ORIGINAL RESEARCH ARTICLE
published: 26 June 2013
doi: 10.3389/fchem.2013.00008

Catalytic conversion of carbon dioxide into dimethylcarbonate using reduced copper-cerium oxide catalysts as
low as 353 K and 1.3 MPa and the reaction mechanism
Seiki Wada , Kazuki Oka , Kentaro Watanabe and Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University, Chiba, Japan

Edited by:
Viswanathan Balasubramanian,
Indian Institute of Technology, India
Reviewed by:
Kannan Srinivasan, CSIR-Central Salt
and Marine Chemicals ResearchInstitute, India
Krishnamurthy K. Ramaswamy,
Indian Institute of Technology, India*Correspondence:
Yasuo Izumi, Department ofChemistry, Graduate School of
Science, Chiba University,
Yayoi 1–33, Inage-ku, Chiba
263–8522, Japane-mail: yizumi@faculty.chiba-u.jp

Synthesis of dimethyl carbonate (DMC) from CO2 and methanol under milder
reaction conditions was performed using reduced cerium oxide catalysts and reduced
copper-promoted Ce oxide catalysts. Although the conversion of methanol was low(0.005–0.11%) for 2 h of reaction, DMC was synthesized as low as 353 K and at
total pressure of as low as 1.3 MPa using reduced Cu–CeO2 catalyst (0.5 wt% of Cu).
The apparent activation energy was 120 kJ mol−1 and the DMC synthesis rates were
proportional to the partial pressure of CO2 . An optimum amount of Cu addition to CeO2
was 0.1 wt% for DMC synthesis under the conditions at 393 K and total pressure of
1.3 MPa for 2 h (conversion of methanol: 0.15%) due to the compromise of two effects of
Cu: the activation of H2 during reduction prior to the kinetic tests and the block (cover) of
the surface active site. The reduction effects in H2 were monitored through the reductionof Ce4+ sites to Ce3+ based on the shoulder peak intensity at 5727 eV in the Ce L3 -edge
X-ray absorption near-edge structure (XANES). The Ce3+ content was 10% for reduced
CeO2 catalyst whereas it increased to 15% for reduced Cu–CeO2 catalyst (0.5 wt% of Cu).
Moreover, the content of reduced Ce3+ sites (10%) associated with the surface O vacancy(defect sites) decreased to 5% under CO2 at 290 K for reduced Cu–CeO2 catalyst (0.1 wt%
of Cu). The adsorption step of CO2 on the defect sites might be the key step in DMC
synthesis and thus the DMC synthesis rate dependence on the partial pressure of CO2
was proportional. Subsequent H atom subtraction steps from methanol at the neighboring
surface Lewis base sites should combine two methoxy species to the adsorbed CO2 toform DMC, water, and restore the surface O vacancy.
Keywords: CO2 , dimethyl carbonate, cerium oxide, partial reduction, X-ray absorption near-edge structure(XANES), oxygen vacancy, hydrogen subtraction, environmental catalyst

INTRODUCTION
Carbon dioxide is one of major green house gases. The conversion
of CO2 has been widely investigated to reduce the atmosphericconcentration of CO2 (Izumi, 2013). In the viewpoint of global
warming, fixation methods of CO2 and/or converted compounds
from CO2 are also critical. Transferring captured CO2 to thebottom of the sea in a supercritical state is partially in practical use, but it incurs huge investment costs. The conversion of
CO2 to dimethyl carbonate (DMC) is attractive because DMC
can be used as an electrolytic solution of lithium ion battery,
methylating reagent, and feedstock for engineering plastics (Ono,
1997).
DMC has been conventionally synthesized starting from
phosgene, carbon monoxide, or oxirane, but these materials
are toxic and/or explosive. From CO2 and methanol/acetals,
DMC was synthesized using homogeneous Sn catalysts at
10–30 MPa and 353–453 K (Sakakura et al., 1998, 1999,
2000; Kalhor et al., 2011) and using homogeneous Ni catalysts at 353 K and 1.0 MPa (Shi et al., 2013). Catalyst separation was improved for DMC synthesis using heterogeneous CeO2 (Yoshida et al., 2006), ZrO2 (Tomishige et al.,www.frontiersin.org

