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

Abstract

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°).

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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, flavoring 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.^2,5–9 Phosgenation process is now abandoned because of the hazards associated with it. 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 environment-friendly nature, associated green chemistry, low cost and easy availability of the materials.^2,10–12 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 and heterogeneous catalysts have been studied for increasing the DMC yield during conversion of CO₂ with methanol to produce DMC.^15–17 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 (CeO₂),^20,21 copper nickel/graphite,²² Cu–Fe,²³ Ce_0.5Zr_0.5O₂,^24,25 metal oxide/Ce_0.6Zr_0.4O₂,²⁶ Al₂O₃–CeO₂,²⁷ ZrO₂–MgO,²⁸ SnO₂/SiO₂,²⁹ ZrO₂/SiO₂,²⁹ and Cu_1.5PMo₁₂O₄₀;³⁰ basic compounds such as KOH,³¹ K₂CO₃,³¹ and CH₃OK;³¹ heteropoly acids H₃PW₁₂O₄/Ce_xZr_1−xO₂,³² H₃PW₁₂O₄₀/Ce_xTi_1−xO₂ heteropolyoxometalates,³³etc. have been used as catalysts for DMC synthesis from CO₂.^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_xZr_1−xO₂ mixed oxide spheres was carried out using exo- and endo-templating method. The synthesized catalysts were characterized by X-ray diffraction (XRD), N₂-sorption, scanning electron microscopy (SEM) and CO₂- and NH₃-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-fluid reaction condition so as to calculate Gibbs free energy change and the heat of reaction .

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 (Brunauer–Emmett–Teller surface area = 1748 m² g⁻¹; Barrett–Joyner–Halenda volume = 2 cm³ g⁻¹; diameter = 0.45–0.5 mm), whereas Pluronic F-127 was purchased from BASF, Germany. All chemicals used were of analytical grade.

Catalyst preparation

Cerium–zirconium mixed oxide catalysts with different Ce_xZr_1−xO₂ (x = 0 to 1) molar ratios were synthesized using exo- and endo-templating method. For this, Ce(NO₃)₂ and Zr(NO₃)₃ were dissolved separately in 100 mL in double-distilled water and were further mixed together in the desired molar proportion Ce_xZr_1−xO₂ (x = 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 thereafter it was filtered. The precipitate retained on the filter was washed with double-distilled water until the pH of the filtrate became neutral. Finally, the filter 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. Thereafter, 2.5 mL HNO₃ (65 wt% in H₂O) 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) polymer-based 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 flow (40 cm³ min⁻¹) with heating rate of 3 °C min⁻¹ 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 Ce_xZr_1−xO₂ mixed oxide spheres were obtained. For characterization and catalytic experiments, Ce_xZr_1−xO₂ 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 CuKα radiation (λ = 1.5406 Å) 0.02 step size over 2θ scan range 5 ≤ 2θ ≤ 100° were obtained. PANalytical X'pert high score was used for the 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 adsorption of N₂ gas at its boiling temperature. TPD was used to study the binding interaction of adsorbate CO₂ or NH₃ 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. 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) 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. After that, the prepared samples were used for taking image using FE-SEM at 20 kV under vacuum. Thereafter, the energy-dispersive X-ray spectroscopy (EDX) was carried out to 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.). 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 ± 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 per ramp for 20 min, hold for 30 min and cooled 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 the 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 (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 filled with the required amount of 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 reaction to proceed. After (6–48 h), the reactor was cooled down so that the product mixture is brought to at <−20 °C by using an 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 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 after each cycle. Similarly, molecular sieve was activated at 240 °C for 4 h after each cycle.

