Dimethyl carbonate synthesis from carbon dioxide using ceria-zirconia catalysts prepared using a templating method: Characterization, parametric optimization and chemical equilibrium modeling

Article abstract of DOI:10.1039/c6ra22643d

In this paper, a series of Ce_xZr_1-xO₂ solid solution spheres were synthesized by exo- and endo-templating methods and tested for dimethyl carbonate (DMC) synthesis using direct conversion of CO₂. The synthesized catalysts were characterized by X-ray diffraction (XRD), N₂-physisorption, scanning electron microscopy (SEM), and CO₂/NH₃-temperature-programmed desorption (TPD). Formation of Ce_xZr_1-xO₂ solid solutions with tetragonal and cubic crystal structures depending on cerium/zirconium compositions was confirmed by XRD analysis. The specific surface area of the mixed oxide decreased and the average pore diameter increased with an increase in the ceria content, with the exception of the mixed oxides with x = 0.4-0.5 i.e. Ce_0.4Zr_0.6O₂ and Ce_0.5Zr_0.5O₂. The basic and acidic site density of the synthesized catalysts was in the order: ZrO₂ < CeO₂ < Ce_0.5Zr_0.5O₂, and the basic and acidic site density per unit area followed the same order. The best Ce_0.5Zr_0.5O₂ catalyst was further used for the optimization of reaction conditions such as reaction time, reaction temperature, catalyst dose and reusability for DMC synthesis. Furthermore, study of chemical equilibrium modeling was done using the Peng-Robinson-Stryjek-Vera equation of state (PRSV-EoS) along with the van der Waals one-fluid reaction condition so as to calculate change of Gibbs free energy (ΔG°) and heat of reaction (ΔH°).

Full text of DOI:10.1039/c6ra22643d

View Article Online
View Journal
RSC Advances
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: P. Kumar, P. With,
V. C. Srivastava, K. Shukla, R. Gläser and I. M. Mishra, RSC Adv., 2016, DOI: 10.1039/C6RA22643D.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
rsc.li/rsc-advances

Page 1 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
Dimethyl carbonate synthesis from carbonꢀdiꢀoxide by ceriaꢀzirconia catalysts prepared
using templating method: Characterization, parametric optimization and chemical
equilibrium modeling
Praveen Kumara,b, Patrick Withb,c, Vimal Chandra Srivastavaa,*, Kartikeya Shuklaa, Roger
Gläserb, Indra Mani Mishraa,d
aDepartment of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee
247667, Uttarakhand, India
bInstitute of Chemical Technology, Universität Leipzig, Linnéstraße 3, 04103 Leipzig,
Germany
cChemical Division, Leibniz Institute for Surface Modification, Permoserstr. 15, 04318
Leipzig, Germany
dDepartment of Chemical Engineering, Indian Institute of Technology (ISM), Dhanbad,
826004, India
*Corresponding Author: Phone: +91–1332–285889; fax: +91–1332–276535;
E–mails: vimalcsr@yahoo.co.in, vimalfch@iitr.ac.in (V.C. Srivastava); praveen.singh@daadꢀ
alumni.de, praveen.zon@gmail.com (P. Kumar); patrick.with@iomꢀleipzig.de (P. With);
kartikeyashr@gmail.com
(KS);
roger.glaeser@uniꢀleipzig.de
(Roger
Gläser);
immishra49@gmail.com, mishra.im.che@ismdhanbad.ac.in (I.M. Mishra)
Abstract
In this paper, a series of CexZr1ꢀxO2 solid solutions spheres were synthesized by exoꢀ
and endoꢀtemplating method and tested for dimethyl carbonate (DMC) synthesis using direct
conversion of CO2. The synthesized catalysts were characterized by Xꢀray diffraction (XRD),
N2ꢀphysisorption, scanning electron microscopy (SEM), CO2/NH3ꢀ temperatureꢀprogrammed
desorption (TPD). Formation of CexZr1ꢀxO2 solid solutions with tetragonal and cubic crystal
structures depending on cerium/zirconium compositions was confirmed by XRD analysis.
The specific surface area of the mixed oxide decreased and the average pore diameter
increased with an increase in the ceria content, with the exception of the mixed oxides with
x=0.4−0.5 i.e. Ce0.4Zr0.6O2 and Ce0.5Zr0.5O2. The basic and acidic site density of the
synthesized catalysts was in the order: ZrO2 < CeO2 < Ce0.5Zr0.5O2, and the basic and acidic
site density per unit area followed the same order. The best Ce0.5Zr0.5O2 catalyst was further
used for the optimization of reaction conditions such as reaction time, reaction temperature,
1

RSC Advances
Page 2 of 38
View Article Online
DOI: 10.1039/C6RA22643D
catalysts dose and reuseability for DMC synthesis. Further, study of chemical equilibrium
modeling was done using Peng–Robinson–Stryjek–Vera equation of state (PRSV−EoS) along
with the van der Waals one−fluid reaction condition so as to calculate change of Gibbs free
energy (∆G°) and heat of reaction (∆H°).
Keywords: CO2 conversion; CexZr1ꢀxO2 solid solutions spheres; Dimethyl carbonate; NH3ꢀ &
CO2ꢀTemperature programmed desorption; Thermodynamic analysis.
Introduction
Dimethyl carbonate (DMC) production is green chemical with growing research
interest in past decade. It is an environment friendly raw material and an alternative for many
toxic, carcinogenic and highly corrosive reagents such as phosgene, chloromethane, dimethyl
sulfate, alkyl halide which are used in alkylation and carbonylation reactions.1,2 It is widely
used for the synthesis of various chemical such as pharmaceuticals, polymers, foodstuff,
agrochemicals, antioxidants, flavoring agents, dyestuff, solvents in electrolytes in lithium ion
batteries, adhesives and coating due to its low toxicity, excellent biodegradability, high
versatility and bioaccumulation. DMC is also used as an fuel additive as an octane enhancer
due to the fact that its high oxygen content three times higher as compared to methyl tertiary
butyl ether (MTBE).3,4
Various traditional and developing methods are used for the synthesis of DMC. The
methanol oxidative carbonylation, methylnitrite carbonylation and phosgenation processes
are fullꢀscale commercial methods used for the DMC synthesis.2,5ꢀ9 Phosgenation process is
now abandoned because of the hazards associated. Transesterification of ethylene carbonate
and urea with methanol and conversion of CO2 are in the developing stage for the synthesis
of DMC. Consequently, direct CO2 conversion reaction is being developed for its industrial
feasibility. It is the most desired method for the synthesis of DMC due to its environmentꢀ
2

