Preparation and characterization of H3PW12O40/ZrO2 catalyst for carbonation of methanol into dimethyl carbonate

Article abstract of DOI:10.1007/s11164-018-3383-3

A H₃PW₁₂O₄₀/ZrO₂ catalyst for effective dimethyl carbonate (DMC) formation via methanol carbonation was prepared using the sol–gel method. X-ray photoelectron spectra showed that reactive and dominant (63%) W(VI) species, in WO₃ or H₂WO₄, enhanced the catalytic performances of the supported ZrO₂. The mesoporous structure of H₃PW₁₂O₄₀/ZrO₂ was identified by nitrogen adsorption–desorption isotherms. In particular, partial sintering of catalyst particles in the duration of methanol carbonation caused a decrease in the Brunauer–Emmett–Teller surface area of the catalyst from 39 to 19?m²/g. The strong acidity of H₃PW₁₂O₄₀/ZrO₂ was confirmed by the desorption peak observed at 415?°C in NH₃ temperature-programmed desorption curve. At various reaction temperatures (T = 110, 170, and 220?°C) and CO₂/N₂ volumetric flow rate ratios (CO₂/N₂ = 1/4, 1/7, and 1/9), the calculated catalytic performances showed that the optimal methanol conversion, DMC selectivity, and DMC yield were 4.45, 89.93, and 4.00%, respectively, when T = 170?°C and CO₂/N₂ = 1/7. Furthermore, linear regression of the pseudo-first-order model and Arrhenius equation deduced the optimal rate constant (4.24 × 10^?3?min^?1) and activation energy (E_a = 15.54?kJ/mol) at 170?°C with CO₂/N₂ = 1/7 which were favorable for DMC formation.

Full text of DOI:10.1007/s11164-018-3383-3

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3383-3
Preparation and characterization of H3PW12O40/ZrO2
catalyst for carbonation of methanol into dimethyl
carbonate
Chao Lung Chiang1 Kuen Song Lin1 Ssu-Han Yu1
•
•
Received: 8 September 2016 / Accepted: 20 February 2017
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract A H3PW12O40/ZrO2 catalyst for effective dimethyl carbonate (DMC)
formation via methanol carbonation was prepared using the sol–gel method. X-ray
photoelectron spectra showed that reactive and dominant (63%) W(VI) species, in
WO3 or H2WO4, enhanced the catalytic performances of the supported ZrO2. The
mesoporous structure of H3PW12O40/ZrO2 was identified by nitrogen adsorption–
desorption isotherms. In particular, partial sintering of catalyst particles in the
duration of methanol carbonation caused a decrease in the Brunauer–Emmett–Teller
surface area of the catalyst from 39 to 19 m2/g. The strong acidity of H3PW12O40/
ZrO2 was confirmed by the desorption peak observed at 415 °C in NH3 temperature-
programmed desorption curve. At various reaction temperatures (T = 110, 170, and
220 °C) and CO2/N2 volumetric flow rate ratios (CO2/N2 = 1/4, 1/7, and 1/9), the
calculated catalytic performances showed that the optimal methanol conversion,
DMC selectivity, and DMC yield were 4.45, 89.93, and 4.00%, respectively, when
T = 170 °C and CO2/N2 = 1/7. Furthermore, linear regression of the pseudo-first-
order model and Arrhenius equation deduced the optimal rate constant
(4.24 9 10-3 min-1) and activation energy (Ea = 15.54 kJ/mol) at 170 °C with
CO2/N2 = 1/7 which were favorable for DMC formation.
Keywords Carbon dioxide Á Methanol Á Carbonation Á Dimethyl carbonate Á
H3PW12O40/ZrO2 catalyst Á Greenhouse effect gas
&
Kuen Song Lin
kslin@saturn.yzu.edu.tw
1
Department of Chemical Engineering and Materials Science/Environmental Technology
Research Center, Yuan Ze University, Chungli District, Taoyuan City 32003, Taiwan, ROC
123

