Efficient and Greener Synthesis of Propylene Carbonate from Carbon

Article pubs.acs.org/IECR

Efficient and Greener Synthesis of Propylene Carbonate from Carbon Dioxide and Propylene Oxide Adegboyega Isaac Adeleye,† Dipesh Patel,† Debdarsan Niyogi,‡ and Basudeb Saha*,† †

Centre for Green Process Engineering, Department of Applied Sciences, Faculty of Engineering, Science and The Built Environment, London South Bank University, 103 Borough Road, London SE1 0AA, United Kingdom ‡ Global Consultancy Practice, Tata Consultancy Services Ltd., 33 Grosvenor Place, London SW1X 7HY, United Kingdom ABSTRACT: Several heterogeneous catalysts have been investigated for solvent-free synthesis of propylene carbonate (PC) for cycloaddition reaction of propylene oxide (PO) and carbon dioxide (CO2). The characterization of different heterogeneous catalysts has been successfully carried out using Raman spectroscopy, scanning electron microscopy, and X-ray diffraction analysis. Batch cycloaddition reaction of PC and CO2 has been conducted in a high pressure reactor. The effect of various parameters that could influence the conversion of PO and the selectivity and yield of PC such as catalyst types, catalyst loading, CO2 pressure, reaction temperature, and reaction time has been studied to find the optimum conditions and the best preferred catalyst for this reaction. Ceria and lanthana doped zirconia (Ce−La−Zr−O) catalyst has been found to be the most active and selective for synthesis of PC as compared to other heterogeneous catalysts that were tested as part of this research. Catalyst reusability studies have been conducted to investigate the long-term stability of the best performed catalyst for synthesis of PC, and it has been found that the catalyst could be reused several times without losing its catalytic activity. An artificial neural network (ANN) model has been developed for PC synthesis by cycloaddition reaction of PO and CO2 using Ce−La−Zr−O catalyst to compare the experimental data and the predicted results by ANN model. The ANN model predicted values are in good agreement with the experimental results.

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INTRODUCTION The prerequisite for sustainable environment and green chemical process is the utilization of renewable raw materials. The readily available renewable carbon source is carbon dioxide (CO2) that has the advantage of being nontoxic, abundant, and economical.1−3 CO2 is an environmentally friendly chemical that plays an important role in numerous industrial applications.4−7 The reaction of CO2 and epoxides to produce organic carbonates including cyclic carbonates and polycarbonates shows promising industrial applications that have gained considerable attention in recent years.8−10 The reaction of epoxides and CO2 to synthesize cyclic carbonates is one of the few catalytic processes that have been commercialized.11 The reaction of epoxides and CO2 produces two major valuable products, cyclic carbonates and polycarbonates.12 The selection of suitable catalyst and reaction conditions favors one or the other types of organic carbonates. The synthesis of cyclic carbonate is thermodynamically favored as compared to the polymerization process of polycarbonate.13 In addition to biodegradability, cyclic carbonate possess high solvency.14 Propylene carbonate (PC) finds numerous applications in chemicals, cosmetic and personal care, pharmaceutical, automobile, paint, optical media, and electronics industries.15,16 PC is an important reactive intermediate in the synthesis of valuable chemicals such as poly(propylene carbonate) and dimethyl carbonate (DMC). It is also an important feedstock in the production of modern lithium ion batteries.17,18 The conventional process for synthesis of organic carbonates employs the use of highly toxic raw materials such as phosgene and iso-cyanate.19−22 Several different routes for synthesis of © XXXX American Chemical Society