Page 3 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
friendly nature associated green chemistry, low cost and easy availability of the materials.2,10ꢀ
12
However, this method has difficulties such as activation of methanol/CO2 and chemical
equilibrium.13 To overcome these problems it is essential to develop effective catalysts which
can relax chemical equilibrium and help in activation of methanol/CO2.14
Various homogeneous catalysts and heterogeneous have been studied for increasing
the DMC yield during conversion of CO2 with methanol to produce DMC.15ꢀ17 On the other
hand, some acidic compounds such as phosphoric acid H3PO4−ZrO2 or H3PW12O4−ZrO2,18
and ZrO2−KCl;19 active metal catalysts such as zirconium oxide (ZrO2),20 cerium oxide
20,21
(CeO2),
copper nickel/graphite,22 CuꢀFe,23 Ce0.5Zr0.5O2,24,25 metal oxide/Ce0.6Zr0.4O2,26
Al2O3−CeO2,27 ZrO2−MgO,28 SnO2/SiO2,29 ZrO2/SiO2,29 and Cu1.5PMo12O40;30 basic
compounds such as KOH,31 K2CO3,31 and CH3OK;31 heteropoly acids H3PW12O4/CeXZr1ꢀ
XO2,32 H3PW12O40/CeXTi1−XO2 heteropolyoxometalates,33 etc. have been used as catalysts for
DMC synthesis from CO2.34,35 Among these catalysts, acid−base biꢀfunctional catalysts have
been found more effective at different pressure and temperature. However, most of these
studies were conducted with only aim for catalyst preparation, characterization and
preliminary testing. Studies on important chemical engineering aspects such as chemical
equilibrium and thermodynamics analysis are scarce.
In the present study, composition of ceriumꢀzirconium mixed oxide has been
optimized as well as the operating conditions for the reaction have been optimized. The
synthesis of CexZr1ꢀxO2 mixed oxide spheres was carried out using exoꢀ and endoꢀtemplating
method. The synthesized catalysts were characterized by Xꢀray diffraction (XRD), N2ꢀ
sorption, scanning electron microscopy (SEM) and CO2ꢀ and NH3ꢀ temperatureꢀprogrammed
desorption (TPD). The optimum catalyst was further used under constant pressure for
optimizing reaction conditions such as reaction time, reaction temperature and catalysts dose.
Reusability of the catalyst was also studied. Further, chemical equilibrium modeling was
3

RSC Advances
Page 4 of 38
View Article Online
DOI: 10.1039/C6RA22643D
done using Peng–Robinson–Stryjek–Vera equation of state (PRSV−EoS) along with the van
o
der Waals one−fluid reaction condition so as to calculate Gibbs free energy change (
)
ꢁGr
and heat of reaction ( o ).
ꢁHr
Experimental
Materials.
Dimethyl carbonate ((CH3O)2CO) ≥ 99%, methanol 99.0%, zirconium (IV)
oxychloride octahydrate (ZrOCl2.8H2O) 99.0% and cerium (III) nitrate hexahydrate
(Ce(NO3)3.6H2O) 99.0%, were purchased from Sigma Aldrich Chemicals, GmbH. Ammonia
solution (25 wt % in H2O) and nitric acid (65 wt % in H2O) were purchased from Merck
GmbH. Carbon spheres were kindly supplied from Blücher GmbH (BrunauerꢀEmmettꢀTeller
surface area = 1748 m2/g; BarrettꢀJoynerꢀHalenda volume = 2 cm3/g; diameter = 0.45 − 0.5
mm), whereas Pluronic Fꢀ127 was purchased from BASF, Germany, and all chemicals used
were of analytical grade.
Catalyst Preparation
Ceriumꢀzirconium mixed oxide catalysts with different CexZr1ꢀxO2 (x=0 to 1) molar
ratios were synthesized using exoꢀ and endoꢀtemplating method. For this, Ce(NO3)2 and
Zr(NO3)3 were dissolved separately in 100 mL in doubleꢀdistilled water and were further
mixed together in the desired molar proportion CexZr1ꢀxO2 (x=0 to 1) under continuous
stirring at room temperature. Liquid ammonia solution was added dropꢀbyꢀdrop to the
precursor solution over a period of 0.5 h until the pH reached ~9.5 and a white/light yellow
precipitate was formed. The mixture was aged for 2 h under continuous stirring, and
thereafter it was filtered. The precipitate retained on the filter was washed with doubleꢀ
4

Page 5 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
distilled water until the pH of the filtrate became neutral. Finally, the filter cake was
transferred to a 200 mL polypropylene (PP) bottle and doubleꢀdistilled water was added to it
until the total weight of the mixture became 30 g. Thereafter, 2.5 mL HNO3 (65 wt.% in H2O)
was added to the mixture. The PPꢀbottle was transferred to an ultrasonic bath (Sonorex RK1,
Fa. Bandelin) where it was kept for 4 h until a clear sol was formed. Pluronic Fꢀ127 as
triblockcopolymer (TBC) was added such that the molar ratios of TBC to cerium along with
zirconium (nTBC/nCe+Zr) become 0.017. This ratio was the optimum to the sol as an endoꢀ
template.36,37 This mixture was again kept in the ultrasonic bath for 3 h for dissolving the
Pluronic Fꢀ127. 4.42 g preactivated (for 24 h at 110°C) polymerꢀbased spherical activated
carbon (PBSAC) was added to the nanoparticle sol and was further dried at 50°C for 12 h and
added drop of water then dry for 6 h. The prepared catalyst was activated at 600°C for 5 h
under air flow (40 cm3/min) with heating rate of 3°C/min from room temperature to 600°C
with a holding time of 1 h at 100°C and 5 h at 600°C. After calcination, light yellow CexZr1ꢀ
xO2 mixed oxide spheres were obtained. For characterization and catalytic experiments,
CexZr1ꢀxO2 mixed oxides were sieved to obtain the sphere in the size range of 0.2ꢀ0.4 mm.
Catalyst characterization
XRD was used to study the molecular structure, atoms and crystalline nature. For this
purpose, samples were crushed with a mortar before testing. Xꢀray diffractograms (Brueker
AXS, Germany) at 40 kV/30 mA with CuKα radiation (λ=1.5406Å) 0.02 step size over 2θ
scan range 5≤ 2θ ≤100°. PANalytical X'pert high score used for identification of crystalline
phase with International Centre for Diffraction Data (ICDD) database. N2 sorption isotherms
utilize the principle of physical adsorption to get the information about BET surface area,
pore volume and pore size distribution of the solid materials. Textural properties were
estimated using N2 sorption measurements at −197°C (Micromeritics ASAP 2020). BET
isotherm was used for calculating the surface area of the porous material by physical
5

RSC Advances
Page 6 of 38
View Article Online
DOI: 10.1039/C6RA22643D
adsorption of N2 gas at its boiling temperature. TPD was used to study the binding interaction
of adsorbates CO2 or NH3 on the catalyst surface and provides the information regarding the
adsorbate bound on the surface. It is known that the high temperature desorption peak has
stronger bonding of the adsorbates on the catalyst surface. In the TPD study, initially a
sample is saturated with the reactant gas, and then physisorbed fraction of the reactant gas is
desorbed with the help of inert a gas such as helium. After that the temperature of the sample
is increased linearly at a particular heating rate. During this process, an inert carrier gas is
passed through the sample at a particular flow rate. The amount of the desorbed CO2 or NH3
is quantified with the help of a thermal conductivity detector (TCD). Acidic and basic nature
of the synthesized catalyst was determined by the TPD (Micromeritics Chemisorb 2720) of
NH3 and CO2, respectively. To investigate the morphology of the synthesized catalysts,
scanning electron microscope (SEM) was used. Elemental composition and morphology of
the cerium/zirconium synthesized materials was investigated by quanta 200 FEG (FEI
Netherlands). Initially, the prepared sample was spread on the sample holder and then the
samples were gold coated using sputter coater (Edwards S150) to increase the conductivity of
the preliminary materials. After that, the prepared samples were used for taking image using
FEꢀSEM at 20 kV under vacuum. Thereafter, the energyꢀdispersive X−ray spectroscopy
(EDX) was carried out find out the metal content of the sample. Elemental mapping was used
for understanding the metal distribution in the prepared catalysts. Metal loading on the
catalysts was determined by AAS and ICP−OES. For determining the elemental ratio of the
catalysts, 1.0 g of the catalyst was soaked in 10 ml 65% nitric acid for 24 h at room
temperature so as to dissolve the metals from the catalysts. The solutions were filtered and
the filtrate was used for the determination of the metal concentration by AAS (Avanta M by
GBC Scientific Equipment Pvt Ltd.). ICPꢀOES supplied by OPTIMA 8000 von Perkin Elmer
was also used for determining the amount of metals dispersed some catalysts. For sample
6