1999), solid solution of ZrO2 and CeO2 (Tomishige et al.,
2001; Zhang et al., 2011b), Ga2 O3 /Ce0.6 Zr0.4 O2 (Lee et al.,2011), Cex Zr0.9–x Y0.1 O2 (Zhang et al., 2011a), SnO2 –ZrO2 /SiO2(Ballivet-Tkatchenko et al., 2011), Co1.5 PW12 O40 (Aouissi et al.,2010), H3 PW12 O40 /Cex Ti1–x O2 (La et al., 2007), Cu–KF/MgSiO(Li and Zhong, 2003), Cu–Ni–diatomite (Chen et al., 2012),
Cu–Ni–graphite (Bian et al., 2009a), Cu–Ni–V2 O5 –active carbon(Bian et al., 2009b), and Cu–Ni–V2 O5 –SiO2 (Wu et al., 2006;
Wang et al., 2007) at 353–453 K and 0.1–60 MPa. The conversionof methanol to DMC was as much as 7.9% for 24 h (Zhang et al.,2011b). In the viewpoint of global environment and the reductionof CO2 , it is desirable to synthesize DMC from CO2 under mild
reaction conditions.
In this context, the conversion of CO2 and methanol to
DMC under milder conditions was studied and the mechanism was investigated by X-ray absorption near-edge structure(XANES). Methanol could be synthesized photocatalytically fromCO2 ((Ahmed et al., 2011, 2012); Morikawa et al. under review).
In future, the DMC synthesis reported in this work could be
combined with photocatalysis to synthesize DMC from CO2 asa single starting material.

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Wada et al.

MATERIALS AND METHODSPREPARATION OF CeO2

Cerium oxide samples were prepared from cerium nitrate hexahydrate (Wako Pure Chemical, >98.0%). It was dissolved in deionized water (<0.06 μS cm−1 ) to make the concentration to 0.2 mol
L−1 . A 5% ammonia aqueous solution (Wako Pure Chemical)
was added to the solution to reach the pH 10. Obtained yellow precipitate was filtered using a polytetrafluoroethene-based
membrane filter (Omnipore JGWP04700, Millipore) with a poresize of 0.2 μm and washed several times with deionized water. The
obtained powder was calcined in air at 673 K for 4 h. Then, thepowder was connected to a vacuum system using rotary and diffusion pumps (10−6 Pa) and the temperature was elevated at a
ramping rate of 5 K min−1 from 290 to 673 K and kept at 673 K
for 1 h.
A part of freshly-prepared CeO2 above was reduced under
25 kPa of hydrogen. The temperature was elevated from 290 to
673 K with the ramping rate of 10 K min−1 and maintained at
673 K for 1 h.
PREPARATION OF Cu–CeO2

3.8–950 mg of copper nitrate trihydrate (Wako Pure Chemical,>99.9%) was dissolved in 10 mL of deionized water. The Cu2+
solution was added to 1.0 g of CeO2 powder prepared in section
Preparation of CeO2 . Then, 25% of ammonia aqueous solution
was added to the suspension until the pH reached 9.5. The mixture was reacted at 290 K for 1 h with magnetically stirred at a rateof 300 rpm. The color of precipitate was yellow, yellow green, anddark brown when the Cu content was 0.1, 1, and 20 wt%, respectively. The precipitate was filtered using JGWP04700 membrane
and washed by several times with deionized water. The obtained
powder was dried at 353 K for 12 h and denoted Cu–CeO2 . The
Cu–CeO2 samples were treated under H2 (25 kPa). The temperature was elevated from 290 to 673 K with the ramping rate of
10 K min−1 , maintained at 673 K for 1 h, and evacuated (10−6 Pa)
at 673 K for 30 min.

