Direct synthesis of dimethyl carbonate from CO2 and methanol over CeO2 catalysts of different morphologies

Article abstract of DOI:10.1007/s12039-016-1094-0

The direct synthesis of dimethyl carbonate (DMC) from carbon dioxide (CO₂) and methanol is an attractive approach towards conversion of the greenhouse gas - CO₂ into value-added chemicals and fuels. Ceria (CeO₂) catalyzes this reaction. But the conversion efficiency of CeO₂ is enhanced when the byproduct water in the reaction medium is separated by employing trapping agents like 2-cyanopyridine (2-CP). In this work, the influence of morphology of CeO₂ on the direct synthesis of DMC in presence of 2-CP is reported. CeO₂ catalysts of cube, rod, spindle and irregular morphology (Ce - C, Ce - R, Ce - S and Ce - N, respectively) were prepared, characterized and studied as catalysts in the said reaction conducted in a batch mode. Among all, Ce - S shows superior catalytic performance with nearly 100 mol% of DMC selectivity. Catalytic activity correlates with the concentration of acid and base sites of medium strength as well as defect sites. Ce - S has an optimum number of these active sites and thereby shows superior catalytic performance. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-016-1094-0

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-016-1094-0
Direct synthesis of dimethyl carbonate from CO2 and methanol
over CeO2 catalysts of different morphologies
UNNIKRISHNAN Pa,b and SRINIVAS DARBHAa,b,∗
aCatalysis and Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune 411 008, India
bAcademy of Scientific and Innovative Research (AcSIR), New Delhi, India
e-mail: d.srinivas@ncl.res.in
MS received 4 January 2016; revised 11 April 2016; accepted 11 April 2016
Abstract. The direct synthesis of dimethyl carbonate (DMC) from carbon dioxide (CO2) and methanol is
an attractive approach towards conversion of the greenhouse gas - CO2 into value-added chemicals and fuels.
Ceria (CeO2) catalyzes this reaction. But the conversion efficiency of CeO2 is enhanced when the byproduct
water in the reaction medium is separated by employing trapping agents like 2-cyanopyridine (2-CP). In this
work, the influence of morphology of CeO2 on the direct synthesis of DMC in presence of 2-CP is reported.
CeO2 catalysts of cube, rod, spindle and irregular morphology (Ce - C, Ce - R, Ce - S and Ce - N, respectively)
were prepared, characterized and studied as catalysts in the said reaction conducted in a batch mode. Among
all, Ce - S shows superior catalytic performance with nearly 100 mol% of DMC selectivity. Catalytic activity
correlates with the concentration of acid and base sites of medium strength as well as defect sites. Ce - S has an
optimum number of these active sites and thereby shows superior catalytic performance.
Keywords. CO2 utilization; dimethyl carbonate; ceria; acid-base catalysis; influence of morphology.
from methanol and CO2 is atom-efficient and attractive.8,9
Zirconia and ceria-based catalysts were most effective
for this reaction.5,10–14 However, the direct synthesis is
low yielding and equilibrium limited. Removal of water
co-generated in the reaction using traps is necessary
to overcome those limitations. Recently, we reported
the application of calcined zirconium phenylphospho-
nate phosphite as a catalyst for the direct synthe-
sis of DMC.15 Molecular sieve (3A) was used as a
water trap. Methanol conversion of 31.8 mol% with
DMC yield of 2.35 g/g-catalyst was achieved. In recent
times, the research interest on the direct synthesis of
DMC is mainly focused on ceria (CeO2)-based cata-
lysts due to their excellent activity as catalyst and cat-
alyst support.16–21 Wang et al.,14 reported morphology
control of CeO2 nanocrystals on the direct synthesis of
DMC. A synergism among the exposed (111) plane,
defect sites, and acid-base sites was proposed to be cru-
cial for getting high DMC formation. Spindle-shaped
CeO2 showed the highest DMC yield followed by
nano-rods, nano-cubes and nano-octahedrons. Spindle-
shaped CeO2 enabled DMC yield of 1.382 mmol/g cat-
alyst (reaction conditions: methanol amount = 15 mL,
CO2 = 5 MPa, reaction temperature = 140◦C, reaction
time = 2 h, catalyst weight = 0.1 g). DMC forms an a-
zeotrope with methanol at atmospheric pressure, which
demands distinct separation steps through extractive
1. Introduction
Climate change due to increasing amount of carbon
dioxide (CO2) emissions into atmosphere has become
a major issue, for which much attention is being paid
globally to control these emissions by way of con-
verting CO2 into fuels or chemicals. The Paris Cli-
mate Conference-2015 (COP21) recommended to limit
the global temperature increase to less than 2◦C above
the present level by 2100.1 This would be possible
only if substantial (40–70%) reduction in CO2 emis-
sions is achieved by different countries by 2050. CO2
is a cheap, non-toxic, non-flammable and abundant C1
feedstock.2–4 An efficient catalyst is needed for its acti-
vation and utilization due to its high thermodynamic
stability and kinetic inertness. Conversion of CO2 to
dimethyl carbonate (DMC) has attracted interest as
DMC is a green reagent/solvent, a raw material for
manufacturing electrolytes used in lithium ion batteries
and a potential gasoline additive with an annual demand
of about 30 million tons.5 DMC is a precursor in
the synthesis of polycarbonates and isocyanates.6,7 The
traditional methods for DMC synthesis (phosgenation
and oxidative carbonylation of methanol) are unsecure.