Page 7 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
preparation, the sample (50.0±0.1) was dissolved by a microwave assisted digestion
(Multiwave 3000 from Anton Paar) using 2 ml HF (48 wt.%, Suprapur, Merck), 2 ml HNO3
(69 wt.%, Supra, Roth), 2 ml HCl (35 wt.%, supra, Roth) and 3 ml H2SO4 (85 wt.%,
suprapur, Merck). Microwave conditions were: 1100 Watt/ramp for 20 min hold for 30 min
cool for 15 min. After the microwave digestion, 12 ml H3BO4 (for complexation of HF) and 1
ml HlO4 were added. Afterwards, the sample was digested for a second time using
microwave conditions as used earlier. After the microwave treatment, H2O was added until a
volume of 50 ml was obtained. The eight samples were prepared as known concentration for
preparation of the calibration curve. Concentration of unknown solution was estimated using
this calibration curve.
Catalytic activity
Catalytic conversion of CO2 with methanol to produce DMC was performed in the
reaction autoclave (i.e. batch reactor) made by Berghof, Germany (ModelꢀBHLꢀ800). A
magnetic stirrer was used to make the reactant mixture homogeneous during the reaction. A
rubber made Oꢀring was used in between the reaction autoclave and the head of the
instrument panel to bind them smoothly and to make the reaction chamber air tight. Initially,
reactor was filled with the required methanol and the catalysts. The reactor was heated to the
reaction temperature (100−180°C) and pressurized with CO2 up to a pressure of 150 bar and
maintained for 6−48 h for the reactor to proceed. After (6−48 h), the reactor was cool down
the product mixture to at < −20°C with using ice bath, and thereafter, centrifugation was used
for removing catalyst from the product mixture. All the reactions were studied in the presence
of activated molecular sieve 3A as a dehydrating agent and at constant stirrer speed of 600
revolutions/min. Catalyst was washed with methanol and dried at 150°C for 12 h and then
was activated at 500°C for 4 h after each cycle. Similarly, molecular sieve was activated at
240°C for 4 h after each cycle.
7

RSC Advances
Page 8 of 38
View Article Online
DOI: 10.1039/C6RA22643D
Results and discussion
Catalyst characterization
Xꢀray diffraction. XRD profiles of CexZr1ꢀxO2 (x=0 to 1) catalysts are given in Figure 1a. No
separate peak is found in the ceriumꢀzirconium mixed oxide. Pure zirconia (x=0) tetragonal
phase showed the characteristic (111) reflection at 2θ=30°. With an increase in ceria amount,
the reflex at 2θ=30° shifted towards lower 2θ values. For pure ceria, a major peak at 2θ=28°
was observed, which is typical of the cubic fluorite structure of ceria.38ꢀ40 For ceria content of
60 mol.%, the crystal structure was tetragonal. The two peaks at 2θ=29° and 35° for the two
samples of Ce0.4Zr0.6O2 and Ce0.5Zr0.5O2 showed much lower intensity than that for other
mixed oxides with tetragonal and cubic crystal structures. This is because of the crystallite
formation for the samples having cerium/zirconium in the molar ratio ≈1.41 This would
explain the sudden increase in the specific surface area of these two samples. XRD of
Ce0.5Zr0.5O2 synthesized using exotemplate and endoꢀ/exoꢀtemplate method
(nTBC/nCe+Zr=0.017) is shown in Figure 1b. It may be seen that the reflexes of Ce0.5Zr0.5O2,
synthesized with endoꢀ/exoꢀtemplate are more intense than that with exoꢀtemplate. This
suggests that in the presence of larger particles, endoꢀtemplates arise. This hypothesis is
supported by the lower values of the specific surface and the specific pore volume (Table 1).
At the same time, the mean pore diameter is larger. Substitution of metals in the crystal lattice
increases the oxygen vacancies which in turn help in increasing the reactive catalytic
sites.42,43 Also, the cations of octahedral sites help in generation of other active/intermediate
species via conjugation of redox pairs.44 High catalytic activity in these doped catalysts is due
to improved electron transfer mechanism and due to more oxygen vacancies.45,46 The ZrO2
incorporation in CeO2 improves the thermal resistance and more importantly the redox
8

Page 9 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
capacity of CeO2−ZrO2 mixed metal oxide. As Ce4+ (1.01 Å) has a larger ionic radius than
Zr4+ (0.80 Å), shrinkage of the lattice due to the replacement of Ce+4 with Zr+4 affects the
lattice structure. This lowers the energy for Ce4+ reduction and enhances the CeO2
reducibility.47ꢀ49
Surface morphology and elemental analysis. SEM micrographs (Figure 2) of CeO2, ZrO2
and Ce0.5Zr0.5O2 with particles size distribution in the range of dP=0.2−0.4 mm. The EDX
analysis of the Ce0.5Zr0.5O2 catalyst is shown in the Figure 2. Analysis has also been carried
out by ICPꢀOES. The structural chemical compositions of the Ce0.5Zr0.5O2 and Ce0.4Zr0.6O2
catalysts are shown in Table 2. The compositions of the synthesized catalysts are similar to
the desired initial metal composition.
Textural properties. The nitrogen sorption results of ceriumꢀzirconium mixed oxides
CexZr1ꢀxO2 (x=0 to1) are summarized in Table 1. Among all the synthesized catalysts,
Ce0.4Zr0.6O2 and Ce0.5Zr0.5O2 were found to possess highest BET surface area of 123 and 121
m2/g and minimum pore diameter of 11.9 and 10.0 nm, respectively. It can be seen from the
Figure 3, that the specific surface area of the mixed oxides is a function of the CeO2 content
in the synthesized catalyst. Adsorption/desorption isotherm and the pore volume distribution
of CeO2, Ce0.5Zr0.5O2, ZrO2 are shown in Figure 4a and Figure 4b, respectively. All the
sorption isotherms of are type IV isotherm with the hysteresis loop, typical of mesoporous
systems.50,51 Peak corresponding to maximum pore volume shifts towards higher pore with
for mixed CeꢀZr oxide as compared to pure CeO2 or ZrO2. Mixed oxide exhibit specific
surface areas between 112 m2/g (pure zirconia) and 28 m2/g (pure ceria). With an increase in
the content of ceria, the specific surface area of the mixed oxide decreased and the average
pore diameter increased with an increase in the ceria content, with the exception of the mixed
oxides with x=0.4−0.5 i.e. Ce0.4Zr0.6O2 and Ce0.5Zr0.5O2 (Figure 3). These two catalysts
exhibit specific surface areas > 120 m2/g with the mean pore diameter of ~10 nm. At these
9