Results and discussion

Catalyst characterization

X-ray diffraction. XRD profiles of Ce_xZr_1−xO₂ (x = 0 to 1) catalysts are given in Fig. 1a. No separate peak is found in the cerium–zirconium 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.^38–40 For ceria content of 60 mol%, the crystal structure was tetragonal. The two peaks at 2θ = 29° and 35° for the two samples of Ce_0.4Zr_0.6O₂ and Ce_0.5Zr_0.5O₂ 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 Ce_0.5Zr_0.5O₂ synthesized using exotemplate and endo-/exo-template method (n_TBC/n_Ce+Zr = 0.017) is shown in Fig. 1b. It may be seen that the reflexes of Ce_0.5Zr_0.5O₂, 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 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 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 Ce⁴⁺ with Zr⁴⁺ affects the lattice structure. This lowers the energy for Ce⁴⁺ reduction and enhances the CeO₂ reducibility.^47–49

Fig. 1 (a) XRD patterns Ce_xZr_1−xO₂ (x = 0 to 1) with exotemplate, (b) XRD patterns of the Ce_0.5Zr_0.5O₂ with exotemplate and endo-/exotemplate (n_TBC/n_Ce+Zr = 0.017).

Table 1 N₂ sorption of cerium–zirconium mixed oxides catalysts

CexZr1−xO2	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
x = 0.0	112	0.42	15.4
x = 0.1	81	0.42	21.5
x = 0.2	69	0.43	22.9
x = 0.3	71	0.47	24.3
x = 0.4	121	0.33	10.0
x = 0.5	123	0.40	11.9
x = 0.6	59	0.39	24.8
x = 0.7	54	0.35	28.7
x = 0.8	49	0.36	30.1
x = 0.9	31	0.21	30.6
x = 1.0	28	0.20	30.4

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Surface morphology and elemental analysis. SEM micrographs (Fig. 2) of CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ with particles size distribution in the range of d_P = 0.2–0.4 mm and the EDX analysis of the Ce_0.5Zr_0.5O₂ catalyst are shown in Fig. 2. Analysis has also been carried out by ICP-OES. The structural chemical compositions of the Ce_0.5Zr_0.5O₂ and Ce_0.4Zr_0.6O₂ catalysts are shown in Table 2. The compositions of the synthesized catalysts are similar to the desired initial metal composition.

Fig. 2 FE-SEM images and EDX of (a) CeO₂, (b) ZrO₂, (c) Ce_0.5Zr_0.5O₂, (d) EDX spectra of Ce_0.5Zr_0.5O₂.

Table 2 Elemental analysis of Ce_0.5Zr_0.5O₂ and Ce_0.4Zr_0.6O₂ catalysts

Catalysts	Nominal value of metals		Actual values of metals from ICP-OES analysis		Chemical formula
	Ce	Zr	Ce	Zr
Ce0.5Zr0.5O2	0.5	0.5	0.48	0.52	Ce0.48Zr0.52O2
Ce0.4Zr0.6O2	0.4	0.6	0.39	0.61	Ce0.39Zr0.61O2

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Textural properties. The nitrogen sorption results of cerium–zirconium mixed oxides Ce_xZr_1−xO₂ (x = 0 to 1) are summarized in Table 1. Among all the synthesized catalysts, Ce_0.4Zr_0.6O₂ and Ce_0.5Zr_0.5O₂ were found to possess highest BET surface area of 123 and 121 m² g⁻¹ and minimum pore diameter of 11.9 and 10.0 nm, respectively. It can be seen from Fig. 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, Ce_0.5Zr_0.5O₂, ZrO₂ 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 shifts towards higher pore width for mixed Ce–Zr oxide as compared to pure CeO₂ or ZrO₂. Mixed oxides exhibit specific surface areas between 112 m² g⁻¹ (pure zirconia) and 28 m² g⁻¹ (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. Ce_0.4Zr_0.6O₂ and Ce_0.5Zr_0.5O₂ (Fig. 3). These two catalysts exhibit specific surface area >120 m² g⁻¹ 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 specific surface area of 49, 47, and 46.5 m² g⁻¹ 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 specific surface area of 135, 120, and 115 m² g⁻¹ with Ce/Zr molar ratio 1/3, 1/1, and 3/1, respectively and Laosiripojana and Assabumrungrat,⁵⁶ shows ≈20 m² g⁻¹ with 5% Ni on Ce/Zr molar ratio 1/3, 1/1, and 3/1, respectively.