Among several eco-friendly routes, its direct synthesis
∗For correspondence

Unnikrishnan P and Srinivas Darnha
distillation, liquid–liquid extraction, evaporation, etc. 2. Experimental
In order to overcome such technical hurdles, a process
which provides complete conversion of methanol to
DMC is desired.
2.1 Catalyst preparation
Honda et al.,22 reported, for the first time, that CeO2
along with benzonitrile as water trapping agent pro-
motes the DMC yield even at CO2 pressures as low as
0.1 MPa. The yields of DMC based on methanol and
CO2 were as high as 47% and 70%, respectively. Later
the same group23 reported the use of 2-cyanopyridine
(2-CP) instead of benzonitrile and obtained quantita-
tive conversion of methanol and CO2 with the yield of
DMC being 94% (scheme 1). Recently, Wang et al.,24
reported the influence of morphologies of CeO2 with
different crystal faces (1 1 0), (1 0 0) and (1 1 1) on the
hydrolysis of 2-CP. CeO2-rods with a crystal face of (1
1 0) exhibited better activity than CeO2-cube (1 0 0) and
CeO2-octahedron (1 1 1). Their study did not include
the spindle morphology studied in the present investiga-
tion. Recently, Asahi Kasei Chemicals Corp.25 declared
construction of a validation plant for dialkyl carbonates
to produce diphenyl carbonate using their proprietary
catalyst. In view of the industrial importance of DMC, it
is of interest to know the factors governing the catalytic
activity/selectivity of CeO2 in the direct synthesis route
using 2-CP as a dehydrating agent. In this study, CeO2
catalysts of cube, rod, spindle and irregular shape mor-
phology (Ce - C, Ce - R, Ce - S and Ce - N, respectively)
are prepared, characterized and studied as catalysts in
the said reaction conducted in a batch reactor. The influ-
ence of structure of CeO2 on the catalytic conversion of
CO2 was investigated.
Ceria samples of different morphology were prepared
by non-template hydrothermal synthesis methods as
follows.14,26
2.1a Ceria - spindles (Ce - S): A solution of 1.042 g
of Ce(NO3)3.6H2O and 0.384 g of urea dissolved in
80 mL of distilled water was taken in a 100 mL Teflon-
lined stainless-steel autoclave. The reactor was moun-
ted in a rotating hydrothermal synthesizer (Hiro Co.,
Japan) and heated at 120◦C for 8 h (stirring speed =
30 rpm). The precipitate obtained was separated by fil-
tration and washed with distilled water and ethanol sev-
eral times. After drying at 80◦C for a day, the product
was calcined at 600◦C for 5 h.
2.1b Ceria - cubes (Ce - C): 1.736 g of Ce(NO3)3.
6H2O and 19.2 g of NaOH were dissolved separately in
10 and 70 mL of distilled water, respectively. These two
solutions were mixed while stirring for 15 min at 25◦C
to get a purple-colored slurry which was then trans-
ferred into a 100 mL Teflon-lined stainless autoclave.
The reactor was mounted in a rotating hydrothermal
synthesizer (Hiro Co., Japan) and heated at 180◦C for
24 h (stirring speed = 30 rpm). The precipitate obtained
was separated by filtration and washed with distilled
water and ethanol several times (until the pH of washing
was neutral). After drying at 80◦C for 1 day, the product
was calcined at 600◦C for 5 h.