Moderate DMC synthesis from CO2(Tsukuba, Japan) on beamline 9C and also in SPring-8 (Sayo,Japan) on beamline 01B1. The X-ray energy was calibrated
at the first intense peak top energy (5731.1 eV) for the Ce
L3 -edge spectrum of CeO2 . The XAFS data were analyzed
with a software package XDAP version 2.2.7 (Vaarkamp et al.,2006).
DMC SYNTHESIS TESTSAn autoclave (Taiatsu Glass Kogyo, inner volume 120 mL; Model
TVS–N2–120) was used for the DMC synthesis tests using CeO2
and Cu–CeO2 catalysts. The inner space of autoclave was purged
with argon gas (>99.998%) at a rate of 300 mL min−1 . 10 mLof dehydrated methanol (Kanto Chemical, 99.8%) and 100 mg
of catalyst were introduced in the autoclave with the flow of Ar,
not in contact with air. Next, CO2 was flowed at a rate of 300 mL
min−1 for 15 min. The outlet valve of reactor was closed andthe interior pressure was increased to 2.0, 1.0, 0.50, 0.10 MPa at
290 K. Then, the temperature of the reactor was elevated from
290 to 393, 373, or 353 K with the ramping rate of 4 K min−1 ,and maintained at the destination temperature for 2–6 h. As acomparison, 10 mL of dehydrated methanol, 50 mg of catalyst,
and 3.6 MPa of CO2 were introduced in the autoclave in similar way at 290 K. Then, the temperature of reactor was elevatedfrom 290 to 403 K with the ramping rate of 4 K min−1 , andmaintained at 403 K for 2 h. The reaction suspension was filtered using a JGWP04700 membrane. The filtrate was analyzed
by gas chromatograph equipped with frame ionization detector(Model GC-18A, Shimadzu) equipped with a capillary column
Ultra ALLOY-5 (Frontier Laboratories; inner diameter 250 μm,
length 30 m). The conversion (%) of methanol to DMC was
defined as
Conversion(%) =2 × molar amount of DMC formed
× 100.
molar amount of methanol introduced

RESULTSCHARACTERIZATIONDMC SYNTHESIS FROM CO2 AND METHANOL

Nitrogen adsorption isotherm measurements were performed at
77 K within the pressure range 1.0–90 kPa in a vacuum system
connected to diffusion and rotary pumps (10−6 Pa) and equippedwith a capacitance manometer (Models CCMT-1000A and GM2001, ULVAC). The Brunauer-Emmett-Teller (BET) surface area
(SBET ) was calculated on the basis of eight-point measurements
between 10 and 46 kPa (P/P0 = 0.10–0.45) on the adsorption
isotherm. The sample was evacuated at 423 K for 90 min before
the measurements.
The electronic state of cerium in catalysts was investigatedby the synchrotron X-ray measurements. The catalyst powder
samples were prepared in vacuum (10−6 Pa) and transferred
directly to a Pyrex glass cell equipped with 50 μm-thick Kapton(Dupont) windows on both sides. The samples in N2 (60 kPa)
or CO2 gas (60 kPa) were sealed with fire and transported tobeamline.
Ce L3 -edge X-ray absorption fine structure (XAFS) spectra
were measured at 290 K in a transmission mode in the Photon
Factory at the High-Energy Accelerator Research Organization

Pretreatment effects in H2

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In the test under the 2.8 MPa CO2 (initial pressure) at 393 K
for 6 h, DMC formation rate using incipient CeO2 catalyst was
0.44 mmol h−1 gcat −1 and that for reduced CeO2 was 0.70 mmol
h−1 gcat −1 (Table 1A). Total initial pressure of CO2 and methanol
was 3.5 MPa at 393 K. By the pretreatment effect in H2 at
673 K, the synthesis rate increased by a factor of 1.6 times. The
conversion (%) of methanol to DMC was improved to 0.33%(Table 1A).
Effects of the Cu addition