C. L. Chiang et al.
Introduction
In recent decades, natural gas, coal, and petroleum have provided more than 85% of
the energy used by industry globally. The growing demand for energy raises CO2
emissions yearly, causing serious global warming and climate change [1]. In order
to reduce CO2 emissions, several solutions have been proposed, including
improving energy utilization efficiency, replacement with renewable energy, CO2
storage/capture, and CO2 conversion [1–3]. For the purpose of CO2 reutilization,
CO2 conversion is the most appropriate solution, since it results in an appreciable
profit for chemical and petrochemical industries [4–7]. In chemical and petrochem-
ical industrial processes, tons of CO2 can be directly converted to valuable
hydrocarbons, such as urea, salicylic acid, dimethyl carbonate (DMC), cyclic
carbonates, and polycarbonates via chemical conversions [8]. Notably, in compar-
ison with other traditional agents used in carbonation, methanation, and esterifi-
cation processes, such as methyl iodide, dimethyl sulfate, phosgene, and
chloromethane, DMC is considered a low-toxicity biodegradable green chemical
[9–11].
DMC has previously been produced from phosgene [12], esters [13, 14], CO2
[15–18], and methanol [19], respectively. However, DMC production by CO2
conversion is exothermic and non-spontaneous with negative enthalpy/entropy, and
a high Gibbs energy, which restrict its application [20]. To remove the limitations of
CO2 conversion, several catalysts have been prepared to raise the reaction entropy
and lower the Gibbs energy. Aymes et al. [21] prepared unsupported SnO2 and ZrO2
with highly effective active sites that were favorable for methanol carbonation. In
addition, ZrO2 can also act as a catalyst for interactions involving CO, H2, CH4,
water, and CO2 due to its flexible lattice structure and excellent redox
properties [22, 23]. Tomishige et al. [15] prepared a ZrO2 catalyst at different
calcination temperatures to carbonate methanol into DMC at 5 MPa and 463 K. The
mechanisms of DMC formation via methanol carbonation on ZrO2 surfaces with
tetragonal and monoclinic structures have been well studied [16]. It reveals that an
increase in Brønsted basicity of hydroxyl groups and Lewis acid/base pairs may
improve the catalytic performance of ZrO2. On this basis, Jiang et al. [17]
synthesized H3PW12O40/ZrO2 using the sol–gel method, which showed nine times
the catalytic performance of ZrO2 due to its raised Brønsted basicity. Moreover, the
catalytic activation of ZrO2 can be improved by adding H3PW12O40 or CexTi1-xO2
(x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0) as a precursor during catalyst preparation
[18, 24]. The H3PW12O40 could also be incorporated on the exfoliated montmo-
rillonite for the production of alcohol, ketone, and oxime from cyclic aromatic
hydrocarbons substrates, such as cyclohexanol [25].
Therefore, the crystal structures, morphologies, chemical compositions, and
catalytic activities of H3PW12O40/ZrO2 prepared via the sol–gel method were
investigated in present study. DMC formation in a fixed-bed reactor at various
reaction temperatures (T = 110, 170, and 220 °C) and CO2/N2 volumetric flow rate
ratios (CO2/N2 = 1/4, 1/7, and 1/9) was analyzed by on-line Fourier-transform
infrared spectroscopy (FTIR) and gas chromatography (GC). Based on the results,
123

