Efficient synthesis of dimethyl carbonate via transesterification of ethylene carbonate with methanol over binary zinc-yttrium oxides

Article abstract of DOI:10.1016/j.catcom.2011.09.004

The binary zinc-yttrium oxides were prepared by co-precipitation method, characterized and tested in the synthesis of DMC via transesterification of ethylene carbonate with methanol. The catalytic results showed that the catalyst with Zn/Y molar ratio of 3 and calcined at 400°C exhibited superior catalytic activity, corresponding to TOF of 236 mmol/g_cat h. Appropriate content of yttrium in the catalyst enhanced the catalytic activity remarkably. Moreover, the abundance of medium basic sites (7.2 < H- < 9.8, as determined by Hammett indicator method) was considered to be responsible for the superior catalytic activity.

Full text of DOI:10.1016/j.catcom.2011.09.004

Catalysis Communications 16 (2011) 45–49
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Efficient synthesis of dimethyl carbonate via transesterification of ethylene carbonate
with methanol over binary zinc-yttrium oxides
Liguo Wang a,b, Ying Wang a,b, Shimin Liu a,b, Liujin Lu a, Xiangyuan Ma a, Youquan Deng a,
⁎
a
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b
Graduate University of Chinese Academy of Sciences, Beijing 100039, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 June 2011
Received in revised form 25 August 2011
Accepted 6 September 2011
Available online 14 September 2011
The binary zinc–yttrium oxides were prepared by co-precipitation method, characterized and tested in the
synthesis of DMC via transesterification of ethylene carbonate with methanol. The catalytic results showed
that the catalyst with Zn/Y molar ratio of 3 and calcined at 400 °C exhibited superior catalytic activity, corre-
sponding to TOF of 236 mmol/gcat h. Appropriate content of yttrium in the catalyst enhanced the catalytic ac-
tivity remarkably. Moreover, the abundance of medium basic sites (7.2bH_b9.8, as determined by Hammett
indicator method) was considered to be responsible for the superior catalytic activity.
Keywords:
© 2011 Elsevier B.V. All rights reserved.
Dimethyl carbonate
Ethylene carbonate
Transesterification
Binary zinc–yttrium oxide
1. Introduction
from epoxides, CO2 and methanol [13, 14], however, undesired side
reaction such as ring opening of epoxides by methanol led to relative-
As the lowest homologue in the family of dialkyl carbonates, di-
methyl carbonate (DMC) has drawn specific attention as a non-toxic
and environmentally friendly building block for a variety of chemicals
[1]. DMC is a promising substitute for phosgene in the syntheses of ar-
omatic polycarbonates [2], carbamates [3], isocyanates [4], etc. As a
safe and environmental-friendly alternative, it can be also used as al-
kylation reagent for carbon, nitrogen, oxygen and sulfur centers [5].
Additionally, DMC has potential applications in the fields of fuel addi-
tive [6], lithium batteries electrolyte [7].
The industrial manufacture of DMC via phosgenation [8] and oxi-
dative carbonylation of methanol [9] involved high-risk compounds
of COCl2 and CO. The chemical utilization of CO2 as the raw material
to synthesize useful chemical feedstocks has drawn much attention
in recent years [10–12]. The production of DMC via transesterification
of cyclic carbonates with methanol was a suitable synthetic pathway
because the syntheses of raw material cyclic carbonates, such as eth-
ylene carbonate (EC), propylene carbonate, from epoxides and carbon
dioxide were well established [10, 11]. Therefore, the transesterifica-
tion route aforementioned has the merits of cleanness and high effi-
ciency for indirect utilization of CO2 to synthesize DMC. It should be
noted that DMC can also be synthesized via one-pot reaction directly
ly lower DMC selectivity.