The DMC synthesis rate using reduced Cu–CeO2 (0.5 wt% Cu)
was compared to that using reduced CeO2 at 393 K for 2 h. The
rates were 1.8 and 1.5 mmol h−1 gcat −1 , respectively (Table 1B).
By the inclusion of 0.5 wt% of Cu in the catalyst, the rate increased
by a factor of 1.2 times. The conversion of methanol to DMC was
improved to 0.32% (Table 1B).
Next, the effects of Cu addition were investigated by progressively changing the Cu content between 0 and 20 wt% underJune 2013 | Volume 1 | Article 8 | 2

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Condition
T. (K)(wt%)

Reducedb00Reducedd

0.5

0Reduced

Reduced

Reduced

Reduced

Reduced

Reduced

Reduced

f

g

h

i

j

k

l

20

10

5

1

0.5

0.3

0.1

0

0.5

0.1

0.5

Reduced

Reduced

Reduced2

Reduced2

h

n

o

p

Reduced

Reduced

q

r

0.5

0.5

0.5
353

373

393

403

403

393

393

393

393

393

393

393

393

393

393

393

393

393

393

393

1,2 Catalyst charged: 100 mg except for entries o and p (50 mg).

Reducedd(E) REACTION TEMPERATURE EFFECTS

0.5

0.5

Reducedm

0.5

Reducedd(D) REACTION PRESSURE EFFECTS

Reducede(C) Cu EFFECTS AT LOWER PRESSURE

Reducedc(B) Cu EFFECTS AT HIGHER PRESSURE

Incipienta393

Reaction

Cu content(A) PRETREATMENT EFFECTS IN H2

Entry

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

6

6

time (h)Reaction

2.0

2.0

2.0

3.6

3.6

0.10

0.50

1.0

2.0

0.50

0.50

0.50

0.50

0.50

0.50

0.50

0.50

2.0

2.0

2.0

2.0

(MPa)

at 290 K

CO2

2.5

2.6

2.8

5.8

5.8

0.13

0.67

1.4

2.8

0.67

0.67

0.67

0.67

0.67

0.67

0.67

0.67

2.8

2.8

2.8

2.8

T. (MPa)at Reac.

0.18

0.35

0.64

0.84

0.84

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

0.64

2.7

2.9

3.5

6.6

6.6

0.77

1.3

2.0

3.5

1.3

1.3

1.3

1.3

1.3

1.3

1.3

1.3

3.5

3.5

3.5

3.5

Reac. T. (MPa)Reac. T. (MPa)0.64

Total atMethanol at

Initial pressure

Table 1 | Conditions and results of DMC synthesis from methanol and CO2 over Ce oxide and Cu–promoted Ce oxide catalysts1 .

Yield

0.006

0.056

0.40

0.22

0.031

0.24

1.8

1.9

3.1

<0.015<0.003
0.29

0.60

0.79

1.8

0.39

0.19

0.41

0.43

0.60

0.64

0.95

0.41

1.8

1.5

0.70

0.005

0.045

0.32

0.17

0.23

<0.002

0.11

0.14

0.32

0.063

0.031

0.064

0.071

0.11

0.10

0.15

0.071

0.32

0.27

0.33

0.24

methanol (%)

0.44

Conversion toSynthesis rate
(mmol h−1 gcat −1 )

0.13

0.18

0.40

0.078

0.038

0.079

0.088

0.13

0.13

0.19

0.087

0.40

0.33

0.41

0.29

(mmol)

DMC

Wada et al.
Moderate DMC synthesis from CO2

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Wada et al.

Moderate DMC synthesis from CO2

FIGURE 1 | The dependence of DMC synthesis rates on the copper
content in reduced Cu–CeO2 catalysts under total initial pressure of
1.3 MPa at 393 K.