2.1c Ceria - rods (Ce - R): This sample was pre-
pared following the same procedure as that of Ce - C
except that the synthesis temperature was 100◦C instead
of 180◦C.
Desired Reactions:
2 CH3OH + CO2
CH3OC(O)OCH3 + H2O
DMC
2.1d CeO2– irregular morphology (Ce - N): For
comparison, CeO2 with irregular morphology
(Ce - N) was also prepared. In a typical synthesis,
6.31 g of Ce(NO3)3.6H2O was dissolved in 180 mL of
distilled water and transferred into a triple neck, glass,
round-bottom flask fitted with water-cooled condenser
and placed in an oil bath. The temperature of the oil
bath was maintained at 80◦C. In next step, 0.1 M aque-
ous NaOH solution was added into it drop-wise for 1 h
with constant stirring till the pH was 10. Stirring was
continued for another 3 h. It was then cooled to 25◦C.
The precipitate formed was separated by filtration and
washed several times with distilled water until the pH
of washings was 7. The solid obtained was dried at
80◦C for 1 day and calcined at 600◦C for 5 h.
+ H2O
NH2
N
N
CN
O
2-CP
2-PA
Undesired Reactions:
2-PA
+ CH3OH / DMC
+ NH3 / CH3OC(O)NH2
OMe
N
O
2-MP
CH3OC(O)NH2 + CH3OH
MC
DMC + NH3
Scheme 1. Products in the direct synthesis of DMC from
CO2 and methanol in presence of 2-CP.

Synthesis of dimethyl carbonate from CO2-methanol
standard, respectively. Further, the mixture was stirred
2.2 Characterization techniques
for 10 min to dissolve the solid 2-picolinamide (2-PA)
completely in the liquid portion. The catalyst was sep-
arated out of the liquid portion by centrifugation fol-
lowed by filtration. The liquid product was analyzed
and quantified with a Varian 3400 gas chromatograph
(GC) equipped with a flame ionization detector and CP-
SIL5CB column (60 m long and 0.32 mm i.d.). Stan-
dard mixtures of methanol, DMC, 2-CP, 2-PA, 2-methyl
picolinate (2-MP) and methyl carbamate (MC) were
prepared, calibration plots were drawn and response
factors of the reactants and products were determined.
These were then used in quantifying the products. The
following are the formulae used to describe conversion
and product selectivity.
The X-ray powder diffraction (XRD) patterns of the
CeO2 samples were recorded on a Philips X’pert Pro
diffractometer using Cu-Kα radiation and a proportional
counter detector. The diffraction patterns were recorded
in the 2θ range of 10–90◦ at a scan rate of 4◦/min.
The step size in the measurements was 0.02◦. High
resolution transmission electron microscopic (HRTEM)
images of the samples were collected on a FEI Technai-
F30 instrument with a 300 kV field emission gun. FT-
Raman spectra were recorded on a Horiba JY LabRa-
man HR 800 Micro Raman spectrometer with 632.8 nm
wavelength generated by a He-Ne laser. Specific sur-
face area (SBET) of the samples was determined from
the nitrogen adsorption measurements carried out at
−196◦C using a Quadrasorb SI automated surface area
and pore size analyzer (Quanta Chrome Instrument).
Before N2 adsorption, the samples were evacuated at
200◦C. SBET was determined from the linear part of
the BET equation (p/p◦ = 0.05–0.31). The pore size
distribution was calculated with the Barrett–Joyner–
Halenda (BJH) method. Acidity and basicity of the cat-
alysts were determined by a temperature-programmed
desorption (TPD) technique using ammonia and carbon
dioxide as probe molecules, respectively (Micromerit-
ics Auto Chem 2910 instrument). About 0.1 g of the
catalyst sample was activated at 200◦C under a He gas
flow (40 mL/min). The sample was cooled to 50◦C and
NH3 (or CO2) was adsorbed at a rate of 40 mL/min
for 30 min. Then the temperature was raised to 100◦C
followed by purging with a He gas to remove the
physisorbed probe molecule. After the stabilization of
base line, desorption of probe molecules was followed
in the temperature range 100–900◦C at a ramp rate
of 10◦C/min. The TPD plot was deconvoluted and the
amounts of acid/base sites of different strength were
determined.