Preparation and characterization of H3PW12O40/ZrO2…
the optimal methanol conversion, DMC selectivity, and DMC yield were calculated
and obtained. The rate constant (k) and activation energy (Ea) of methanol
carbonation were evaluated by linear regression of the pseudo-first-order model and
Arrhenius equation, respectively. The dependence of DMC formation on temper-
ature and CO2/N2 volumetric flow rate ratio have also been discussed.
Experimental
Catalyst preparation via sol–gel method
The catalyst precursor was prepared by dissolving ZrOCl2Á8H2O and H3PW12-
O40Á12H2O in a 1:1 molar ratio in ethanol (12.5 mL) at 70 °C with stirring for
complete dissolving, and then cooling slowly to room temperature. High-
concentration NH3OH solution was added dropwise to adjust the pH to 7.0 0.2,
followed by stirring for 30 min. Pure gels were obtained by filtering, drying, and
then washing with hot water until Cl anions were completely removed.
Eventually, H3PW12O40/ZrO2 powder was afforded after calcining at 500 °C in
a flowing airstream for 4 h.
Catalyst characterization
X-ray diffraction (XRD) patterns of the catalyst samples were recorded at a scan
rate of 4°(2h)/min using monochromatic Cu Ka radiation (MXP18; MAC Science)
at 30 kV and 20 mA. The recorded specific peak intensities and 2h values were
further identified using an electronic database system (JCPDS). The morphologies,
microstructures, and particle sizes of fresh and used H3PW12O40/ZrO2 were
investigated by field-emission scanning electron microscopy (FE–SEM; S–4700
Type II; Hitachi). The chemical compositions of fresh and used H3PW12O40/ZrO2
were determined by X-ray photoelectron spectroscopy (XPS; ESCA PHI 1600;
Physical Electronic ) at the excitation energy of Al Ka (1486.6 eV). The C 1s
(284.5 eV) peak was used as a calibration standard for tungsten and zirconium
species (e.g., WO3, H2WO4, WCl2, ZrOCl2, and ZrO2) and their wide-region
spectra. XPS signals of the above species were recorded using a cylindrical mirror
analyzer. Specific surface areas and pore diameters were obtained from nitrogen
adsorption–desorption isotherms (ASAP 2020; Micromeritics) using the Brunauer–
Emmett–Teller (BET) equation. In addition, the acidity of H3PW12O40/ZrO2 was
investigated by NH3 temperature-programmed desorption (TPD; ChemiSorb 2750;
Micromeritics) at 25–700 °C.
Catalytic performance calculation
The catalytic activity of these as-synthesized H3PW12O40/ZrO2 catalysts were eval-
uated in a fixed-bed stainless steel column reactor (CatRea2 v.1.02; JNP Tech.) with
an inner diameter of 10 mm (catalyst loading: 1.0–1.5 g). Operating conditions,
including the temperature/pressure in the fixed-bed reactor, and the flow rates of
123

C. L. Chiang et al.
inlet gases, were controlled by a computer (with instilled LabVIEW software)
connected to the above reactor. Methanol (0.05 mL/min) and CO2/N2 gas mixtures
[volumetric flow rate ratio (CO2/N2) = 1/3, 1/7, and 1/9] were supplied to the fixed-
bed zone at 40.0 mL/min and 50 bar under various reaction temperatures (110, 170,
and 220 °C) for 10 h. FTIR was conducted on a Nicolet Magna-IR 830 spectrometer
using the attenuated total reflectance method; 32 scans were collected to produce a
spectrum, and background noise correction was performed. A GC–TCD (China
Chromatography 9800) instrument connected to the FTIR spectrometer was used to
measure the concentrations of unreacted methanol, DMC, and other byproducts.
Ultimately, methanol conversion and DMC selectivity/yield using as-synthesized
H3PW12O40/ZrO2 were calculated using Eqs. (1)–(3), which have been modified
from related literature [20, 26, 27]:
½MeOHŠ0À½MeOHŠ
Methanol conversion ðCÞ ¼
DMC selectivity ðSÞ ¼
 100ð%Þ
ð1Þ
½MeOHŠ
0
½DMCŠ
½DMCŠ þ ½H2OŠ
 100 ð%Þ
ð2Þ
ð3Þ
DMC yield ðYÞ ¼ C Â S ð%Þ
Kinetic analysis
The experimental equilibrium constant (k) and activation energy (Ea) were
evaluated stepwise using Eqs. (4) and (5). In order to obtain k, we used a fixed
methanol flow rate, allowing the methanol concentration to be considered constant,
and thus simplifying and integrating the consumption rate of CO2 to Eq. (4). It is
called the pseudo-first-order model and is used for the simple analysis of reactant
consumption rate [28, 29]. The activation energy of methanol carbonation was
calculated using the linear form of the Arrhenius equation [Eq. (5)].
½CO2Š
C0
C
0
ln
¼ ln
¼ kðt À t0Þ
ð4Þ
ð5Þ
½CO2Š
ꢀ ꢁ
Ea
1
ln k ¼ À
þ ln A
R
T
Results and discussion
Crystal structures and morphologies of H3PW12O40/ZrO2
The effects of methanol carbonation on the crystal structure of H3PW12O40/ZrO2 are
displayed in Fig. 1. Diffraction peaks of ZrO2 (2h = 30.2°, 34.6° and 50.0°) and
WO3 (2h = 60.0°) were confirmed using the JCPDS database. The existence of
123