lower initial pressure (0.67 MPa) of CO2 at 393 K. By the inclusion of 0.1 wt% of Cu in Cu–CeO2 catalyst, the DMC synthesis rate increased 2.3-fold higher: from 0.41 mmol h−1 gcat −1(reduced CeO2 ) to 0.95 mmol h−1 gcat −1 (Table 1C; Figure 1).
However, further increase of Cu content between 0.3 and 1 wt%
in Cu–CeO2 catalysts was not effective compared to the test
results for the Cu–CeO2 catalyst, 0.1 wt% of Cu (Table 1C).
When the Cu content was between 1 and 20 wt%, the synthesis rates gradually approached to constant, similar to theone for undoped CeO2 (0.41 mmol h−1 gcat −1 ) (Table 1C;
Figure 1).
Reaction pressure effects

In the reaction tests at 393 K using reduced Cu–CeO2 catalyst(0.5 wt% Cu), partial pressure of CO2 introduced at 290 K wasvaried between 2.0 and 0.10 MPa. The partial (initial) pressureof CO2 increased to between 2.8 and 0.13 MPa at reaction temperature of 393 K (Table 1D). The DMC synthesis rates wereplotted as a function of initial pressure of CO2 and initial totalpressure of CO2 + methanol at 393 K (Figure 2). The DMC synthesis was possible under the total pressure of 1.3 MPa, but theamount of produced DMC was below detection limit (3 μmol)
under the total pressure of 0.77 MPa at 393 K (Table 1D). The
DMC synthesis rates were proportional to partial pressure of CO2(Figure 2).
The reaction pressure effects were also tested under severer
reaction conditions: at 403 K and 6.6 MPa using Cu–CeO2 catalysts (0.1 and 0.5 wt% Cu) (Table 1o,p). The synthesis rates(3.1–1.9 mmol h−1 gcat −1 ) were higher by a factor of 3.3–3.2
times compared to corresponding test results at 393 K and
1.3 MPa (Table 1C,D). Thus, major reason of relatively low DMC
synthesis rates in this paper was mild reaction conditions at
393–353 K and 3.5–1.3 MPa.

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FIGURE 2 | The dependence of DMC synthesis rates on the partialpressure of CO2 and total pressure using Cu–CeO2 catalyst (0.5 wt%
of Cu) at 393 K.

Reaction temperature effects

Further, the reaction temperature was varied between 393 and353 K using reduced Cu–CeO2 catalyst (0.5 wt% Cu) under theCO2 partial pressure of 2.0 MPa introduced at 290 K. The CO2
pressure increased to between 2.8 and 2.5 MPa at reaction temperatures of 393–353 K (Table 1E). Under the total pressure of
2.7 MPa, DMC synthesis was possible as low as 353 K: 0.031 mmol
h−1 gcat −1 (Table 1E; Figure 3). The apparent activation energy
was estimated to 120 kJ mol−1 based on the Arrhenius plot
(Figure 3, inset).
BET SURFACE AREA AND Ce L3 -EDGE XANESThe BET surface area was 78 and 94 m2 gcat −1 for Cu–CeO2
samples consisting of 0.1 and 0.5 wt% Cu, respectively (Table 2).
Ce L3 -edge XANES spectra taken for Ce–based catalysts andalso standard Ce compounds were depicted in Figure 4. Twinpeaks appeared at 5731 and 5738 eV in the XANES spectrum foras-synthesized CeO2 (spectrum a), indicating that valence state of
Ce4+ was predominant (Zhang et al., 2004).
When the CeO2 was reduced under hydrogen at 673 K (spectrum b), a shoulder peak gradually grew at 5727 eV, on the lower
side of peak at 5731 eV. As an intense whiteline peak appeared at5726.7 eV in the spectrum for CeIII (NO2 )3 ·6H2 O (Figure 4A-f),
the shoulder peak for spectrum b suggested the partial reduction of initial Ce4+ to Ce3+ . The partial reduction of CeO2 istraditionally known to promote electron-donating catalysis, e.g.,ammonia synthesis (Izumi et al., 1996; Aika et al., 1997). The
spectrum b was fitted with the spectrum a for fresh CeO2 andspectrum f for Ce(NO2 )3 ·6H2 O by changing the mixing ratioof standard spectra (Izumi and Nagamori, 2000; Izumi et al.,June 2013 | Volume 1 | Article 8 | 4

Wada et al.

