Coupling of CH 3 OH and CO 2 with 2-cyanopyridine for enhanced yields of dimethyl carbonate over ZnO – CeO 2 catalyst

Article abstract of DOI:10.1007/s12039-019-1651-4

Abstract: The present work is aimed to produce dimethyl carbonate by coupling of CH ₃OH and CO ₂ with 2-cyanopyridine over ZnO–CeO ₂ catalysts prepared by co-precipitation method. These catalysts were characterized by XRD, TEM, UV-Vis DRS, BET surface area, CO ₂ and NH ₃-TPD techniques and applied for the titled reaction. Among the investigated catalysts 10ZnO–90CeO ₂ catalyst with CeO ₂ crystallite size 8.0?nm exhibited 96% conversion of methanol with 99% selectivity to dimethyl carbonate. The superior catalytic activity is a unified effect of crystalline size of CeO ₂ and presence of an optimum number of acidic and basic sites. This protocol offers enhanced conversion of methanol with the simultaneous conversion of 2-cyanopyridine into 2-picolinamide by removing water molecules formed in the reaction. Graphic Abstract: Incorporation of ZnO with CeO ₂ enhanced the number of active sites, i.e., acidic and basic sites due to synergetic effect between ZnO and CeO ₂. The role of 2-cyano pyridine is to act as a dehydrating agent for the removal of H ₂O. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-019-1651-4

J. Chem. Sci.
(2019) 131:86
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-019-1651-4
REGULAR ARTICLE
Coupling of CH3OH and CO2 with 2-cyanopyridine for enhanced
yields of dimethyl carbonate over ZnO–CeO2 catalyst
PRATHAP CHALLAa,b, VENKATA RAO Mb, NAGAIAH Pb, NAGU Aa,b, DAVID RAJU Ba,b and
RAMA RAO K Sa,b,∗
aCSIR- Academy of Scientific and Innovative Research (CSIR-AcSIR), New Delhi, India
bCatalysis and Fine Chemicals Department, CSIR-Indian Institute of Chemical Technology, Hyderabad,
Telangana 500 007, India
E-mail: ksramaraoiict@gmail.com
MS received 2 March 2019; revised 20 May 2019; accepted 21 May 2019
Abstract. The present work is aimed to produce dimethyl carbonate by coupling of CH3OH and CO2
with 2-cyanopyridine over ZnO–CeO2 catalysts prepared by co-precipitation method. These catalysts were
characterized by XRD, TEM, UV-Vis DRS, BET surface area, CO2 and NH3-TPD techniques and applied for
the titled reaction. Among the investigated catalysts 10ZnO–90CeO2 catalyst with CeO2 crystallite size 8.0nm
exhibited 96% conversion of methanol with 99% selectivity to dimethyl carbonate. The superior catalytic activity
is a unified effect of crystalline size of CeO2 and presence of an optimum number of acidic and basic sites.
This protocol offers enhanced conversion of methanol with the simultaneous conversion of 2-cyanopyridine into
2-picolinamide by removing water molecules formed in the reaction.
Keywords. Dimethyl carbonate; methanol; carbon dioxide; 2-cyano pyridine; acid-base property.
1. Introduction
catalyst (Scheme 1) has been represented as a green
chemical process.6 Because of the thermodynamic
constraints, the reaction between methanol and CO2
results in low yields of DMC. However, higher pressure
and instant removal of water produced by employing
suitable dehydrating agents can shift the equilibrium
towards products side.7 However CO2 is kinetically
inert and thermodynamically stable. Hence, CO2 activa-
tion is the key problem in its conversion to DMC.8 The
reaction between CH3OH and CO2 is slightly exother-
mic, with the heat of reaction of 23 kJ/mol. Hence
to reach a higher amount of DMC yields, a lower
reaction temperature is preferable. The low yields of
DMC due to the generation of H2O as a by-product.9
Currently, CeO2, ZrO2,10 CeO2-ZrO2,11 K2CO3,12 Cu-
Ni/SBA-15, Nb2O5/CeO2, Cu-Ni/graphite,13 metaltetra
Dimethyl carbonate (DMC) has been considered as a
green chemical because it is projected as the substituent
for toxic and corrosive chemicals like dimethyl sul-
fate and phosgene which are used as carbonylation and
methylation agents respectively in organic synthesis.1
Apart from that, DMC is a good solvent and a fuel addi-
tive. Because of its high dielectric constant, DMC is
used as an electrolyte in lithium batteries.2 The exis-
tenceofhighamountofoxygeninDMCanditslowtoxic
and biodegradable nature makes it as a suitable replace-
ment of methyl tert.butyl ether (MTBE), a gasoline
oxygenate. It also acts as a promising octane booster.3,4
Among the greenhouse gases, CO2 is the main con-
tributor. So, the utilization of CO2 is very important.