RSC Advances
Page 10 of 38
View Article Online
DOI: 10.1039/C6RA22643D
values of x, the cause of increase in surface area is the formation of structurally homogeneous
solid solution.52,53 Laosiripojana et al.54 shows the specific surface area of 49, 47, and 46.5
m2/g with average particle size of 50–80 nm using Ce/Zr molar ratio 1/3, 1/1, and 3/1,
respectively. Shotipruk et al.55 shows the specific surface area 135, 120, and 115 m2/g with
Ce/Zr molar ratio 1/3, 1/1, and 3/1, respectively and Laosiripojana and Assabumrungrat,56
shows ≈ 20 m2/g with 5% Ni on Ce/Zr molar ratio 1/3, 1/1, and 3/1, respectively.
CO2–TPD. The basic properties of CeO2, Ce0.5Zr0.5O2 and ZrO2 catalysts were determined
from the CO2ꢀTPD profile (Figure 5a) and the results are given in Table 3. Basic properties of
the catalysts depend upon the temperature profile in the weak region (< 200°C), moderate
region (200−450°C) and the strong region (> 450°C). Weak basic sites are due to the
interaction between the surface and the OH groups and the formation of bicarbonate;
moderate basic sites are due to the sites Mx+–O2− pairs and the formation of biꢀdentate and
bridged carbonates; and the strong basic sites are due to the low coordination O2− ions and the
formation of uniꢀdentate carbonates.57 In the synthesized catalysts, basicity was found in the
week and strong regions corresponding to ∼115 and ∼717°C. Basic site density of the
synthesized catalysts was in the order: ZrO2 (0.40 mmol/g) < CeO2 (0.41 mmol/g) <
Ce0.5Zr0.5O2 (1.93 mmol/g), and the basic site density per unit area followed the same order
(Table 3). In ceria catalyst, Ce may have variable valency (Ce3+ and Ce4+). Moreover, ceria is
also a typical Lewisꢀbase catalyst, which is responsible for its application in several baseꢀ
catalyzing processes. Thus, the mixed metal oxides possess higher basic site density as
compared to single oxide catalysts.33 Zhang et al.16 and Lee et al.17 reported maximum basic
site density of 0.276 and 0.017 mmol/g, respectively, for Ce0.6Zr0.4O2 catalysts.
NH3–TPD. NH3ꢀTPD spectra of CeO2, Ce0.5Zr0.5O2 and ZrO2 catalysts are shown in Figure
5b and the results are summarized in Table 3. Desorption peaks of NH3 are in the temperature
range of 50−900°C. The NH3 desorption peaks at 110°C and 667°C for CeO2 and 156°C and
10

Page 11 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
643°C for ZrO2 were observed in the week and strong regions. Ce0.5Zr0.5O2 catalyst shows
peaks in all the three regions at 106°C, 294°C and 666°C. Acidic site density of synthesized
catalysts is found to be: CeO2 (0.94 mmol/g) < ZrO2 (1.52 mmol/g) < Ce0.5Zr0.5O2 (2.48
mmol/g). Thus, the Ce0.5Zr0.5O2 catalyst has the highest acidic site density and the CeO2 has
the lowest acidic site density. Thus, the Ce0.5Zr0.5O2 catalyst has the highest density of basic
and acidic sites as compared to other catalysts. Therefore, this catalyst can act as an acidꢀbase
biꢀfunctional catalyst. It has been reported that both the basic and acidic sites are required for
the direct conversion of CO2 to produce DMC.16,17,20
Catalytic activity of catalysts for DMC synthesis
The direct catalytic conversion of CO2 with methanol for the synthesis of DMC was
studied in the presence of CeO2, ZrO2 and Ce0.5Zr0.5O2 catalysts. Negligible conversion of
CO2/methanol to DMC after for 24 h at a pressure of 120°C temperature and 150 bar were
observed in the blank experiment without any catalyst. In the presence of a Ce0.5Zr0.5O2
catalyst, methanol is activated to form CH3O− and H+ in the presence of basic sites and
CH+3
and OH− in the presence of acidic site present on the surface of the catalyst. Methoxy species
(CH3O−) react with CO2 in the presence of basic site to form methoxyl carbonyl ions.
Methanol is activated at the acidic site to form CHꢀꢁ and OH− ions. Methoxyl carbonyl ion
reacts with + to form DMC, and OH─ reacts with H+ to form water (Figure 6). As such,
CH3
higher basicity and acidity in the catalysts facilitates DMC synthesis from CO2 and
methanol.44 Reaction mechanism for DMC synthesis from direct conversion of CO2 with
methanol in the presence of catalyst is shown in Figure 7. Ce0.5Zr0.5O2 catalyst showed better
activity as compared to CeO2, and ZrO2 (Figure 7). The order of the activity of the catalysts
followed: ZrO2 (0.912 mmol DMC/g.cat.) < CeO2 (1.384 mmol DMC/g.cat.) < Ce0.5Zr0.5O2
(2.921 mmol DMC/g.cat.). Best active Ce0.5Zr0.5O2 catalyst was further used for the
11

RSC Advances
Page 12 of 38
View Article Online
DOI: 10.1039/C6RA22643D
optimization of the reaction conditions such as reaction temperature, catalyst dose and
reaction time for CO2 conversion.
The influences of reaction time for DMC synthesis in the presence of Ce0.5Zr0.5O2
catalyst is shown in Figure 8a. It can be seen from the Figure 8a that the DMC formation
(1.989−2.921 mmol/g.cat.), methanol conversion (0.644−0.945%) and CO2 conversion
(1.567−2.310%) increased with an increase in reaction time up to 24 h. Further increase in the
reaction time showed a decrease in the DMC yield and CO2/methanol conversion. It may be
because of the saturation of the molecular sieves due to the adsorption of water.
The effect of catalyst dose on the DMC yield and the CO2 conversion is shown in
Figure 8b. It can be seen in that the maximum DMC yield and CO2 conversion were obtained
at a catalyst dose of 1.25 g. Further increase in the catalyst dose diminished the DMC yield.
This may be because of the formation of agglomerates at higher catalyst doses in the reaction
mixture.
The influence of the reaction temperature for the DMC synthesis and CO2/methanol
conversion is shown in Figure 8c. Initially, the DMC yield (1.021−2.9212 mmol/g cat),
methanol conversion (0.331−0.945 mmol/g cat) and CO2 conversion (0.804−2.300 mmol/g
cat.) increased with an increase in the reaction temperature in the range of 80−120°C. Above
120°C, an increase in the reaction temperature quickly decreased the DMC yield
(2.125−0.8924 mmol/g cat), methanol conversion (0.687−0.2888 mmol/g cat) and the CO2
conversion (1.673−0.7026 mmol/g cat). Thus, the optimum DMC yield was obtained at
120°C. The decrease in the DMC yield, methanol and CO2 conversion may be because of the
poor solubility of CO2 in methanol and due to decomposition of DMC.58
It can be seen that the acidic and basic properties directly influence the catalytic
activity of DMC synthesis. The reuse of the Ce0.5Zr0.5O2 catalyst was investigated at optimum
reaction conditions (T=120°C, t=24 h and catalyst amount=1.25 g) in five consecutive batch
12