Over the past decades many efforts were devoted for synthesizing
DMC via transesterifcation of cyclic carbonates with methanol, and sever-
al homogenous or heterogeneous catalytic systems have been reported
previously, such as Mg–Al-hydrotalcite [15], Fe–Zn double metal cyanide
[16], semectite [17], mesoporous CaO–ZrO2 nano-oxides [18], activated
dawsonites [19], polymer [20], or Au/CeO2 [21]. However, most catalytic
systems suffer from low catalytic activity, high reaction temperature or
involvement of noble metals. Therefore, the development of an effective
and stable heterogeneous catalyst is still highly desirable for this impor-
tant process due to the significance both in fundamental research and in-
dustrial manufacture. The reaction is shown in Scheme 1.
In this study, binary zinc–yttrium oxides were prepared by co-
precipitation method. These catalytic materials were found to cata-
lyze the DMC synthesis from EC and methanol with high efficiency.
The catalysts were characterized by means of nitrogen adsorption,
XRD, XPS, FE-SEM, and Hammett indicator methods in detail. Influ-
ence of the preparation parameters (Zn/Y molar ratio, catalyst activa-
tion), and the effect of reaction parameters (MeOH/EC, reaction time)
were also investigated in a batch mode.
2. Experimental
2.1. Materials
Zn(OAc)2·2H2O and Y(NO3)3·6H2O were both of analytical grades,
and were purchased from Sinopharm Chemical Reagent Co., Ltd.
⁎
Corresponding author.
E-mail address: ydeng@licp.cas.cn (Y. Deng).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.09.004

46
L. Wang et al. / Catalysis Communications 16 (2011) 45–49
O
O
OH
O
O
H3C
CH3
+
2CH3OH
+
HO
O
O
Scheme 1. Transesterification of ethylene carbonate and methanol to dimethyl carbonate.
calcination temperature had a strong influence on the catalyst textures.
Notably, with this preparation method, catalytic materials with rela-
tively high surface area and mesostructured pores could be obtained.
2.2. Catalyst preparation
A mixture of given amount of Zn(OAc)2·2H2O and Y(NO3)3·6H2O in
designated ratios was dissolved into distilled water under stirring. Sub-
sequently, 1 M aqueous solution containing sodium hydroxide and so-
dium carbonate was added dropwise until the pH of mixed solution
reached 11. The resulting precipitate then was aged, filtrated, washed,
and calcined in static air for 4 h. The obtained white solid was denoted
as ZnxY-T, where the x represents the Zn/Y molar ratio, and T is the cal-
cination temperature.
3.1.2. Crystal structure
The X-ray diffractograms of the ZnO, Y2O3 and Zn3Y binary oxides
are shown in Fig. 1. Diffractograms of Y2O3 sample mainly consisted
of amorphous phase, and poorly crystalline body-centered cubic yt-
trium oxide was identifiable (JCPDS 89–5592). The diffractograms
for ZnO were dominated by hexagonal structure of crystalline zinc
oxide (JCPDS 89–1397). In contrast, the intensities of XRD lines spe-
cific to Y2O3 in the Zn3Y-400 binary oxide were higher than that of
pure Y2O3, suggesting the involvement of zinc element improves
the crystallization of Y2O3. The characteristics of Y2O3 peaks became
sharper with increase of calcination temperature or after reuse for
six times. It suggested that the higher calcination temperature and
calcination procedure between each recycling run to regenerate the
catalytic activity promoted the crystallization of Y2O3, which was
also consistent with the view in reference [23].
2.3. Catalyst characterization
N2 adsorption and desorption isotherms at 77 K were measured
on a Micromeritics ASAP 2010 surface analyzer.
Powder X-ray diffraction (XRD) was measured on a Siemens
D/max-RB powder X-ray diffractometer.
X-ray photoelectron spectroscopy (XPS) analysis was performed
with a VG ESCALAB 210 instrument.
The morphological structures of the zinc–yttrium precipitate and the
ones calcined at the temperature region of 300–600 °C were examined
by field emission scanning electron microscopy (FE-SEM, JSM-6701F).
Hammett indicator method was used to measure the basic strength
distribution of the catalyst according to the literature [22]. In a typical
measurement, a mixture of 50 mg catalyst and 5 ml dry methanol was
stirred for 2 h and the resultant solution was titrated against the stan-
dard benzene carboxylic acid solution.