Preparation and characterization of H3PW12O40/ZrO2…
(101)
(112)
(110)
(622)
(b)
(a)
20
30
40
50
60
70
80
Fig. 1 XRD patterns and FE–SEM microphotos of a fresh and b used H3PW12O40/ZrO2 for DMC
formation via methanol carbonation
WO3 was attributed to the oxidization of H3PW12O40 on the catalyst surface during
the calcination procedure. The position and intensity of diffraction peaks in XRD
patterns of fresh and used H3PW12O40/ZrO2 catalysts were unchanged. Based on the
morphology of H3PW12O40/ZrO2, the particle size was observed and it increased
from 194 to 270 nm due to the slight sintering of catalyst particles during DMC
formation.
Chemical composition on H3PW12O40/ZrO2 surface
The chemical composition of as-synthesized H3PW12O40/ZrO2, measured by using
XPS, is shown in Fig. 2. The signals for the Zr, W, and O elements in Fig. 2a
indicate their existence on the catalyst surface. For W species, the valences were
identified as W(VI) and W(II), as shown in Fig. 2b. WO3 and H2WO4 were
produced from H3PW12O40Á12H2O, while WCl2 was formed by the combination of
W cations and residual Cl anions. Most importantly, the total integrated peak area
percentage of W(VI) was 66.3%. It infers that it was the dominant species can
induce methanol carbonation. Furthermore, the Zr species were identified as ZrOCl2
and ZrO2, and their integrated peak area percentages were 38.9 and 61.1%,
respectively. The XPS spectra of W and Zr species showed that the percentage of
reactive W species was over 60%, and residual Cl anions lowered the ratio of
reactive W species.
Nitrogen isotherms and NH3-TPD measurement of H3PW12O40/ZrO2
As shown in Fig. 3a, nitrogen isotherms of fresh and used H3PW12O40/ZrO2 are
recognized as Type IV-H3 (mesopore with slit arrangement) according to the
standard isotherms of the International Union of Pure and Applied Chemistry [30].
123

C. L. Chiang et al.
Fig. 2 a Full-scanned wide
region, b W 4f5, and c Zr 3d3
XPS spectra of as-synthesized
H3PW12O40/ZrO2 for DMC
formation via methanol
carbonation
(a)
O
Zr
Zr
O
Zr
W
1400 1200 1000 800
600
400
200
0
Binding Energy (eV)
(b)
Raw
Fitting Curve
W6+ (WO3)
W6+ (H2WO4)
W2+ (WCl2)
39.0%
33.7%
27.3%
37
36
35
34
Binding Energy (eV)
(c)
Raw
Fitting Curve
Zr4+ (ZrOCl2)
Zr2+ (ZrO2)
61.1%
38.9%
184
183
182
181
Binding Energy (eV)
The BET surface area (pore volume) of H3PW12O40/ZrO2 decreased from 39 m2/g
(0.24 cm3/g) to 19 m2/g (0.04 cm3/g), as shown in Table 1. It can be seen that the
mesopore percentage in the catalyst decreased from 89.4 to 82.2% with an
123

Preparation and characterization of H3PW12O40/ZrO2…
(a) 160
Fresh H3PW12O40/ZrO2 (Adsorption)
140
120
100
80
Fresh H3PW12O40/ZrO2 (Desorption)
Used H3PW12O40/ZrO2 (Adsorption)
Used H3PW12O40/ZrO2 (Desorption)
60
40
Type IV-H3
20
0
Type IV-H3
0.8 1.0
0.0
0.2
0.4
0.6
Relative Pressure (P/P0)
(b)
H3PW12O40/ZrO2
415 °C
100
200
300
400
500
600
700
Temperature (°C)
Fig. 3 a Nitrogen isotherms and b NH3-TPD curve of fresh and used H3PW12O40/ZrO2
Table 1 Pore textural properties of as-synthesized H3PW12O40/ZrO2
Sample
(H3PW12O40
ZrO2)
BET
surface
area
(m2/g)
Langmuir
surface
area (m2/
g)
Pore
volumea
(cm3/g)
Saturated nitrogen
adsorbed amount
(cm3/g)—77 K
Pore size distributiona
/
Vmirco
(%)
Vmeso
(%)
Vmarco
(%)
Fresh
Used
39
19
64
30
0.24
0.04
154
28
1.9
89.4
82.2
8.7
3.3
14.5
aAverage pore volume and pore size were measured by the BJH desorption method
123