Moderate DMC synthesis from CO2

FIGURE 4 | (A) Ce L3 -edge XANES for as-synthesized CeO2 (a), CeO2
reduced at 673 K (b), Cu–CeO2 (0.5 wt% Cu) reduced at 673 K (c),Cu–CeO2 (0.1 wt% Cu) under N2 after reduction at 673 K (d), Cu–CeO2(0.1 wt% Cu) under CO2 after reduction at 673 K (e), andCe(NO3 )3 ·6H2 O (f). (B) Fits to spectra (b) and (c) with the combinationof spectra (a) and (f). Best-fit results were shown by changing themixing ratio of data (a) and (f).
FIGURE 3 | DMC synthesis rates over Cu–CeO2 catalyst (0.5 wt% of Cu)
at 393–353 K. Initial partial pressure of CO2 was 2.0 MPa at 290 K. (Inset)
The associated Arrhenius plot and the fit to the equation.

Table 2 | BET surface area of Cu–promoted Ce oxide catalysts.
Cu content (wt%)

SBET (m2 gcat −1 )

0.1

78

0.5

94

2007). Spectra a and f were used as models of Ce4+ and Ce3+
states, respectively. The goodness of fit was evaluated based on
the residual-factor (Rf )2  sample dataχ(k) − χreference data (k) dk
Rf =


χsample data (k)2 dk
The spectrum b was best fitted with the mixing ratio of
Ce4+ :Ce3+ = 90 : 10 (Figure 4B-b).
The shoulder peak at 5727 eV also appeared in the XANES
spectrum for reduced Cu–CeO2 (0.5 wt% Cu; spectrum c), andthe intensity was greater than that in spectrum b for reduced
CeO2 . The spectrum c was also fitted with the spectrum a (Ce4+ )and spectrum f (Ce3+ ) by changing the mixing ratio. The best
fit was realized with the mixing ratio was Ce4+ :Ce3+ = 85:15(Figure 4B-c).
The Ce L3 -edge XANES spectrum for reduced Cu–CeO2 catalyst (0.1 wt% Cu; spectrum d) was essentially identical with thatfor reduced CeO2 catalyst (spectrum b). The best-fit ratio with themixing standard spectrum component of Ce4+ and Ce3+ was 90
and 10%. However, the shoulder peak at 5727 eV in the spectrum
significantly weakened when 60 kPa of CO2 was introduced to the
reduced Cu–CeO2 (0.1 wt% Cu) at 290 K (spectrum e). The bestfit ratio with the mixing standard spectrum component of Ce4+

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and Ce3+ was 95 and 5%. The decrease of shoulder peak intensity
at 5727 eV suggested re-oxidation of Ce3+ to Ce4+ by the reaction
with CO2 .

DISCUSSIONDMC SYNTHESIS UNDER MILDER CONDITIONSDMC synthesis from CO2 and methanol was reported at a synthesis rate of 1.8–5.1 mmol h−1 gcat −1 using CeO2 at 403 K and
8.7 MPa for 2–4 h (Yoshida et al., 2006). DMC synthesis rate from
CO2 and methanol in this work using reduced CeO2 at 393 K
and 3.5 MPa for 6 h was lower: 0.70 mmol h−1 gcat −1 (Table 1A)
due to lower reaction temperature and lower pressure. Because
the forward reaction reduces the molar amount of materials in
system from three to two and is uphill reaction (Pacheco andMarshall, 1997) (Equation 1), reaction conditions of lower reaction temperature and lower pressure are disadvantageous forthe DMC synthesis reaction. The synthesis rate was enhanced
by a factor of 1.6 times by the pre-reduction in H2 for CeO2
(Table 1A).
CO2 + 2CH3 OH  OC(OCH3 )2 + H2 O
Gr = 51.0 kJmol−1 (373K)(1)

