Zn/Fe mixed oxide: Heterogeneous catalyst for the synthesis of dimethyl carbonate from methyl carbamate and methanol

Article abstract of DOI:10.1016/j.catcom.2009.11.016

A series of Zn-Fe-O mixed oxides were prepared for the synthesis of dimethyl carbonate (DMC) from methyl carbamate and methanol. X-ray diffraction revealed that zinc ferrite crystal phase appeared and changed with different Zn/Fe molar ratio. The DMC yield could reach 30.7% under suitable conditions. In addition, elemental chemical analysis and the reusability test indicated that these catalysts presented good stability.

Full text of DOI:10.1016/j.catcom.2009.11.016

Catalysis Communications 11 (2010) 430–433
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Zn/Fe mixed oxide: Heterogeneous catalyst for the synthesis of dimethyl
carbonate from methyl carbamate and methanol
a,
Dengfeng Wang a,b, Xuelan Zhang a,b, Yangyan Gao a,b, Fukui Xiao a, Wei Wei a, , Yuhan Sun
*
*
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 Zn–Fe–O mixed oxides were prepared for the synthesis of dimethyl carbonate (DMC) from
methyl carbamate and methanol. X-ray diffraction revealed that zinc ferrite crystal phase appeared
and changed with different Zn/Fe molar ratio. The DMC yield could reach 30.7% under suitable conditions.
In addition, elemental chemical analysis and the reusability test indicated that these catalysts presented
good stability.
Received 28 September 2009
Received in revised form 13 November 2009
Accepted 19 November 2009
Available online 26 November 2009
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Dimethyl carbonate
Methyl carbamate
Methanol
Zn/Fe mixed oxide
1. Introduction
speaking, urea can be easily converted to ammonia and isocyanic
acid, and the latter can react with methanol to produce MC with
As an environmental benign chemical product, dimethyl car-
bonate (DMC) has been used widely in fields of chemical industry.
It is mainly used as methylation and carbonylation reagent to re-
place dimethyl sulphate, chloromethane, and phosgene without
environmental pollution [1–4]. Besides, because of its versatile
chemical properties, it is also used as electrolyte, foodstuff flavor-
ing agent, and potential additive for gasoline [5,6]. Since DMC was
first produced from methanol and phosgene in 1910s [7], there are
several environmentally compatible DMC synthesis routines
including oxidative carbonylation of methanol, ester exchange,
and esterification of carbon dioxide with methanol. However, all
these techniques suffer some corresponding drawbacks such as
explosion hazards, reaction routine complex and low conversation,
respectively [8–10].
high selectivity and yield even without catalysts. Nevertheless,
MC to DMC is more difficult than the first step. This is because
the ammonia produced in the first step can restrict the reaction
shift to the DMC preparation. Consequently, the second step is
the key and rate-control step for this approach [12–14]. Thus, the
key to improve the DMC synthesis by urea methanolysis is to de-
velop a catalyst with high activity towards the reaction of MC
and methanol.
ZnO presented high catalytic performance for the reaction be-
tween urea and methanol [15]. Unfortunately, for the reaction of
MC and methanol, it didn’t exhibit satisfactory property. In fact,
ZnO was a precursor of homogeneous catalyst for the DMC syn-
thesis from urea and methanol [16]. As to the isolated second
reaction, solid bases were tested in a batch reactor, but the
DMC yield was still far from satisfaction [12]. Our group reported
that using ZnCl2 as catalyst, the DMC yield could reach 33.6% [17].
However, it was famous that the catalyst recovery was a great
inconvenience for industry production using homogeneous cata-
lyst. Therefore, it was of significance to explore highly efficient
heterogeneous catalytic materials for the DMC production from
MC and methanol.
Thus, in order to avoid all shortcomings mentioned above, a
new process for synthesis of DMC from methanol and urea
has been investigated [11]. It can be divided into two steps as
follows:
NH2CONH2 þ CH3OH ! NH2COOCH3 þ NH3
NH2COOCH3 þ CH3OH ! CH3OCOOCH3 þ NH3
ð1Þ
ð2Þ
Here, a series of zinc/iron mixed oxides were prepared and
tested by the DMC synthesis from MC and methanol. The effect
of Zn/Fe molar ratio was also discussed in detail on their cata-
lytic performance. Based on the results of element analysis and
The intermediate methyl carbamate (MC) is first formed, and
further converted to DMC by reaction with methanol. Generally
reusability test,
explored.
a heterogeneous catalyst was successfully
* 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).
1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.11.016