Availability of CO2 in surplus quantity at cheap cost,
hence it can be utilized as a precursor for synthe-
sis of various chemicals and chemical intermediates.5
Dimethyl carbonate (DMC) is one such an important
chemical derived from CO2.
14
alkoxides, H3PW12O40/ZrO2 are some of the major
catalyst compositions for DMC synthesis. Among the
heterogeneous catalysts, ZrO2-based and CeO2-based
catalysts exhibit highest activities. Ki et al., reported
that CeO2-ZnO catalysts are active in the synthe-
sis of DMC from methanol and CO2.6 However, the
conversion of methanol in their study is limited to
1% only and also there are no results regarding the
There are so many routes to synthesize dimethyl
carbonate. Among these, the production of DMC by
combining methanol and carbon dioxide on a proper
*
For correspondence
0123456789().: V,-vol

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influence of reaction temperature, pressure on the syn- 2. Experimental
thesis of DMC from methanol and CO2. Since the syn-
2.1 Catalyst preparation
thesis of DMC from methanol and CO2 is equilibrium
controlled reaction, the removal of water is pre-requisite
for the enhanced yield of DMC. Under pressurized reac-
tion conditions, the mechanical removal of water is so
difficult that the addition of dehydrating agent which
doesn’t disturb the targeted reaction is desirable. In this
context, the addition of 2-cyanopyridine to the reaction
would be an added advantage where 2-cyanopyridine
converted to 2-picolinamide (an industrially interested
ZnO-CeO2 mixed oxide catalysts were prepared by the
co-precipitation method. In a typical procedure, an aque-
ous solution of (NH4)2Ce (NO3)6 and Zn (NO3)2 was
co-precipitated with (5%) aqueous NH4OH solution at pH
maintained at around 9–9.5. The resulting precipitate after
ageing for 24 h was subsequently filtered with deionized
water, washed several times until the pH of the filtrate become
◦
neutral. The resulting solid was dried in an oven at 100 C
chemical) by consuming the water molecules. Here for 12 h followed by calcination at 773 K for 5 h in
static air. In the same manner, pure CeO2 and ZnO were
prepared by precipitation method. Finally, obtained cata-
lysts named as (a) pure CeO2 (b) 10wt%ZnO-90wt%CeO2
(c) 30wt%ZnO-70wt%CeO2 (d) 50wt%ZnO- 50wt%CeO2
(e) 70wt%ZnO-30wt%CeO2 (f) 90wt%ZnO-10wt%CeO2 (g)
Pure ZnO.
some extent of methanol is reacted with 2-picolinamide
to form methyl picolinate as described in Scheme 4.
Withthesepreliminaryunderstandings, thepresentwork
has been undertaken to (i) apply ZnO-CeO2 mixed oxide
catalysts for the synthesis of dimethyl carbonate with
high yield by coupling methanol and CO2 reaction with
the hydration of 2-cyanopyridine (Schemes 1, 2, 3) and
optimizing reaction parameters for the best yield (ii) to
establish a correlation between the catalytic activity and
the characterization results.
2.2 Catalyst characterization
The X-ray powder diffraction (XRD) patterns of the CeO2
samples were recorded on Ultima-IV X-ray diffractometer
(M/s. Rigaku Corporation, Japan) with Cu Ka radiation oper-
ated at 40 kV and 20 mA. The diffraction patterns were
The synthesis of DMC from methanol and CO2
requires a catalyst with acidic and basic sites. The mech-
anism for the synthesis of DMC from CH3OH and CO2 recorded in the 2θ range of 10–80◦ at a scan rate of 4◦/min.
over the acid-base catalyst is shown in Scheme 5.15
The average crystallite size (D) was calculated using the
Scherrer equation:
Both the basic sites and acidic sites play a key role
in the activation of methanol to methoxy species and
methyl species respectively. Basic sites adsorb CO2 and
adsorbed CO2 attacks on methoxy group to get methoxy
carbonate anion. The reaction between methoxy carbon-
ate anion with methyl species results in the formation
of DMC.