mmol of MeOH consumed
% MeOH conversion =
mmol MeOH taken
×100
mmol of 2 − CP consumed
% 2 − CP conversion =
mmol of 2 − CP taken
×100
mmol of DMC formed
% DMC yield =
× 100
× 100
(mmol of MeOH taken/2)
mmol of 2 − PA formed
mmol of 2 − CP taken
% 2 − PA yield =
mmol of DMC formed
% DMC selectivity =
(mmol of MeOH consumed/2)
×100
mmol of 2 − PA formed
% 2 − PA selectivity =
mmol of 2 − CP consumed
×100
2.3 Reaction procedure and product analysis
Catalytic reactions were conducted in a 100 mL
stainless-steel Parr pressure reactor. In a typical reac-
tion, 3.2 g of methanol (100 mmol), 5.2 g of 2-CP
(50 mmol) and 0.1 g of ceria catalyst were taken in the
reactor which was then pressurized with CO2 to 5 MPa.
The temperature of the reactor was raised to a specific
value (120–160◦C) and the reaction was conducted for
a known period of time (1–6 h) while stirring at a speed
of 600 rpm. At the end of the reaction, the reactor was
cooled to 25◦C and the unreacted CO2 was vented out.
Then, 30 mL of ethanol and 0.2 g of nonan-1-ol were
added to the liquid portion as a solvent and internal
mmol of MC formed
% MC selectivity =
× 100
mmol of MeOH consumed
mmol of MP formed
% MP selectivity =
% MP selectivity =
mmol of MeOH consumed
×100 (based on MeOH)
mmol of MP formed
mmol of 2 − CP consumed
×100 (based on 2 − CP)

Unnikrishnan P and Srinivas Darnha
⎛
⎜
⎞
planes with d-spacing of 0.311 and 0.270 nm, respec-
tively (figure 2d). Ceria-spindles (Ce - S) were large in
mmol of unreacted MeOH
⎟
+2 x mmol of DMC formed
⎜
⎟
⎝
⎠ dimension. They had a length of 3–12 μm and width
of 0.8–2.5 μm. The ends of the spindles were sharp
and edges were curved indicating the existence of
defect sites (figure 2e). They had exposed (111) planes
(figure 2f). Ceria with irregular morphology (Ce - N)
+mmol of MC formed
+ mmol of MP formed
MeOH mass balance =
2 − CP mass balance =
mmol of MeOH taken
×100
had a particle size in the range 10–16 nm (Supplemen-
tary Information, figure S1).
⎛
⎞
mmol of unreacted 2 − CP
⎝
⎠
+mmol of 2 − PA formed
+mmol of MP formed
FT-Raman spectrum showed an intense band at
463 cm−1 attributable to the F2g mode of ceria fluorite
phase and weak bands at 250, 600 and 1179 cm−1 cor-
responding to second-order transverse acoustic mode
(2TA), defect-induced mode and longitudinal optical
mode, respectively (figure 3).27,28 The intensity of the
defects peak (at 600 cm−1) was small due to its weaker
sensitivity to visible Raman spectroscopy. The concen-
tration of the defect sites (ratio of the area of Raman
band due to defect-induced mode to the band due to the
F2g mode) was determined and found that Ce - S sample
has higher amount of defect sites (1.1%) than Ce - N
(0.8%), Ce - R (0.5%) and Ce - C (0.2%). The specific
surface area (SBET) of CeO2 samples decreased in order:
Ce - R (105 m2/g) >Ce - N (77 m2/g) >Ce - C (43 m2/g)
>Ce - S (36 m2/g). This variation is in near agreement
with that expected from their particle sizes.