D. Wang et al. / Catalysis Communications 11 (2010) 430–433
431
ðstarting MCꢀMCÞ
2. Experimental
MC conversion ¼
DMC yield ¼
ðmolÞ ꢁ 100%
starting MC ðmolÞ
DMC ðmolÞ
ðmolÞ ꢁ 100%
starting MC
NMMCꢁ2 ðmolÞ
2.1. Catalysts preparation
NMMC yield ¼
ðmolÞ ꢁ 100%
starting MC
Zinc/iron catalyst precursors were prepared by co-precipita-
tion method as following: according to desired metals proportion
(Zn/Fe = 2, 4, 8, 10), a 100 mL aqueous solution containing x mol
Zn(NO3)3 and y mol Fe(NO3)3 (x + y = 0.1) and another 100 mL
aqueous solution containing 0.15 mol (NH4)2CO3 was prepared
separately. Then, these two solutions were added dropwise with
stirring to 100 mL deionized water simultaneously. During this
process, pH value of the mixed solution was maintained close
to 8.0 using ammonia solution. After that, the mother liquid
was aged at 65 °C with stirring overnight. The as-prepared prod-
ucts were obtained after being filtered and washed with deion-
ized water until pH was 7.0. After being dried at 80 °C for 12 h,
all samples were calcined at 600 °C for 5 h. They were called
ZFO-X, in which X was corresponding to the initial zinc/iron
atomic ratio. Besides, zinc oxide was obtained via the same pro-
cedure and method as that of the zinc/iron mixed oxides in the
present case.
Zinc ferrite material was prepared as the description in the lit-
erature elsewhere [18]. In brief, zinc and iron nitrates were dis-
solved with a Zn:Fe ratio = 2. Under vigorous stirring, aqueous
ammonia solution was added slowly to the mixed solution until
the pH was 8.0. Then the resulting sol obtained was aged at
60 °C for 24 h. The powers were separated by centrifugation,
washed repeatedly with deionized water, further filtered and dried
at 100 °C overnight. Finally, the ZnFe2O4 sample was obtained by
calcining at 600 °C for 5 h.
3. Results and discussion
3.1. Catalysts structure
The chemical analysis revealed that the Zn/Fe molar ratio in
mixed oxide samples deviated from that in mother solutions. Be-
sides, the deviation gradually increased with the rise of the Zn/Fe
molar ratio. It was well known that the low pH value disfavored
the incorporation of Zn2+ into precipitation due to the relatively
high solubility of Zn(OH)2 (Ksp = 6.8 ꢁ 10-17) in comparison of
Fe(OH)3 (Ksp = 2.8 ꢁ 10ꢀ39) [18].
XRD patterns of solid catalysts with different Zn/Fe molar ratio
were given in Fig. 1. Each phase was identified by its characteristic
diffraction peak using JCPDS. All of the mixed oxides prepared dis-
played two phases, ZnFe2O4 and ZnO. It should be mentioned that
diffraction peaks of the two crystal phases presented obvious
change with different Zn/Fe molar ratio. ZFO-2 exhibited almost
all peaks ascribed to ZnFe2O4, for there were peaks existed at 2h
of 18.2°, 30.5°, 35.4°, 36.9°, 43.1°, 53.4°, 56.9° and 62.6°. Only
one ZnFe2O4 peak at 35.5° was observed for ZFO-10. Interestingly,
all peak intensity attributed to ZnO became stronger and stronger
Δ
Ο: ZnO Δ: ZnFe O
2 4
Other chemicals were analytic reagent from chemical products
corporation without any purification.
Δ
Δ
2.2. Characterization
Δ
Δ
Δ
Δ
Power X-ray diffraction (XRD) experiments were performed on
a Rifaku D max III VC instrument with k = 0.1541 nm, Cu-K
ation in the 2h range of 5–70°.
Elemental chemical analysis of solid catalysts and the methanol
solution after reaction was carried out using the inductively cou-
pled plasma-optical (ICP) emission spectroscopy on a Perkin Ele-
mer 3000 equipment.
The surface area of catalysts was determined with the BET
method using N2 on a Micromeritics ASAP-2000 instrument.
Ο
Ο
Δ
a
a radi-
Δ
Ο
Ο
Δ
Ο
Δ
Ο
Ο
Ο
Ο
Ο
Δ
Δ
Δ
Ο
Ο
Δ
Δ
Δ
b
Δ
Ο
Ο
Δ
Ο
Δ
Ο
Δ
2.3. Catalytic test
b'
Δ
Ο
Reaction of MC and methanol was carried out in a stainless steel
autoclave of 300 mL with a magnetic stirrer and a reflux column. In
a typical experiment, 7.5 g MC, 64 g methanol and 1.0 g catalyst
were put into the reactor. Then the reactor was heated to 190 °C
and kept for 10 h with stirring. After the reaction, the reactor
was cooled down to room temperature. Products in liquid phase
after centrifugal separation were analyzed by a gas chromatogra-
phy (GC) equipped with TCD. It should be mentioned that the
by-product dimethyl ether could not be detected by chromatogra-
phy because it was easy to volatilize when the autoclave was
opened.
Ο
Ο
Ο
Δ
Ο
Ο
Ο
Ο
Ο
Ο
Ο
Δ
Ο
Ο
c
Ο
Ο
Ο
Δ
d
15 20 25 30 35 40 45 50 55 60 65 70
2 Theta (Cu Ka)
2.4. Calculations
The MC conversion, DMC and NMMC yield were calculated by
the following formulas, respectively:
Fig. 1. XRD patterns of zinc/iron mixed oxides: (a) ZFO-2, (b) ZFO-4 before reaction,
(b0) ZFO-4 after reaction, (c) ZFO-8 and (d) ZFO-10.

