Direct synthesis of dimethyl carbonate from methanol and carbon dioxide over Ga2O3/Ce0.6Zr0.4O2 catalysts: Effect of acidity and basicity of the catalysts

Article abstract of DOI:10.1007/s10562-010-0544-4

Ce _X Zr_1-X O₂ catalysts with different cerium content (X) (X = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0) were prepared by a sol-gel method for use in the direct synthesis of dimethyl carbonate from methanol and carbon dioxide. Among these catalysts, Ce_0.6Zr _0.4O₂ was found to show the best catalytic performance. In order to enhance the acidity and basicity of Ce_0.6Zr _0.4O₂ catalyst, Ga₂O₃ was supported on Ce_0.6Zr_0.4O₂ (XGa₂O ₃/Ce_0.6Zr_0.4O₂ (X = 1, 5, 10, and 15)) by an incipient wetness impregnation method with a variation of Ga ₂O₃ content (X, wt%). Effect of acidity and basicity of Ga₂O₃/Ce_0.6Zr_0.4O₂ on the catalytic performance in the direct synthesis of dimethyl carbonate was investigated using NH₃-TPD and CO₂-TPD experiments. Experimental results revealed that both acidity and basicity of the catalysts played a key role in determining the catalytic performance in the direct synthesis of dimethyl carbonate from methanol and carbon dioxide. Large acidity and basicity of the catalyst facilitated the formation of dimethyl carbonate. The amount of dimethyl carbonate produced over XGa₂O ₃/Ce_0.6Zr_0.4O₂ catalysts increased with increasing both acidity and basicity of the catalysts. Among the catalysts tested, 5Ga₂O₃/Ce_0.6Zr_0.4O ₂, which retained the largest acidity and basicity, showed the best catalytic performance in the direct synthesis of dimethyl carbonate from methanol and carbon dioxide. Graphical Abstract: In the direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide over Ga ₂O₃/Ce_0.6Zr_0.4O₂ catalysts, the amount of DMC showed a volcano-shaped curve with respect to Ga₂O₃ content. The amount of DMC increased with increasing both acidity and basicity of the catalysts [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-010-0544-4

Catal Lett (2011) 141:531–537
DOI 10.1007/s10562-010-0544-4
Direct Synthesis of Dimethyl Carbonate from Methanol
and Carbon Dioxide over Ga2O3/Ce0.6Zr0.4O2 Catalysts:
Effect of Acidity and Basicity of the Catalysts
•
•
•
Hye Jin Lee Sunyoung Park In Kyu Song
Ji Chul Jung
Received: 15 December 2010 / Accepted: 29 December 2010 / Published online: 14 January 2011
ꢀ Springer Science+Business Media, LLC 2011
Abstract CeXZr1-XO2 catalysts with different cerium
content (X) (X = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0) were
prepared by a sol–gel method for use in the direct synthesis
of dimethyl carbonate from methanol and carbon dioxide.
Among these catalysts, Ce0.6Zr0.4O2 was found to show the
best catalytic performance. In order to enhance the acidity
and basicity of Ce0.6Zr0.4O2 catalyst, Ga2O3 was supported
on Ce0.6Zr0.4O2 (XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and
15)) by an incipient wetness impregnation method with a
variation of Ga2O3 content (X, wt%). Effect of acidity and
basicity of Ga2O3/Ce0.6Zr0.4O2 on the catalytic perfor-
mance in the direct synthesis of dimethyl carbonate was
investigated using NH3-TPD and CO2-TPD experiments.
Experimental results revealed that both acidity and basicity
of the catalysts played a key role in determining the cata-
lytic performance in the direct synthesis of dimethyl car-
bonate from methanol and carbon dioxide. Large acidity
and basicity of the catalyst facilitated the formation of
dimethyl carbonate. The amount of dimethyl carbonate
produced over XGa2O3/Ce0.6Zr0.4O2 catalysts increased
with increasing both acidity and basicity of the catalysts.
Among the catalysts tested, 5Ga2O3/Ce0.6Zr0.4O2, which
retained the largest acidity and basicity, showed the best
catalytic performance in the direct synthesis of dimethyl
carbonate from methanol and carbon dioxide.