mmol of 2 − CP taken
×100
3. Results and Discussion
3.1 Catalyst characterization
XRD analysis showed characteristic peaks at 28.6, 33.1,
47.6, 56.6, 59.2, 69.6, 76.8, 79.3 and 88.6◦, which could
be indexed to CeO2 with a cubic fluorite structure (unit
cell parameter: a = 0.540 nm; space group of Fm3−m;
figure 1). The average crystallite size (determined from
the full width at half maximum) and crystallinity (deter-
mined from the intensity of XRD peaks) of CeO2 sam-
ples decreased in the order: Ce - C >Ce - S >Ce - N
>Ce - R (table 1). HRTEM revealed that CeO2 sam-
ple with a cubic morphology (Ce - C) contains parti-
cles of 20–140 nm size (figure 2a). Inter planar spacing
(d-spacing) of 0.273 nm confirmed the exposed (100)
planes in the cubic sample. A few exposed (111) planes
were also traced (figure 2b). Ceria nano rods (Ce - R)
have a diameter of 6.8 1 nm and length of 50–100 nm
(figure 2c). They contained exposed (111) and (200)
NH3-TPD showed three overlapping desorption
peaks with peak maximum at 150–200◦C, 320–360◦C
and 430–570◦C, respectively (figure 4). While the first
peak corresponded to weak Lewis acid sites, the second
and third peaks are attributed to medium and strong acid
sites. CO2-TPD also exhibited three overlapping peaks
at 150–160◦C, 310–330◦C and 480–550◦C (figure 4)
attributable to weak, medium and strong basic sites,
respectively. The total acidity of the catalysts decreased
in the order: Ce - C (454 μmol/g) >Ce - N (423 μmol/g)
>Ce - S (285 μmol/g) >Ce - R (252 μmol/g) and
basicity in the order: Ce - N (90 μmol/g) >Ce - S
(85 μmol/g) >Ce - R (83 μmol/g) >Ce - C (65 μmol/g)
(table 1). While Ce - C contained more amount of
strong acid and base sites, the other samples contained
a large amount of medium acid and base sites. The
amount of strong acid sites was nearly the same for Ce
- R, Ce - S and Ce - N. But the amount of strong basic
sites was lower for Ce - R and Ce - S than for Ce - N
and Ce - C. The concentration of medium acid and base
sites decreased in the order: Ce - N >Ce - S >Ce - R
>Ce - C. It should, however, be noted that the position
of the desorption peak of medium and strong acid/base
Ce - N
(422)
(111)
(200)
(311)
(222)
(220)
(331)
(400)
(420)
Ce - C
Ce - R
Ce - S
sites shifted to higher temperature as the morphology
changed from cubes to rods to irregular shape to spin-
dles. Thus, ceria with spindle morphology contained
medium and strong acid/base sites of higher strength
30
40
50
60
70
80
90
2 (degree)
Figure 1. XRD patterns of CeO2 with different morphologies.

Synthesis of dimethyl carbonate from CO2-methanol
Table 1. Textural, acid-base and catalytic properties of CeO2 with different morphologies.
Average
crystallite size
Sample (m2/g) (nm, XRD)
SBET
Acidity (μmol/g; NH3-TPD)a Basicity (μmol/g; CO2-TPD)a
Particle size
(μm; HRTEM)
Weak
Medium
Strong
Weak
Medium
Strong
Ce-N
Ce-C
Ce-R
Ce-S
77
43
105
36
12.0
23.7
11.2
17.6
–
106
85
240
128
138
189
77
241
72
16
12
23
18
51
25
47
53
23
28
13
14
0.02–0.14
0.05–0.1 × 0.007 0.001 42
3–12 × 0.8–2.5 23
73
aWeak (<200◦C), medium (200–400◦C) and strong (>400◦C).
Figure 2. HRTEM images of CeO2 samples: (a & b) Ce - C, (c & d) Ce - R and (e & f) Ce - S.
than those of rods, cubes and irregular morphology >Ce - C (table 2). Methanol and 2-CP conversions were
(figure 4).
63.0 mol% over Ce - S catalyst and DMC and 2-PA
formed with 97 mol% selectivity (table 2). With other
catalysts, the yield of DMC was lower (≤35.4 mol%),
and 2-methylpicolinate (2-MP) and methyl carba-
3.2 Catalytic Activity
Controlled experiments (at 150◦C for 2 h with CO2 of mate (MC) were detected in more amount (≥6.8 and
5 MPa) revealed that the reaction of methanol with CO2 1.4 mol%, respectively). When the amount of the cat-
producing DMC (in presence of 2-CP as a dehydrat- alyst was raised from 0.1 to 0.3 g while keeping the
ing agent) did not occur without a CeO2 catalyst. In other conditions the same, a quantitative conversion of
agreement with the findings of others, the morphology methanol was observed over Ce - S. The influence of
of CeO2 had a marked effect on the catalytic activ- morphology of CeO2 on product distribution (in mmol)
ity. The activity of ceria samples (methanol conver- and mass balance (liquid product) is reported in table 3.
sion) decreased in the order: Ce - S >Ce - N >Ce - R While the mass balance was 100% with Ce - S, it was

Unnikrishnan P and Srinivas Darnha
study reveals that ceria sample of spindle morphology
is highly active and selective.