Fig. 3 Surface area of the Ce_xZr_1−xO₂ mixed oxides from the synthesis depending on the CeO₂-content.

Fig. 4 (a) N₂ adsorption/desorption isotherm CeO₂, Ce_0.5Zr_0.5O₂, ZrO₂; ZrO₂, Ce_0.5Zr_0.5O₂, CeO₂. (b) Pore diameter distributions of CeO₂, Ce_0.5Zr_0.5O₂, ZrO₂. (1) CeO₂, (2) Ce_0.5Zr_0.5O₂, and (3) ZrO₂.

CO₂-TPD. The basic properties of CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂ catalysts were determined from the CO₂-TPD profile (Fig. 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 M^x+–O²⁻ pairs and the formation of bi-dentate and bridged carbonates; and the strong basic sites are due to the low coordination O²⁻ 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: ZrO₂ (0.40 mmol g⁻¹) < CeO₂ (0.41 mmol g⁻¹) < Ce_0.5Zr_0.5O₂ (1.93 mmol g⁻¹), 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 Lewis-base catalyst, which is responsible for its application in several base-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⁻¹, respectively, for Ce_0.6Zr_0.4O₂ catalysts.

Fig. 5 (a) CO₂-TPD of the synthesized CeO₂, Ce_0.5Zr_0.5O₂, ZrO₂ catalysts, (b) NH₃-TPD of the synthesized CeO₂, Ce_0.5Zr_0.5O₂, ZrO₂ catalysts. (1) CeO₂, (2) ZrO₂, and (3) Ce_0.5Zr_0.5O₂.

Table 3 TPD analysis using absorbed CO₂ and NH₃ for determining basic and acidic properties of CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂^a

Catalyst	TPD analysis of absorbed CO2 (mmol g^−1)			Total evolved CO2 (mmol g^−1)	Basic site density (μmol m^−2)
	Weak (<200 °C)	Moderate (200–450 °C)	Strong (>450 °C)
a Temperature (°C) at maxima is given in brackets.
CeO2	0.41 (117)	0	0	0.41	14.64
C0.5Zr0.5O2	0.45 (113)	0.17 (345)	1.31 (717)	1.93	15.69
ZrO2	0.37 (100)	0	0.03 (846)	0.40	3.89

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Catalyst	TPD analysis of absorbed NH3 (mmol g^−1)			Total evolved NH3 (mmol g^−1)
	Weak (<200 °C)	Moderate (200–450 °C)	Strong (>450 °C)
CeO2	0.81 (110)	0	0.13 (667)	0.94
C0.5Zr0.5O2	0.99 (106)	0.27 (294)	1.49 (666)	2.48
ZrO2	1.31 (156)	0	0.21 (643)	1.52

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NH₃-TPD. NH₃-TPD spectra of CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂ catalysts are shown in Fig. 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 at 156 °C and 643 °C for ZrO₂ were observed in the week and strong regions. Ce_0.5Zr_0.5O₂ 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⁻¹) < ZrO₂ (1.52 mmol g⁻¹) < Ce_0.5Zr_0.5O₂ (2.48 mmol g⁻¹). Thus, the Ce_0.5Zr_0.5O₂ catalyst has the highest acidic site density and the CeO₂ has the lowest acidic site density. Thus, the Ce_0.5Zr_0.5O₂ 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 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 Ce_0.5Zr_0.5O₂ catalysts. Negligible conversion of CO₂/methanol to DMC after 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_0.5Zr_0.5O₂ catalyst, methanol is activated to form CH₃O⁻ and H⁺ in the presence of basic sites and CH₃⁺ 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 CH₃⁺ and OH⁻ ions. Methoxyl carbonyl ion reacts with CH₃⁺ to form DMC, and OH⁻ reacts with H⁺ to form water (Fig. 6). As such, higher basicity and acidity in the catalysts facilitate DMC synthesis from CO₂ and methanol.⁴⁴ Reaction mechanism for DMC synthesis from the direct conversion of CO₂ with methanol in the presence of the catalyst is shown in Fig. 7. Ce_0.5Zr_0.5O₂ catalyst showed better activity as compared to CeO₂, and ZrO₂ (Fig. 7). The order of the activity of the catalysts followed: ZrO₂ (0.912 mmol DMC per g cat.) < CeO₂ (1.384 mmol DMC per g cat.) < Ce_0.5Zr_0.5O₂ (2.921 mmol DMC per g cat.). Best active Ce_0.5Zr_0.5O₂ catalyst was further used for the optimization of the reaction conditions such as reaction temperature, catalyst dose and reaction time for CO₂ conversion.