Page 13 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
cycles (Figure 9). Almost similar DMC yield was found in all the batch cycles. Thus, the
Ce0.5Zr0.5O2 catalyst is an effective catalyst with long life and can be used a number of times.
Chemical equilibrium modeling
Synthesis of DMC from direct conversion of CO2 and methanol can be related to the
equilibrium constant as shown below:
aDMC
a2MeOH
a
H 2
O
K eq (T) =
a
CO2
1
X2
1ꢀ0.5yMeOH,0Xeq,MeOH
2
0
CO2
ϕM0 eOH
)
ϕ
0
(
)
×

 

ϕDMC
ϕ
(
eq,MeOH






P
H2O
2
=



 
 
eq 



2
1
2
(
ϕMeOH
)
2ϕCO
ϕD0MCϕH0 O
P


2
2
yMeOH,o 1ꢀX
Θ
ꢀ
X
(
)
eq


CO2

eq 
(6)
where,
.
Θ
= y
yMeOH,o
CO2 ,0
CO2
The Peng–Robinson–Stryjek–Vera equation of state (PRSV−EoS)59 along with the
van der Waals one−fluid (1PVDW) mixing rule,60,61 were used to calculate the rules fugacity
coefficient of species in the mixture. PRSV−EoS is given as:
RT
aα(T)
P =
−
(7)
V−b
V(V+ b) + b(V−b)
where,
a = 0.45724 R 2 Tc2 Pc
(8)
(9)
b = 0.0778 RTc/Pc
2
(10)
1 2
r

α = 1 + m(ω) 1 − T
(
)


m(ω) = κ + κ 1+T1 2 0.7 − T
(11)
(
)
(
)
0
1
r
r
κ0 = 0.378893 + 1.4897153ω − 0.1713184ω2 + 0.0196554ω3
(12)
κ
where, Pc and Tc are the critical pressure and temperature, respectively,
is a specific pure
ω
ω
κ as
compound parameter and
is the accentric factor. The values of Tc, Pc,
and
13

RSC Advances
Page 14 of 38
View Article Online
DOI: 10.1039/C6RA22643D
obtained from the literature are compiled in Table S1. van der Waals One−Fluid Model
(1PVDW) gives the following sets of equations which were used to obtain data of the
quadratic mixture:
1
2
a =
b =
y y 1– k a a
(13)
(14)
(
ij )(
)
∑∑
i
j
i
j
i
j
b + b


j 

i
y y 1–l
(
)
∑∑

i
j
ij
2
i
j

where, lij and kij are the single binary interaction parameters, which are used to determine the
mixture parameters a, and b in the PRSV−EOS. Values of lij and kij were obtained from the
ꢁHor
literature and are given in Table S2. Assuming that the heat of reaction
is constant
within the temperature range of 100−180°C, the equilibrium constant Keq can be related to the
T by the classical van’t Hoff equation:
o
ꢀH ro
RT
ꢀH r − ꢀGro






(15)
ln Keq ,T = −
+
RT o
Equations 6 to 14 were solved simultaneously using the parameters given in the
Tables S1 and S2 to calculate the values of Keq at various temperatures. The values Keq at
373, 393 413, 433 and 453 K were found to be 6.811×10ꢀ8, 3.629×10ꢀ7, 1.713×10ꢀ7, 1.823×10ꢀ
8
and 1.327×10ꢀ9 l/mol, respectively (Table S3). The values of
and
ꢀHro
ꢀGro
(using eq. 15)
for Ce0.5Zr0.5O2 using the data points at T=120−180°C were found to be −139.76 kJ/mol and
1.54 kJ/mol, respectively. and values of −15.259 kJ/mol and 29.583 kJ/mol,
ꢁHor
ꢀGro
respectively have been reported in the literature.62 Table 4 compares the equilibrium, kinetic
and thermodynamic parameters as obtained in the present study with those reported in the
literature.63ꢀ68 It seems that a direct comparison of these parameters for different catalysts is
not possible as these were prepared using different methods and evaluated under different
operating conditions.
14

Page 15 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
Conclusions
In this paper, porous and spherically shaped ceriumꢀzirconium catalysts (CexꢀZr1ꢀxO2)
with different molar ratios were synthesized using an exoꢀ and endoꢀtemplating method using
PBSAC as exoꢀtemplate, and pluronic Fꢀ127 as endoꢀtemplate. XRD pattern showed the
reflexes of cubic phase in CeO2, tetragonal phase in ZrO2 and Ce0.5Zr0.5O2. The synthesized
catalysts showed BET surface between 28ꢀ112 m2/g and pore volume in the range of 0.2ꢀ0.42
cm3/g. An increase in the ceria content was found to decrease the specific surface area of the
mixed oxides except for x=0.4ꢀ0.5. At these values of x, formation of structurally
homogeneous solid solution increased in surface area. These catalysts were tested for direct
conversion of CO2 with methanol for the production DMC in a batch reactor. The CexꢀZr1ꢀx (x
= 0.5) catalyst was found to posses highest amount of basic and acidic sites among all the
catalysts, and gave highest DMC yield. At optimized condition (pressure=150 bar,
temperature=120°C, reaction time=24 h, catalysts dose=1.25 g), the activity of the catalysts
was in the following order: ZrO2 (0.912 mmol DMC/g.cat.) < CeO2 (1.384 mmol
DMC/g.cat.) < Ce0.5Zr0.5O2 (2.921 mmol DMC/g.cat.). During five consecutive reuse cycles
of Ce0.5Zr0.5O2 catalyst, only marginal change in DMC yield and methanol conversion was
ꢀHro
ꢀGro
for Ce0.5Zr0.5O2 catalyst were found to be −139.76
observed. The values of
and
kJ/mol and 1.54 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.
15

RSC Advances
Page 16 of 38
View Article Online
DOI: 10.1039/C6RA22643D
References
1. Pacheco MA, Marshall CL. Review of dimethyl carbonate (DMC) manufacture and its
characteristics as a fuel additive. Energ. Fuels 1997;11:2–29.
2. Keller N, Rebmann G, Keller V. Catalysts, mechanisms and industrial processes for the
dimethyl carbonate synthesis. J. mol. Catal. A: Chem. 2010;317:1–18.
3. Santos BAV, Silva VMTM, Loureiro JM, Rodrigues AE. Review for the direct synthesis
of dimethyl carbonate. Chem. Bio. Eng. Rev. 2014;1:214–229.
4. Kumar P, Srivastava VC, Mishra IM. Dimethyl carbonate synthesis from propylene
carbonate with methanol using Cu−Zn−Al catalyst. Energy Fuels 2015;29:2664−2675.
5. Zhang P, Wang S, Chen S, Zhang Z, Ma X. The effects of promoters over PdCl2ꢀ
CuCl2/HMS catalysts for the synthesis of diethyl carbonate by oxidative carbonylation of
ethanol. Chem. Eng. J. 2008;143:220–224.
6. Zhang Z, Ma X, Zhang P, Li Y, Wang S. Effect of treatment temperature on the crystal
structure of activated carbon supported PdCl2ꢀCuCl2 catalysts in the oxidative
carbonylation of ethanol to diethyl carbonate. J. Mol. Catal. A: Chem. 2007;266:202–206.
7. Yan B, Huang S, Meng Q, Shen Y, Wang S, Ma X. Ordered mesoporous carbons
supported wackerꢀtype catalyst for catalytic oxidative carbonylation. AIChE J.
2013;59:3797ꢀ3805.
8. Razzaq R, Li C, Amin N, Zhang S, Suzuki K. Coꢀmethanation of carbon oxides over
nickelꢀbased CexZr1–xO2 catalysts. Energy Fuels 2013:27:6955−6961.
9. Li C, Zhang X, Zhang S. Environmental benign design of DMC production process.
Chem. Eng. Res. Des. 2006;84:1–8.
10. Zeng HY, Deng X, Wang YJ, Liao KB. Preparation of MgꢀAl hydrotalcite by urea
method and its catalytic activity for transesterification. AIChE J. 2009;55:1229ꢀ1235.
16