D = k / β cos
λ
θ
Where λ is X-ray wavelength, K constant of proportional-
ity taken as 0.94, β is the full width at half maximum of
the maximum intensity peak and θ is the diffraction angle.
The BET surface areas of all catalysts were determined by
nitrogen adsorption on an Autosorb Instrument (M/s. Quan-
tachrome, USA). UV-Vis diffused reflectance spectra was
recorded on a UV Win Lab spectrometer (M/s. Perkin Elmer,
Germany) with an integrating sphere reflectance accessory in
a range of 200–800 nm. The acidic and basic strengths of
the samples were obtained by subjecting the catalyst sample
Scheme 1. Direct synthesis of DMC from CO2 and
methanol.
Scheme 4. Reaction of methanol with 2-picolinamide to
form methyl picolinate.
Scheme 2. Hydration of 2-cyanopyridine to
2-picolinamide.
Scheme 3. Coupling of methanol and 2-cyanopyridine.

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86
Scheme 5. Mechanism for the synthesis of DMC from CH3OH and CO2 over acid-base catalyst.16
to the temperature-programmed desorption of NH3 and CO2
respectively on an Auto chem 2010 instrument (M/s.
Micromeritics Instruments, USA). A sample of 0.25 g was
placed in a U shaped quartz tube. The catalyst was activated
bydryingunder heliumflow(20mL/min) followedbyadsorp-
tion of CO2 or NH3 gas at 373 K for 30 min. The desorption
of CO2 or NH3 takes place from 373 K to 873 K at a heating
rate of 10 K/min.
2.3 Catalytic activity
The synthesis of DMC from CO2 and CH3OH was conducted
in a pressure autoclave reactor over ZnO-CeO2 catalyst. The
reaction was coupled with hydration of 2-cyanopyridine. A
stoichiometric amount of methanol (100 mmol), CO2 (70
mmol), 2-cyanopyridine (50 mmol) and 0.6 g of ZnO-CeO2
catalyst were charged into the reactor. The reactor was purged
with CO2 and pressurized to 4 MPa and then heated to a tem-
Figure 1. XRD of (a) Pure CeO2, (b) 10Z-90C,
perature of 393 K. After the completion of the reaction, the
(c) 30Z-70C, (d) 50Z-50C, (e) 70Z-30C, (f)
reactor was cooled to room temperature and depressurized.
90Z-10C and (g) Pure ZnO catalysts.
The reaction mixture was analyzed on a gas chromatograph,
GC-17A (M/s. Shimadzu Instruments, Japan) equipped with
flame ionization detector and OV-1 capillary column (30 m
(g) CeO2 catalysts. CeO2 exhibited diffraction peaks due
length, 0.53 mm id) and the product components were con-
to the fluorite phase, while ZnO showed the characteris-
firmed by GCMS, QP 5050 (M/s. Shimadzu Instruments,
tic diffraction peaks for the wurtzite phase. In the XRD
Japan).
patterns of ZnO-CeO2 catalysts, the diffraction peaks
at 2θ values of 31.7, 34.4, 36.2 and 63◦ corresponding
to ZnO and the diffraction peaks at 2θ values of 28.7
3. Results and Discussion
and 33.2◦ corresponds to CeO2. Furthermore, a mixed-
phase of CeO2 and ZnO was observed with 2θ values
3.1 X-ray diffraction analysis
of 47.6 and 56.4◦, which are in good agreement with
Figure 1 shows the XRD patterns of (a) ZnO (b) 90Z- the reported results.16 The crystalline size of CeO2 in
10C (C) 70Z-30C (d) 50Z-50C (e) 30Z-70C (f) 10Z-90C ZnO-CeO2 catalysts & BET surface area of catalysts are

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Table 1. Textural features of CeO2, ZnO and ZnO-CeO2
catalysts.
