Efficient ceria-zirconium oxide catalyst for carbon dioxide conversions: Characterization, catalytic activity and thermodynamic study

Article abstract of DOI:10.1016/j.jallcom.2016.10.293

In this study, ceria-zirconia based catalysts (CeO₂, ZrO₂and Ce_0.5Zr_0.5O₂) were synthesized by hydrothermal method and characterized by N₂-sorption, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Acidity and basicity of synthesized catalysts were investigated by NH₃[sbnd] and CO₂[sbnd] temperature-programmed desorption (TPD). Brunauer-Emmett-Teller (BET) surface area of CeO₂, Ce_0.5Zr_0.5O₂and ZrO₂were found to be 88, 117 and 70?m²?g^?1and average crystallite sizes was 9.48, 7.09 and 9.45?nm, respectively. These catalysts were further used for direct conversion of CO₂with methanol for the synthesis of dimethyl carbonate (DMC). DMC yield was found to be highly dependent upon the both basicity and acidity of catalysts. Ce_0.5Zr_0.5O₂catalysts showed better activity as compared to CeO₂and ZrO₂catalysts. Effect of reaction conditions (such as catalyst dose, reaction temperature and reaction time) and catalyst reusability was studied with Ce_0.5Zr_0.5O₂catalyst. The optimum operating condition for direct conversion of CO₂into DMC at constant pressure of 150 bar were found to be reaction time?=?24?h, catalyst dose?=?1.25?g and temperature?=?120?°C. Moreover, chemical equilibrium modeling was performed using Peng–Robinson–Stryjek–Vera equation of state (PRSV-EoS) along with the van der Waals one-fluid (1PVDW) mixing rule to calculate the heat of reaction and Gibbs free energy change.

Full text of DOI:10.1016/j.jallcom.2016.10.293

Accepted Manuscript
Efficient ceria-zirconium oxide catalyst for carbon dioxide conversions:
Characterization, catalytic activity and thermodynamic study
Praveen Kumar, Patrick With, Vimal Chandra Srivastava, Roger Gläser, Indra Mani
Mishra
PII:
S0925-8388(16)33430-2
10.1016/j.jallcom.2016.10.293
JALCOM 39462
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 1 May 2016
Revised Date: 13 September 2016
Accepted Date: 29 October 2016
Please cite this article as: P. Kumar, P. With, V.C. Srivastava, R. Gläser, I.M. Mishra, Efficient ceria-
zirconium oxide catalyst for carbon dioxide conversions: Characterization, catalytic activity and
thermodynamic study, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.293.
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ACCEPTED MANUSCRIPT
Efficient ceria-zirconium oxide catalyst for carbon dioxide conversions:
characterization, catalytic activity and thermodynamic study
a,c*
b
a
c
Praveen Kumar , Patrick With , Vimal Chandra Srivastava , Roger Gläser , Indra Mani
a,d
Mishra
a
Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee
2
47667, Uttarakhand, India
b
Leibniz-Institutes für Oberflächenmodifizierung e. V. (IOM), Permoserstr. 15, D-04318
Leipzig, Germany
c
Institute of Chemical Technology, Universität Leipzig, Linnéstraße 3, 04103 Leipzig,
Germany
d
Department of Chemical Engineering, Indian Institute of Technology (Indian School of
Mines), Dhanbad - 826004, Jharkhand, India
*
Corresponding Author: Phone: +91–1332–285889; fax: +91–1332–276535. E–mail
addresses: praveen.zon@gmail.com, praveen.singh@daad-alumni.de (P. Kumar)
Abstract
In this study, ceria-zirconia based catalysts (CeO , ZrO and Ce Zr O ) catalysts
2
2
0.5 0.5
2
were synthesized by hydrothermal method and characterized by N -sorption, X-ray
2
diffraction (XRD), scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). Acidity and basicity of synthesized catalysts have been investigated by
NH - and CO - temperature-programmed desorption (TPD). Brunauer-Emmett-Teller (BET)
3
2
2
-1
surface area of CeO , Ce Zr O and ZrO were found to be 88, 117 and 70 m g and
2
0.5 0.5
2
2
average crystalline sizes was 9.48, 7.09 and 9.45 nm, respectively. These catalysts were
further used for direct conversion of CO with methanol for the synthesis of dimethyl
2
carbonate (DMC). DMC yield was found to be highly dependent upon the both basicity and
acidity of catalysts. Ce Zr O catalysts showed better activity as compared to CeO and
0
.5 0.5
2
2
ZrO catalyst. Effect of reaction conditions (such as catalysts dose, reaction temperature and
2
reaction time) and catalyst reusability was studied with Ce Zr O catalyst. The optimum
0
.5 0.5
2
operating condition for direct conversion of CO into DMC at constant pressure 150 bar,
2
reaction time=24 h, catalyst dose=1.25 g and temperature=120 °C. Moreover, chemical
1

