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

Department of Chemical Engineering, Indian 247667, Uttarakhand, India. E-mail: vim praveen.singh@daad-alumni.de; praveen.zo immishra49@gmail.com; mishra.im.che@is Tel: +91-1332-285889 Institute of Chemical Technology, Universit Germany. E-mail: patrick.with@iom-leipzig. Chemical Division, Leibniz Institute for Sur Leipzig, Germany Department of Chemical Engineering, India 826004, India † Electronic supplementary informa 10.1039/c6ra22643d Cite this: RSC Adv., 2016, 6, 110235


Introduction
Dimethyl carbonate (DMC) production is green, with growing research interest in the past decade. It is an environmentally friendly raw material and an alternative for many toxic, carcinogenic and highly corrosive reagents such as phosgene, chloromethane, dimethyl sulfate, and alkyl halide which are used in alkylation and carbonylation reactions. 1,2 It is widely used for the synthesis of various chemicals such as pharmaceuticals, polymers, foodstuffs, agrochemicals, antioxidants, avoring agents, dyestuffs, solvents in electrolytes in lithium ion batteries, adhesives and coatings due to its low toxicity, excellent biodegradability, high versatility and bioaccumulation. DMC is also used as a fuel additive as an octane enhancer due to the fact that its oxygen content is three times higher than that of 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 full-scale commercial methods used for the DMC synthesis. 2,[5][6][7][8][9] Phosgenation process is now abandoned because of the hazards associated with it. Transesterication of ethylene carbonate and urea with methanol and conversion of CO 2 are in the developing stage for the synthesis of DMC. Consequently, direct CO 2 conversion reaction is being developed for its industrial feasibility. It is the most desired method for the synthesis of DMC due to its environment-friendly nature, associated green chemistry, low cost and easy availability of the materials. 2,[10][11][12] However, this method has difficulties such as activation of methanol/CO 2 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/ CO 2 . 14 Various homogeneous and heterogeneous catalysts have been studied for increasing the DMC yield during conversion of CO 2 with methanol to produce DMC. [15][16][17] On the other hand, some acidic compounds such as phosphoric acid H 3 PO 4 -ZrO 2 or H 3 PW 12 O 4 -ZrO 2 , 18 and ZrO 2 -KCl; 19 active metal catalysts such as zirconium oxide (ZrO 2 ), 20 cerium oxide (CeO 2 ), 20,21 copper nickel/graphite, 22 Cu-Fe, 23 33 etc. have been used as catalysts for DMC synthesis from CO 2 . 34,35 Among these catalysts, acid-base bi-functional 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 thermodynamic analysis are scarce.
In the prthe esent study, composition of cerium-zirconium mixed oxide as well as the operating conditions for the reaction have been optimized. The synthesis of Ce x Zr 1Àx O 2 mixed oxide spheres was carried out using exo-and endo-templating method. The synthesized catalysts were characterized by X-ray diffraction (XRD), N 2 -sorption, scanning electron microscopy (SEM) and CO 2 -and NH 3 -temperature-programmed desorption (TPD). The optimum catalyst was further used under constant pressure for optimizing reaction conditions such as reaction time, reaction temperature and catalyst dose. Reusability of the catalyst was also studied. Further, chemical equilibrium modeling was done using Peng-Robinson-Stryjek-Vera equation of state (PRSV-EoS) along with the van der Waals one-uid reaction condition so as to calculate Gibbs free energy change ðDG r Þ and the heat of reaction ðDH r Þ.