Catalyst
Crystallite size (nm)a
Surface area (m2/g)
CeO2
7.1
8.0
10.2
11.4
18.6
25.2
54.2
76.0
71.2
60.8
48.2
37.2
23.4
8.9
10Z-90C
30Z-70C
50Z-50C
70Z-30C
90Z-10C
ZnO
aCrystallite sizes of CeO2 are measured using Scherrer equa-
tion from (111) plane at 2θ= 33.7◦ and for pure ZnO from the
diffraction peak at 2θ= 36.2◦.
illustrated in Table 1. In the XRD patterns of ZnO-CeO2
catalysts, up to 50 wt % loading of ZnO, there are no sig-
nificant peaks representing ZnO. Hence, the ZnO over
CeO2 exists either in amorphous form or in crystalline
form with below the detection limit of the instrument.
However, at higher loadings of ZnO on CeO2 surface,
high intense ZnO peaks were observed due to high crys-
talline nature of ZnO. Moreover higher ZnO loading is
not good to well disperse on the surface of CeO2. As
well as the mutual interface between Ce and Zn oxide
particles results in an increase in the surface areas with
decreased crystalline sizes in the catalysts.
Figure 2. UV-Vis DRS patterns of (a) Pure
CeO2, (b) Pure ZnO, (c) 10Z-90C, (d) 30Z-70C,
(e) 50Z-50C and (f) 70Z-30C catalysts.
Table 2. Band gap ener-
gies calculated from Tauc
Plot method.
Catalyst
Band gap (eV)
CeO2
2.74
2.81
2.82
2.85
2.94
3.11
10Z-90C
30Z-70C
50Z-50C
70Z-30C
ZnO
3.2 BET surface analysis
The BET-surface areas of the pure ZnO, pure CeO2 and
ZnO/CeO2 (10Z-90C, 30Z-70C, 50Z-50C, 70Z-30Cand
90Z-10C) catalysts were shown in Table 1. Among
all the synthesized catalysts, 10Z-90C, 30Z-70C were
found to possess the highest BET surface area of 71.2
m2/g, and 60.8 m2/g respectively. With the increase in
ZnO content from 10 wt% to 90 wt% the surface area
species. It is noticed that there is a good correlation
between the total adsorption capacity of CO2, NH3 and
specific surface area of the catalysts.
3.3 UV-Vis-DRS studies
of ZnO-CeO2 catalysts decreases. It was majorly due The UV-Vis DRS patterns of CeO2 and ZnO-CeO2 cat-
to ceria species significantly facilitating enough space alyst were depicted in Figure 2. From this study, it is
for the homogeneous dispersion of ZnO species accord- found that a signal at 280 nm in the pattern of pure
inglyhighsurfaceemergedinallcatalysts. ButhighZnO CeO2 and ZnO-CeO2 is due to the intrinsic character of
content in the catalysts play a negative role in view of CeO2 17 whileabsorptionedgeat413nminCeO2 isblue-
the formation of agglomerated particles on the surface shifted for ZnO-CeO2 mixed oxides which may be due
of the catalysts, which is responsible for less surface to the presence of solid solution between Ce-O-Zn. The
area and high crystalline size depicted in Table 1. It was absorption bands at 256 and 296 nm are due to charge
further investigated from XRD patterns, high-intensity transfer transitions of O2− → Ce3+ and O2− → Ce4+.18
peaks observed in 90Z-10C catalyst rather than mild Addition of CeO2 to ZnO changes the absorption fea-
intensity peaks in the 10Z-90C catalyst. For that reason, tures indicating bands at 230, 280 and 340 nm in the
10Z-90C catalyst possesses the highest surface area of UV region of the spectra due to localized transitions.
71.2 m2/g, whereas 90Z-10C presents the lowest surface Suchnarrowbandsarereportedinthespectraofultrafine
area of only 23.4 m2/g. Therefore, the highest surface alignment of CeO2 particles in SiO2 and Al2O3 matri-
area in the 10Z-90C catalyst was provided by uniform ces.19,20 The bandgap energy from UV-Vis DRS spectra
dispersion of zinc oxide particles on the surface of ceria was calculated using the Tauc plot method and presented

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86
Figure 3. (a) TEM, (b) SAED images of 10Z- 90C catalyst indicates the circular planes which correspond
to the existed peaks in the XRD patterns.