Keywords Dimethyl carbonate Á Methanol Á Carbon
dioxide Á Gallium oxide Á Ceria-zirconia
1 Introduction
Dimethyl carbonate (DMC) has attracted much attention as
a green chemical that can replace dimethyl sulfate and
phosgene, which are highly toxic and corrosive methyla-
tion and carbonylation agents [1, 2]. In addition, DMC has
been widely employed as a good polar solvent and a
chemical intermediate in the synthesis of various organic
compounds and functional resins [3, 4]. Due to its high
oxygen content and good blending nature with octane,
DMC can also be used as a fuel additive [4, 5].
Several commercialized processes for the synthesis of
DMC have been reported, including methanolysis of
phosgene [3], oxidative carbonylation of methanol [4],
oxidative carbon monoxide-methyl nitrite processes [5],
and transesterification of alkene carbonate with methanol
[6, 7]. However, these conventional processes involve
many drawbacks in an environmental point of view,
because toxic, flammable, explosive, and corrosive gases
such as phosgene, hydrogen chloride, nitric oxide, and
carbon monoxide are employed in the processes. There-
fore, direct synthesis of DMC from methanol and carbon
dioxide has been recognized as a green chemical process
for the synthesis of DMC [8–12].
H. J. Lee Á S. Park Á I. K. Song
According to the mechanism for the direct synthesis of
DMC from methanol and carbon dioxide [2, 8], methanol is
activated to methyl species and methoxy species on the
acid and base sites of the catalyst, respectively [1, 2].
Methoxy carbonate anion is then formed by the reaction of
methoxy species with carbon dioxide adsorbed on base
sites of the catalyst [2]. Methoxy carbonate anion further
School of Chemical and Biological Engineering,
Institute of Chemical Processes, Seoul National University,
Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea
J. C. Jung (&)
Department of Chemical Engineering, Myongji University,
Yongin 449-728, South Korea
e-mail: jcjung@mju.ac.kr
123

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H. J. Lee et al.
reacts with methyl species on the acid sites of the catalyst
to produce DMC [2, 8]. This implies that both acid and
base sites of the catalyst play an important role in the direct
synthesis of DMC from methanol and carbon dioxide.
Therefore, it is expected that acid–base bifunctional cata-
lysts will show an excellent catalytic activity in this reac-
tion. Various catalysts have been utilized for the direct
synthesis of DMC from methanol and carbon dioxide,
including organometallic compounds [9, 10], metal tetra-
alkoxides [11], potassium carbonate [12], Ni(CH3COO)2
[13], zirconia [8], CeO2–ZrO2 [14, 15], H3PW12O40/ZrO2
[16], and H3PW12O40/CeXTi1-XO2 [17]. Among these
catalysts, CeO2–ZrO2 showed a considerable catalytic
performance in the direct synthesis of DMC from methanol
and carbon dioxide due to its acid–base bifunctional
property [14, 15].
0.6, 0.8, and 1.0), according to the similar method in lit-
erature [23]. A known amount of Ce(NO3)3•6H2O (Sigma-
Aldrich) and ZrO(NO3)•xH2O (Sigma-Aldrich) was dis-
solved in distilled water. A known amount of citric acid
(C6H8O7, Sigma-Aldrich) was separately dissolved in dis-
tilled water. The citric acid solution was then added to the
solution containing cerium and zirconium precursors. After
stirring the mixed solution at 80 ꢁC for 3 h, it was evap-
orated to obtain a gel. The gel was then dried at 100 ꢁC for
24 h. After grinding the dried gel, it was finally calcined at
500 ꢁC for 3 h in an air stream to yield CeXZr1-XO2
catalysts.
Ga2O3/Ce0.6Zr0.4O2 catalysts were prepared by an
incipient wetness impregnation method using an aqueous
solution of Ga(NO3)3ÁxH2O (Sigma-Aldrich) with a varia-
tion of Ga2O3 content. Ga2O3 content in the Ga2O3/
Ce0.6Zr0.4O2 catalysts was adjusted to 1, 5, 10, and 15 wt%.