Fig. 6 (a) Methanol conversion and DMC yield over CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂ catalysts; reaction conditions: (methanol = 25.03 mL, catalyst dose = 1.25 g, P = 150 bar, T = 120 °C, t = 24 h); DMC yield (mmol g⁻¹ cat.), % methanol conversion, % CO₂ conversion.

Fig. 7 Mechanism of DMC synthesis from direct conversion of CO₂ and methanol.

The influences of reaction time for DMC synthesis in the presence of Ce_0.5Zr_0.5O₂ catalyst is shown in Fig. 8a. It can be seen from Fig. 8a that the DMC formation (1.989–2.921 mmol g⁻¹ cat.), methanol conversion (0.644–0.945%) and CO₂ 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 CO₂/methanol conversion. It may be because of the saturation of the molecular sieves due to the adsorption of water.

Fig. 8 Effect of various parameters for direct conversion of CO₂ with methanol for DMC synthesis; (a) effect of reaction time at methanol = 25.03 mL, catalyst dose = 1.25 g, P = 150 bar, T = 120 °C; (b) effect of catalyst dose at Methanol = 25.03 mL, P = 150 bar, T = 120 °C, t = 24 h; (c) effect of temperature at methanol = 25.03 mL, catalyst dose = 1.25 g, P = 150 bar. CO₂ conversion, DMC yield, methanol conversion.

The effect of catalyst dose on the DMC yield and the CO₂ conversion is shown in Fig. 8b. It can be seen 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 Fig. 8c. Initially, the DMC yield (1.021–2.9212 mmol g⁻¹ cat.), methanol conversion (0.331–0.945 mmol g⁻¹ cat.) and CO₂ conversion (0.804–2.300 mmol g⁻¹ 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⁻¹ cat.), methanol conversion (0.687–0.2888 mmol g⁻¹ cat.) and the CO₂ conversion (1.673–0.7026 mmol g⁻¹ cat.). Thus, the optimum DMC yield was obtained at 120 °C. The decrease in the DMC yield, methanol and CO₂ conversions may be because of the poor solubility of CO₂ in methanol and also 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 Ce_0.5Zr_0.5O₂ catalyst was investigated at optimum reaction conditions (T = 120 °C, t = 24 h and catalyst amount = 1.25 g) in five consecutive batch cycles (Fig. 9). Almost similar DMC yield was found in all the batch cycles. Thus, the Ce_0.5Zr_0.5O₂ catalyst is found to be an effective catalyst with long life and can be used in a number of cycles.

Fig. 9 Reusability of Ce_0.5Zr_0.5O₂ catalyst DMC synthesis from direct conversion of CO₂ with methanol (methanol = 25.03 mL, catalyst dose = 1.25 g, P = 150 bar, T = 120 °C, t = 24 h); DMC yield (mmol g⁻¹ cat.), % methanol conversion, % CO₂ conversion.

Chemical equilibrium modeling

Synthesis of DMC from direct conversion of CO₂ and methanol can be related to the equilibrium constant as shown below:where, Θ_CO2 = y_CO2,0/y_MeOH,0.