Catalyst preparation
Cerium-zirconium mixed oxide catalysts with different Ce x Zr 1Àx O 2 (x ¼ 0 to 1) molar ratios were synthesized using exoand endo-templating method. For this, Ce(NO 3 ) 2 and Zr(NO 3 ) 3 were dissolved separately in 100 mL in double-distilled water and were further mixed together in the desired molar proportion Ce x Zr 1Àx O 2 (x ¼ 0 to 1) under continuous stirring at room temperature. Liquid ammonia solution was added drop-by-drop 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 thereaer it was ltered. The precipitate retained on the lter was washed with double-distilled water until the pH of the ltrate became neutral. Finally, the lter cake was transferred to a 200 mL polypropylene (PP) bottle and double-distilled water was added to it until the total weight of the mixture became 30 g. Thereaer, 2.5 mL HNO 3 (65 wt% in H 2 O) was added to the mixture. The PP-bottle was transferred to an ultrasonic bath (Sonorex RK1, Fa. Bandelin) where it was kept for 4 h until a clear sol was formed. Pluronic F-127 as triblock copolymer (TBC) was added such that the molar ratios of TBC to cerium along with zirconium (n TBC /n Ce+Zr ) become 0.017. This ratio was the optimum to the sol as an endo-template. 36,37 This mixture was again kept in the ultrasonic bath for 3 h for dissolving the Pluronic F-127. 4.42 g preactivated (for 24 h at 110 C) polymerbased spherical activated carbon (PBSAC) was added to the nanoparticle sol and was further dried at 50 C for 12 h. The prepared catalyst was activated at 600 C for 5 h under air ow (40 cm 3 min À1 ) with heating rate of 3 C min À1 from room temperature to 600 C with a holding time of 1 h at 100 C and 5 h at 600 C. Aer calcination, light yellow Ce x Zr 1Àx O 2 mixed oxide spheres were obtained. For characterization and catalytic experiments, Ce x Zr 1Àx O 2 mixed oxides were sieved to obtain the spheres in the size range of 0.2-0.4 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. X-ray diffractograms (Bruker AXS, Germany) at 40 kV/30 mA with CuKa radiation (l ¼ 1.5406Å) 0.02 step size over 2q scan range 5 # 2q # 100 were obtained. PANalytical X'pert high score was used for the identication of crystalline phase with International Centre for Diffraction Data (ICDD) database. N 2 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 N 2 sorption measurements at À197 C (Micromeritics ASAP 2020). BET isotherm was used for calculating the surface area of the porous material by physical adsorption of N 2 gas at its boiling temperature. TPD was used to study the binding interaction of adsorbate CO 2 or NH 3 on the catalyst surface and to provide 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 an inert gas such as helium. Aer 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 ow rate. The amount of the desorbed CO 2 or NH 3 is quantied 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 NH 3 and CO 2 , respectively. To investigate the morphology of the synthesized catalysts, scanning electron microscope (SEM) were 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. Aer that, the prepared samples were used for taking image using FE-SEM at 20 kV under vacuum. Thereaer, the energy-dispersive X-ray spectroscopy (EDX) was carried out to nd 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 ltered and the ltrate was used for the determination of the metal concentration by AAS (Avanta M by GBC Scientic Equipment Pvt Ltd.). ICP-OES supplied by OPTIMA 8000 von Perkin Elmer was also used for determining the amount of metals dispersed on the catalysts. For sample preparation, the sample (50.0 AE 0.1) was dissolved by a microwave assisted digestion (Multiwave 3000 from Anton Paar) using 2 mL HF (48 wt%, Suprapur, Merck), 2 mL HNO 3 (69 wt%, Supra, Roth), 2 mL HCl (35 wt%, Supra, Roth) and 3 mL H 2 SO 4 (85 wt%, Suprapur, Merck). Microwave conditions were: 1100 watt per ramp for 20 min, hold for 30 min and cooled for 15 min. Aer the microwave digestion, 12 mL H 3 BO 4 (for complexation of HF) and 1 mL HlO 4 were added. Aerwards, the sample was digested for a second time using microwave conditions as used earlier. Aer the microwave treatment, H 2 O 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 the unknown solution was estimated using this calibration curve.

Catalytic activity
Catalytic conversion of CO 2 with methanol to produce DMC was performed in the reaction autoclave (i.e. batch reactor) made by Berghof, Germany (Model-BHL-800). A magnetic stirrer was used to make the reactant mixture homogeneous during the reaction. A rubber made O-ring 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, the reactor was lled with the required amount of methanol and the catalysts. The reactor was heated to the reaction temperature (100-180 C) and pressurized with CO 2 up to a pressure of 150 bar and maintained for 6-48 h for the reaction to proceed. Aer (6-48 h), the reactor was cooled down so that the product mixture is brought to at <À20 C by using an ice bath, and thereaer, 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 a constant stirrer speed of 600 revolutions per min. Catalyst was washed with methanol and dried at 150 C for 12 h and then was activated at 500 C for 4 h aer each cycle. Similarly, molecular sieve was activated at 240 C for 4 h aer each cycle.

Results and discussion
Catalyst characterization X-ray diffraction. XRD proles of Ce x Zr 1Àx O 2 (x ¼ 0 to 1) catalysts are given in Fig. 1a. No separate peak is found in the cerium-zirconium mixed oxide. Pure zirconia (x ¼ 0) tetragonal phase showed the characteristic (111) reection at 2q ¼ 30 .