ACCEPTED MANUSCRIPT
equilibrium modeling has been performed using Peng–Robinson–Stryjek–Vera equation of
state (PRSV-EoS) along with the van der Waals one-fluid (1PVDW) mixing rule to calculate
heat of reaction and Gibbs free energy change.
Keywords: DMC; direct conversion of CO ; Ce Zr O catalyst; NH - & CO -TPD.
2
0.5 0.5
2
3
2
1
. Introduction
Dimethyl carbonate (DMC) is a promising chemical in alkyl carbonate series, due to
its non-toxic and biodegradable nature. It is high versatile in terms of its usage and is widely
used for green chemical synthesis [1-3]. It can be used as a solvent for synthesis of various
useful chemicals such as copolymers, phenol carbonate [4], polycyclic aromatic [5], etc. It is
also used in electrolytic solution in lithium battery due to high dielectric constant [5]. DMC is
used as a replacement of non-biodegradable and toxic methyl tert-butyl ether (MTBE)
additive in the gasoline based fuels [6]. DMC has high oxygen content (53.3%) as compared
to other commercial fuel additives such as methyl tert-butyl ether (MTBE) (17.6%), ethanol
(34.89%) and methanol (50%) and is consequently used as octane booster in gasoline based
fuels [1-3]. Traditionally, DMC was synthesized by various methods such as phosgenation
process, oxycarbonylation and methyl nitrite carbonylation process [1-3,7-8]. Other methods
used for DMC synthesis such as the direct synthesis from CO , transesterification of
2
propylene carbonate and urea methanolysis process are also used for synthesis of DMC [9-
1
3].
In the last few years, CO is considered as an abundant, cheap, non-toxic and
2
inexpensive renewable resource which can be utilized for the synthesis of a variety of
chemicals. However, the transformation of CO to other valuable products requires high
2
energy [14-16]. Consequently various catalysts are being used to lower this energy
requirement and enhance the productivity using different routes [14-17]. Methods are also
2

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being developed for transformation of CO into DMC in the presence of a catalyst. However,
2
this route of CO utilization requires a lot of research attention due to very low DMC yield
2
because of thermodynamic restrictions (∆H=16.74 kJ/mol) [18]. Various homogeneous
catalysts such as thallium (I) hydroxide, tin (IV) tetralkoxides, dialkytin dialkoxides, bases,
C,N-chelated organotin (IV) trifluoro methane sulfonates and titanium (IV) tetralkoxides
have been used for the synthesis of DMC from CO [19-25]. Deactivation of catalyst, difficult
2
separation process, and recovery of catalyst, etc. are some of the drawbacks of homogeneous
systems. Subsequently, various heterogeneous catalyst have been investigated for DMC
synthesis such as CeO [26], ZrO [26], Ce Zr O [27-28], Gd-Ce Zr O [28], Cu-CeO
2
2
2
0.5 0.5
2
0.4 0.6
2
[
29], Sn-SBA-15 [30], and C,N-chelated organotin (IV) trifluoro methane sulfonates [19]
have been reported for DMC synthesis. Few investigators have previously used CeO , ZrO2
2
and combined CeO -ZrO catalysts for DMC synthesis from CO [26-28]. Zhang et al. [27]
2
2
2
synthesized and characterized Ce Zr O catalyst and used it for DMC synthesis from CO .
0
.5 0.5
2
2
However, Lee et al. [28] synthesized Ce Zr O catalysts by sol-gel method and metal
0
.4 0.6
2
oxides (Ga O , La O , Ni O , Fe O , Y O , Co O , and Al O ) were further supported on
2
3
2
3
2
3
2
3
2
3
3
4
2
3
Ce Zr O by an incipient wetness impregnation method and used as catalysts for DMC
0
.6 0.4
2
synthesis. Authors correlated the amount of DMC formed mmol/g-catalyst with basicity and
acidity of the catalysts. Chen et al. [26] used CeO and ZrO catalysts for direct conversion of
2
2
CO with methanol into DMC and studied their reaction mechanism. Synthesis of DMC from
2
CO is one of the primary utilization routes and interest CeO -ZrO solid solutions have been
2
2
2
demonstrated in the literature to have good catalytic properties.
It is known that for commercialization of this route is essential to optimize the
operating parameters such as catalysts dose, reaction temperature and reaction time and study
the reusability of the catalysts. All these factors have been studied in the present study and
3

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discussed with respect to catalyst characterization. Moreover, present study reports chemical
equilibrium modeling which is essential for reactor design.
In the present work, synthesis of DMC from direct conversion of CO with methanol
2
was studied using ceria, zirconia and ceria-zirconia catalysts prepared by hydrothermal
method. Synthesized catalysts were characterized by liquid N -sorption, X-ray diffraction
2
(XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and
transmission electron microscopy (TEM). Acidity and basicity of synthesized catalysts have
been investigated by NH - and CO -temperature-programmed desorption (TPD). Effect of the
3
2
reaction condition such as catalysts dose, reaction temperature, reaction time and reusability
of the catalyst was studied with chemical equilibrium modeling using best performing
catalyst.
2
. Experimental
2
.1 Materials
Cerium (III) nitrate hexahydrate (Ce(NO ) .6H O) 99.0%, and zirconium (IV)
3
3
2
oxychloride octahydrate (ZrOCl .8H O) 99.0% were purchased from Sigma Aldrich
2
2
Chemicals, GmbH. Ammonia solution (25 wt% in H O) was purchased from Merck GmbH.
2
Dimethyl carbonate (DMC) 99.9% and methanol (99.0%) were purchased from Sigma
Aldrich Chemicals, GmbH. All chemicals used were of analytical grade (AR) without further
purification. Deionized water was obtained from Milli-Q water filtration station (Millipore).
2
.2. Catalyst Preparation
CeO , Ce Zr O and ZrO catalyst were synthesized from Ce(NO ) .6H O and
2
0.5 0.5
2
2
3 3
2
ZrOCl .8H O salts using hydrothermal method. Ce(NO ) .6H O (0, 6, 12 mmol) and
2
2
3 3
2
3
ZrOCl .8H O (12, 6, 0 mmol) were dissolved separately in 50 cm ethanol and further mixed
2
2
together in desired molar ratio under continuous stirring at room temperature. Liquid
4