in Table 2. The band gap energy is increased from 2.74 are observed. The low-temperature peak is ascribed
eV to 2.94 eV with doping of ZnO to CeO2. It means, the most easily removable surface capping oxygen of
with a decrease in CeO2 content the band gap energy is CeO2, while the high-temperature signal is caused by
increased. For pure ZnO, the band gap energy is 3.11 eV. the removal of bulk oxygen. The H2-TPR profiles of the
The increase in band gap energies resulted in a decrease mixed oxides ZnO-CeO2 catalysts show a main broad
in surface oxygen vacancies.21
reduction in the region between 763–833 K with differ-
ent Tmax values. For pure CeO2, two reduction peaks at
around 853 and 923 K were observed, which ascribed
to the reduction of surface and bulk oxygen of CeO2
respectively. It was interesting to note that two reduc-
tion peaks at 769 and 833 K of the ZnO-CeO2 mixed
oxide shifted to lower temperatures extremely relative
to those of pure CeO2. It was due to effective syner-
gistic interface between ceria and zinc oxide species
consequently exhibited higher catalytic activity than
pure CeO2 and ZnO catalysts, which is reliable with
the H2-TPR results.25 It is due to the beneficial inter-
actions between ceria and zinc leads to facial reduction
properties of mixed oxide catalyst rather than individual
oxides. Moreover, some extent of ZnO particles might
be inserted into the CeO2 lattice, these properties make
the better catalytic activity.
3.4 Transmission electron spectroscopy (TEM)
TEM studies were performed on 10Z-90C catalyst.
The representative images were displayed in Figure 3.
The HR-TEM and selected-area electron diffraction
(SAED) patterns further confirmed the structure of the
desired solid oxide materials. The HR-TEM image in
Figure 3(a) clearly shows an interface between CeO2
and ZnO species. The d-spacing values were calcu-
lated on the basis of the HR-TEM image. The planes
existed in SAED image are in correlation with peaks in
XRD patterns in Figure 1, and reported studies.22,23 The
calculated d-spacing values show that the ZnO species
exhibited a hexagonal structure and that CeO2 exhibited
a cubic structure which is close agreement with the XRD
results.
3.6 X-ray photoelectron spectroscopy studies
3.5 Temperature programmed reduction studies
The XPS analysis spectrum is shown in Figure 5. The
The redox properties of the ZnO-CeO2 catalysts were ZnO-CeO2 surface is composed of Zn, Ce, O, and C.
evaluated by temperature-programmed reduction (H2- The binding-energy values at 1021.61 eV and 1044.61
TPR). Figure 4 depicts the H2-TPR profiles of the eV represent Zn 2p3/2 and Zn 2p1/2 in the ZnO-CeO2
ZnO-CeO2 catalysts. As we know that cerium can exist mixed oxide catalyst as shown in Figure 5.23 The labels
as Ce3+ and Ce4+ ions while zinc exists as Zn+2 only. used in identifying Ce 3d XPS peaks were established
Thereby CeO2 has high oxygen storage capacity and by Burroughs et al.,26 where V and U indicate the spin-
possesses a larger number of oxygen vacancies.24 In orbit coupling 3d5/2 and 3d3/2, respectively. The peaks
case of pure CeO2 catalyst typically two reduction peaks corresponding to V, Vꢀꢀ, Vꢀꢀꢀ, U, Uꢀꢀ, Uꢀꢀꢀ represent the

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presence of Ce4+, whereas the peaks V0, Vꢀ, U0, Uꢀ arising from ionization of Ce 3d5/2, while the signals Uꢀꢀ
represent Ce3+ ions.27 As shown in the figure, the and Uꢀꢀꢀ are accounting for ionization of Ce 3d3/2.28 The
Ce 3d spectrum is integrated to subsequent peaks at satellite peak Uꢀꢀꢀ associated to the Ce 3d3/2 is a charac-
about 881 (V0), 882.3 (V), 888.9 (Vꢀꢀ), 897.5 (Vꢀꢀꢀ), 899 teristic aspect hinting to the presence of tetravalent Ce
(U0), 901.2 (U), 907.0(Uꢀꢀ), and 918 eV (Uꢀꢀꢀ). The pri- ions in ceria compounds which is in good agreement
mary peaks of 3d5/2 and 3d3/2 are located at 882.3 and with literature reports.29
901.2 eV, respectively. The peak Vꢀꢀ is the satellite peak
3.7 Temperature programmed desorption (TPD) of
NH3
Figure 6 shows the NH3 -TPD profiles of the cata-
lysts. The synthesis of DMC from CH3OH and CO2
involves the activation of CH3OH to CH+3 species on
the acidic sites of the catalyst. So, higher acidity of
the catalyst is helpful to get more yield of DMC.