After drying the impregnated catalysts overnight at 80 ꢁC,
they were calcined at 500 ꢁC for 3 h in an air stream. The
prepared catalysts were denoted as XGa2O3/Ce0.6Zr0.4O2
(X = 1, 5, 10, and 15), where X represented wt% of Ga2O3
in XGa2O3/Ce0.6Zr0.4O2.
It is known that gallium oxide (Ga2O3) retains both acid
and base properties [18, 19]. It has also been reported that
gallium oxide supported on metal oxide can modify the
acidity and basicity of the support [20]. Therefore, many
supported gallium oxide catalysts have been investigated
for various acid–base catalytic reactions [19–22]. To our
best knowledge, however, gallium oxide supported on
metal oxide has never been applied to the direct synthesis
of DMC from methanol and carbon dioxide. Therefore, a
systematic investigation on the supported gallium oxide as
a feasible catalyst for the direct synthesis of DMC would
be worthwhile.
2.2 Catalyst Characterization
Crystalline phases of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
(X = 1, 5, 10, and 15) catalysts were investigated by XRD
measurements (Rigaku, D-MAX2500-PC) using Cu-Ka
˚
In this work, CeXZr1-XO2 catalysts were prepared by a
sol–gel method with a variation of cerium content in order
to find an appropriate support for Ga2O3. Among the
CeXZr1-XO2 catalysts, Ce0.6Zr0.4O2 was found to show the
best catalytic activity in the direct synthesis of DMC from
methanol and carbon dioxide. Based on this result, Ga2O3
was supported on Ce0.6Zr0.4O2 by an incipient wetness
impregnation method with a variation of Ga2O3 content to
enhance the acidity and basicity of Ce0.6Zr0.4O2. The pre-
pared Ga2O3/Ce0.6Zr0.4O2 catalysts were applied to the
direct synthesis of DMC from methanol and carbon diox-
ide. Acidity and basicity of Ga2O3/Ce0.6Zr0.4O2 catalysts
were measured by NH3-TPD and CO2-TPD experiments,
respectively, with an aim of elucidating the effect of acidity
and basicity of the catalysts on the catalytic performance in
this reaction.
radiation (k = 1.54056 A) operated at 50 kV and 100 mA.
Surface areas of the catalysts were measured using a BET
apparatus (Micromeritics, ASAP 2010). Chemical compo-
sitions of the catalysts were determined by ICP-AES
(Shimadz, ICP-1000IV) analyses.
Acidity of the catalysts was measured by NH3-TPD
experiments. Each catalyst (0.2 g) 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 mole-
cules. 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 NH3.
The physisorbed NH3 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 500 ꢁC at a heating rate of 5 ꢁC/min
under a flow of helium (10 mL/min). The desorbed
ammonia was detected using a GC-MSD (Agilent,
5975MSD-6890N GC). Basicity of the catalysts was
measured by CO2-TPD experiments. The experimental
procedures for CO2-TPD were identical to those for NH3-
TPD, except that CO2 instead of NH3 was employed as a
probe molecule.
2 Experimental
2.1 Catalyst Preparation
CeXZr1-XO2 catalysts were prepared by a sol–gel method
with a variation of cerium content (X) (X = 0, 0.2, 0.4, 0.5,
123

Effect of Acidity and Basicity of the Catalysts
533
2.3 Direct Synthesis of DMC from Methanol
and Carbon Dioxide
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Direct synthesis of DMC from methanol and carbon
dioxide was carried out in a stainless steel autoclave reactor
with a volume of 75 mL. Methanol (30 mL) and catalyst
(0.7 g) were charged into the autoclave, and the reactor
was then purged with carbon dioxide. After heating the
reactor to the reaction temperature with constant stirring,
the autoclave was pressurized up to 60 bar using carbon
dioxide. Catalytic reaction was carried out at 170 ꢁC for
3 h. After the reaction, the reactor was cooled to room
temperature and depressurized. Reaction products were
sampled and analyzed using a gas chromatograph (HP 5890
II).
0
0.2
0.4
0.6
0.8
1.0
X in CeXZr1-XO2
3 Results and Discussion
Fig. 1 Catalytic performance of CeXZr1-XO2 (X = 0, 0.2, 0.4, 0.5,
0.6, 0.8, and 1.0) in the direct synthesis of DMC from methanol and
carbon dioxide at 170 ꢁC after a 3 h-catalytic reaction
3.1 Catalytic Performance of CeXZr1-XO2 Catalysts
presented in Fig. 2. CeO2 showed the peaks corresponding
to (111), (200), (220), and (311) planes of cubic fluorite
phase, while ZrO2 exhibited the peaks for tetragonal phase.