With an increase in ceria amount, the reex at 2q ¼ 30 shied  towards lower 2q values. For pure ceria, a major peak at 2q ¼ 28 was observed, which is typical of the cubic uorite structure of ceria. [38][39][40] For ceria content of 60 mol%, the crystal structure was tetragonal. The two peaks at 2q ¼ 29 and 35 for the two samples of Ce 0.4 Zr 0.6 O 2 and Ce 0.5 Zr 0.5 O 2 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 z1. 41 This would explain the sudden increase in the specic surface area of these two samples. XRD of Ce 0.5 Zr 0.5 O 2 synthesized using exotemplate and endo-/exo-template method (n TBC /n Ce+Zr ¼ 0.017) is shown in Fig. 1b. It may be seen that the reexes of Ce 0.5 Zr 0.5 O 2 , synthesized with endo-/exo-template are more intense than that with exo-template. This suggests that in the presence of larger particles, endo-templates arise. This hypothesis is supported by the lower values of the specic surface and the specic 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 ZrO 2 incorporation in CeO 2 improves the thermal resistance and more importantly the redox capacity of CeO 2 -ZrO 2 mixed metal oxide. As Ce 4+ (1.01Å) has a larger ionic radius than Zr 4+ (0.80Å), shrinkage of the lattice due to the replacement of Ce 4+ with Zr 4+ affects the lattice structure. This lowers the energy for Ce 4+ reduction and enhances the CeO 2 reducibility. [47][48][49] Surface morphology and elemental analysis. SEM micrographs (Fig. 2) of CeO 2 , ZrO 2 and Ce 0.5 Zr 0.5 O 2 with particles size  Table 2. The compositions of the synthesized catalysts are similar to the desired initial metal composition.
Textural properties. The nitrogen sorption results of ceriumzirconium mixed oxides Ce x Zr 1Àx O 2 (x ¼ 0 to 1) are summarized in Table 1. Among all the synthesized catalysts, Ce 0.4 Zr 0.6 O 2 and Ce 0.5 Zr 0.5 O 2 were found to possess highest BET surface area of 123 and 121 m 2 g À1 and minimum pore diameter of 11.9 and 10.0 nm, respectively. It can be seen from Fig. 3, that the specic surface area of the mixed oxides is a function of the CeO 2 content in the synthesized catalyst. Adsorption/desorption isotherm and the pore volume distribution of CeO 2 , Ce 0.5 Zr 0.5 O 2 , ZrO 2 are shown in Fig. 4a and b, respectively. All the sorption isotherms are of type IV isotherm with the hysteresis loop, typical of mesoporous systems. 50,51 Peak corresponding to maximum pore volume shis towards higher pore width for mixed Ce-Zr oxide as compared to pure CeO 2 or ZrO 2 . Mixed oxides exhibit specic surface areas between 112 m 2 g À1 (pure zirconia) and 28 m 2 g À1 (pure ceria). With an increase in the content of ceria, the specic 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 ¼ 0.4-0.5 i.e. Ce 0.4 Zr 0.6 O 2 and Ce 0.5 Zr 0.5 O 2 (Fig. 3). These two catalysts exhibit specic surface area >120 m 2 g À1 with the mean pore diameter of $10 nm. At these values of x, the cause of increase in surface area is the formation of structurally homogeneous solid solution. 52, 53 Laosiripojana et al. 54 showed the specic surface area of 49, 47, and 46.5 m 2 g À1 with the average particle size of 50-80 nm using Ce/Zr molar ratio of 1/3, 1/1, and 3/1, respectively. Shotipruk et al. 55 showed the specic surface area of 135, 120, and 115 m 2 g À1 with Ce/Zr molar ratio 1/3, 1/1, and 3/1, respectively and Laosiripojana and Assabumrungrat, 56 shows z20 m 2 g À1 with 5% Ni on Ce/Zr molar ratio 1/3, 1/1, and 3/1, respectively. CO 2 -TPD. The basic properties of CeO 2 , Ce 0.5 Zr 0.5 O 2 and ZrO 2 catalysts were determined from the CO 2 -TPD prole (Fig. 5a) and the results are given in Table 3. Basic properties of the catalysts depend upon the temperature prole 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 M x+ -O 2À pairs and the formation of bi-dentate and bridged carbonates; and the strong basic sites are due to the low coordination O 2À ions and the formation of uni-dentate 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: ZrO 2 (0.40 mmol g À1 ) < CeO 2 (0.41 mmol g À1 ) < Ce 0.5 Zr 0.5 O 2 (1.93 mmol g À1 ), and the basic site density per unit area followed the same order (Table 3). In ceria catalyst, Ce may have variable valency (Ce 3+ and Ce 4+ ). Moreover, ceria is also a typical Lewis-base catalyst, which is responsible for its application in several basecatalyzing 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 À1 , respectively, for Ce 0.6 Zr 0.4 O 2 catalysts. NH 3 -TPD. NH 3 -TPD spectra of CeO 2 , Ce 0.5 Zr 0.5 O 2 and ZrO 2 catalysts are shown in Fig. 5b and the results are summarized in Table 3. Desorption peaks of NH 3 are in the temperature range of 50-900 C. The NH 3 desorption peaks at 110 C and 667 C for CeO 2 and at 156 C and 643 C for ZrO 2 were observed in the week and strong regions. Ce 0.5 Zr 0.5 O 2 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: CeO 2 (0.94 mmol g À1 ) < ZrO 2 (1.52 mmol g À1 ) < Ce 0.5 Zr 0.5 O 2 (2.48 mmol g À1 ). Thus, the Ce 0.5 Zr 0.5 O 2 catalyst has the highest acidic site density and the CeO 2 has the lowest acidic site density. Thus, the Ce 0.5 Zr 0.5 O 2 catalyst has the highest density of basic and acidic sites as compared to other catalysts. Therefore, this catalyst can act as an acid-base bi-functional catalyst. It has been reported that both the basic and acidic sites are required for the direct conversion of CO 2 to produce DMC. 16,17,20 Catalytic activity of catalysts for DMC synthesis The direct catalytic conversion of CO 2 with methanol for the synthesis of DMC was studied in the presence of CeO 2 , ZrO 2 and Ce 0.5 Zr 0.5 O 2 catalysts. Negligible conversion of CO 2 /methanol to DMC aer for 24 h at 120 C temperature and 150 bar pressure were observed in the blank experiment without any catalyst. In the presence of a Ce 0.5 Zr 0.5 O 2 catalyst, methanol is activated to  with H + to form water (Fig. 6). As such, higher basicity and acidity in the catalysts facilitate DMC synthesis from CO 2 and methanol. 44 Reaction mechanism for DMC synthesis from the direct conversion of CO 2 with methanol in the presence of the catalyst is shown in Fig. 7. Ce 0.5 Zr 0.5 O 2 catalyst showed better activity as compared to CeO 2 , and ZrO 2 (Fig. 7). The order of the activity of the catalysts followed: ZrO 2 (0.912 mmol DMC per g cat.) < CeO 2 (1.384 mmol DMC per g cat.) < Ce 0.5 Zr 0.5 O 2 (2.921 mmol DMC per g cat.). Best active Ce 0.5 Zr 0.5 O 2 catalyst was further used for the optimization of the reaction conditions such as reaction temperature, catalyst dose and reaction time for CO 2 conversion. The inuences of reaction time for DMC synthesis in the presence of Ce 0.5 Zr 0.5 O 2 catalyst is shown in Fig. 8a. It can be seen from Fig. 8a that the DMC formation (1.989-2.921 mmol g À1 cat.), methanol conversion (0.644-0.945%) and CO 2 conversion (1.567-2.310%) increased with an increase in the reaction time up to 24 h. Further increase in the reaction time showed a decrease in the DMC yield and CO 2 /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 CO 2 conversion is shown in Fig. 8b. It can be seen that the maximum DMC yield and CO 2 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 inuence of the reaction temperature for the DMC synthesis and CO 2 /methanol conversion is shown in Fig. 8c. Initially, the DMC yield (1.021-2.9212 mmol g À1 cat.), methanol conversion (0.331-0.945 mmol g À1 cat.) and CO 2 conversion (0.804-2.300 mmol g À1 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 À1 cat.), methanol conversion (0.687-0.2888 mmol g À1 cat.) and the CO 2 conversion (1.673-0.7026 mmol g À1 cat.). Thus, the optimum DMC yield was obtained at 120 C. The decrease in the DMC yield, methanol and CO 2 conversions may be because of the poor solubility of CO 2 in methanol and also due to decomposition of DMC. 58 It can be seen that the acidic and basic properties directly inuence the catalytic activity of DMC synthesis. The reuse of the Ce 0.5 Zr 0.5 O 2 catalyst was investigated at optimum reaction conditions (T ¼ 120 C, t ¼ 24 h and catalyst amount ¼ 1.25 g) in ve consecutive batch cycles (Fig. 9). Almost similar DMC yield was found in all the batch cycles. Thus, the Ce 0.5 Zr 0.5 O 2 catalyst is found to be an effective catalyst with long life and can be used in a number of cycles.