During the reaction, the decomposition of CH3OH
molecule is higher on 10Z-90C catalyst due to the
higher amount of acidic sites present in ZnO along
with a small fraction of acidic sites in CeO2. Among
these catalysts, 10Z-90C possesses a higher amount
of acidic sites which could be responsible for the
better catalytic performance with respect to higher
production of DMC from CH3OH and CO2. The acid-
ity of the catalysts increased as the content of CeO2
Figure 4. TPR patterns of (a) pure CeO2, (b)
increased, whereas the acidity decreased at an exces-
70Z-30C, (c) 50Z-50C, (d) 30Z-70C and (e)
sive amount of CeO2 content. Cerium oxide is more
10Z-90C.
Figure 5. XPS spectra of (a) Ce 3d in 10Z-90C, (b) Zn 2p in 10Z-90C and (c) Ce 3d in pure CeO2 samples calcined at
773K.

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86
Table 3. The total acidity and basicity of the catalysts.
Number of acidic Acidic site density
sites (mmol/g)
Number of basic Basic site density
(×10−2 mmol/m2) sites (mmol/g)
(×10−2 mmol/m2)
Catalyst
CeO2
0.56
0.75
0.59
0.51
0.47
0.37
0.19
0.73
1.1
1.0
1.05
1.26
1.58
2.13
1.497
1.775
1.635
1.322
1.183
0.666
0.379
1.96
2.49
2.68
2.74
3.18
2.84
4.25
10Z-90C
30Z-70C
50Z-50C
70Z-30C
90Z-10C
ZnO
Figure 7. Temperature programmed desorption
(TPD) patterns of CO2: (a) pure ZnO, (b) 10Z-90C,
(c) 70Z-30C, (d) pure CeO2, (e) 30Z-70C, (f)
50Z-50C and (g) 90Z-10C.
Figure 6. Temperature programmed desorption
(TPD) patterns of NH3: (a) pure CeO2, (b)
10Z-90C, (c) 30Z-70C, (d) 50Z-50C, (e) 70Z-30C,
(f) 90Z-10C and (g) pure ZnO.
yielding better catalytic activity towards the synthesis
of DMC from CH3OH and CO2. The introduction of
an appropriate amount of Ce into ZnO could increase
the number of basic sites in the catalyst, whereas
an excessive amount of Ce could lead to a decrease
in the amount of basic sites. The reduction in the
amount of basic sites may be due to the aggrega-
tion of CeO2 particles. It was reported that the CeO2
species were exhibited better O2 storage and releasing
capacity which leads to adsorption of more number of
CO2 molecules in view of higher number of methoxy
carbonate molecule formation during the reaction.30
On 10Z-90C catalyst, we speculate that there could
basic oxide and therefore, a high amount of CeO2 in the
catalyst may result in a decrease of ammonia
adsorption. In the present study based on catalytic activ-
ity, we assumed that there might be the considerable
synergetic interface between Lewis acidity of ZnO and
Lewis basicity of CH3OH. The amount of acidity of
catalysts calculated from NH3-TPD peak area is sum-
marized in Table 3.
3.8 Temperature programmed desorption (TPD) of
CO2
Not only acidic sites but also basic sites of the cata- be better synergetic interactions between CeO2 and
lyst can also play an important role in the synthesis CO2 molecules which lead to the better conversion
of DMC. Basic sites of the catalyst are more account- of CH3OH molecules. Basicity of catalysts obtained
able for the formation of methoxy carbonate through a from CO2-TPD is described in Table 3. As can be
reaction between methoxy species and CO2 molecules. seen from Scheme 5, both acidic and basic sites are
Figure 7 shows the CO2-TPD profiles of different cat- major responsible for the better catalytic activity. Thus
alysts. Among these catalysts, 10Zn-90C possesses the amount of DMC obtained could be proportional
more number of basic sites which could be useful in to the high concentration of acidic and basic sites of

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Figure 8. (a) Effect of reaction temperature (b) CO2 Pressure on the DMC yield over 10Z-90C catalyst after
12 h catalytic reaction. Reaction conditions: Catalyst amount: 0.6 g, Methanol: 100 mmol, 2-cyanopyridine:
50 mmol.
the catalysts. Based on Figures 6 and 7, DMC yields
are typically depends on more number of acidic and
basic sites of the catalysts, which are close correlation
with CO2 and NH3 TPD analysis shown in Figures 6
and 7. The surface acidic/basic site densities of the
catalysts were calculated and presented in Table 3.