Figure 5 demonstrates the influence of reaction tem-
perature and reaction time on methanol conversion
and DMC yield over Ce - S catalyst. Conversion of
methanol increased with temperature and remained
unaltered above 150◦C. The yield of DMC reached a
maximum at 150◦C and above that, it decreased. This
behavior is due to decomposition of DMC above 150◦C.
Methanol conversion and DMC yield increased with
reaction time and remained constant after 3 h. The ini-
tial rate of the reaction over ceria-spindles was found to
be 10.6×10−3 mmol.sec−1. This reaction is exothermic
in nature (ꢀH = −27.90 kJ.mol−1).29 Hence, higher
temperature is unfavourable. However, to prevent poi-
soning of the surface active sites with CO2, 2-CP and
by-product water, and also to activate CO2 for the reac-
tion to occur on the solid surface, a temperature of about
150◦C seems optimum.
Ce-S
Ce-R
Ce-C
Ce-N
ex=630 nm
400
200
600
800
1000
1200
Raman shift (cm-1)
Figure 3. FT-Raman spectra of CeO2 samples of different
morphologies.
Methanol conversion and DMC yield increased with
increasing amount of medium type acid as well as base
sites (figure 6). Deviation from linearity is perhaps due
to difference in the strengths of acid/base sites of dif-
ferent ceria catalysts. The desorption occurred at higher
temperature for Ce - S than for the other catalysts indi-
cating that the strength of acid/base sites is higher for
Ce - S. While the earlier report by Wang et al., dis-
closed that the catalytic activity depends on acidity and
only 91% with Ce - R and Ce - C and 86% with Ce - N
catalysts. Conversion of 2-CP was higher than the con-
version of methanol. This suggests that a part of 2-CP
got decomposed. Such decomposition was less for Ce - S
than for other CeO2 catalysts. Gas analysis pointed out
that a part of DMC decomposed into dimethyl ether
over Ce - R, Ce - C and Ce - N. This was not the
case with Ce - S catalyst. In other words, the present
(a)
(b)
343
358
(d)
(c)
329
164
324
437
169
195
150
502
564
517
100
200
300
400
500
600
700
100
700
200
300
400
500
600
700
100
200
300
400
500
600
700
100
200
300
400
500
600
Temperature ( C)
Temperature ( C)
Temperature ( C)
Temperature ( C)
161
(e)
(f)
(g)
317
(h)
333
332
337
151
156
166
483
508
517
554
100
200
300
400
500
600
700 100
200
300
400
500
600
700 100
200
300
400
500
600
700 100
200
300
400
500
600
700
Temperature ( C)
Temperature ( C)
Temperature ( C)
Temperature ( C)
Figure 4. NH3-TPD (top) and CO2-TPD (bottom) profiles of Ce - C (a & e), Ce - R (b & f), Ce - S (c & g) and
Ce - N (d & h).

Synthesis of dimethyl carbonate from CO2-methanol
Table 2. Catalytic activity data CeO2 of different morphologies for the direct synthesis of DMC
Product selectivity
(mol%; based on
Product selectivity
(%; based on 2-CP)
Conversion (mol%)
Yield (mol%)
methanol)
Catalyst
Methanol
2-CP
DMC
2-PA
DMC
MC
2-MP
2-PA
2-MP
Ce-N
Ce-C
Ce-R
Ce-S
51.2
37.3
50.0
63.0
64.6
49.6
61.8
63.0
31.6
31.4
35.4
61.0
44.0
33.2
45.4
60.8
61.7
84.2
70.8
97.0
2.1
1.4
2.2
1.2
6.8
9.4
7.0
2.9
68.1
66.9
73.5
96.5
10.8
14.1
11.3
5.4
Reaction conditions: methanol = 3.2 g (100 mmol), 2-cyanopyridine (2-CP) = 5.2 g (50 mmol), CO2 pressure = 5 MPa,
catalyst = 0.1 g, reaction temperature = 150◦C, reaction time = 2 h.
Table 3. Influence of ceria morphology on the product distribution and mass balance in the direct synthesis of DMC.
