Synthesis of dimethyl carbonate from methyl carbamate and methanol catalyzed by mixed oxides from hydrotalcite-like compounds

Article abstract of DOI:10.1016/j.jpcs.2009.12.005

A series of mixed oxides calcined from hydrotalcite-like compounds with different cations were prepared and their catalytic activities were studied by the synthesis of dimethyl carbonate (DMC) from methyl carbamate and methanol. Among them, ZnFe mixed oxide possessed the best catalytic ability. Furthermore, the zinc-based mixed oxides as well as the corresponding hydrotalcite-like compounds were characterized by using ICP, TGA, CO₂-TPD and N₂ adsorption/desorption techniques.

Full text of DOI:10.1016/j.jpcs.2009.12.005

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Journal of Physics and Chemistry of Solids 71 (2010) 427–430
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Synthesis of dimethyl carbonate from methyl carbamate and methanol
catalyzed by mixed oxides from hydrotalcite-like compounds
Dengfeng Wang a,b, Xuelan Zhang a,b, Wenbo Zhao a,b, Weicai Peng a, Ning Zhao a, Fukui Xiao a,
Wei Wei a,n, Yuhan Sun a,n
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, PR China
b Graduate University of Chinese Academy of Sciences, Beijing, 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of mixed oxides calcined from hydrotalcite-like compounds with different cations were
prepared and their catalytic activities were studied by the synthesis of dimethyl carbonate (DMC) from
methyl carbamate and methanol. Among them, ZnFe mixed oxide possessed the best catalytic ability.
Furthermore, the zinc-based mixed oxides as well as the corresponding hydrotalcite-like compounds
were characterized by using ICP, TGA, CO2-TPD and N2 adsorption/desorption techniques.
& 2009 Elsevier Ltd. All rights reserved.
Received 17 May 2009
Received in revised form
24 September 2009
Accepted 26 September 2009
Keywords:
A. Multilayers
A. Oxides
B. Chemical synthesis
C. X-ray diffraction
1. Introduction
effective heterogeneous catalysts for this process are highly
demanded.
As a type of layered solids, hydrotalcite-like compounds (HTlc),
are also called as layered double hydrotalcites (LDHs) or
Dimethyl carbonate (DMC) is attracting broad attentions in
recent years from researchers owing to its use in replacing
environmentally unfriendly compounds. It can be used as
methylation and carbonylation reagents to replace dimethlsulfate,
methyl halides, and phosgene [1]. It is also considered as an
ideal additive for gasoline due to its high oxygen content [2].
Moreover, DMC is a promising electrolytic solution for the
secondary lithium battery [3]. Up to now, several methods of
DMC synthesis have been proposed [4,5]. Among them, urea
methanolysis has drew extensive attentions because of low-cost
and facile production separation. In this process, the intermediate
methyl carbamate (MC) was produced first and further converted
to DMC by consecutive reaction with methanol, which was the
rate-control step [6–8]. So, detailed studies on the second step
would be useful for the DMC synthesis via this new routine.
As a precursor of homogeneous catalyst for urea methanolysis,
ZnO could be used as a catalyst for the DMC preparation from MC
and methanol, but the DMC yield was less than 5%. Zhao et al.
reported that zinc compounds, especially ZnCl2, presented high
catalytic performance with the DMC yield of 33% [9–11].
However, these homogeneous catalysts suffered from the draw-
backs of poor catalyst recovery and product separation. So,
anionic clays with a general formula, which could be deter-
II
1Àx
mined as: [M
MIxII(OH)2]x+(Ax/n
)
nÀ Á mH2O, where M(II) is a
divalent metal cation; M(III) represents a trivalent metal cation,
AnÀ is an anion, often carbonate in nature, m is the number of
molars of co-intercalated water per formula weight of the
compound, and x is the number of molars of M(III) per formula
weight of the compounds [12]. They constitute a brucite-like
net-work where the divalent cations are partially replaced by
the trivalent ones. In other words, the HTlc are prepared by
positive charged metal hydroxide layers, separated by interlayer
regions containing anions and water molecules. Due to the
character of the LDH compounds, the metal mixed oxides or
spinel-like structures that obtained from thermal decomposition
of LDHs usually present the advantages of thermal stability.
Furthermore, highly dispersed metal clusters could also be
obtained by dedicated activation treatments. Thus, calcined
layered materials are often viewed as promising catalysts in
many catalytic reactions.
The object of this work is to prepare and investigate the
catalytic performance of mixed oxides derived from calcined
hydrotalcite-like compounds for the DMC synthesis from MC and
methanol. The results revealed that the zinc-containing compo-
site oxides, especially zinc–iron mixed oxide, presented high
catalytic activity.
n Corresponding authors. Tel.: +86 351 4049612; fax: +86 351 4041153.
E-mail addresses: weiwei@sxicc.ac.cn (W. Wei), yhsun@sxicc.ac.cn (Y. Sun).
0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2009.12.005