The surface acidic site densities are increased from
0.73 x 10−2 mmol/m2 (for pure CeO2) to 1.58 ×10−2
mmol/m2 (for 90Z-10C) with increase in ZnO loading.
The surface basic site densities are also followed the
same trend as surface acidic site density (1.96 x10−2
mmol/m2 (for pure CeO2) to 2.84 x10−2 mmol/m2 (for
90Z-10C)). These results indicate that, the incorpora-
tion of ZnO in CeO2 enhances the available surface
acidic and basic sites per unit surface area of the cat-
alyst.
Figure 9. Catalytic performance of ZnO–CeO2
mixed oxide catalysts. Reaction conditions: Cat-
alyst amount: 0.6 g, Methanol: 100 mmol,
2-cyanopyridine: 50 mmol, Reaction temperature:
393 K, Pressure: 40 bar.
3.9 Activity studies
3.9a Effect of reaction temperature and pressure:
Even though DMC synthesis is slightly exothermic, the
high reaction temperature is needed for the activation
of CH3OH and CO2. However, the activity decreases
beyond 393 K due to considerable CO2 and CH3OH
(Figure 8 (b)). There is a decrease in the yield of DMC
over 40 bar due to the dilution effect.
desorption.31 The selectivity to DMC also declines with 3.9b Effect of loading: The amount of formation of
an increase in temperature due to the decomposition of DMC over CeO2, ZnO and ZnO-CeO2 mixed oxide cat-
DMC (Figure 8 (a)). DME observed as a side product alytic systems was shown in Figure 9. It is found that
is due to the decomposition of dimethyl carbonate.32 ZnOcatalystexhibitedloweractivity(14%CH3OHcon-
It can also be obtained from MeOH via intermolecular version) than that of CeO2 (84% conversion of CH3OH)
dehydration in the presence of an acid-base catalyst at even though both catalysts are exhibited similar higher
high temperature but this is very low when compared selectivity to DMC. Catalysts with 10 and 30% ZnO
to the decomposition of DMC. The yield and selectivity exhibitedhigheractivitythanthatofCeO2. Amongthese
of DMC increase with increasing pressure up to 40 bar two, 10Z-90C exhibited higher activity (96% CH3OH

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86
the catalytic activity is decreased at high crystalline
sizes of metal oxides. In the present case, the best
catalytic activity is obtained over the 10Z-90C cata-
lyst with CeO2 crystalline size of 8.0 nm even though
the crystalline size of pure CeO2 is 7.1 nm. It indi-
cates that the catalytic activity of the catalyst can’t
be dependent alone on the crystalline size of CeO2.
In addition, the presence of surface acidic and basic
sites is responsible for achieving the best yield of
DMC. In particular, the optimum numbers of sur-
face acidic and basic site densities along with small
crystallites of CeO2 are key contributors for the cat-
alytic activity in the synthesis of DMC from CO2 and
methanol.
The reaction conditions for different catalysts used
for the synthesis of DMC are reported in Table 4.
Various catalysts have been reported for the synthesis
of DMC from CO2 and CH3OH such as pure metal
oxide, mixed metal oxides and bimetallic supported
catalysts. Ahmed et al., reported 7.6% CH3OH con-
version and 87% DMC selectivity over Co1.5PW12O40
catalyst.4 Over Cu-Ni/graphite, 10.2% CH3OH conver-
sion and 88% DMC selectivity were observed from
the report of Bian et al.14 Yingjie et al., reported
5.3% CH3OH conversion and 86% DMC selectivity
over Cu–Fe/SiO2 catalyst.33 Qinghai et al., reported
16.2% CH3OH conversion and 100% DMC selectiv-
ity over CH3OK catalyst.34 Over Ce-Zr/Graphene, 58%
of CH3OH conversion and 57% of DMC selectivity
were reported by Rim et al.35 Over LG-HTO catalyst,
15.9% CH3OH conversion and 99.9% DMC selectiv-
ity were observed from the report of Stoian et al.32
Zhongweietal., reported5.38%CH3OHconversionand
83.1% DMC selectivity over Ti0.04Ce0.96O2 catalyst.36
63% CH3OH conversion and 97% DMC selectivity over
Figure 10. Loading of ZnO against the crystalline size of
CeO2 and DMC yield.
conversion with 99% selectivity to DMC) (Figure 9).