3.1.3. XPS analysis
The chemical state and surface composition of the Zn3Y samples as a
function of calcination temperature are shown in Table 2. The peaks
with binding energy (BE) located at 1021.7, 156.3–157.9, 529.3 and
531.3 eV can be attributed to Zn 2p3/2, Y 3 d5/2, O1at 1 s (lattice oxygen)
and Oad 1 s (adsorbed oxygen), respectively. BE of Y 3 d3/2 gradually
shifts from 157.9 to 156.3 eV with calcination temperature increased
from 300 to 600 °C. The shift toward lower BE value indicated that
Y3+ ionic state possessed enhanced electron density with the increase
of calcination temperatures, and a possible reason was that the electron
transfer from oxygen vacancy to metal atoms of yttrium probably oc-
curred as reported in the literature [24]. Percentage of the surface lattice
oxygen (Olat), or together with Zn2+ and Y3+ as total amount both
reached maximum for the sample calcined at 400 °C. The concentration
of Olat species decreased with further elevating calcination temperature
to 500 or 600 °C. Subsequently, the concentrations of the Olat follow the
order: Zn3Y-400NZn3Y-500~Zn3Y-600NZn3Y-300.
2.4. Catalytic testing
In a typical reaction procedure, 2.64 g EC (30 mmol), 7.6 g methanol
(240 mmol) and 0.07 g binary zinc–yttrium oxide catalyst (2.5 wt.% re-
spect to the amount of EC) were added into a 50 ml flask. The reaction
mixture was heated up to 65 °C and the reaction was conducted at at-
mospheric pressure. After the completion of reaction, the catalyst was
recovered by centrifugation, and the quantitative analysis of the prod-
uct was determined by Agilent 6820 GC (FTD detector) with octane as
internal standard.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Nitrogen physisorption
The textural properties for the binary zinc–yttrium oxides were
shown in Table 1. The higher BET surface area of 182.5 m2/g could
be obtained when the sample calcined at 400 °C is compared to that
of 300 or 600 °C. On the other hand, relatively lower pore volume
and diameter size were obtained simultaneously. It indicated that the
Table 1
Textural properties of the binary zinc–yttrium oxides as a function of calcination
temperature.
Catalyst
SBET (m2/g)
P (cm3/g)
d (nm)
Zn3Y-300
Zn3Y-400
Zn3Y-600
65.7
182.5
20.5
0.232
0.108
0.107
10.2
2.4
17.9
Fig. 1. Power XRD patterns of Y2O3, ZnO, and binary zinc–yttrium oxide samples. (■)
Hexagonal phase of ZnO. (▼) Body-centered cubic phase of Y2O3.

L. Wang et al. / Catalysis Communications 16 (2011) 45–49
47
Table 2
to act as Lewis base sites [25]. Therefore, it can be proposed that the
lat species plays an important role in the Lewis basicity.
Binding energies of core electrons and XPS atomic ratios of binary zinc–yttrium oxides
as a function of calcination temperature.
O
Samples
Binding energy (eV)
Surface
percentage (at.%)
Surface atom
ratio of Olat
(Zn2++Y3+
3.1.5. SEM study
/
The representative morphology of Zn3Y precipitate and morphol-
ogy evolution of Zn3Y samples along with the increase of calcination
temperature (300–600 °C) were studied by means of FE-SEM, as
shown in Fig. 2. The Zn3Y precipitate exhibits hexagonal rod-like
morphology. Upon calcination at 300 °C, the morphology with the
poriform and discriminable sheet-like features were observed. Inter-
estingly, after calcining the precursor at 400 °C, the resulting Zn3Y-
400 sample primarily consists of nanostructured sheet-like morphol-
ogy with the thickness ~100 nm. By contrast, the compact aggregated
palates with irregular size were observed after calcination at higher
temperature of 600 °C, suggesting that the grains sintered to form
the larger particles. In brief, besides the uncalcined catalyst with hex-
agonal rod-like morphology, the calcination of the catalyst precursor
at varied temperatures resulted in different morphologies, but not
prominent.