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ammonia solution was added drop-wise in the mixed metal solution over a period until the
pH reached ~9.5. Further, the formed solution was aged for 4 h. Yellow mixed solution
obtained after the reaction were introduced in 100 mL teflon-lined autoclave and kept in 120
°
C for 24 h, then cooled to room temperature naturally. Yellow colour slurry obtained was
washed several times with double distilled water with acetone until pH became neutral.
Resulting precipitate was dried at 110 °C for 24 h in air. Finally, the fresh samples were
calcined at 500 °C for 4 h in air atmosphere to obtain the CeO , Ce Zr O and ZrO
2
0.5 0.5
2
2
catalysts. The synthesized catalysts were denoted as CeO , Ce Zr O and ZrO catalyst
2
0.5 0.5
2
2
having pure CeO , mixed CeO -ZrO and pure ZrO , respectively.
2
2
2
2
2
.3 Catalyst characterization
Synthesized catalysts were characterized by X-ray diffraction pattern (XRD), N2-
sorption, temperature programmed desorption (TPD) of CO and NH , scanning electron
2
3
microscope-energy disperse X-ray (SEM-EDX).
The crystallographic structure of samples was examined using Siemens D5000
o
diffractometer with Cu-Kα radiation (λ=0.15406 nm) over 2θ range of 5° - 80 with a step
size of 0.02° at 40 kV. Crystalline phases were analyzed using PANalytical X'pertHigh Score
software with the database in the JCPDS (Joint Committee of Powder Diffraction Standards).
Scherrer equation used for the calculating the average crystalline sizes.
K
λ
L =
(
β
sam ple
−
β
standard
)
cosθ
(1)
where, L is the average particle size, K is the Scherrer’s constant (0.94), λ is the X-ray
radiation of wavelength of which is equal to 0.154051 nm, β is the full width of the reflection
at half maximum (FWHM) and θ is the Bragg angle. High grade silicon powder was used as
an internal standard for the instrument broadening correction [31].
Morphology of the sample and its elemental compositions were analyzed using
QUANTA, Model 200 FEG, Netherlands equipped with EDX on gold coated samples. The
5

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2
TEM characterization was study using TECNAI G 20S-TWIN, FEI Netherlands with LaB6
as cathode (resolution: point 0.24 nm and line 0.14 nm) at 200 kV. The samples were
dispersed with ethanol and sonicated for 60 min and then drop deposited on the copper
micro-grid and dried at room temperature for 2 h.
Textural properties of the samples were measured at -195 °C using Micromeritics
ASAP 2020 apparatus by N adsorption/desorption. Sample was degassed for 8 h at 200 °C
2
under vacuum to remove any absorbed impurities before performing the experiments.
Brunauer-Emmett-Teller (BET) method used for determined the surface area of the sample in
the range of relative pressure 0.05 to 0.35 [32] and Barrett-Joyner-Halenda (BJH) method
used for calculating the pore area and pore volume [33] using the desorption section of
nitrogen isotherms.
Basicity of the samples were determined using CO -TPD experiment using
2
Micromeritics Chemisorb 2720 instrument coupled with a thermal conductivity detector
3
-1
(
TCD). The sample (100 mg) pretreated at 200 °C under helium flow (20 cm min ) for 6 h
-1
and then exposed to CO stream with a flow rate 20 ml min for 30 min at room temperature
2
and physisorbed CO was removed by helium purging for 1 h at room temperature. Then, the
2
-1
3
-1
temperature was increased with heating rate of 10 °C min under helium flow (20 cm min )
from 50 - 700 °C and the desorbed CO was monitored by TCD, which was calibrated by
2
injections of pure CO pulses. Similarly, same process was applied for NH -TPD to
2
3
determine the acidity and acidic strength of the sample.
Morphology and grain size distribution of the catalysts was also studied using atomic
force microscopy (AFM) by NT-MDT M/s Molecular Tools and Devices for Nanotechnology
and NOVA software used for image analysis. Sample were prepared in ethanol, the small
amount of sample was dispersed in ethanol solution and solution was sonicated for 180 min.
6