The disadvantage of moderate reaction conditions was compensated by the Cu addition to CeO2 catalysts. At 393 K and 3.5 MPafor 2 h, the DMC synthesis rates increased to 1.8 mmol h−1 gcat −1by the addition of 0.5 wt% of Cu (Table 1B).
The effects of Cu addition to the DMC synthesis rates werecompared at even milder reaction conditions: at 393 K and
1.3 MPa for 2 h (Table 1C). Under the reaction conditions, the0.1 wt% of Cu was most effective and it promoted the synthesis rate by a factor of 2.3 times (Figure 1). One of the plausible
explanations is that the positive effects to induce the Ce4+ site
reduction to facilitate H2 dissociation and spillover on the catalyst

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Wada et al.

surface and negative effects to block (cover) the surface active
sites for DMC synthesis, e.g., the adsorption/activation sites formethanol, compromised to make a synthesis rate maximum at
the Cu amount of 0.1 wt%.
The maximal DMC synthesis rate using Cu–CeO2 catalyst(0.1 wt% Cu) at 393 K and 1.3 MPa was 0.95 mmol h−1 gcat −1(Table 1f), but the rate at 403 K and 6.6 MPa was quite higher(3.1 mmol h−1 gcat −1 ), nearly equivalent to those in literatureusing CeO2 (1.8–5.1 mmol h−1 gcat −1 ) at even severe conditions (403 K and 8.7 MPa) (Yoshida et al., 2006), demonstrating
the effects of pre-reduction and/or the Cu addition to CeO2
found in this work. The SBET values (78–94 m2 gcat −1 ; Table 2)
for Cu–CeO2 catalysts (0.1–0.5 wt% Cu) were also similar to
those for CeO2 catalyst reported (80 m2 gcat −1 ) (Yoshida et al.,2006).
The effects of ZrO2 mixed with CeO2 (Tomishige et al., 2001;
Zhang et al., 2011b) were also interpreted to enhance the redox
chemistry between Ce4+ and Ce3+ . In this sense, the redox of Cu+
and Cu2+ may enhance the redox between Ce4+ and Ce3+ . We
tested Co–CeO2 catalyst under the reaction condition of Table 1d.
The conversion of methanol to DMC was 0.23% (not listed),slightly inferior to Cu–CeO2 catalyst. Furthermore, in our preliminary results, the conversions to DMC using Fe–CeO2 andNi–CeO2 catalysts were nearly equivalent to that using Co–CeO2
catalyst. Thus, the effects of hydrogen activation and/or redox ofadded metal (Fe, Co, Ni, or Cu) to CeO2 may work in similar way:enhancing effects of mixed metal ions and/or activating effects ofhydrogen during pretreatment.
The reactions at lower reaction temperatures were tested(Table 1E). At 2.7 MPa using Cu–CeO2 catalyst (0.5 wt% of Cu),
DMC was formed at as low as 353 K (Table 1E). The temperaturedependence of DMC synthesis rates nicely followed the Arrhenius
equation to give the apparent activation energy: 120 kJ mol−1(Figure 3, inset). Similar range of apparent activation energy(107 kJ mol−1 ) was obtained using homogeneous Sn catalysts inthe temperature range of 357–403 K (Kalhor et al., 2011). The
dependences of DMC synthesis rates on the CO2 pressure andtotal reactant pressure were also investigated at 393 K using Cu–
CeO2 catalyst (0.5 wt% of Cu; Table 1D). DMC was synthesized
as low as 1.3 MPa.
The progress of catalysts to synthesize DMC from CO2 isquite fast, especially under relatively mild conditions (see theIntroduction section). The Cu–CeO2 catalysts in this study
are one of the good catalysts to work at relatively mild conditions. The dependence of synthesis rates on pressure andtemperature (Table 1) was interpreted based on X-ray spectroscopy in next section by monitoring the oxygen defect sites
and Ce3+ .
ACTIVE SITES OF DMC SYNTHESISThe dependence of DMC synthesis on the CO2 pressure (previous
section) was proportional (Figure 2). This fact suggested that the
key reaction step of DMC synthesis depended linearly on the CO2
concentration.
To provide the insight into the surface reaction mechanism,
the electronic state and structure for Ce sites were investigated using Ce L3 -edge XANES. 10–15% of the Ce4+ sites of