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2. Experimental
the reactor, and then the bath was heated to the desired reaction
temperature (controlling to a range of 72 K) in a desired time
with stirring (revolving at a rate of 600750 rpm). After reaction,
the reactor was cooled to room temperature. The liquid products
were analyzed on a gas chromatography (GC) equipped with TCD
after weighed and centrifugal separation from the catalyst.
2.1. Synthesis of the samples
Preparation method of hydrotalcite-like compounds precursor has
been described elsewhere [13]. Typically, solution A (100 mL) was
obtained by mixing solution of metal nitrates with the divalent/
trivalent atomic ratio of 3. Solution B was prepared by dissolving
suitable amount sodium carbonate in 100 mL deionized water. Then
the two solutions were added dropwise into a beaker containing
100 mL deionized water under constant stirring. And the pH of the
solution was kept closed to 10.0 by addition of appropriate amounts of
a 2 M sodium hydroxide. After that, the precipitates were aged in
mother liquid at 343 K overnight. The solids were obtained separated
by filtration and washed several times with deionized water until the
filtrate showed no presence of Na+. And the precipitates were dried at
353 K for about 12 h. A series mixed oxides were obtained by calcining
the result hydroltalcite-like compounds in oven at 773 K for 5 h under
a flowing stream of pure nitrogen. The hydrotalcite-like compounds
synthesized here were referred as M(II) M(III)-xHT, where x standed
for M(II)/ M(III) molar ratio. And the samples after calcined were
labeled as M(II)M(III)O. For example, ZnFe-3HT and ZnFeO represented
the ZnFe hydrotalcite-like sample and Zn/Fe mixed oxide, respectively.
3. Results and discussion
3.1. Texture and structure of zinc-containing mixed oxides and
corresponding LDHs compounds
The results of chemical analysis revealed that the Zn/Al, Zn/Cr
and Zn/Fe molar ratio in ZnAlO, ZnCrO and ZnFeO were all
different from the corresponding divalent and trivalent metal
molar ratio in mother solutions of 3.0 (Table 1). The molar ratio
between divalent and trivalent cations in the mixed oxides
0.1
100
0.0
90
-0.1
2.2. Characterization
-0.2
80
-0.3
Power X-ray diffraction (XRD) experiments were carried out
-0.4
70
on a Rifaku D Max III VC instrument with Ni filtered Cu-K
a
˚
300
400
500
600
700
800
radiation (l=1.5405 A) at room temperature.
The specific surface area was obtained by N2 adsorption–
desorption method on a Micromeritics ASAP-2000 instrument
(Norcross, GA).
The elemental chemical analysis was performed using the
inductively coupled plasma-optical (ICP) emission spectroscopy
on a Perkin Elemer 3000 equipment.
Thermogravimetric analysis (TGA) was conducted on TGA-92
instrument under nitrogen with the temperature range from 303
to 1073 K at the heating rate of 5 K/min.
The total basicity and base strength of the samples were
determined by temperature-programmed desorption of absorbed
CO2 (CO2-TPD). Catalysts (0.1 g, 40–60 mesh) were placed in the
quartz reactor bed. After 5 h pretreatment at 873 K in Ar flow and
cooled to room temperature, CO2 was pulsed to the reactor using a
6-way valve until saturation was reached. The TPD procedure was
started with a heating rate of 10 K/min under nitrogen flow. And
the effluent was measured using a BALZAQ-mass spectrometer.
Temperature (K)
0.1
100
90
0.0
-0.1
-0.2
-0.3
-0.4
80
70
300
400
500
600
700
800
Temperature (K)
0.2
0.1
0.0
100
90
-0.1
-0.2
-0.3
-0.4
80
2.3. Catalytic test
70
The reaction of MC and methanol was carried out in a stainless
steel autoclave of 300 mL with strong magnetic stirring and a
reflux column. A typical experiment procedure was as follows: a
desired amount of catalyst, MC and methanol were charged into
300
400
500
600
700
800
Temperature (K)
Fig. 1. TG-DTG curves of (a) ZnAl-3HT; (b) ZnFe-3HT; (c) ZnCr-3HT.
Table 1
Phisicochemical properties of the Zn-based mixed oxides and hydrotalcite-like compounds
a
Sample
Zn/M(III) in solids
Surface area
(m2/g)
CO2 uptake (mmol/g)
Lattice parameters
˚
˚
a (A)
c (A)
ZnAlO
ZnFeO
ZnCrO
2.82
2.81
2.86
50
52
72
1.72E–3
1.03E–2
1.18E–2
3.06
3.07
3.08
22.55
22.76
22.61
a
The lattice parameters were belonged to corresponding hydrotalcite-like compounds