organic cyclic carbonates have been reported including oxidative carboxylation of alkenes,23 phosgene and oxetanes, oxidative carbonylation of alcohol and phenol, and reaction of urea and alcohol/phenol.20,24 These methods use toxic raw materials and produce carcinogenic byproducts that could have serious impact on the environment and the human health. The drawback of the current process remains the major challenge. Catalyst is an important tool in designing a greener process for the synthesis of valuable chemicals. The catalyst required for organic carbonate synthesis not only should be limited to enhancement of activity and selectivity toward the required organic carbonate products but also should fulfill the requirement of a greener and sustainable chemical process.13 Homogeneous catalysts such as salt and metal halide,25−30 salen metal complex,31−34 and ionic liquid35,36 have been the most employed catalysts for the reaction of epoxide and CO2. However, homogeneous catalysts suffer from numerous drawbacks including high cost of catalyst production, multistep process, high cost of separation of product from the reaction mixture, potential production of toxic species, use of cosolvent, problem of catalyst reusability, and instability of the catalyst under room conditions.37,38 In recent years, several methods have been developed to fabricate a stable and reusable heterogeneous system by immobilizing or grafting ionic liquids and salts into solid materials such as polymers, molecular sieve Special Issue: Ganapati D. Yadav Festschrift Received: January 24, 2014 Revised: May 14, 2014 Accepted: May 15, 2014

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dx.doi.org/10.1021/ie500345z | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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(99.8% (w/w)) were procured from Sigma-Aldrich Co. LLC. The catalysts including Ce−La−Zr−O, Ce−Zr−O, La−Zr−O, and Zr−O were supplied by Magnesium Elektron Ltd. (MEL) Chemicals UK. La−O catalyst was procured from SigmaAldrich. The liquid CO2 cylinder equipped with a dip tube was purchased from BOC Ltd. UK. All chemicals and catalysts were used without further purification or pretreatment. Supercritical fluid (SCF) pump (model SFT-10) was procured from Analytix Ltd., U.K. A Parr high pressure stainless steel autoclave reactor (model 5500) of 100 mL capacity equipped with a reactor controller (model 4848) was purchased from SCIMED (Scientific and Medical Products Ltd.), U.K. Catalyst Characterization. Metal oxide and mixed metal oxide catalysts are commonly employed heterogeneous catalysts for the synthesis of cyclic organic carbonates. La−O, Zr−O, and doped zirconium oxides such as Ce−Zr−O, La− Zr−O, and Ce−La−Zr−O have been used in several chemical processes because of their promising catalytic properties. Surface and structural characteristics of Zr−O (cubic, monoclinic, and tetragonal zirconia) depend on the calcination process. Calcination process at 773 K identical to amorphous monoclinic zirconia at 873 K connotes the tetragonal zirconia.47 The processing methods including calcination temperature and incorporation of dopants have a strong influence on the physiochemical properties of the catalysts. Different combinations of these properties are required for a number of chemical catalytic applications. Large surface area and a high porosity of the catalysts are prerequisites for most of the heterogeneous catalytic applications. The activity and selectivity of the solid catalyst for the synthesis of PC depends on the structural characteristics, particle size, texture, and active sites present on the surface area of the catalyst. In this work, several catalysts have been tested and analyzed based on their activity and selectivity toward the production of PC. The morphology of the solid catalysts was determined by scanning electron microscopy (SEM), and structural characteristics of the catalysts were investigated by Raman spectroscopy and X-ray diffraction (XRD). The solid catalysts (except La−O which was procured from Sigma-Aldrich) used in this research were prepared by proprietary processes developed by MEL Chemicals that have been optimized to produce specific particle size distributions and different textural properties (surface area and pore volume). SEM analysis was carried out using FEI Inspect F and Quanta 3D FEG with a gold plated sample holder. XRD patterns were investigated on a Siemens D5000 X-ray powder diffractometer analyzer operating in θ/2θ geometry. Raman spectra were recorded using a Renishaw Ramascope 1000 (model 52699) analyzer connected to an optical microscope and a modu-laser source. All catalyst characterization results and subsequent catalyst activity studies are presented in detail in ‘Results and Discussion’ section. Experimental Procedure. Cycloaddition reaction of PO and CO2 was carried out in a mechanically stirred Parr high pressure reactor, as shown in Figure 1. The reactor was equipped with a pressure gauge and a thermocouple that monitor the pressure and temperature inside the reactor at any given time. The heating of the reactor was provided by an aluminum cylindrical heater that is connected to the controller unit. The gas inlet valve at the top of the reactor was connected to a liquid CO2 cylinder via SCF pump that allows liquid CO2 into the reactor at a desired rate.