C. L. Chiang et al.
increasing micropore percentage. This indicated that the catalyst particles were
partially sintered, such that the pore size became smaller for DMC formation via
methanol carbonation. The acid characteristic of the as-synthesized H3PW12O40/
ZrO2 was analyzed by NH3-TPD, as shown in Fig. 3b. Generally, desorption
temperatures of NH3 peaks are proportional to the acidity [31], and the integrated
peak area of NH3 desorption is related to the amount of acidic sites on the catalyst
surface. A single large NH3 desorption peak appeared at 415 °C, indicating a
strongly acidic surface. Thus, the activities of acidic sites on the catalyst surface
favored DMC formation via methanol carbonation.
Catalytic performances of H3PW12O40/ZrO2
During DMC formation via methanol carbonation, the gaseous products were
analyzed by in situ FTIR, as shown in Fig. 4. The intensities of specific absorption
peaks for DMC (1705, 687 cm-1), methanol (2974, 1053 cm-1), and CO2 (3730,
2345 cm-1) increased markedly with reaction time, suggesting the formation of
DMC. Especially, the absorption peak of water was undetected in the FTIR spectra
because of the lower feeding rate of methanol. This implied that DMC formation
may occur continuously toward the positive direction. The DMC yield was likely to
exceed the equilibrium conversion of methanol, which was attributed to the
effective prevention of water formation. Apart from the kinds of products, the
effects of reaction temperature and CO2/N2 volumetric flow rate ratio on methanol
conversion, DMC selectivity, and DMC yield were also studied. As shown in
Fig. 5a–c, the optimal temperature and CO2/N2 volumetric flow rate ratio for
methanol conversion were 170 °C and 1/7, respectively, which gave the highest
methanol conversion (4.45%). At CO2/N2 = 1/4 with a fixed methanol flow rate, the
60 min
300 min
540 min
120 min
360 min
600 min
180 min
420 min
240 min
480 min
3OH
2
3OH
2
4000
3500
3000
2500
2000
1500
1000
500
(cm-1)
Fig. 4 Online FTIR spectra during DMC formation by using H3PW12O40/ZrO2 for 10 h
123

Preparation and characterization of H3PW12O40/ZrO2…
5.0
(a)
110 °
170 °
220 °
4.0
3.0
2.0
1.0
0.0
2/N2=1/4
500 600
0
100
200
300
400
(min)
(b)
5.0
4.0
3.0
2.0
1.0
0.0
110
170
220
°
°
°
2/N2=1/7
500 600
0
100
200
300
400
(min)
5.0
4.0
3.0
2.0
1.0
0.0
(c)
110 °
170 °
220 °
2/N2=1/9
500 600
0
100
200
300
400
(min)
Fig. 5 Methanol conversions at various temperatures in CO2/N2 volumetric flow rate ratios of a 1/4,
b 1/7, and c 1/9, for DMC formation via methanol carbonation with H3PW12O40/ZrO2
123