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429
gradually decreased in the order of ZnCrO 4 ZnAlO 4 ZnFeO.
Such phenomenon might be ascribed to the different
precipitation degrees for the three trivalent cations above-
mentioned.
noted that a steady increase of a was observed in the following
order: ZnAl-3HToZnFe-3HToZnCr-3HT. This trend was in
agreement with the ionic radii of the metal ions that in the
a
follow of Al3+(0.50 A)4Fe (0.64 A)4Cr (0.69 A) [12]. Fig. 2(b)
revealed the XRD patterns of HTls after calcination. It was found
that all the hydrotalcite structure was destroyed and some new
crystal phases were formed after calcination. According to Joint
Committee on Powder Diffraction System (JCPDS), zinc oxide
(ZnO) was the main content for the three mixed oxides, and new
phases such as franklinite (ZnFe2O4), gahnite (ZnAl2O4) and
picotite (ZnCr2O4) also appeared for ZnFeO, ZnAlO, and ZnCrO
mixed oxides, respectively.
3+
3+
˚
˚
˚
The thermal decomposition behaviors of the three samples
were illustrated in Fig. 1. All the samples showed two typical
weight-loss patterns. The first one, at low temperature
(about 373 K), was due to the removal of the water in the
interlayer without structure collapse, whereas the second one, at
higher temperature (about 500 K), was assigned to the
dehydroxylation of layers as well as decarbonation of anions
leading to collapse of the layered network. It must be noted that
for ZnCr-3HT material, Cr(III) ion can change to species with other
chemical valencies due to the oxidation [14]. However, samples
were all treated under an inert nitrogen condition in the present
case. So, the second weight-loss peak of the ZnCr-3HT was
also result in the decomposition of hydroxycarbonates to
amorphous phases [15]. As a result, the three mixed oxides
containing zinc were synthesized successfully by decomposition
of corresponding HTls.
Fig. 3 illustrated the CO2-TPD profiles of zinc-based mixed
oxides prepared in this work. For the mixed oxide samples,
basicity (basic sites amount per unit weight of catalysts) and basic
strength were two important parameters that affected their
catalytic performance. The baisicity of the three mixed oxides
were listed in Table 1. Three desorption peaks were detected at
573, 533 and 463 K for ZnFeO, ZnCrO and ZnAlO, respectively
(see Fig. 3). This implied that the basic strength of these catalysts
decreased in the following sequence: ZnFeO4ZnCrO4ZnAlO.
Fig. 2(a) showed the X-ray diffraction of as-synthesized
hydrotalcite-like materials. The XRD patterns of all the samples
revealed the peaks characteristic of LDH phases with the presence
of sharp and intense lines at low angle values as well as less
3.2. Catalytic performance of mixed oxides
intense and asymmetric lines at higher values of 2y[14–16]. It was
Usually, the synthesis of DMC from MC and methanol took
place as follows: DMC was first produced as the methoxy group
derived from methanol substituting the amino of MC. In order to
improve the formation of DMC, ammonia should be removed in
time. On the one hand, DMC could further react with MC to
produce the by-product N-methyl methyl carbamate (NMMC).
On the other hand, it also decomposed into dimethyl ether
and carbon dioxide over solid basic and acidic oxides at high
temperature [17,18]. Therefore, high temperature, long reaction
time and excessive catalysts would have negative effects on
the DMC synthesis. because It should be noted that dimethyl
ether cannot be detected by gas chromatography, because it was
easy to volatilize when the autoclave was opened. Therefore, all
the catalysts were evaluated based on MC conversion, DMC and
NMMC yield in the present work.
well known that the hydrotalcite could be described by two
important values, a and c. The parameter a decided by the (1 1 0)
reflection (a=2d110), was related to the unit cell dimension and
depended on the divalent and trivalent metal element molar ratio.
The peaks at 11.61 and 23.41 ascribed to the (0 0 3) and (0 0 6)
reflections could be used to calculate the interlayer distance c,
where c=3d003, and further relied on the electrostatic force in the
hydrotalcite interlayer [12,13,15,16]. The calculation results of
the three hydrotalcite samples were given in Table 1. It should be
Zn-Fe-3HT
Zn-Cr-3HT
Zn-Al-3HT
Table 2 showed typical results of MC methanolysis over
various catalysts. It was observed that the DMC yield was only
2.58% in the absence of catalyst (Entry 1). ZnO, as an ideal catalyst
for production of DMC from urea methanolysis, exhibited low
10
20
30
40
50
60
70
2 Theta (Cu Ka)
Ο: ZnO Δ: ZnFe2O4 #: ZnAl2O4 *: ZnCr2O4
Δ
Δ
Ο
Ο
Ο
Ο
Ο
Ο
Ο
Δ
ZnFeO
ZnCrO
Ο
Δ
*
Ο
*
Ο
Ο
*
#
a
#
Ο
Ο
Ο
b
c
Ο
#
Ο
ZnAlO
20
30
40
50
60
70
400
500
600
700
800
2 Theta (Cu Ka)
Desorption tempreature (K)
Fig. 2. XRD patterns of samples: (a) Before calcination (b) after calcination at
Fig. 3. CO2-TPD profiles of (a) ZnAlO; (b) ZnFeO; (c) ZnCrO catalysts.
773 K for 5 h.