Catalysts with higher ZnO content (>50%) showed
lower activity when compared to bare CeO2 because
of high crystalline ZnO species existed on the surface
of the CeO2 leads to the mild surface area. CeO2 -
ZnO mixed oxide catalysts with more number of acidic
and basic sites in the moderate region (ranging from
573–773 K temperature) showed enhanced catalytic
activity.
From Figure 10, it is clear that the crystalline size
of CeO2 and as well as ZnO have a profound effect
in governing the yield of DMC. The crystalline sizes
of CeO2 in ZnO-CeO2 are increased with the incor-
poration of ZnO in CeO2 and vice versa. In general,
Table 4. Comparison of the activity of the present catalyst with the reported catalysts.
Catalyst
Temperature (K)
Pressure (bar)
CH3OH Conversion (%)
DMC Selectivity (%)
References
4
Co1.5PW12O40
Cu-Ni/graphite
LG-HTO
Cu-Fe/SiO2
CH3OK
Ce-Zr/Graphene
Ti0.04Ce0.96O2
CeO2-spindles
CeO2
473
383
403
393
353
383
373
413
393
443
393
393
1
12
-
7.6
10.2
15.9
5.3
16.2
58
83.1
63
97
87
88
99.9
86
100
57
5.3
97
14
31
32
33
34
35
36
37
6
12
73
275
10
50
50
70
40
40
99
ZnO-CeO2
CeO2-ZnO
*CeO2-ZnO
<1
2.7
96
100
100
99
This work
This work
* With addition of 2-cyanopyridine.

86 Page 10 of 11
J. Chem. Sci.
(2019) 131:86
CeO2 spindles were reported by Unnikrishnan et al.37
Masayoshi et al., reported 97% CH3OH conversion and
99% DMC selectivity over CeO2 catalyst for 16 h.38
In the present work, we report 96% conversion of
methanol with 99% selectivity to DMC by coupling
of methanol and CO2 with 2-cyanopyridine. Since it is
equilibrium controlled reaction, removal of H2O, the
by-product is necessary to get better methanol conver-
sion. We achieved this, by coupling of CH3OH with
2-cyanopyridine which can be beneficial to get more
DMC yield. Hence coupling with 2-cyanopyridine has
been used and afforded good activity results.
5. Jun B, Min X, Shuanjin W, Xiaojin W, Yixin L and
Yuezhong M 2009 Highly effective synthesis of dimethyl
carbonate from methanol and carbon dioxide using a
novel copper–nickel/graphite bimetallic nanocomposite
catalyst Chem. Eng. J. 147 287
6. Ki Hyuk K, Wangrae J, Chang Hoon L, Mieock K, Dong
Baek K, Boknam J and In Kyu S 2013 Direct synthesis
of dimethyl carbonate from methanol and carbon diox-
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Nanotechnol. 13 8116
7. Hye Jin L, Sunyoung P, Ji Chul J and In Kyu S 2011
Direct synthesis of dimethyl carbonate from methanol
andcarbondioxideoverH3PW12O40/CeX Zr1−X O2 cat-
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4. Conclusions
In conclusion, ZnO–CeO2 catalysts prepared by co-
precipitation method were found to be active in the
synthesis of dimethyl carbonate via coupling of CH3OH
and CO2 with 2-cyanopyridine. UV-Vis DRS patterns
evidenced the formation of Ce-O-Zn solid solution. 11. Tomishige K and Kunimori K 2002 Catalytic and direct
synthesis of dimethyl carbonate starting from carbon
The synergetic interaction between ZnO and CeO2 had
dioxide using CeO2-ZrO2 solid solution heterogeneous
improved the quantity of acidic and basic sites. Among
catalyst: effect of H2O removal from the reaction system
the series of catalysts, 10ZnO–90CeO2catalyst with
Appl. Catal. A Gen. 237 103
CeO2 crystallite size 8.0 nm and surface acid site den-
sity 1.1 x 10−2 mmol/m2 and surface basic site density
2.49 x 10−2 mmol/m2 delivered better results. Further,
the addition of 2-cyanopyridine absorbed the formed
watermoleculesduringthereaction. Duetotheseunified
effects, a maximum 95% yield of dimethyl carbonate
was achieved at 393K.
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