Page 17 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
11. Xu S, Zeng HY, Xheng CR, Duan HZ, Han J, Ding PX, Xiao GF. Mg–Fe mixed oxides
as solid base catalysts for the transesterification of microalgae oil. RSC Adv.
2015;5:71278ꢀ71286.
12. Zeng HY, Xu S, Liao MC, Zhang ZQ, Zhao C. Activation of reconstructed Mg/Al
hydrotalcites in the transesterification of microalgae oil. Appl. Clay Sci. 2014;91–92:16–
24.
13. Honda M, Tamura M, Nakagawa Y, Tomishige K. Catalytic CO2 conversion to organic
carbonates with alcohols in combination with dehydration system. Catal. Sci. Technol.
2014;4:2830–2845.
14. Wang S, Shen H, Fan S, Zhao Y, Ma X, Gong J. Enhanced CO2 adsorption capacity and
stability using CaOꢀbased adsorbents treated by hydration. AIChE J. 2013;59:3586ꢀ3593.
15. Svec P, Olejník R, Padelková Z, Ruzicka A, Plasseraud L. C,Nꢀchelated organotin(IV)
trifluoromethanesulfonates: Synthesis, characterization and preliminary studies of its
catalytic activity in the direct synthesis of dimethyl carbonate from methanol and CO2. J
Organometallic Chem. 2012;708ꢀ709:82ꢀ87.
16. Zhang ZF, Liu ZT, Liu ZW, Lu J. DMC formation over Ce0.5Zr0.5O2 prepared by
complexꢀdecomposition method. Catal. Lett. 2009;129:428–436.
17. Lee HJ, Joe W, Song IK. Direct synthesis of dimethyl carbonate from methanol and
carbon dioxide over transition metal oxide/Ce0.6Zr0.4O2 catalysts: Effect of acidity and
basicity of the catalysts. Korean J. Chem. Eng. 2012;29:317ꢀ322.
18. Ikeda Y, Sakaihori T, Tomishige K, Fujimoto K. Promoting effect of phosphoric acid on
zirconia catalysts in selective synthesis of dimethyl carbonate from methanol and carbon
dioxide. Catal. Lett. 2000;66:59−62.
17

RSC Advances
Page 18 of 38
View Article Online
DOI: 10.1039/C6RA22643D
19. Eta V, MakiꢀArvela P, Leino AR, Kordas K, Salmi T, Murzin DY, Mikkola JP. Synthesis
of dimethyl carbonate from methanol and carbon dioxide: Circumventing thermodynamic
limitations. Ind. Eng. Chem. Res. 2010;49:9609−9617.
20. Chen L, Wang S, Zhou J, Shen Y, Zhao Y, Ma X. Dimethyl carbonate synthesis from
carbon dioxide and methanol over CeO2 versus over ZrO2: comparison of mechanisms.
RSC Adv. 2014;4:30968–30975.
21. Kumar P, With P, Srivastava VC, Gläser R, Mishra IM. Conversion of carbon dioxide
along with methanol to dimethyl carbonate over ceria catalyst. J. Environ. Chem. Eng.
2015;3:2943–2947.
22. Bian J, Xiao M, Wang S, Wang X, Lu Y, Meng Y. Highly effective synthesis of dimethyl
carbonate from methanol and carbon dioxide using a novel copper nickel/graphiteꢀ
bimetallic nanocomposite catalyst. Chem. Eng. J. 2009;147:287ꢀ296.
23. Zhou Y, Wang S, Xiao M, Han D, Lu Y, Meng Y. Novel CuꢀFe bimetal catalyst for the
formation of dimethyl carbonate from carbon dioxide and methanol. RSC adv.
2012;2:6831–6837.
24. Kumar P, With P, Srivastava VC, Gläser R, Mishra IM. Efficient ceriaꢀzirconium oxide
catalyst for carbon dioxide conversions: characterization, catalytic activity and
thermodynamic study. J. Alloys Comp. 10.1016/j.jallcom.2016.10.293.
25. Zhang ZF, Liu ZT, Liu ZW, Lu J. DMC formation over Ce0.5Zr0.5O2 prepared by
complexꢀdecomposition method. Catal. Lett. 2009;129:428–436.
26. Lee HJ, Joe W, Song IK. Direct synthesis of dimethyl carbonate from methanol and
carbon dioxide over transition metal oxide/Ce0.6Zr0.4O2 catalysts: Effect of acidity and
basicity of the catalysts. Korean J. Chem. Eng. 2012;29:317ꢀ322.
27. Aresta M, Dibenedetto A, Pastore C, Angelini A, Aresta B, Pápai I. Influence of Al2O3 on
the performance of CeO2 used as catalyst in the direct carboxylation of methanol to
18

Page 19 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
dimethylcarbonate and the elucidation of the reaction mechanism. J. Catal. 2010;269:44–
52.
28. Eta V, MäkiꢀArvela P, Salminen E, Salmi T, Murzin DY, Mikkola JP. The Effect of
alkoxide ionic liquids on the synthesis of dimethyl carbonate from CO2 and methanol
over ZrO2–MgO. Catal. Lett. 2011;141:1254–1261.
29. BallivetꢀTkatchenko D, Chambrey S, Keiski R, Ligabue R, Plasseraud L, Richard P,
Turunen H. Direct synthesis of dimethyl carbonate with supercritical carbon dioxide:
Characterization of a key organotin oxide intermediate. Catal. Today 2006;115:80ꢀ87.
30. Allaoui L, Aouissi A. Effect of the Bronsted acidity on the behavior of CO2 methanol
reaction. J. Mol. Catal. A: Chem. 2006;259:281–285.
31. Cai Q, Lu B, Guo L, Shan Y. Studies on synthesis of dimethyl carbonate from methanol
and carbon dioxide. Catal. Commun. 2009:10:605–609.
32. Lee HJ, Park S, Jung JC, Song IK. Direct synthesis of dimethyl carbonate from methanol
and carbon dioxide over H3PW12O40/CexZr1–xO2 catalysts: Effect of acidity of the
catalysts. Korean J. Chem. Eng. 2011, 28, 1518– 1522
33. La KW, Jung JC, Kim H, Baeck SH, Song IK. Effect of acid–base properties of
H3PW12O40/CexTi1−xO2 catalysts on the direct synthesis of dimethyl carbonate from
methanol and carbon dioxide: A TPD study of H3PW12O40/CexTi1−xO2 catalysts. J. Mol.
Catal. A: Chem. 2007;269:41–45.
34. Sheintuch M, MatatovꢀMeytal YI. Comparison of catalytic processes with other
regeneration methods of activated carbon. Catal. Today 1999;53:73–80.
35. MatatovꢀMeytal U, Sheintuch M. Activated carbon clothꢀsupported Pd–Cu catalyst:
Application for continuous water denitrification. Catal. Today 2005;102–103:121–127.
19