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 fugacity coefficient of species in the mixture. PRSV-EoS is given as:

where,

a = 0.45724R²T_c²/P_c (3)

b = 0.0778RT_c/P_c (4)

α = [1 + m(ω)(1 − T_r^1/2)]² (5)

m(ω) = κ₀ + κ₁(1 + T_r^1/2)(0.7 − T_r) (6)

κ₀ = 0.378893 + 1.4897153ω − 0.1713184ω² + 0.0196554ω³ (7)

where, P_c and T_c are the critical pressure and temperature, respectively, κ is a specific pure compound parameter and ω is the acentric factor. The values of T_c, P_c, ω and κ as 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: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 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 × 10⁻⁸, 3.629 × 10⁻⁷, 1.713 × 10⁻⁷, 1.823 × 10⁻⁸ and 1.327 × 10⁻⁹ L mol⁻¹, respectively (Table S3†). The values of and (using eqn (10)) for Ce_0.5Zr_0.5O₂ using the data points at T = 120–180 °C were found to be −139.76 kJ mol⁻¹ and 1.54 kJ mol⁻¹, respectively. and values of −15.259 kJ mol⁻¹ and 29.583 kJ mol⁻¹, respectively have been reported in the literature.⁶²Table 4 compares the equilibrium, kinetic and thermodynamic parameters as obtained in the present study with those reported in the literature.^63–68 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.

Table 4 Literature review of kinetic models used for the chemical equilibrium and thermodynamics analysis

Catalyst	T (°C)	P (bar)	Kinetics models	Activation energy, Ea (kJ mol^−1)	K eq (m s^−1)	Entropy (ΔS°) (J mol^−1 K^−1)	Gibbs free energy (ΔG°) (kJ mol^−1)	Enthalpy (ΔH°) (kJ mol^−1)	Reference
K2CO3, KOH, CH3OK	80	73	–	—	9.4 × 10^−8	—	36.31	−28.8	63
CeO2	130	200	Soave–Redlich–Kwong equation of state coupled with five different mixing rules	—	0.26 ± 0.04 × 10^−2	—	−454.93	−570.70^@25 °C	64
CeO2	105–135	150–200	Langmuir–Hinshelwood and Eley–Rideal mechanisms	106–117	—	—	31.5	−20.1	65
CeO2	40–70	200	Mathematical model	—	3.3–1.1^@40–70 °C	−69 ± 4^@H2O & −60 ± 4^@DMC	−4 ± 6^@25 °C H2O & −2 ± 6^@25 °C DMC	−25 ± 6^@H2O & −25 ± 6^@DMC	66
CeO2	90–130	300	Mathematical model using PROMS model builder (version 3.6.0)	106	—	−69 ± 4^@H2O & −60 ± 4^@DMC	—	−25 ± 6^@H2O & −25 ± 6^@DMC	67
Ceria nanorod	125, 140	138	Langmuir–Hinshelwood mechanisms	65 ± 18	—	—	—	—	68
Ce0.5Zr0.5O2	100–180	150	Peng–Robinson–Stryjek–Vera equation with van der Waals one-fluid reaction	—	3.629 × 10^−7	—	1.54	−139.76	This paper

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Conclusions

In this paper, porous and spherically shaped cerium–zirconium catalysts (Ce_x–Zr_1−xO₂) 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 reflexes of cubic phase in CeO₂, tetragonal phase in ZrO₂ and Ce_0.5Zr_0.5O₂. The synthesized catalysts showed BET surface between 28–112 m² g⁻¹ and pore volume in the range of 0.2–0.42 cm³ g⁻¹. An increase in the ceria content was found to decrease the specific surface area of the mixed oxides except for x = 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 CO₂ with methanol for the production DMC in a batch reactor. The Ce_x–Zr_1−x (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 per g cat.) < CeO₂ (1.384 mmol DMC per g cat.) < Ce_0.5Zr_0.5O₂ (2.921 mmol DMC per g cat.). During five consecutive reuse cycles of Ce_0.5Zr_0.5O₂ catalyst, only marginal change in DMC yield and methanol conversion was observed. The values of and for Ce_0.5Zr_0.5O₂ catalyst were found to be −139.76 kJ mol⁻¹ and 1.54 kJ mol⁻¹, respectively.

Acknowledgements

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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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22643d

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