432
D. Wang et al. / Catalysis Communications 11 (2010) 430–433
Table 1
with increase of Zn/Fe molar ratio. While, that of zinc ferrite
tended to weaker and weaker. As a result, the formation of crystal
phase of catalysts was correlated with Zn/Fe molar ratio, and might
further display influence on the catalytic performance of the cata-
lysts. More important, Fig. 1 illustrated that when Zn/Fe atomic ra-
tio was 4, not only the number of characteristic peaks for both ZnO
and ZnFe2O4, but also the peak intensity was in a temperate degree
among these catalysts.
Typical XRD patterns of the as-synthesized ZnFe2O4 and ZnO
materials were shown in Fig. 2. It could be observed that the char-
acteristic crystallographic peaks such as (1 1 1), (2 2 0), (3 1 1),
(4 0 0), (4 2 2), (5 1 1), and (4 4 0) of the spinal structure were
well-assigned to these of standard ZnFe2O4 (Franklinite, JCPDS
22-1012) [19,20]. Thus, the zinc ferrite was successfully synthe-
sized in the present case.
Catalytic performance of catalystsa.
Entry Catalyst
MC conversion (%) DMC yield (%) NMMC yield (%)
1
2
3
4
5
6
7
8
9
None
ZnO
4.1
7.1
2.6
3.8
2.6
0
0
0
0
7.1
5.9
5.4
0
Fe2O3
9.1
4.3
b
ZnO-Fe2O3
ZFO-4
ZFO-2
ZFO-8
ZFO-10
ZnFe2O4
3.0
46.1
28.8
29.9
24.7
10.86
30.7
16.0
15.5
7.9
6.4
0
a
Reaction conditions: reaction temperature, 190 °C; reaction time, 10 h; catalyst
amount, 1.0 g; MC, 7.5 g; methanol, 64 g.
b
Metal oxides were physically mixed and molar ratio of ZnO/Fe2O3 was 4.
3.2. Performance of catalysts
[12,17]. Furthermore, the surface areas of the Zn/Fe mixed oxides
are listed in Table 2. It could be seen that their surface areas de-
creased in the order of ZFO-10 > ZFO-4 > ZFO-8 > ZFO-2. However,
their catalytic activity followed this order: ZFO-4 > ZFO-2 > ZFO-
8 > ZFO-10. Thus, surface area of catalysts was not a decisive factor
for their catalytic ability in the present work. Interestingly, accord-
ing to the discussions mentioned above, both ZnO and ZnFe2O4
proportion of ZFO-4 were in a temperate state among these cata-
lysts. Therefore, Zn/Fe molar ratio was a crucial factor for their cat-
alytic performance. More important, such obviously high catalytic
ability of Zn–Fe–O catalyst compared ZnO could be attributed to
the appearance of ZnFe2O4.
In order to eliminate that the catalytic ability of ZFO-4 origi-
nated from ZnO and Fe2O3 interaction, a mixture of the two oxides
with molar ratio of 4 was evaluated and it did not presented good
performance (Entry 4). Besides, a further investigation on the per-
formance of ZnFe2O4 was conducted. Obviously, it showed a little
catalytic activity (Entry 9). This also confirmed the role of interac-
tion between ZnFe2O4 and ZnO for catalyzing this reaction.
As a multiple system, synthesis of DMC from MC and methanol
could be proposed as follows:
NH2COOCH3 þCH3OH!CH3OCOOCH3 þNH3
NH2COOCH3 þCH3OCOOCH3 !CH3NHCOOCH3 þCH3OHþCO2 ð4Þ
CH3OCOOCH3 !CH3OCH3 þCO2 ð5Þ
ð3Þ
First, methoxy group in methanol substituted amino group of
MC to produce DMC. Moreover, the by-product N-methyl methyl
carbamate (NMMC) produced from the reaction of MC and DMC
when the concentration of DMC reached a certain extent [17]. Be-
sides, DMC could decompose into dimethyl ether and carbon diox-
ide at high temperature [17,21].
The catalytic results of different catalysts were showed in Table
1. It was found that the yield of DMC was only 2.6% without any
catalysts (Entry 1). As an ideal catalyst for the DMC synthesis from
urea methanolysis, ZnO was inactive for this reaction (Entry 2).
This was consistent with the results reported previously [15,16].
For Fe2O3, the catalytic activity was also very low (Entry 3). Among
mixed oxides, ZFO-4 exhibited the best catalytic performance and
the DMC yield could reach 30.7% (Entry 5). Obviously, it was more
active than base catalyst and its activity was even as the same as
that of ZnCl2, a typical homogeneous catalyst for this reaction
3.3. Stability of Zn/Fe mixed oxides
According to former studies reported, Zn2+ could activate MC
via coordination with the nitrogen atom in MC molecule [17].
Therefore, ZnCl2 exhibited the best catalytic performance, just be-
cause it could solve in methanol completely even at room temper-
ature. Wu et al., proposed Zn-based catalyst to catalyze urea
alcoholysis. Unfortunately, its catalytic ability was very low com-
pared to that of ZnO [15,22]. Therefore, on the one hand, a model
heterogeneous catalyst preparing DMC during this routine should
be insoluble in methanol solution at certain temperature. On the
other hand, the catalytic ability of the heterogeneous catalyst
was similar to that of the homogenous one.
Table 2 showed element analysis results of methanol solutions
after reaction in different systems. It could be found that divalent
zinc ion concentration was very low, when catalysts were ZnO
and Zn/Fe mixed oxides. Because there was lack of Zn2+ in the
reaction system, ZnO presented no significant activity for the
DMC production from MC and methanol. However, the high cata-
lytic performance of mixed oxides, especially ZFO-4, was obviously
not related to the metallic ions that dissolved in solution. In other
words, there was no species that could dissolve in methanol. It also
provided evidence that the excellent performance of these mixed
oxides originated from the interaction of ZnO with ZnFe2O4.
A further study on the catalytic reusability of ZFO-4 was carried
out. After the first run, used ZFO-4 sample was separated by filtrat-
ing, and washed with methanol several times, dried and reused in
another two catalytic cycles. Fig. 3 presented that it still showed
high catalytic performance after used three times. Additionally,
a
b
20
30
40
50
60
2 Theta (Cu Ka)
Fig. 2. XRD pattern of (a) zinc ferrite, and (b) zinc oxide.