RSC Advances
Page 20 of 38
View Article Online
DOI: 10.1039/C6RA22643D
36. Kumar P, With P, Srivastava VC, Gläser R, Mishra IM. Glycerol carbonate synthesis by
hierarchically structured catalysts: catalytic activity and characterization. Ind. Eng. Chem.
Res. 2015;54:2543–12552.
37. With P, Heinrich A, Lutecki M, Fichtner S, Böhringer B, Gläser R. Zirconia with defined
particle morphology and hierarchically structured pore system synthesized via combined
exoꢀ and endotemplating. Chem. Eng. Technol. 2010;33:1712–1716.
38. Epifani M, Andreu T, AbdollahzadehꢀGhom S, Arbiol J, Morante JR. Synthesis of ceria–
zirconia nanocrystals with improved microstructural homogeneity and oxygen storage
capacity by hydrolytic sol–gel process in coordinating environment. Adv. Funct. Mat.
2012;22:2867–2875.
39. Bharali P, Saikia P, Katta L, Reddy BM. Enhancement in CO oxidation activity of
nanosized CexZr1ꢀxO2 solid solutions by incorporation of additional dopants. J. Ind. Eng.
Chem. 2013;19:327–336.
40. Zhu H, Razzaq R, Li C, Muhmmad Y, Zhang S. Catalytic methanation of carbon dioxide
by active oxygen material CexZr1−xO2 supported NiꢀCo bimetallic nanocatalysts. AIChE J.
2013;59;2567ꢀ2576.
41. Deshpande AS, Niederberger M. Synthesis of mesoporous ceria zirconia beads.
Micropor. Mesopor. Mat. 2007;101:413–418.
42. Satyle TXT, Parker SC, Catlow CRA. Surface segregation of metal ions in cerium
dioxide. J. Phy. Chem. 1994;98:13625–13630.
43. Wu L, Dey S, Gong M, Liu F, Castro RHR. Surface segregation on manganese doped
ceria nanoparticles and relationship with nanostability. J. Phys. Chem. C 2014;
118:30187–30196.
44. Jacobs JP, Maltha A, Reintjes JGH, Drimal J, Ponec V, Brongersma HH. The surface of
catalytically active spinels. J. Catal. 1994;147:294–300.
20

Page 21 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
45. Sun Z, Wang X, Liu Z, Zhang H, Yu P, Mao L. PtꢀRu/CeO2/Carbon nanotube
nanocomposites: An efficient electrocatalyst for direct methanol fuel cells. Langmuir
2010;26:12383–12389.
46. Wang L, Wang Y, Liu S, Lu L, Ma X, Deng Y. Efficient synthesis of dimethyl carbonate
via transesterification of ethylene carbonate with methanol over binary zincꢀyttrium
oxides. Catal. Commun. 2011;16:45–49.
47. Rivas BD, LópezꢀFonseca R, GutiérrezꢀOrtiz MA, GutiérrezꢀOrtiz JI. Structural
characterisation of Ce0.5Zr0.5O2 modified by redox treatments and evaluation for
chlorinated VOC oxidation. Appl. Catal. B: Environ. 2011;101:317–325.
48. Christou SY, Efstathiou AM. The effects of Pꢀpoisoning of CexZr1ꢀxO2 on the transient
oxygen storage and release kinetics. Top. Catal. 2013;56:232–238.
49. Wang W, Wang S, Ma X, Gong J. Crystal structures, acid–base properties, and
reactivities of CexZr1ꢀxO2 catalysts. Catal. Today 2009;148:323–328.
50. Abir H, Sheintuch M. Atomistic calculation of adsorption in activated carbon with poreꢀ
size distribution. J. Coll. Interf. Sci. 2010;342:445–454.
51. Taubert M, Beckmann J, Lange A, Enke D, Klepel O. Attempts to design porous carbon
monoliths using porous concrete as a template. Micropor. Mesopor. Mat. 2014;197:58–
62.
52. Fuentes RO, Baker RT. Synthesis of nanocrystalline CeO2ꢀZrO2 solid solutions by a
citrate complexation route: A thermochemical and structural study. J. Phys. Chem. C
2009;113:914–924.
53. Li Y, Wang L, Yan R, Hana J, Zhang S. Gold nanoparticles supported on Ce–Zr oxides
for the oxidative esterification of aldehydes to esters. Catal. Sci. Technol. 2015;5:3682–
3692.
21

RSC Advances
Page 22 of 38
View Article Online
DOI: 10.1039/C6RA22643D
54. Laosiripojana N, Kiatkittipong K, Assabumrungrat S. Partial oxidation of palm fatty acids
over CeꢀZrO2: Roles of catalyst surface area, lattice oxygen capacity and mobility. AIChE
J. 2011;57:2861ꢀ2869.
55. Shotipruk A, Assabumrungrat S, Pavasant P, Laosiripojana N. Reactivity of CeO2 and
Ce–ZrO2 toward steam reforming of palm fatty acid distilled (PFAD) with coꢀfed oxygen
and hydrogen. Chem. Eng. Sci. 2009;64:459ꢀ466.
56. Laosiripojana N, Assabumrungrat S. Methane steam reforming over Ni/Ce–ZrO2 catalyst:
Influences of Ce–ZrO2 support on reactivity, resistance toward carbon formation, and
intrinsic reaction kinetics. Appl. Catal. A: Gen. 2005;290:200–211
57. Liu Z, Yi Y, Zhang S, Zhu T, Zhu J, Wang J. Selective catalytic reduction of NOx with
NH3 over MnꢀCe mixed oxide catalyst at low temperatures. Catal. Today 2013;216:76–
81.
58. Kumar S, Jain SL. Polyethylene glycol enfolded KBr assisted base catalyzed synthesis of
dimethyl carbonate from methanol and carbon dioxide. Ind. Eng. Chem. Res.
2014;53:15798−15801.
59. Stryjek R, Vera JH. PRSV: An improved Peng—Robinson equation of state for pure
compounds and mixtures. Can. J. Chem. Eng. 1986;64:323–333.
60. Piñeroa P, Garcíaa J, Sokolovab M, Cocero MJ. Modelling of the phase behaviour for the
direct synthesis of dimethyl carbonate from CO2 and methanol at supercritical or near
critical conditions. J. Chem. Thermodyn. 2007;39:536–549.
61. Bustamente F, Orrego AF, Villegas S, Villa AL. Modeling of chemical equilibrium and
gas phase behavior for the direct synthesis of dimethyl carbonate from CO2 and methanol.
Ind. Eng. Chem. Res. 2012;51:8945−8956.
22