Ce0.6Zr0.4O2 catalyst retained a single cubic fluorite phase
without a detectable tetragonal phase, although the char-
acteristic XRD peaks of Ce0.6Zr0.4O2 slightly shifted to
higher angles compared to the peaks for cubic fluorite
phase of CeO2. The shift of XRD peaks was due to the
shrinkage of lattices originated from the replacement of
Ce4? (ionic radius = 0.098 nm) with a smaller Zr4? (ionic
radius = 0.084 nm) in the Ce0.6Zr0.4O2 catalysts [24, 25].
The above results are in good agreement with the previous
reports [26, 27], indicating successful formation of
Ce0.6Zr0.4O2 in this work.
In order to find an appropriate support for Ga2O3 catalyst in
the direct synthesis of DMC from methanol and carbon
dioxide, CeXZr1-XO2 samples were prepared by a sol–gel
method with a variation of cerium content (X) (X = 0, 0.2,
0.4, 0.5, 0.6, 0.8, and 1.0). Figure 1 shows the catalytic
performance of CeXZr1-XO2 catalysts in the direct syn-
thesis of DMC from methanol and carbon dioxide at
170 ꢁC after a 3 h-catalytic reaction. In the catalytic
reaction, CeXZr1-XO2 catalysts were highly selective for
the formation of DMC without any by-products. The
amount of DMC increased with increasing reaction time,
but no significant increase was observed after 3 h. What is
interesting is that the amount of DMC produced over
CeXZr1-XO2 catalysts showed a volcano-shaped curve with
respect to cerium content (X). As mentioned earlier, both
acid and base sites of the catalyst are required for the direct
synthesis of DMC from methanol and carbon dioxide [8].
This implies that acid–base properties of CeXZr1-XO2
catalysts were different depending on cerium content (X),
leading to the different catalytic activity in the direct
synthesis of DMC from methanol and carbon dioxide.
Among the CeXZr1-XO2 catalysts, Ce0.6Zr0.4O2 showed the
best catalytic performance in the reaction. Therefore,
Ce0.6Zr0.4O2 was chosen as an efficient support for further
investigation of Ga2O3/Ce0.6Zr0.4O2 catalysts in the direct
synthesis of DMC from methanol and carbon dioxide.
The formation of XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10,
and 15) catalysts was confirmed by XRD and ICP-AES
analyses. For XRD measurement of Ga2O3, Ga2O3 was
prepared by the thermal treatment of Ga(NO3)3•xH2O
(Sigma-Aldrich) at 500 ꢁC for 3 h in an air stream. The
XRD pattern of Ga2O3 was presented in Fig. 2. It is
noteworthy that no characteristic peaks for Ga2O3 were
found in the XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15)
catalysts. This indicates that Ga2O3 was finely dispersed on
the surface of Ce0.6Zr0.4O2 catalyst. This result is well
consistent with the previous works [28, 29] reporting that
no characteristic XRD peaks for Ga2O3 were detected for
impregnated Ga2O3 samples such as Ga2O3/Al2O3 and
Ga2O3/Nb2O5. Ga2O3 contents (X) in the XGa2O3/
Ce0.6Zr0.4O2 catalysts determined by ICP-AES analyses
were in good agreement with the designed values, although
these are not shown here. This result indicates that
XGa2O3/Ce0.6Zr0.4O2 catalysts were successfully prepared
3.2 Catalyst Characterization
Figure 2 shows the XRD patterns of Ce0.6Zr0.4O2 and
XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15) catalysts. For
comparison, XRD patterns of CeO2 and ZrO2 are also
123

534
H. J. Lee et al.
CeO2 (220) CeO2 (311)
CeO2 (111) CeO2 (200)
C- Cubic-spinel Ga2O3
catalysts showed a better catalytic performance than
Ce0.6Zr0.4O2 catalyst. This indicates that XGa2O3/
Ce0.6Zr0.4O2 catalysts are more efficient than Ce0.6Zr0.4O2
catalyst in the direct synthesis of DMC from methanol and
carbon dioxide. As shown in Fig. 3, the amount of DMC
produced over XGa2O3/Ce0.6Zr0.4O2 catalysts showed a
volcano-shaped curve with respect to Ga2O3 content (X).