Product distribution (mmol)
Mass balance (%)
Catalyst
Methanol
2-CP
DMC
2-PA
MC
2-MP
Based on methanol
Based on 2-CP
Ce-N
Ce- C
Ce-R
Ce-S
48.8
62.7
50.0
40.4
17.7
25.2
19.1
18.5
15.8
15.7
17.7
30.5
22.0
16.6
22.7
30.4
1.1
0.5
1.1
0.7
3.5
3.5
3.5
1.7
85
98
90
86
91
91
104
101
Reaction conditions: Methanol = 3.2 g (100 mmol), 2-cyanopyridine = 5.2 g (50 mmol), CO2 pressure = 5 MPa, catalyst =
0.1 g, reaction temperature = 150◦C, reaction time = 2 h.
70
60
60
50
50
MeOH conversion (mol%)
DMC yield (mmol)
MeOH conversion (mol%)
DMC yield (mmol)
40
30
20
10
40
30
20
120
130
140
150
160
0
1
2
3
4
5
6
Reaction temperature (°C)
Reaction time (h)
Figure 5. Influence of, (left) reaction temperature, and (right) reaction time on DMC yield over Ce - S
catalyst. Reaction conditions: methanol = 3.2 g, 2-CP = 5.2 g, CO2 pressure = 5 MPa and catalyst = 0.1 g
for, (a) t = 2 h, and for (b) T = 150◦C.
basicity, the present study narrows it down specifically
to medium type acid/base site.
Methanol conversion (mol%) should be equal to
2-CP conversion. But, an excess amount of 2-CP got
While the basic sites are responsible for formation of converted on Ce - N, Ce - R and Ce - C catalysts
methoxide ions, the acid sites assist in generating CH+3 (table 2). This is due to a large percentage of strong
ions from methanol. A perfect tuning of the acidic and acid sites present on those catalysts that decompose a
basic sites is crucial for achieving 100% DMC selectiv- part of 2-CP taken in the reaction. DMC yield corre-
ity. Ce - S has a right balance of these active sites and lated with the concentration of defect sites as well. The
thereby shows higher conversion and DMC selectivity. defect sites act as a source to activate CO2. The high
A tentative mechanism for DMC over CeO2 catalysts is defect morphology (Ce - S in the present study) adsorbs
presented in figure 7.
more CO2, which is further activated by the base sites

Unnikrishnan P and Srinivas Darnha
The most stable surface (111) is the active surface
Acidity (medium; mol/g)
140 160 180
for this reaction. Ceria sample with spindle morphology
has a large amount of these exposed facets. Thus, acid-
base sites of medium strength, defect sites, exposed
(111) facet and morphology control the catalytic activ-
ity and selectivity of CeO2 in the direct synthesis of
DMC from methanol and CO2.
120
60
200
60
45
30
15
45
30
15
4. Conclusions
The direct synthesis of DMC from methanol and
CO2 in presence of 2-CP (dehydrating/water trapping
agent) was studied over CeO2 samples of different mor-
phologies. CeO2 with spindle morphology was found
superior to rods, cubes and irregular shape morpholo-
gies. Quantitative conversion of methanol with nearly
100 mol% selectivity for DMC was obtained over
Ce - S catalyst. Methanol conversion and DMC yield
correlates with the concentration of acid/base sites of
medium strength, exposed (111) facets and defect sites.
The highest catalytic activity of Ce - S is attributed
24
32
40
48
56
Basicity (medium; mol/g)
Figure 6. Correlation between catalytic activity (methanol
conversion and DMC yield) and density of acid/base sites
of medium strength of CeO2 catalysts. Reaction conditions:
same as in table 2.
of medium strength present on the catalyst. In addition to its spindle morphology having a right balance of
to acid/base and defect sites, exposed facets may also medium acid-base sites and defect centers. While this
control the catalytic activity.
study confirms some of the earlier findings, it points out
Figure 7. Possible mechanism for DMC synthesis from methanol and CO2 over CeO2 catalysts.

Synthesis of dimethyl carbonate from CO2-methanol
10. Eta V, Mäki-Arvela P, Wärnå J, Salmi T, Mikkola J P
the importance of acid/base sites of medium strength in
the catalytic reaction.
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Supplementary Information (SI)
HRTEM images of Ce - N sample and catalytic activity
data of Ce - S as a function of reaction temperature and
reaction time are available as supporting information on
the website of Journal of Chemical Sciences at www.
ias.ac.in/chemsci.
Acknowledgements
P.U. acknowledges CSIR, New Delhi for the fellowship.
This work forms a part of the Project “TapCoal (CSC
0102)” sponsored by CSIR.
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