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Afterwards, small amount of solution was placed on a glass plate and dried at room
temperature for 12 h. Then this glass plate was used for AFM analysis.
2
.4. Catalytic activity
Catalytic activity of the synthesized catalysts was checked for the synthesis of DMC
from direct conversion of CO with methanol in a stainless-steel autoclave reactor. Reactor
2
was initially filled with required amounts of methanol and the catalysts. The reactor was
heated at reaction temperature (100-180 °C) and pressurizing with CO with the reaction
2
pressure (150 bar). After (6-48 h), the reactor was kept in an ice bath so as to cool down the
product mixture to (<−20 °C) and thereafter catalyst was separated from the mixture by
centrifugation. All the reactions were studied in the presence of activated molecular sieve 3A
-1
as a dehydrating agent and at constant stirrer speed of 600 min . Catalyst was washed with
°
methanol and dried at 150 °C for 12 h and further activated at 500 C for 4 h after each cycle.
Similalrly, molecular sieves was activated at 240 °C for 4 h after each cycle.
The product was analyzed by gas chromatograph (GC) HP 5890 equipped with a
capillary column HP-5 (5%-phenyl)-methyl polysiloxane with a length of 30 m, an inner
diameter of 0.25 mm and a film thickness of 1.8 microns with a flame ionization detector
(FID). Injector and detector temperature were 220 and 250 °C, respectively. Helium gas was
used as a carrier gas with flow rate 30 ml/min. Initial column temperature was 40 °C with
holding time 5 min. After that, temperature was ramped to 100 °C where it was held for 5
min thereafter temperature was ramped to 220 °C at the rate of 10 °C/min where it was held
for 5 min.
3
. Results and Discussion
3
.1 Catalyst Characterization
7

ACCEPTED MANUSCRIPT
XRD profiles of CeO , Ce Zr O and ZrO catalysts are presented in Fig. 1. XRD
2
0.5 0.5
2
2
pattern of CeO catalysts shows the reflections of cubic phase (JPDS File No. 01-074-1145)
2
with space group Fm3m (225), ZrO catalysts shows the reflections of tetragonal phase (JPDS
2
File No. 01-080-2155) with space group P42/nmc (137) and Ce Zr O reflections show
0
.5 0.5
2
tetragonal phase (JPDS File No. 00-038-1436) with space group P42/nmc (137). Only pure
phases were found in CeO and ZrO . Ce Zr O catalysts showed the presence of
2
2
0.5 0.5
2
Ce Zr O (JPDS File No. 00-038-1436). XRD reflections get shifted in the mixed solid
0
.5 0.5
2
4
+
4+
solution due to difference in the ionic radii of Ce (0.097 nm) and Zr (0.084 nm). Average
crystallite sizes of CeO , ZrO and Ce Zr O were determined from the most intensive
2
2
0.5 0.5
2
reflections at 2θ=28.5°, 30.2° and 29.2°, respectively. Average crystalline sizes of the CeO2,
Ce Zr O and ZrO catalysts were 9.48, 7.09 and 9.45 nm, respectively. Additionally, a
0
.5 0.5
2
2
TEM image of the Ce Zr O catalysts shows spherical particles in the range of 7-12 nm.
0
.5 0.5
2
SAED indexing pattern confirms the presence of the crystalline phases identified using XRD
in Ce Zr O . Several investigators have previously reported similar XRD results for CeO ,
0
.5 0.5
2
2
ZrO and Ce Zr O . Chen et al. [26] reported cubic phase of CeO and tetragonal phase of
2
0.5 0.5
2
2
ZrO whereas Zhang et al. [27] reported that the Ce Zr O samples calcined at 773 K were
2
0.5 0.5
2
quite similar (cubic phase with crystal sizes of ≈2.6 nm), while the samples calcined at 1273
contained both cubic and tetragonal phases, irrespective of the complexing agents used.
Fuentes and Baker [31] reported either a cubic (Fm3m) or a tetragonal (P42/nmc) phase
depending on Ce content in Ce Zr O (x = 0.1, 0.25, 0.5, 0.75, and 0.9) prepared using a
x
1-x
2
citrate complexation technique. Similarly, Si et al. [34] and Roh et al. [35] reported average
crystalline size of 5-6 with cubic and tetragonal mixed phase of Ce Zr O . Atribak et al.,
0
.5 0.5
2
[
36, 37] have been reported average crystalline sizes 11 nm of mixed phase of Ce0.76Zr0.24O2
catalyst. In the present study, the Ce/Zr molar ratios in the Ce Zr O mixed oxides catalyst
0
.5 0.5
2
were determined by ICP-OES analysis and EDX analysis. Structural chemical compositions
8

ACCEPTED MANUSCRIPT
of the catalysts are shown in Table S1 (supporting information). Formation of chemical
compositions of synthesized catalysts was relatively close to the initial mixed metal
composition.
SEM was performed on CeO , Ce Zr O and ZrO synthesized catalysts and
2
0.5 0.5
2
2
obtained micrographs are shown in Fig. 4. All the catalysts show spherical morphology.
Average particle size of the all the catalysts as investigated from FE-SEM is found to be in
the range of 5-10 nm. ZrO is of smaller size as compared to Ce Zr O catalyst (Fig. 4).
2
0.5 0.5
2
Ce Zr O mapping shown in Fig. S1 shows homogeneous dispersion of metals in all the
0
.5 0.5
2
catalysts. The surface morphology was further investigated by AFM. AFM (1D and 3D)
images along with grain size distribution of the Ce Zr O catalyst is shown in Fig. S2.
0
.5 0.5
2
Average grain size was found to be ~4.5 nm. Surface roughness values obtained from AFM
analysis was 0.945 nm.
The pore surface area of catalysts was calculated according to the BET methods and
pore size and pore volume was calculated by BJH method. N adsorption-desorption isotherm
2
and pore size distribution are shown in Fig. 2. Textural data of the catalysts are given in
Table 1. BET surface area of CeO , Ce Zr O and ZrO were found to be 88, 117 and 70
2
0.5 0.5
2
2
2
-1
m g , respectively, BJH desorption pore volume and average pore diameter was in the range
3
-1
of 0.12 - 0.237 cm g and 5 - 8.41 nm, respectively. Ce Zr O catalyst was found to posses
0
.5 0.5
2
the highest surface area, specific pore volume and average pore diameter among all the
2
synthesized catalysts. Different authors have reported varied BET surface area (5-134 m /g)
of Ce Zr O catalysts [27-28, 31, 34-38]. All catalysts showed type VI isotherms according
0
.5 0.5
2
to IUPAC classification CeO catalyst shows H hysteresis loop, Ce Zr O shows H
1
2
2
0.5 0.5
2
hysteresis loop and ZrO catalyst shows H hysteresis loop in the relative pressure (P/P )
2
2
0
range of 0.4−0.95 characteristic for mesoporous materials [39-42].
9