Among the catalysts tested, 5Ga2O3/Ce0.6Zr0.4O2 catalyst
showed the best catalytic performance in the direct syn-
thesis of DMC from methanol and carbon dioxide.
T- Tetragonal ZrO2
C
C
C
Ga2O3
15Ga2O3/Ce0.6Zr0.4O2
10Ga2O3/Ce0.6Zr0.4O2
5Ga2O3/Ce0.6Zr0.4O2
1Ga2O3/Ce0.6Zr0.4O2
Ce0.6Zr0.4O2
3.4 Effect of Acidity on the Catalytic Performance
of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
CeO2
ZrO2
T
T
T
T
In order to investigate the effect of acid properties on the
catalytic performance of Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15) catalysts, NH3-TPD
experiments were conducted over the catalysts. Figure 4
shows the NH3-TPD profiles of Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 catalysts. All the catalysts showed a broad
NH3-TPD peak. Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
catalysts exhibited a significant difference in acidity (peak
area) with a variation of Ga2O3 content (X). Ce0.6Zr0.4O2
and XGa2O3/Ce0.6Zr0.4O2 catalysts also showed a slight
difference in acid strength (peak temperature) with a var-
iation of Ga2O3 content (X). We attempted to correlate the
catalytic performance with the acid properties (acidity and
acid strength) of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
catalysts. Unfortunately, acid strength was not directly
correlated with the catalytic performance of Ce0.6Zr0.4O2
and XGa2O3/Ce0.6Zr0.4O2. However, a reliable correlation
between acidity and catalytic activity of the catalysts was
observed. Acidity of Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 catalysts calculated from NH3-TPD peak area
is summarized in Table 1. Acidity of the catalysts
decreased in the order of 5Ga2O3/Ce0.6Zr0.4O2 [ 1Ga2O3/
Ce0.6Zr0.4O2 [10Ga2O3/Ce0.6Zr0.4O2 [15Ga2O3/Ce0.6Zr0.4
O2 [ Ce0.6Zr0.4O2.
10
20
30
40
50
60
70
2 Theta (degree)
Fig. 2 XRD patterns of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
(X = 1, 5, 10, and 15) catalysts
in this work. BET surface areas of XGa2O3/Ce0.6Zr0.4O2
catalysts are summarized in Table 1. BET surface areas of
XGa2O3/Ce0.6Zr0.4O2 catalysts slightly decreased with
increasing Ga2O3 content (X).
3.3 Catalytic Performance of XGa2O3/Ce0.6Zr0.4O2
Catalysts
Figure 3 shows the catalytic performance of Ce0.6Zr0.4O2
and XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15) in the
direct synthesis of DMC from methanol and carbon dioxide
at 170 ꢁC after a 3 h-catalytic reaction, plotted as a func-
tion of Ga2O3 content (X). No by-products were observed
in the reaction over Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
catalysts, indicating that both Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 were highly selective for the formation of
DMC. It is noticeable that all XGa2O3/Ce0.6Zr0.4O2
Table 1 Surface area, acidity, and basicity of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15) catalysts
Catalyst
Surface
area (m2/g)a
Acidity
(lmol-NH3/g-catalyst)b
Basicity
(lmol-CO2/g-catalyst)c
Ce0.6Zr0.4O2
53.8
51.6
50.0
43.5
34.9
85.7
170.5
226.3
143.7
110.7
17.0
50.9
1Ga2O3/Ce0.6Zr0.4O2
5Ga2O3/Ce0.6Zr0.4O2
10Ga2O3/Ce0.6Zr0.4O2
15Ga2O3/Ce0.6Zr0.4O2
a
121.8
45.4
36.7
Calculated by the BET (Brunauer–Emmett–Teller) equation
b
c
Determined by NH3-TPD measurement
Determined by CO2-TPD measurement
123

Effect of Acidity and Basicity of the Catalysts
535
5Ga2O3/Ce0.6Zr0.4O2
2.4
2.0
1.6
1.2
0.8
0.4
2.5
2.0
1.5
1.0
0.5
0
1Ga2O3/Ce0.6Zr0.4O2
10Ga2O3/Ce0.6Zr0.4O2
15Ga2O3/Ce0.6Zr0.4O2
Ce0.6Zr0.4O2
60
90
120
150
180
210
240
0
5
10
15
Acidity ( mol-NH3/g-catalyst)
Content of Ga2O3 in Ga2O3/Ce0.6Zr0.4O2 (wt%)
Fig. 5 Correlation between acidity and catalytic performance of
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15)
Fig. 3 Catalytic performance of Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15) in the direct synthesis of DMC
from methanol and carbon dioxide at 170 ꢁC after a 3 h-catalytic
reaction
indicates that acidity of the catalyst plays an important role
in determining the catalytic performance in this reaction.