C. L. Chiang et al.
methanol conversion became stable after 420 min (Fig. 5a), revealing that the
methanol had been totally consumed as a limiting reagent (Eq. 6).
2CH3OH þ CO2 ! ðCH3OÞ2CO þ H2O
ð6Þ
At the ratio of CO2/N2 = 1/9, the CO2 concentration was too low (limiting agent)
and was insufficient for DMC formation. Therefore, it may reduce the final
methanol conversion. In similar, the dependence on temperature, CO2/N2 ratio, and
DMC selectivity is displayed in Fig. 6a–c. For example, the optimal temperature
and CO2/N2 volumetric flow rate ratio for DMC selectivity were 170 °C and 1/7,
respectively, giving the highest DMC selectivity (89.93%). It shows that most of the
methanol feedstock was converted to DMC. Finally, DMC yields at various tem-
peratures and CO2/N2 volumetric flow rate ratios (Fig. 7a–c) were consistent with
the catalytic performances in methanol conversion and DMC selectivity, showing
that the highest DMC yield (4.00%) was obtained at 170 °C and CO2/N2 = 1/7.
Based on these calculations, it is likely that the reaction temperature and CO2/N2
volumetric flow rate ratio were dominant factors in the catalytic performance of
H3PW12O40/ZrO2. Furthermore, DMC formation was seen to occur in the positive
direction at a medium CO2/N2 volumetric flow rate ratio (1/7), thus increasing the
final catalytic performance according to Le Chatelier’s principle [28, 29, 32, 33].
However, variety tendencies of these catalytic performances were confirmed that
corresponds to the results of kinetic analyses.
Kinetics analysis of DMC formation via methanol carbonation
According to Eq. (4), the slope of the linear regression lines increased with the rate
constant (k) of methanol carbonation, as shown in Fig. 8a–c and Table 2. At various
CO2/N2 volumetric flow rate ratios, it is apparent that the slope of linear regression
lines at 170 °C was larger than those at 110 or 220 °C. The results of linear
regression for the pseudo-first-order model revealed that the rate constant (k) at
170 °C was higher than those at 110 or 220 °C. Notably, CO2/N2 volumetric flow
rate ratios also affected the k value. In the duration of DMC formation, a relative
high CO2 concentration led to methanol carbonation occurring in the positive
direction until an equilibrium state was reached. Afterwards, carbon from the
reduction of CO2 was deposited on the catalyst surface, causing catalyst
deactivation. For instance, the rate constant at 110 °C using CO2/N2 = 1/7 was
3.25 9 10-3 min-1, which is larger than that for CO2/N2 = 1/4 (k = 3.03 9 10-3
min-1) and 1/9 (k = 2.51 9 10-3 min-1), as shown in Table 2. The linear
regression data of the pseudo-first-order model for the rate constant corresponded
with the previously calculated catalytic performances. Similarly, the slope of linear
regression of Arrhenius equation also increased with the activation energy (Ea) of
methanol carbonation, as shown in Fig. 8d–f and Table 2. Table 2 displays that the
activation energy in CO2/N2 = 1/7 (Ea = 15.54 kJ/mol) was lower than those in
CO2/N2 = 1/4 (Ea = 20.56 kJ/mol) and 1/9 (Ea = 28.17 kJ/mol). This indicated that
the optimal reaction temperature and CO2/N2 volumetric flow rate ratio for DMC
formation were 170 °C and 1/7, respectively, giving optimal k (4.24 9 10-3 min-1
)
123

Preparation and characterization of H3PW12O40/ZrO2…
(a) 70
110 °
170 °
220 °
60
50
40
30
20
2/N2=1/4
500 600
0
100
200
200
200
300
300
300
400
(min)
100
90
80
70
60
50
(b)
110 °
170 °
220 °
2/N2=1/7
0
100
400
500
600
(min)
80
70
60
50
40
(c)
110 °
170 °
220
°
2/N2=1/9
0
100
400
500
600
(min)
Fig. 6 DMC selectivities at various temperatures in CO2/N2 volumetric flow rate ratios of a 1/4, b 1/7,
and c 1/9 for DMC formation via methanol carbonation with H3PW12O40/ZrO2
123

C. L. Chiang et al.
(a) 3.0
2.0
110
170
220
°
°
°
1.0
2/N2=1/4
500 600
0.0
0
100
200
300
400
(min)
5.0
4.0
3.0
2.0
1.0
0.0
(b)
110
170
220
°
°
°
2/N2=1/7
500 600
0
100
200
300
400
(min)
(c)
4.0
3.0
2.0
1.0
0.0
110
170
220
°
°
°
2/N2=1/9
600
0
100
200
300
400
500
(min)
Fig. 7 DMC yields at various temperatures in CO2/N2 volumetric flow rate ratios of a 1/4, b 1/7, and
c 1/9 for DMC formation via methanol carbonation with H3PW12O40/ZrO2
123

Preparation and characterization of H3PW12O40/ZrO2…
3.5
(a)
(d)-5.3
110 °
170 °
220 °
(
110
°
°
°
)
)
)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(170
(220
-5.4
-5.5
-5.6
-5.7
-5.8
-5.9
-6.0
2/N2=1/4
2/N2=1/4
0
100
200
300
400
500
600
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
(min)
1/T × 103 (1/K)
-5.6
-5.7
-5.8
-5.9
-6.0
-6.1
-6.2
-6.3
(e)
2.5
(b)
110
(
(
(
110
170
220
°
°
°
)
)
)
°
°
°
170
220
2.0
1.5
1.0
0.5
0.0
2/N2=1/7
2/N2=1/7
500 600
0
100
200
300
400
(min)
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
1/T × 103 (1/K)
-5.4
-5.5
-5.6
-5.7
-5.8
-5.9
-6.0
-6.1
2.5
(f)
(c)
110
170
220
(170
)
)
)
°
°
°
°
°
°
(110
2.0
1.5
1.0
0.5
0.0
(220
2/N2=1/9
2/N2=1/9
0
100
200
300
400
500
600
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
1/T × 103 (1/K)
(min)
Fig. 8 Linear regression of a–c pseudo–first-order reaction and d–f Arrhenius equations at various
temperatures in CO2/N2 volumetric flow rate ratios of 1/4, 1/7, and 1/9, for DMC formation via methanol
carbonation with H3PW12O40/ZrO2
and Ea (15.54 kJ/mol) values. In comparison of these linear regressions and
catalytic performances, it implies that DMC formation via methanol carbonation
was preferred at a medium CO2/N2 volumetric flow rate ratio (1/7) and reaction
temperature (170°C). At these reaction conditions, it allows the reaction to readily
proceed in the positive direction until equilibrium was reached. After that, the
catalyst particles were slightly sintered until the reactive W species was totally
poisoned, causing the rate constant to decrease with larger Ea. In summary, the
catalytic performances of H3PW12O40/ZrO2 at reaction temperature (110°C) and
CO2/N2 feeding ratio (1/9) were the lowest one due to the lower CO2 concentration
123