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Basic properties of CeO , Ce Zr O and ZrO of synthesized catalysts were
2
0.5 0.5
2
2
characterized by CO -TPD experiments. Results are shows in Fig 3. Basic properties of
2
catalysts depend upon the temperature profile in weak, moderate and strong range: < 200 °C,
2
00−450 °C, > 450 °C, respectively. Since all the CeO , Ce Zr O and ZrO synthesized
2 0.5 0.5 2 2
catalysts show main peaks in the range of 78−117 °C, they contain predominantly week basic
sites as shown in Fig 3. Basic sites density (per unit mass of catalysts) of synthesized
catalysts was in the order: ZrO (0.2512 mmol/g) < CeO (0.4154 mmol/g) < Ce Zr O
2
2
2
0.5 0.5
(0.6487 mmol/g) and basic site density per unit area follows the same order (Table 1). Thus
mixed metal oxides are found to posses the basic site density as compared to single oxide
catalysts [43]. Zheng et al. [27] and Lee et al. [28] reported maximum CO desorption of
2
0
.276 and 0.017 mmol/g, respectively, for Ce Zr O catalysts.
0.6 0.4 2
Acidic properties of the CeO , Ce Zr O and ZrO synthesized catalysts were
2
0.5 0.5
2
2
investigated using NH -TPD. Results are shown in Fig. 4b. NH desorption peaks are in
3
3
temperature region of 50-550 °C. All the catalysts found to contain only week acidic sites.
Total desorbed amount of NH was found to be in the following order: Ce Zr O > CeO >
3
0.5 0.5
2
2
ZrO . The acidic sites density (per unit mass) of synthesized catalysts was in the order: ZrO2
2
(
0.511 mmol/g) < CeO (0.793 mmol/g) < Ce Zr O (1.893 mmol/g). Respective value of
2 0.5 0.5 2
2
2
acid site density (per unit area) were ZrO (7.3 mmol/m ) < CeO (9.012 mmol/m ) <
2
2
2
Ce Zr O (16.18 mmol/m ). Therefore, Ce Zr O was found to have highest acidic site
0
.5 0.5
2
0.5 0.5
2
and ZrO found to have lowest acidic sites in the catalyst. Lee et al. [28] reported much lower
2
NH desorption of 0.0857 mmol/g for Ce Zr O catalyst. It may be seen that the
3
0.6 0.4
2
Ce Zr O catalyst has highest surface area, acidic and basic site density (per unit mass/per
0
.5 0.5
2
unit area) as compared to other CeO and ZrO catalysts. Prepared Ce Zr O catalysts
2
2
0.5 0.5
2
posses both basic and acidic sites revealing that it can act as base-acid bi-functional catalyst.
1
0

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Few researchers have previously reported that both acidic and basic sites are required for
direct conversion of CO with methanol to produced DMC [26-28].
2
3
.2 Catalytic activity of catalysts for DMC formation using CO2
Direct conversion of CO with methanol to form DMC was investigated at different
2
reaction conditions over CeO , ZrO and Ce Zr O catalysts. It should be mentioned that in
2
2
0.5 0.5
2
the absence of a catalyst, a negligible methanol conversion and DMC formation after 24 h
was observed. Results are presented in Fig. 5a. Three catalysts showed different activity in
the order ZO < CeO < Ce Zr O . Among all the catalysts Ce Zr O catalyst (2.670
2
2
0.5 0.5
2
0.5 0.5
2
mmol DMC/g.cat.) exhibited the best catalytic activity compared to ZrO (0.456 mmol
2
DMC/g. cat.).
In presence of a suitable catalyst (Ce Zr O in the presence case), methanol is
0
.5 0.5
2
-
+
activated to form CH O and H due to acidic and basic site of catalysts [44]. Methoxy species
3
-
(
CH O ) reacts with CO in the presence of basic site and to form methoxyl carbonyl ions.
3
2
Methanol at the acidic site forms CH ꢀꢁ and OH ions. Methoxyl carbonyl ion reacts
—
ꢁ
ꢃ
+
withꢂCH ꢂto form DMC and OH reacts with H to form of water. As such, higher acidic and
ꢀ
basic catalysts facilitate the DMC synthesis from the CO [45]. The reaction mechanism for
2
DMC synthesis from direct conversion of CO with methanol in presence of catalyst can be
2
represented by following equation [45]:
1
1