)
Y3d3/2 Olat
Oad
Zn2p1/2 Olat Zn2+ Y3+
Zn3Y-300 157.9
Zn3Y-400 157.5
Zn3Y-500 156.5
Zn3Y-600 156.3
529.2 531.4 1021.7
529.3 531.5 1021.6
529.3 531.3 1021.7
529.4 531.3 1021.6
4.2
9.1
6.4 0.27
9.4 21.6
5.6 15.0
5.8 10.6
11.3 0.22
9.3 0.39
0.6 0.52
Table 3
Base strength (H_) distribution obtained from Hammett indicator measurements.
Entry
Catalyst
Basicity (mmol/gcat.)
4.2bH_b7.2
7.2bH_b9.8
H_N9.8
Total
1
2
3
4
5
6
ZnO
Y2O3
Zn3Y-300
Zn3Y-400
Zn3Y-500
Zn3Y-600
0.13
0.04
0.10
0.89
0.37
0.29
0.21
0.19
0.31
0.74
0.46
0.37
–
0.34
0.23
0.89
1.81
1.02
0.80
–
0.48
0.18
0.19
0.14
3.2. Catalytic testing
3.1.4. Basicity characterization
The transesterification of EC and methanol for DMC synthesis
was investigated over various catalysts (Table 4). 11–19% of EC
conversions with 10–17% of DMC yields were obtained over Y2O3
and ZnO catalysts, entries 1–2. For conciseness, the catalytic activ-
ity was expressed in the form of DMC yield in the following dis-
cussion. For the physical mixed oxide, entry 3, it exhibited 13%
of DMC yield. Additionally, when Zn3Y precipitate was employed,
it was almost inactive to the DMC synthesis, entry 4. Among the
catalysts calcined at different temperatures, entries 5–8, Zn3Y-
400 exhibited 54% of DMC yield with 51% of slightly lower EG
yield, corresponding to higher TOF of 236 mmol/gcat h compared
to previous reports [15–21]. It indicated that 400 °C was the prop-
er activation temperature for obtaining better catalytic activity.
Higher catalytic activity was obtained over binary zinc–yttrium
oxide than that of the physical mixture. This result demonstrated
The effect of the calcination temperature on the basicity was de-
termined by Hammett indicator method (Table 3). Apparently, the
higher total amount of basicity was obtained for the binary metal
oxide compared to the corresponding single metal oxide, suggesting
that the surface basicity was improved through combing the two
components of zinc and yttrium. Among the binary oxides, the one
calcined at 400 °C with the larger surface area possessed more total
basic sites (1.81 mmol/g), simultaneously with the amount of medi-
um basicity (7.2≤H_≤9.8) up to 0.74 mmol/gcat. Additionally, the
concentration of the medium basic sites follows the trends: Zn3Y-
400NZn3Y-500NZn3Y-600NZn3Y-300. In association of the XPS re-
sults, it can be found that the concentration sequence of the medium
basic sites is essentially consistent with that of the Olat species. For
metal oxides, lattice oxygens on the surface are generally considered
Fig. 2. FE-SEM images of (a) Zn3Y precipitate, (b) Zn3Y-300, (c) Zn3Y-400 and (d) Zn3Y-600.

48
L. Wang et al. / Catalysis Communications 16 (2011) 45–49
Table 4
Results of transesterification of EC and methanol over various catalysts.
Entry
Catalyst
EC Con./%
DMC Yield./%
EG Yield./%
c
1
2
Y2O3
ZnO
11
19
16
8
33
55
48
46
38
46
34
10
17
13
5
31
54
45
41
34
44
33
–
16
12
–
3a
4b
5
ZnO–Y2O3
Zn3Y-uncalcined
Zn3Y-300
Zn3Y-400
Zn3Y-500
Zn3Y-600
ZnY-400
Zn4Y-400
Zn3Y-400
30
51
42
38
30
43
33
6
7
8
9
10
11d
Reaction conditions: EC, 30 mmol; methanol, 240 mmol; catalyst, 0.07 g; 65 °C; 1 h;
a: 0.07 g physically mixed ZnO and Y2O3 (molar ratio of Zn/Y=3/1); b: Zn3Y
precipitate; c: not detectable. d: 20 °C.
