sites. Therefore, the in-situ
IR spectra between reactant and ZCO catalysts were investigated for proposing the
plausible mechanism (Fig. 7).
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The spectra of urea decomposition over ZnO and ZCO-4(1073K) were displayed
(Fig.7A), respectively. The IR band at ~2200 cm-1 was attributed to metal isocyanato
group (O=C=N-M stretching), deriving from the decomposition of urea (Scheme 2).
The shift of isocyanate and OH- peak (3639 cm-1) over ZCO-4(1073K) to lower
frequency was attributed to the enhancement of metal isocyanato bond and weaker
interaction of isocyanate intramolecular bond. The results also indicated that the
acidity and basicity of ZCO catalysts was altered.
(A)
3639
2187
2206
b
a
4000
3800
3600
3400
2400
2200
2000
(B)
~1450
a
d
3643
1012~1059
2826~2981
3000
4000
3500
1600
1400
1200
1000
Wavenumbers (cm-1)
Fig. 7 IR spectra of the adsorption of urea (A) and methanol (B) over catalysts:
(A) decomposition of urea on ZnO (a); decomposition of urea on ZCO-4(1073K) (b) .
(B) adsorption of methanol on ZCO-4(1073K) at 453 K (a); desorption of methanol at 383 K for 5
min (b); desorption of methanol at 473 K for 30 min (c); pure ZCO-4(1073K) (d).
The adsorption and desorption of methanol over ZCO-4(1073K) is also performed
(Fig. 7B). Methanol vapor was introduced into the catalytic system and further heated
to 453 K (Fig. 7B, line a). The sample was then evacuated by vuccum for several
minutes to remove the undissociated methanol (Fig. 7B, line b, c) . It should be
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noticed that methanol could be activated instead of dissociated to form methoxide
species. Therefore, a mechanism for the DMC synthesis could be proposed as follows:
metal isocyanato group was produced due to urea decomposition on the acid sites.
Subsequently, it was nucleophilic attacked by the activated methanol on the basic sites
to form DMC (Scheme 2).
O
C
H OCH
M
N
H
O
3
O
2MeOH
Cat.
O
-Cat.
...
O
O
M
H N
NH
2
2
DMC
NH
3
Cat.
NH
3
2+
2+
M: Zn or Ca
Scheme 2 Plausible mechanism for DMC synthesis over ZCO catalysts
4. Conclusion
ZnO-CaO catalysts were prepared and tested for the DMC synthesis by urea
alcoholysis. The highest DMC yield (41.2 %) was obtained over the catalyst with
Zn/Ca molar ratio of 4. The interesting catalytic activity toward DMC synthesis was
ascribed to the appropriate acidic/basic sites of ZCO catalysts, causing by the
synergistic effect between ZnO and CaO in ZCO catalysts. The formation of DMC
could be involved that urea was activated on the acid sites to produce metal
isocyanato group. Subsequently, metal isocyanato group was attacked by the
nucleophilicity of the activated methanol on the basic sites.
Acknowledgement
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This work is financially supported from State Key Program for Development and
Research of China (No.2006BAC02A08).
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Highlights
 ZnO-CaO catalysts were prepared via the decomposition of hydrotalcite-type
precursors.
 ZnO-CaO catalyst exhibited an interesting DMC yield (41.2 %) with the Zn/Ca
molar ratio of 4:1.
 ZnO-CaO catalysts with suitable acidic/basic sites were beneficial for the DMC
synthesis, attributing to the synergistic effect between ZnO and CaO.
 A bi-functional mechanism for the DMC synthesis over ZnO-CaO catalysts was
proposed.
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Graphical Abstract
O
C
H OCH
M
N
H
3
O
2MeOH
Cat.
O
-Cat.
...
O
O
M
O
H N
NH
2
2
DMC
NH
3
Cat.
NH
3
M: Zn2+ or Ca2+
Plausible mechanism for DMC synthesis over ZCO catalysts
17