ACCEPTED MANUSCRIPT
The relationship between acidic-basic properties and activity of catalysts are shown in
Fig. 5b. It can be seen in the above equation, that the acidic and basic properties directly
influence the catalytic activity of DMC synthesis. ZrO and CeO have smaller number of
2
2
acidic and basic sited as compared to Ce Zr O . It is clearly seen that the DMC yield
0
.5 0.5
2
directly depends upon the acidic-basic properties of catalysts.
The Ce Zr O catalyst was further used to study the effects of the reaction
0
.5 0.5
2
conditions such as amount of catalyst, reaction time and reaction temperature for DMC
synthesis from CO and methanol.
2
The effect of reaction time was studied in the range of 6-48 h using Ce Zr O
2
0
.5 0.5
catalyst with all other reaction conditions being constant. Results are shown in Fig. 6a. It can
be seen that an increase in the reaction time from 6 to 24 h increased the DMC yield from
1
.49 to 2.67 mmol/g. After 24 h, DMC yield was constant. It could be observed in Fig 6a that
the DMC yield and methanol conversion at 24 h is optimum. It may be due to the saturation
of molecular sieve for adsorption of water.
The influence of catalysts dose was studied in the range of 0.62-2.19 g and the results
are shown in Fig. 6b. It can be observed in the Fig. 6b, there are distinct trends in methanol
conversion and DMC yield with increase in the catalyst amount from 0.6-1.25 g. DMC yield
increases from 1.71 to 2.68 mmol/g.cat with increase in the catalyst amount from 0.6-1.25 g.
Hereafter, DMC yield decreases with increases in the catalyst amount from 1.86-2.19 g.
Maximum DMC yield was found with 1.25 g of catalyst.
The effect of reaction temperature was studied in the temperature (Fig. 6c) range of
1
20-180 °C. It can be seen that an increase in reaction temperature from 100 to 120 °C
increased the DMC yield from 1.249 to 2.682 mmol/g. After that DMC yield decreases to
0
.129 mmol/g at 180 °C. Clearly, it can be seen that the optimum in DMC yield was at 120
°
C. Further increase in reaction temperature decreases the DMC yield due to the poor
1
2

ACCEPTED MANUSCRIPT
solubility of CO in methanol at higher temperature and also DMC gets decomposed at higher
2
temperature [46,47].
The reusability of Ce Zr O catalyst was studied at optimum reaction condition
0
.5 0.5
2
such as catalyst amount=1.25 g, T=120 °C and reaction time=24 h. For this, the Ce Zr O
2
0
.5 0.5
catalyst was used in five consecutive batch reactions (Fig. 7). The observed DMC yield and
methanol conversion slightly decreased with an increase in reuse cycles. DMC yield and
methanol conversion in third run was found to be: 2.6789 mmol/g and 0.78454 mmol/g,
respectively. This slight loss of activity of Ce Zr O catalyst may be due to the blockage of
0
.5 0.5
2
the pores and active sites deposition of reaction products.
Table 3 compares various catalysts used for synthesis of DMC by direct conversion of
CO at various reaction conditions in terms of CO conversion (%) and DMC yield (mmol). It
2
2
may be seen that the Ce Zr O catalyst used in the present study show better or comparable
0
.5 0.5
2
yield and CO conversion with those reported in the literature.
2
3
.3 Chemical equilibrium modeling
For the direct synthesis of DMC, equilibrium conversion can be related to the
equilibrium constant as shown below:
a DMC
2
a
H 2
O
K (T) =
eq
a
a
MeOH CO2
1
2
X
(
1-0.5yMeOH,0Xeq,MeOH
)
0
2
0
CO2
0
eq,MeOH



ϕDMC
ϕ
  (
ϕ
)
ϕ

 P 
H O
MeOH
2
2
=
× 
 
 



2
0
0




2
1


(
ϕMeOH
)
ϕCO2
ϕ
ϕ
2
P
DMC H O

eq

yMeOH,o
(
1-Xeq
)


Θ
CO2
- X
2
eq


(2)
where, ΘCO2 = y
yMeOH,o . The Peng–Robinson–Stryjek–Vera equation of state (PRSV-
CO ,0
2
EoS) [48] along with the van der Waals one-fluid (1PVDW) mixing rule [49], were used to
calculate the rules fugacity coefficient of species in the mixture. PRSV-EoS is given as:
RT
aα(T)
P=
−
(3)
V−b
V(V+b) +b(V−b)
1
3