C. L. Chiang et al.
Table 2 Kinetic analysis for DMC formation using H3PW12O40/ZrO2 at various temperatures with CO2/
N2 volumetric flow rate ratios of 1/4, 1/7, and 1/9
CO2/N2
volumetric
flow rate ratio
Pseudo-first-order model
Arrhenius equation
Temp (°C)
Rate constant
(k 3 103, min-1
R2 value
Activation energy
(Ea, kJ/mol)
R2 value
)
1/4
1/7
1/9
110
170
220
110
170
220
110
170
220
3.03
3.49
1.98
3.25
4.24
2.38
2.51
2.90
1.97
0.995
0.999
0.995
0.964
0.974
0.970
0.999
0.999
0.997
20.56
15.54
28.17
0.997
0.999
0.999
and insufficient heat supply. On the other hand, the high reaction temperature
(220°C) and CO2/N2 feeding ratio (1/4) might cause the sintering of H3PW12O40/
ZrO2 catalyst particles and the deposition of carbon over the surface, thus reducing
its catalytic performances. The slight sintering of catalyst particles can also be seen
by FE-SEM microphotos in Fig. 1. According to some related literature [34–37], the
carbon deposition had the possibility to be removed by low-temperature burning
with a flowing O2 or O3 stream. The deposited carbon should be oxidized to CO2
following Eqs. (7) and (8). This method may provide the possible solution to
regenerate the used catalyst by removing the deposited carbon.
C þ O2 ! CO2
ð7Þ
ð8Þ
3C þ 2O3 ! 3CO2
Conclusions
The existence of reactive W species in H3PW12O40/ZrO2 prepared via the sol–gel
method was well characterized by XRD and XPS. The acidic strength of
H3PW12O40/ZrO2 was also shown by NH3-TPD measurement. Additionally,
DMC formation was demonstrated and confirmed by its specific absorption peak
at 1705 cm-1 in online FTIR spectra for 1–10 h. In particular, the formation of
water was effectively prevented by minimizing the feeding rate of methanol, which
could enhance the DMC yield. Optimal reaction temperature and CO2/N2
volumetric flow rate ratio for DMC formation via methanol carbonation were
170 °C and 1/7, respectively, giving the highest methanol conversion (4.45%),
selectivity (89.93%), and yield (4.00%) of DMC. The optimal rate constant
(k = 4.24 9 10-3 min-1) and activation energy (Ea = 15.54 kJ/mol), evaluated
from linear regression of the pseudo-first-order model and Arrhenius equation,
respectively, were also obtained at 170 °C and CO2/N2 = 1/7. These analytical
123

Preparation and characterization of H3PW12O40/ZrO2…
results of linear regression were in agreement with the practical catalytic
performance. In summary, H3PW12O40/ZrO2 was demonstrated to be an effective
catalyst for DMC formation via methanol carbonation. It has the possibility to
extend the application of methanol for CO2 reutilization and global
warming reduction.
Acknowledgements The financial support of the Ministry of Science and Technology (MOST), Taiwan
(MOST 105–3113–E–008–003) is gratefully acknowledged. We also thank Prof. Y. W. Yang, Dr. J. F.
Lee, and M. L. Lu of the National Synchrotron Radiation Research Center (NSRRC) for their helps in the
EXAFS experiments.
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