Fig. 4. The effect of reaction time on the catalytic synthesis of DMC. Reaction conditions:
30 mmol EC; 240 mmol methanol; 0.07 g Zn3Y-400; 65 °C.
that the integration of zinc and yttrium components can remark-
ably improve the catalytic activity and the synergistic effect be-
tween them probably occurred. In some cases reported in the
previous literatures [26, 27], it was demonstrated that the catalyt-
ic materials with different morphologies exhibited distinct catalyt-
ic performance through exposing various reactive crystal planes.
However, in association of the basicity and FE-SEM characteriza-
tions, it can be seen that the catalysts investigated without prom-
inent differences in morphology exhibited distinct variation in
catalytic activity, i.e. 31% and 54% DMC yields for Zn3Y-300 and
Zn3Y-400, respectively. It also can be found that the catalytic se-
quence is essentially consistent with that of medium basicity
(7.2bH_b9.8, as determined by Hammett indicator method). Gen-
erally, it was widely accepted that transesterification reaction was
efficiently catalyzed by acid–base catalysts [28]. Therefore, it is
reasonable to infer that the medium basic sites on the catalyst
surface have a vital influence on the catalytic activity, i.e. Zn3Y-
400 with more medium basic sites demonstrated higher catalytic
activity. Moreover, considering the relationship between the me-
dium basicity and oxygen species, Lewis basic sites of Olat proba-
bly acted as the active sites.
Binary zinc–yttrium oxides with other molar ratio of Zn/Y, such as 1
or 4, were also tested, entries 9–10. However, lower DMC yields were
obtained, especially obvious for the catalyst with Zn/Y molar ratio of
1, entry 9. It indicated that appropriate content of yttrium in the catalyst
was more effective for the DMC synthesis. That is, there was an opti-
mum composition of the catalyst for obtaining higher catalytic activity
and the catalyst with molar ratio of Zn/Y 3 was most active. Notably,
the catalyst exhibited 33% of DMC yield at 20 °C, entry 11, suggesting
the catalyst was active even at room temperature condition.
The influence of MeOH/EC molar ratio on DMC synthesis in the
range of 3–12 was also investigated, as shown in Fig. 3. A significant
increase in DMC yield was observed when the MeOH/EC molar ratio
increased from 3 to 6, and reached the maximum when the ratio
was 8. Further increasing the molar ratio did not improve the DMC
yield apparently within 2 h reaction time. The dependence of DMC
synthesis on the reaction time was also investigated in the range of
0.5–2.5 h, as shown in Fig. 4. A maximum DMC yield of 72% could
be obtained within 2 h. Further prolonging the reaction time has no
positive effect in increasing the DMC yield, indicating that the reac-
tion reached the equilibrium.
3.3. Recyclability of the catalyst
The recycling test for the catalyst was shown in Fig. 5. After a sim-
ple calcination at 350 °C, the catalytic activity can be essentially
regenerated. The catalyst can be reused without significant loss in ac-
tivity in the subsequent runs, and 65% of DMC yield could be obtained
even after six runs. The preservation of the crystal structure measured
by XRD characterization was considered to be partially responsible
for the good recyclability.
4. Conclusions
In summary, binary zinc–yttrium oxides were prepared by co-
precipitation method and Zn3Y-400 was found to be an effective cata-
lyst for the synthesis of DMC via transesterification of EC and methanol,
Fig. 3. The effect of MeOH/EC on the catalytic synthesis of DMC. Reaction conditions:
30 mmol EC; 0.07 g Zn3Y-400; 2 h; 65 °C.
Fig. 5. Recycling tests for DMC synthesis from EC and methanol over Zn3Y-400 catalyst.
(Reaction conditions: EC, 30 mmol; Methanol, 240 mmol; Catalyst, 0.07 g; 2 h; 65 °C.).

L. Wang et al. / Catalysis Communications 16 (2011) 45–49
49
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over, the catalytic activity can be regenerated after a simple calcination
handling.
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