ACCEPTED MANUSCRIPT
where, T and P are the critical temperature and pressure, respectively,
ω
is the accentric
and as
c
c
c
factor and
κ
is a specific pure compound parameter. The values of T , P ,
ω
κ
c
obtained from the literature [49,50] are compiled in Table S2. Values of parameters for
PVDW model are given in Table S3. More details of PRSV-EoS and 1PVDW are given in
supporting information.
PRSV-EoS and 1PVDW model equations along with their parameters and the
1
experimental data obtained in the present study were solved simultaneously to calculate the
values of K at different temperatures and a constant pressure (P=150 bar) for Ce Zr O
eq
0.5 0.5
2
catalyst. The values K are given in Table 2.
eq
o
Assuming that the heat of reaction ꢀHr is constant within the temperature range of
1
00-180 °C, the equilibrium constant K can be related to the T by the classical van’t Hoff
eq
equation:
o
o
r
o
r
∆
H r  ∆H − ∆G 
RT
ln Keq,T = −
+
(4)


o


RT
o
o
The values of ꢀHr and ∆Gr for Ce Zr O using the data points at T=120-160 °C
0.5
0.5
2
were found to be −45.66 kJ/mol and 25.04 kJ/mol, respectively. Kongpanna et al. [51] shows
o
o
the values of ꢀHr and ∆Gr at 25 °C were found to be −15.259 kJ/mol and 29.583 kJ/mol,
respectively.
4
. Conclusion
In the present study, DMC was produced by direct conversion of CO with methanol
2
using three different catalysts namely CeO , ZrO and Ce Zr O catalysts in presence of
2
2
0.5 0.5
2
activated molecular sieve 3A as dehydrating agent. CeO , ZrO and Ce Zr O catalysts
2
2
0.5 0.5
2
were synthesized by hydrothermal method and characterized using various techniques. CeO2
catalysts showed reflections of cubic phase with average crystalline sizes 7.09 nm, ZrO2
1
4

ACCEPTED MANUSCRIPT
catalysts showed reflections of tetragonal phase with average crystalline sizes 9.45 nm and
Ce Zr O showed tetragonal phase with average crystalline sizes 7.09 nm. BET surface
0
.5 0.5
2
2
-1
area of CeO , ZrO and Ce Zr O were found to be 88, 70, and 117 m g , respectively.
2
2
0.5 0.5
2
Ce Zr O catalyst showed the highest surface area, specific pore volume and average pore
0
.5 0.5
2
diameter among all the synthesized catalysts. The basic sites density and acidic site density of
synthesized catalysts was in the order: ZrO < CeO < Ce Zr O ; and that the basic and
2
2
0.5 0.5
2
acidic site density per unit area followed the similar order. DMC production was found to be
highly dependent on the acid and base sites of the catalysts. Under optimized reaction
condition of reaction temperature=120 °C, reaction time=24 h, catalysts dose=1.25 g with
pressure=150 bar, optimum yield of DMC was obtained as 2.56 mmol/g-cat for Ce Zr O .
0
.5 0.5
2
o
o
The values of the heat of reaction (ꢀHr ) and Gibbs free energy change (ꢀGr ) for Ce Zr O
2
0.5
0.5
were found to be -45.66 kJ/mol and 25.04 kJ/mol, respectively.
Acknowledgments
One of the authors, namely Praveen Kumar, is thankful to Deutscher Akademischer
Austausch Dienst (DAAD), Germany, for providing financial support to carry out this work
under a Sandwich Model Scholarship.
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Table 1. Crystallite size, textural CO -TPD and NH -TPD analysis of CeO , Ce Zr O and
2
3
2
0.5 0.5
2
ZrO catalysts.
2
Catalysts Properties
Crystallite size (nm)
CeO2
9.48
0.3124
88
Ce Zr O
ZrO2
0.5
0.5
2
a
7.09
9.45
a
Lattice constant d (nm)
0.3051
117
0.2950
70
2
BET surface area (m /g)
3
-1 b
Specific pore volume (cm g )
0.12
5
0.237
8.41
0.202
7.89
c
Average Pore diameter (nm)
-
1 d
CO adsorption (mmol.g )
0.4154 (117) 0.6487 (88)
0.2512 (78)
3.588
0.511 (93)
7.3
2
-2
d
Basic site density (µmol.m )
4.721
5.550
-
1 e
NH adsorption (mmol.g )
0.793 (110)
9.012
1.893 (96)
16.18
3
-
2 e
Acidic site density (µmol.m )
a
Crystallite size and lattice constant are calculated by Scherrer equation.
b
BJH desorption cumulative pore volume of pores in the range 17 to 3000 Å.
c
BJH desorption average pore diameter.
d
CO -TPD of for basicity and basic site and temperature (°C) at maxima is given in brackets.
2
e
NH -TPD of for acidity and acidic site and temperature (°C) at maxima is given in brackets.
3
2
2

ACCEPTED MANUSCRIPT
Table 2. Values of K for DMC synthesis by direct CO conversion using Ce Zr O
2
eq
2
0.5 0.5
catalyst at different temperatures and a constant pressure, P=150 bar.
T (K)
yo,MeOH
yo, CO2
Xeq,MeOH,
yeq,MeOH
yeq, CO2
yeq,DMC
yeq,H2O
K (l/mol)
eq
experimental
-
-
-
-
8
7
7
7
3
3
3
4
4
4
53 0.686055 0.313945 0.00330459 0.684985 0.313975 0.000519638 0.000519638 3.930×10
73 0.697797 0.302203 0.0072792 0.695485 0.302307 0.001104294 0.001104294 2.221×10
93 0.708692 0.291308 0.00945482 0.705748 0.291483 0.001384505 0.001384505 4.313×10
13 0.718828 0.281172 0.00687782 0.716735 0.281324 0.000970785 0.000970785 2.608×10
-
7
33 0.728283 0.271717 0.0050323 0.726785 0.271843 0.000685724 0.000685724 1.620 ×10
-
8
53 0.737123 0.262877 0.00288836 0.736282 0.262957 0.000380307 0.000380307 6.370×10
2
3