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D. Wang et al. / Journal of Physics and Chemistry of Solids 71 (2010) 427–430
Table 2
followed by nucleophilic attack of carbon atom of MC to produce
Catalytic performance of catalysts
DMC [6]. It could also explain why ZnFeO has the best catalytic
ability in the present case.
Entry Catalyst
MC conversion (%) DM Cyield (%) NMM Cyield (%)
1
–
4.06
2.58
4.52
2.62
4.38
3.70
3.25
7.82
3.21
2.66
13.91
17.04
31.48
0
4. Conclusions
2
ZnO
7.29
0
3
Fe2O3
Al2O3
Cr2O3
ZnO–Fe2O3
MgFeO
NiFeO
CuFeO
ZnAlO
ZnCrO
ZnFeO
9.04
0
4
10.58
9.46
0
Among the calcined hydrotalcite-like compounds, zinc/iron
mixed oxide was an effective catalyst for the synthesis DMC from
MC and methanol. XRD patterns showed that this catalyst showed
both zinc oxide and zinc ferrite phases and they produced a
synergistic effect on its catalytic performance. In addition, the
basic strength of the catalyst was an important factor to activate
methanol and further promoted the reaction between MC and
5
0
a
6
5.62
0
7
24.07
23.69
31.81
24.87
27.64
66.58
0
8
0
9
0
10
11
12
3.64
4.01
9.53
d
+
methanol through the abstraction of H by base sites.
Reaction conditions: MC, 7.5 g; methanol, 64 g; reaction temperature, 463 K;
reaction time 10 h; catalyst amount, 1.0 g.
a
Acknowledgement
The metal oxides were mixed and the molar ratio of ZnO/Fe2O3 was 4.
This work was supported by the State Key Program for
Development and Research of China (No. 2006BAC02A08).
activity for the reaction. This was consistent with the results
reported previously (Entry 2) [9,10]. Moreover, iron-based mixed
oxides, such as MgFeO and NiFeO were also found to be less
activity, with the DMC yield of 7.82% and 3.21%, respectively
(Entries 7, 8). Under the same reaction conditions, the zinc-based
mixed oxides, especially ZnFeO, presented the best catalytic
ability, yielding 31.48% DMC (Entry 12). Based on these as well as
the characterization results, zinc showed a positive effect on
the catalytic performance of these mixed oxide. This further
confirmed by comparing the catalytic behavior of ZnO/Fe2O3
physically mixture (Entry 6). Surprisingly, the activity of ZnCrO
was poor despite its larger surface area compared with that of
ZnFeO, which might suggest that surface area of catalyst is not
the decisive factor for this reaction. It was considered that the
appearance of the new component, especially zinc ferrite could
promote the DMC synthesis. In other words, the activity of
catalysts must be owing to the synergistic effect between the zinc
ferrite and zinc oxide.
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d
+
d
activated by base sites via abstraction of H to form CH3O