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Moderate DMC synthesis from CO2

FIGURE 5 | Proposed reaction mechanism of CO2 conversion to DMC
using partially-reduced Ce oxide surface.

as-prepared CeO2 or Cu–CeO2 (0.5 wt% of Cu) were reduced
to Ce3+ based on the shoulder peak intensity at 5727 eV(Figure 4b,c). Similarly, 10% of the Ce4+ sites of as-prepared
Cu–CeO2 (0.1 wt% of Cu) were reduced to Ce3+ under H2 at
673 K, but a half of the Ce3+ sites were re-oxidized to Ce4+ by
the introduction of CO2 at 290 K (Figure 4d,e). These changes
in the XANES spectra suggested the adsorption of CO2 at thesurface defect sites over the catalyst and the associated, neighboring Ce3+ sites to the defects were re-oxidized to Ce4+ . This
reduction and re-oxidation mechanism was already reported
on CeO2 layers grown over Cu(111) surface (Staudt et al.,2010).
The proposed reaction mechanism was shown in Figure 5.
Based on the dependence of DMC synthesis rates on the CO2
pressure and the change of a shoulder peak at 5727 eV in the Ce
L3 -edge XANES spectra, O vacancy was assumed as defect site
and worked to adsorb CO2 . The population of O vacancy should
increase by the reduction in H2 and/or by the presence of Cu sites
in catalysts. In order to synthesize DMC, surface Lewis base sites
are required to subtract H atom from methanol (Figure 5). If each
H atom was subtracted at the Lewis base site from two methanol
molecules, DMC and water molecules are formed to restore an O
vacancy site.

CONCLUSIONSReduction of CeO2 and Cu–promoted CeO2 catalysts in hydrogen at 673 K was effective to enhance the DMC synthesis from
CO2 and methanol by a factor of 1.6–1.2 times. Added Cu
worked cooperatively with CeO2 catalysts as it accelerated thepartial reduction of Ce4+ sites to Ce3+ . At the same time,
doped Cu sites may block surface active sites. As a compromise,
the DMC synthesis rate was maximal: 0.95 mmol h−1 gcat −1 at
393 K and 1.3 MPa (total pressure) in 2 h when the Cu amount
was 0.1 wt% for reduced Cu–CeO2 catalyst. The DMC synthesis was possible at the reaction temperature as low as 353 K(2.7 MPa) using the reduced Cu–CeO2 catalyst. The apparent
activation energy was calculated to be 120 kJ mol−1 . Based on theJune 2013 | Volume 1 | Article 8 | 6

Wada et al.

Moderate DMC synthesis from CO2

Ce L3 -edge XANES, 10% of Ce sites were reduced to Ce3+
by the reduction in H2 for Cu–CeO2 (0.1 wt% of Cu) while
half of them were re-oxidized to Ce4+ by the introductionof CO2 at 290 K. A linear rate dependence on CO2 pressure
and the re-oxidation in CO2 suggest that the adsorption of
CO2 might be the key step in DMC synthesis. H subtraction from methanol needs to occur at the neighboring sites ofadsorbed CO2 . Two methoxy groups and adsorbed CO2 combine then to form DMC and water and restores surface O vacancy(defect site).

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Frontiers in Chemistry | Green and Environmental Chemistrycommercial or financial relationshipsthat could be construed as a potential
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Received: 15 March 2013; paper pending published: 19 April 2013; accepted:
06 June 2013; published online: 26 June
2013.
Citation: Wada S, Oka K, Watanabe
K and Izumi Y (2013) Catalytic conversion of carbon dioxide into dimethylcarbonate using reduced copper-cerium
oxide catalysts as low as 353 K and
1.3 MPa and the reaction mechanism.

Front. Chem. 1:8. doi: 10.3389/fchem.
2013.00008
This article was submitted to Frontiers inGreen and Environmental Chemistry, aspecialty of Frontiers in Chemistry.
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