Page 23 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
62. Kongpanna P, Pavarajarn V, Gani R, Assabumrungrat S. Technoꢀeconomic evaluation of
different CO2ꢀbased processes for dimethyl carbonate production. Chem. Eng. Res. Des.
2015;93:496–510.
63. Cai Q, Lu B, Guo L, Shan Y. Studies on synthesis of dimethyl carbonate from methanol
and carbon dioxide. Catal. Commun. 2009:10:605–609.
64. Santos BAV, Silva VMTM, Loureiro JM, Barbosa D, Rodrigues AE. Modeling of
physical and chemical equilibrium for the direct synthesis of dimethyl carbonate at high
pressure conditions. Fluid Phase Equilibria 2012;336:41–51
65. Santos BAV, Pereira CSM, Silva VMTM, Loureiro JM, Rodrigues AE. Kinetic study for
the direct synthesis of dimethyl carbonate from methanol and CO2 over CeO2 at high
pressure conditions. Appl. Catal. A: Gen. 2013;455:219–226
66. Santos BAV, Silva VMTM, Loureiro JM, Rodrigues AE. Adsorption of H2O and
Dimethyl Carbonate at High Pressure over Zeolite 3A in Fixed Bed Column. Ind. Eng.
Chem. Res. 2014;53:2473−2483.
67. Santos BAV, Pereira CSM, Silva VMTM, Loureiro JM, Rodrigues AE. Design of a true
moving bed reactor for the direct synthesis of dimethyl carbonate. Chem. Eng. Sci.
2015;123:406–419.
68. Marin CM, Li L, Bhalkikar A, Doyle JE, Zeng XC, Cheung CL. Kinetic and mechanistic
investigations of the direct synthesis of dimethyl carbonate from carbon dioxide over
ceria nanorod catalysts. J. Catal. 2016;340:295ꢀ301.
23

RSC Advances
Page 24 of 38
View Article Online
DOI: 10.1039/C6RA22643D
Table 1. N2 sorption of ceriumꢀzirconium mixed oxides catalysts.
CexZr1ꢀxO2
BET surface area Pore Volume Pore diameter
(m2/g)
112
81
(cm3/g)
0.42
0.42
0.43
0.47
0.33
0.40
0.39
0.35
0.36
0.21
0.20
(nm)
15.4
21.5
22.9
24.3
10.0
11.9
24.8
28.7
30.1
30.6
30.4
x=0.0
x=0.1
x=0.2
x=0.3
x=0.4
x=0.5
x=0.6
x=0.7
x=0.8
x=0.9
x=1.0
69
71
121
123
59
54
49
31
28
24

Page 25 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
Table 2. Elemental analysis of Ce0.5Zr0.5O2 and Ce0.4Zr0.6O2 catalysts.
Catalysts
Nominal value of Actual values of metals Chemical formula
metals
Ce
from ICPꢀOES analysis
Zr
0.5
0.6
Ce
Zr
Ce0.5Zr0.5O2
Ce0.4Zr0.6O2
0.5
0.48
0.39
0.52
0.61
Ce0.48Zr0.52O2
Ce0.39Zr0.61O2
0.4
25

RSC Advances
Page 26 of 38
View Article Online
DOI: 10.1039/C6RA22643D
Table 3. TPD analysis using absorbed CO2 and NH3 for determining basic and acidic
properties of CeO2, Ce0.5Zr0.5O2 and ZrO2.
TPD analysis of absorbed CO2 (mmol/g)
Total
Basic site
density
Catalyst
Weak (< 200°C)
Moderate
Strong (> 450°C) evolved CO2
(200ꢀ450°C)
(mmol/g)
(µmol/m2)
14.64
CeO2
C0.5Zr0.5O2
ZrO2
0.41 (117)
0.45 (113)
0.37 (100)
0
0
0.41
1.93
0.40
0.17(345)
0
1.31 (717)
0.03(846)
15.69
3.89
Catalyst
TPD analysis of absorbed NH3 (mmol/g)
Weak (< 200°C) Moderate (200ꢀ450°C) Strong (> 450°C)
Total evolved NH3
(mmol/g)
0.94
CeO2
C0.5Zr0.5O2
ZrO2
0.81 (110)
0.99 (106)
1.31 (156)
0
0.27 (294)
0
0.13 (667)
1.49 (666)
0.21 (643)
2.48
1.52
Temperature (°C) at maxima is given in brackets.
26

Page 27 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
Table 4. Literature review of kinetics models compared for kinetics study of chemical
equilibrium and thermodynamics analysis.
Catalyst
T
P
Kinetics
Activatio Keq Entrop Gibbs
n energy, (m· y (∆S°) free
Enthalpy Refere
(°C) (bar) models
(∆H°)
nce
Ea
s−1) (J·mol energy
−1·K−1) (∆G°)
(kJ·mol−1)
(kJ·mol−1
)
(kJ·mol−1
)
K2CO3,
KOH,
CH3OK
CeO2
80
73
−
−
−
9.4× −
10−8
36.31
−28.8
63
130 200 Soave–
Redlich–
0.26± −
0.04
×
10−2
−454.93 −570.70@
25°C
64
Kwong
equation of
state coupled
with five
different mixing
rules
CeO2
105ꢀ 150ꢀ Langmuir–
135 200 Hinshelwood
and Eley–
106ꢀ117
−
−
31.5
−20.1
−25 ±
65
Rideal
mechanisms
40ꢀ70 200 Mathematical
model
CeO2
CeO2
−
3.3ꢀ −69±4 −4 ±
66
67
1.1@ @H2O & 6@25°C H2O 6@H2O
&
&
40ꢀ70°C
−60 ± & −2 ±
4@DMC 6@25°C DMC 6@DMC
−25 ±
90ꢀ 300 Mathematical 106
−
−
−69 ±
4@H2O
& −60
±
4@DMC
−
−
−25 ±
6@H2O
−25 ±
6@DMC
130
model using
PROMS Model
Builder (version
3.6.0)
Ceria
nanorod 140
125, 138 Langmuir–
65 ± 18
−
−
−
68
Hinshelwood
mechanisms
Ce0.5Zr0.5 100ꢀ 150 Peng–
O2
3.629 −
×10ꢀ7
1.54
−139.76
This
paper
180
Robinson–
Stryjek–Vera
equation with
van der Waals
one−fluid
reaction
27

RSC Advances
Page 28 of 38
View Article Online
DOI: 10.1039/C6RA22643D
CeO2 (mol %)
(111)
(311)
(220)
(200)
100
(222)
90
80
70
60
50
40
30
20
10
0
10
20
30
40
50
60
70
80
2
θ
(degrees)
(a)
(111)
(200)
Exotemplate
(220)
(311)
Endoꢀ/Exotemplate
10
20
30
40
50
60
70
80
2
θ
(degrees)
(b)
Figure 1. (a) XRD patterns CexZr1ꢀxO2 (x=0 to 1) with exotemplate, (b) XRD patterns of
the Ce0.5Zr0.5O2 with exotemplate and endoꢀ/exotemplate (nTBC/nCe+Zr=0.017).
28

Page 29 of 38
RSC Advances
View Article Online
DOI: 10.1039/C6RA22643D
(a)
(b)
(c)
(d)
Figure 2. FEꢀSEM images and EDX of (a) CeO2, (b) ZrO2, (c) Ce0.5Zr0.5O2, (d) EDX
spectra of Ce0.5Zr0.5O2.
29