Therefore, it is believed that large acidity of the catalyst
was favorable for the activation of methanol to methyl
species, leading to the facile formation of DMC in the
direct synthesis of DMC from methanol and carbon dioxide
[30, 31].
15Ga2O3/Ce0.6Zr0.4O2
10Ga2O3/Ce0.6Zr0.4O2
5Ga2O3/Ce0.6Zr0.4O2
3.5 Effect of Basicity on the Catalytic Performance
of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
According to the mechanism of DMC formation, not only
acid sites but also base sites of the catalyst play an
important role in the direct synthesis of DMC from meth-
anol and carbon dioxide [30, 31]. Therefore, CO2-TPD
experiments were conducted with an aim of investigating
the effect of base properties on the catalytic performance of
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and
15) catalysts. Figure 6 shows the CO2-TPD profiles of
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 catalysts. As simi-
larly observed in the effect of acid properties, no reliable
correlation between base strength (peak temperature) and
catalytic performance of Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 was found, while a good correlation between
basicity (peak area) and catalytic performance of
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 could be estab-
lished. Basicity of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
catalysts calculated from CO2-TPD peak area is listed in
Table 1. Basicity of the catalysts decreased in the order of
5Ga2O3/Ce0.6Zr0.4O2 [ 1Ga2O3/Ce0.6Zr0.4O2 [ 10Ga2O3/
Ce0.6Zr0.4O2 [ 15Ga2O3/Ce0.6Zr0.4O2 [ Ce0.6Zr0.4O2. Inter-
estingly, basicity of XGa2O3/Ce0.6Zr0.4O2 catalysts showed
the same trend as acidity of the catalysts. This implies that
both acidity and basicity of XGa2O3/Ce0.6Zr0.4O2 catalysts
1Ga2O3/Ce0.6Zr0.4O2
Ce0.6Zr0.4O2
0
100
200
300
400
500
Temperature (oC)
Fig. 4 NH3-TPD profiles of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
(X = 1, 5, 10, and 15) catalysts
Figure 5 shows the correlation between acidity and
catalytic performance of Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15). Acidity (NH3-TPD
peak area) was directly correlated with the catalytic per-
formance. The amount of DMC produced over
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 catalysts increased
with increasing acidity of the catalysts. Among the cata-
lysts tested, 5Ga2O3/Ce0.6Zr0.4O2 with the largest acidity
exhibited the best catalytic performance in the direct syn-
thesis of DMC from methanol and carbon dioxide. As
mentioned earlier, methanol is activated to methyl species
on the acid sites for the DMC formation [30, 31]. This
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H. J. Lee et al.
5Ga2O3/Ce0.6Zr0.4O2
2.4
2.0
1.6
1.2
0.8
0.4
15Ga2O3/Ce0.6Zr0.4O2
10Ga2O3/Ce0.6Zr0.4O2
1Ga2O3/Ce0.6Zr0.4O2
5Ga2O3/Ce0.6Zr0.4O2
1Ga2O3/Ce0.6Zr0.4O2
10Ga2O3/Ce0.6Zr0.4O2
15Ga2O3/Ce0.6Zr0.4O2
Ce0.6Zr0.4O2
Ce0.6Zr0.4O2
20
40
60
80
100
120
0
100
200
300
400
500
Basicity ( mol-CO2/g-catalyst)
Temperature (oC)
Fig. 7 Correlation between basicity and catalytic performance of
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15)
Fig. 6 CO2-TPD profiles of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2
(X = 1, 5, 10, and 15) catalysts
reaction. In addition, the amount of DMC produced over
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 catalysts increased
with increasing both acidity and basicity of the catalysts.
