Direct Synthesis of Dimethyl Carbonate
Kang et al.
reported in the literature.23 Known amounts of cerium
OCH3
C
precursor (Ce(NO ꢁ ·6H O, Sigma-Aldrich) and zinc pre-
3
3
2
H
O
M
CH3
O
O
cursor (Zn(NO ꢁ · 6H O, Sigma-Aldrich) were dissolved
3
2
2
O
O
in ethanol under vigorous stirring. Ammonia solution was
then slowly added into the solution containing cerium and
zinc precursors to increase pH value to 10. The precip-
CH OH
CO2
O
3
O
+
H O
2
M
M
M
M
M
M : Metal
(Base site)
Methoxy carbonate
ꢀ
itate was aged for 3 h at 50 C, and it was then fil-
(Base site)
tered and washed with deionized water and ethanol. After
ꢀ
CH OH
drying the resulting product at 100 C for 24 h, it was
3
O
C
H
ꢀ
finally calcined at 500 C for 3 h in an air stream to
yield CeO (0.7)–ZnO(0.3) mixed metal oxide. A series
O
CH3
2
O
M
O
C
of XNiO/CeO (0.7)–ZnO(0.3) (X = 0, 1, 5, 10, and 15)
+
H CO
3
O
M
OH
M
2
O
M
H CO
OCH3
nano-catalysts with different NiO content (X, wt%) were
prepared by a wet impregnation method using an aque-
ous solution of nickel precursor (Ni(NO ꢁ ·6H O, Sigma-
3
Dimethyl carbonate
(Acid site)
3
2
2
ꢀ
Aldrich). The impregnated catalysts were dried at 100 C
Figure 1. Mechanism for the direct synthesis of DMC from methanol
and carbon dioxide over acid-base bifunctional catalyst.
ꢀ
for 24 h, and they were calcined at 500 C for 3 h in the
presence of air to yield XNiO/CeO (0.7)–ZnO(0.3) nano-
2
catalysts.
including organometallic compounds, metal oxides,19–21
1
8
2
2
and bimetallic catalysts. Among these catalysts, mixed
metal oxides are known to be the most efficient
In particular, it is reported that CeO -based
metal oxide showed a superior catalytic activity due to
its excellent acid-base property. Furthermore, it is also
reported that the addition of NiO into metal oxide can
modify the acid-base properties of metal oxide.
previous work, we have developed a CeO (0.7)–ZnO(0.3)
mixed metal oxide catalyst for direct synthesis of DMC
from methanol and carbon dioxide, which showed a con-
siderable catalytic performance in the reaction. In this
work, CeO (0.7)–ZnO(0.3) was chosen as a support for
NiO in order to increase the acid-base properties of the
catalyst. To our best knowledge, NiO catalyst supported
on CeO -based metal oxide has never been applied to the
direct synthesis of DMC. Therefore, developing a NiO
catalyst supported on CeO (0.7)–ZnO(0.3) with enhanced
acid-base properties would be worthwhile.
2.2. Characterization
catalysts.19–21
Crystalline phases of XNiO/CeO (0.7)–ZnO(0.3) nano-
2
2
catalysts were confirmed by XRD measurements (Rigaku,
D-MAX2500-PC) using Cu-Kꢂ radiation (ꢃ = 1ꢄ54056 Å)
operated at 50 kV and 100 mA. Chemical compositions
of the catalysts were measured by inductively coupled
2
1
2
3ꢀ24
In our
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2
IP: 110.36.39.137 On: Thu, 03 Dec 2015 07:26:08
sis (Shimadz, ICP-1000IV). Surface areas of the catalysts
Copyright: American Scientific Publishers
were determined using an ASAP-2010 (Micromeritics)
instrument.
NH -TPD experiment was performed in order to inves-
3
2
tigate acidity of the catalysts. 0.2 g of each catalyst was
charged into the quartz reactor of the conventional TPD
apparatus. It was pretreated at 200 C for 1 h under a
flow of helium (20 ml/min) to remove any physisorbed
organic molecules. 20 ml of ammonia was then pulsed
into the reactor every minute at room temperature under
a flow of helium (5 ml/min), until the acid sites were
saturated with NH . Physisorbed NH was removed by
evacuating the catalyst sample at 50 C for 1 h under
a flow of helium (15 ml/min). Furnace temperature was
increased from room temperature to 900 C at a heating
rate of 5 C/min under a flow of helium (10 ml/min).
Desorbed ammonia was detected using a GC-MSD (Agi-
lent, 5975MSD-6890N GC). Basicity of the catalysts was
ꢀ
2
2
In this work, XNiO/CeO (0.7)–ZnO(0.3) (X = 0, 1, 5,
2
3
3
1
0, and 15) nano-catalysts were prepared by a wet impreg-
ꢀ
nation method with a variation of NiO content (X, wt%),
and they were applied to the direct synthesis of DMC from
methanol and carbon dioxide. Successful formation of
ꢀ
ꢀ
XNiO/CeO (0.7)–ZnO(0.3) nano-catalysts was confirmed
2
by XRD and ICP-AES analyses. NH -TPD (temperature-
3
programmed desorption) and CO -TPD experiments were
conducted to investigate the effect of acidity and basic-
ity of XNiO/CeO (0.7)–ZnO(0.3) on the catalytic perfor-
2
measured by CO -TPD experiment. Experimental proce-
dures for CO -TPD were identical to those for NH -TPD,
except that CO instead of NH was employed as a probe
molecule.
2
2
3
2
2
3
mance in the reaction.
2
. EXPERIMENTAL DETAILS
2.3. Direct Synthesis of DMC from
2
.1. Catalyst Preparation
Methanol and Carbon Dioxide
CeO (0.7)–ZnO(0.3) mixed metal oxide with a fixed
molar composition (value in parenthesis) was prepared
by a co-precipitation method according to the procedures
Direct synthesis of DMC from methanol and carbon diox-
ide was conducted in a stainless steel autoclave reactor
with a volume of 75 ml. 30 ml of methanol and 0.7 g of
2
8
694
J. Nanosci. Nanotechnol. 14, 8693–8698, 2014