D. Wang et al. / Catalysis Communications 11 (2010) 430–433
433
Table 2
4. Conclusions
Composition, BET surface area of the catalysts and the content of cations in solution
after the reactiona.
Zinc/iron mixed oxide showed the best catalytic performance for
DMC synthesis from MC and methanol when the Zn/Fe molar ratio
was 4. The MC conversion and DMC yield at the optimal reaction
conditions were 46.1% and 30.7%, respectively. Elemental chemical
analysis revealed that metal elements in methanol solution after
reaction could be neglected. In addition, based on results of XRD
experiment and the reusability test, its high catalytic performance
and stability originated from the interaction of ZnO with ZnFe2O4.
Catalyst Zn/Fe in
solids
BET surface area
(m2/g)
Zn content
(mg/L)
Fe content
(mg/L)
ZnO
–
17.6
2.0
0.3
0.2
<0.1
<0.1
–
0.2
<0.1
<0.1
<0.1
ZFO-2
ZFO-4
ZFO-8
ZFO-10
1.86
3.77
7.70
9.59
16.90
21.76
20.16
26.95
a
Reaction conditions: reaction temperature, 190 °C; reaction time, 10 h; catalyst
amount, 1.0 g; MC, 7.5 g; methanol, 64 g.
Acknowledgement
This work is financially supported from State Key Program for
Development and Research of China (No. 2006BAC02A08).
NMMC Yield
55
DMC Yield
50
MC Conversion
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the recovered catalyst after reused three times was analyzed using
XRD technique (see Fig. 1b0). Diffraction peak number was as the
same as the fresh one, and the peak intensity didn’t show any sig-
nificant change. This demonstrated that a heterogeneous catalyst
with high catalytic performance and stability for this reaction
was successfully developed.