Therefore, it is concluded that both acidity and basicity of
the catalyst can serve as crucial factors determining cata-
lytic performance in the direct synthesis of DMC from
methanol and carbon dioxide. Among the catalysts tested,
5Ga2O3/Ce0.6Zr0.4O2, which retained the largest acidity
and basicity, exhibited the best catalytic performance in
this reaction.
were simultaneously enhanced by impregnating Ga2O3 on
Ce0.6Zr0.4O2.
Figure 7 shows the correlation between basicity and
catalytic performance of Ce0.6Zr0.4O2 and XGa2O3/
Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15). The correlation clearly
shows that the catalytic performance was closely related to
the basicity of Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 cat-
alysts. The amount of DMC produced over Ce0.6Zr0.4O2
and XGa2O3/Ce0.6Zr0.4O2 catalysts increased with
increasing basicity of the catalysts. Among the catalysts
tested, 5Ga2O3/Ce0.6Zr0.4O2 with the largest basicity
showed the best catalytic performance in the direct syn-
thesis of DMC from methanol and carbon dioxide. It has
been reported that base sites of the catalyst are required for
the formation methoxy carbonate anion through the reac-
tion of carbon dioxide with methoxy species in the direct
synthesis of DMC from methanol and carbon dioxide [1,
2]. The methoxy carbonate anion formed on the base sites
of the catalyst reacts with methyl species on the acid sites
of the catalyst to produce dimethyl carbonate [16].
Therefore, it is believed that large basicity of the catalyst
can facilitate the formation of DMC from methanol and
carbon dioxide [30, 31].
4 Conclusions
Among various CeXZr1-XO2 samples prepared by a sol–gel
method, Ce0.6Zr0.4O2 was found to be the most favorable
support for Ga2O3. On the basis of this result, Ga2O3 was
supported on Ce0.6Zr0.4O2 by an incipient wetness
impregnation method with a variation of Ga2O3 content (X,
wt%) for use in the direct synthesis of DMC from methanol
and carbon dioxide. XGa2O3/Ce0.6Zr0.4O2 showed a better
catalytic performance than Ce0.6Zr0.4O2 due to the
enhanced acid and base sites provided by Ga2O3. Acid and
base sites of the catalyst were responsible for activation of
methanol to methyl species and methoxy species, respec-
tively. Base sites of the catalyst were also required for the
formation of methoxy carbonate anion through the reaction
of carbon dioxide with methoxy species. Experimental
results revealed that the catalytic performance of
Ce0.6Zr0.4O2 and XGa2O3/Ce0.6Zr0.4O2 catalysts was clo-
sely related to the acidity and basicity of the catalysts.
The amount of DMC increased with increasing both acidity
and basicity of the catalyst. Among the catalysts tested,
As shown in Figs. 5 and 7, the catalytic performance of
XGa2O3/Ce0.6Zr0.4O2 was closely related to both acidity
and basicity of the catalyst in the direct synthesis of DMC
from methanol and carbon dioxide. When considering the
fact that all XGa2O3/Ce0.6Zr0.4O2 (X = 1, 5, 10, and 15)
exhibited a better catalytic performance than Ce0.6Zr0.4O2
(Fig. 3) in the direct synthesis of DMC from methanol and
carbon dioxide, it is thought that acid and base sites pro-
vided by Ga2O3 played an important role in enhancing the
catalytic performance of XGa2O3/Ce0.6Zr0.4O2 in this
123

Effect of Acidity and Basicity of the Catalysts
537
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5Ga2O3/Ce0.6Zr0.4O2 with the largest acidity and basicity
showed the best catalytic performance in the direct syn-
thesis of DMC from methanol and carbon dioxide. It is
concluded that both acidity and basicity of the catalyst
played an important role in determining the catalytic per-
formance in the direct synthesis of DMC from methanol
and carbon dioxide.
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Thernochim Acta 79:117–185
Acknowledgements This work was supported by 2010 research
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