ACCEPTED MANUSCRIPT
Table 3. Comparison of various catalysts used for synthesis of DMC by direct
conversion of CO at various reaction conditions.
2
Catalyst @
Reaction
CO2
moles
mmol)
604
%Conv
ersion
DMC
Yield
Reference
[30]
Preparation Method
Condition
(
of CO2 (mmol)
Sonicated Sn-SBA-
P=20 MPa,
T=423 K, t=15h,
m=0.1
0.268
0.81
1
5@ Template
@
Sn-SBA-15@
Template
P=20 MPa,
T=423 K, t=15h,
m=0.1
604
123
0.099
0.324
0.3
[30]
@
Ce Zr O @ Sol-gel
P=6 MPa, T
0.7
[28]
0
.4 0.6
2
=443 K, t=3h,
m=0.7 g
@
@
Gd-Ce Zr O @
P=6 MPa, T=443
K, t=3h, m=0.7 g
P=62.8MPa,
123
134
1.131
0.598
0.7
0.4
[28]
[29]
0
.4 0.6
2
Sol-gel
Cu-CeO @
2
Impregnation
T=393, t=4h,
m=0.1 g
@
@
CeO @ Sol-gel
P=5 MPa, T=413
K, t=2h, m=0.1 g
P=5 MPa, T=413
K, t=2h, m=0.1 g
P=15 MPa,
145.57
145.57
294
0.724
0.028
0.534
0.021
2.04
[26]
[26]
2
ZrO @ Sol-gel
2
CeO @
1.39
0.51
1.82
This work
2
Hydrothermal
T=393K, t=24 h,
m=1.25 g
ZrO @ Hydrothermal
P=15 MPa,
294
294
0.45
2.67
This work
This work
2
T=393K, t=24 h,
m=1.25 g
Ce Zr O @
P=15 MPa,
0
.5 0.5
2
Hydrothermal
T=393K, t=24 h,
m=1.25 g
@
Calculated from conditions given in the respective literature. P: Pressure; T: temperature; t: time; m:
mass of catalyst.
2
4

ACCEPTED MANUSCRIPT
CeO2
Ce0.5Zr0.5O2
ZrO2
20
30
40
50
60
70
80
90
100
2θ (degrees)
(a)
(b)
Fig. 1. (a) XRD pattern of CeO , Ce Zr O and ZrO catalysts and; (b) TEM image of
2
0.5 0.5
2
2
Ce Zr O catalyst with SEAD patterns.
0
.5 0.5
2
2
5

ACCEPTED MANUSCRIPT
0
.007
1
60
40
20
00
(1)
CeO(1)
2
CeO(1)
2
Ce Zr O(2)
1
0.006
0.005
0.5 0.5 2
Ce Zr O (2)
0.5 0.5
2
ZrO(3)
2
1
(3)
ZrO(3)
2
1
0
.004
.003
.002
.001
.000
8
0
0
6
0
(3)
(
2)
0
4
0
(2)
0
2
0
(1)
0
0
0.0
0.2
0.4
0.6
0.8
1.0
1
10
Porewidth(nm)
Relativepressurep/p0
(a)
(b)
Fig. 2. (a) Nitrogen adsorption-desorption isotherms; (b). Variation of pore volume and pore
area with pore diameter of CeO , Ce Zr O and ZrO catalysts.
2
0.5 0.5
2
2
2
6

ACCEPTED MANUSCRIPT
CeO (1)
2
(1)
CeO (1)
2
Ce Zr O (2)
Ce Zr O (2)
0.5
0.5
2
0.5
0.5
2
ZrO (3)
ZrO (3)
2
2
(2)
(2)
(1)
(3)
(2)
50
100 150 200 250 300 350 400 450 500 550
50
100 150 200 250 300 350 400 450 500 550
o
o
Temperature ( C)
Temperature ( C)
(a)
(b)
Fig. 3. (a). CO -TPD; (b). NH -TPD of CeO , Ce Zr O and ZrO catalysts.
2
3
2
0.5 0.5
2
2
2
7

ACCEPTED MANUSCRIPT
CeO2
ZrO2
Ce Zr O
2
0
.5
0.5
Fig. 4 SEM image of CeO , ZrO and Ce Zr O catalysts.
2
2
0.5 0.5
2
2
8

ACCEPTED MANUSCRIPT
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
3
2
1
0
3
2
1
0
1.4
Basicity
Acidity
Ce Zr
0.5
0.
1
1
0
0
0
0
0
.2
5O2
CeO2
.8
.6
.4
.2
ZrO2
DMC Yield (mmol/g.cat)
0
.793
0.2
%
Methanol Conversion
0
.1
0
0
1
2
3
DMC Yield (mmol/g.cat.)
Catalysts
(a)
(b)
Fig. 5. (a) Methanol Conversion and DMC yield over CeO , Ce Zr O and ZrO catalysts;
2
0.5 0.5
2
2
(
b) Correlation between acidic-basic and catalytic activity of CeO , Ce Zr O and ZrO
2 0.5 0.5 2 2
catalysts; Reaction conditions: (Methanol=25.03 ml, mcatalyst dose=1.25 g, P=150 bar, T=120
C, t =24 h).
°
r
2
9