Chemical equilibrium modeling
Synthesis of DMC from direct conversion of CO 2 and methanol can be related to the equilibrium constant as shown below: where, Q CO 2 ¼ y CO 2 ,0 /y MeOH,0 . The Peng-Robinson-Stryjek-Vera equation of state (PRSV-EoS) 59 along with the van der Waals one-uid (1PVDW) mixing rule, 60,61 were used to calculate the fugacity coefficient of species in the mixture. PRSV-EoS is given as: where,  This journal is © The Royal Society of Chemistry 2016 k 0 ¼ 0.378893 + 1.4897153u À 0.1713184u 2 + 0.0196554u 3 (7) where, P c and T c are the critical pressure and temperature, respectively, k is a specic pure compound parameter and u is the acentric factor. The values of T c , P c , u and k as obtained from the literature are compiled in Table S1. † van der Waals one-uid model (1PVDW) gives the following sets of equations which were used to obtain data of the quadratic mixture:  where, l ij and k ij are the single binary interaction parameters, which are used to determine the mixture parameters a, and b in the PRSV-EOS. Values of l ij and k ij were obtained from the literature and are given in Table S2. † Assuming that the heat of reaction DH r is constant within the temperature range of 100-180 C, the equilibrium constant K eq can be related to T by the classical van't Hoff equation: Eqn (1)-(9) were solved simultaneously using the parameters given in the Tables S1 and S2 † to calculate the values of K eq at various temperatures. The values K eq at 373, 393, 413, 433 and 453 K were found to be 6.811 Â 10 À8 , 3.629 Â 10 À7 , 1.713 Â 10 À7 , 1.823 Â 10 À8 and 1.327 Â 10 À9 L mol À1 , respectively (Table S3 †). The values of DH r and DG r (using eqn (10)) for Ce 0.5 Zr 0.5 O 2 using the data points at T ¼ 120-180 C were found to be À139.76 kJ mol À1 and 1.54 kJ mol À1 , respectively. DH r and DG r values of À15.259 kJ mol À1 and 29.583 kJ mol À1 , 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. [63][64][65][66][67][68] 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.

Conclusions
In this paper, porous and spherically shaped cerium-zirconium catalysts (Ce x -Zr 1Àx O 2 ) with different molar ratios were synthesized using an exo-and endo-templating method using PBSAC as exo-template, and Pluronic F-127 as endo-template. XRD pattern showed the reexes of cubic phase in CeO 2 , tetragonal phase in ZrO 2 and Ce 0.5 Zr 0.5 O 2 . The synthesized catalysts showed BET surface between 28-112 m 2 g À1 and pore volume in the range of 0.2-0.42 cm 3 g À1 . An increase in the ceria content was found to decrease the specic surface area of the mixed oxides except for x ¼ 0.4-0.5. At these values of x, formation of structurally homogeneous solid solution increased the surface area. These catalysts were tested for direct conversion of CO 2 with methanol for the production DMC in a batch reactor. The Ce x -Zr 1Àx (x ¼ 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 ¼ 150 bar, temperature ¼ 120 C, reaction time ¼ 24 h, catalysts dose ¼ 1.25 g), the activity of the catalysts was in the following order: ZrO 2 (0.912 mmol DMC per g cat.) < CeO 2 (1.384 mmol DMC per g cat.) < Ce 0.5 Zr 0.5 O 2 (2.921 mmol DMC per g cat.). During ve consecutive reuse cycles of Ce 0.5 Zr 0.5 O 2 catalyst, only marginal change in DMC yield and methanol conversion was observed. The values of DH r and DG r for Ce 0.5 Zr 0.5 O 2 catalyst were found to be À139.76 kJ mol À1 and 1.54 kJ mol À1 , respectively. Table 4 Literature review of kinetic models used for the chemical equilibrium and thermodynamics analysis