(MCM-41), silicon oxide (SiO2), and magnesium oxide (MgO).18,39 These heterogeneous catalysts showed good catalytic activity and selectivity for cycloaddition reaction of organic carbonate synthesis. However, these heterogeneous catalysts failed a reusability test.38 The development of a solvent-free heterogeneous catalytic system plays an important role in a greener process route for valuable chemical synthesis. It offers numerous benefits including the ease of separation of catalyst from the reaction mixture, thereby eliminating the use of a complex separation process by distillation, solubility, and extraction. In addition, it could eliminate the use of toxic catalyst and generation of hazardous byproducts, and increase thermal stability, easy and safer handling and storage, long shelf life, safe disposal, and reusability over the homogeneous counterpart.3 The solventfree heterogeneous catalytic system is therefore an economically viable and safer process that eliminates the risk to human health and the environment.40 On the other hand, there are limitations of the heterogeneous system including the lack of reactor design flexibility and the use of a high amount of catalyst. In recent years, an artificial neural network (ANN) model has been employed to solve the complex modeling problem associated with the conventional method of modeling the kinetic data that involves assuming an empirical mathematical equation and finding the unknown parameters by nonlinear regression analysis.41 ANN is a computational modeling and data processing tool used in several disciplines including science and engineering. In chemical engineering, ANN model has been employed in many applications such as complex chemical and process modeling, chemical reactor modeling, estimation of catalyst deactivation, and design of catalyst.42−44 The ANN model provides accurate solutions to the practically precise and imprecise problems and offers a detailed understanding of experimental results.45 The advantage of ANN modeling is that it does not require theoretical knowledge or human experience during the training process46 and still could accurately mimic the trends using the experimental results satisfactorily. From the environmental, economic, and human health point of view, the solvent-free heterogeneous catalytic system is a potentially greener route that could reduce or eliminate the shortcomings of the current process of cyclic carbonate synthesis. Hence, in this work, several solid catalysts including ceria and lanthana doped zirconia (Ce−La−Zr−O), ceria doped zirconia (Ce−Zr−O), lanthana doped zirconia (La−Zr− O), lanthanum oxide (La−O), and zirconium oxide (Zr−O) have been characterized and tested for the synthesis of PC in the absence of a solvent. In addition, the effect of numerous factors such as catalyst types, catalyst loading, CO2 pressure, reaction temperature, and reaction time has been studied systematically for selecting the best performing catalyst. Catalyst reusability studies have been conducted to investigate the stability of the best performing catalyst for the synthesis of PC. The batch experimental results have been modeled using an ANN to predict the effect of various reaction parameters on PC synthesis for cycloaddition reaction of propylene oxide (PO) and CO2 for the best performing catalyst.

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EXPERIMENTAL METHODS Materials. Acetone (99% (w/w)), iso-octane (99.8% (w/ w)), propylene oxide (99.5% (w/w)), and propylene carbonate (98% (w/w)) were purchased from Fisher Scientific UK Ltd. 2Ethyl-4-methyl-1,3-dioxolane (99% (w/w)) and n-pentane B



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The ANN model was calibrated with a range of randomly selected values from experimental data. The selected values were divided into validation, training, and test sets. The validation sets were used to calibrate the parameter of the model to ensure that the predicted responses are representatives of the experimental results. Training sets were used to adjust the interconnection nodes while test sets were used for testing the robustness of the developed model.48 Method of Analysis. Samples collected from the reactor were analyzed by a Shimadzu GC-2014 gas chromatograph equipped with a capillary column of dimensions (30 m × 320 μm × 0.25 μm) and a flame ionization detector (FID). High purity (99.9%) helium was used as the carrier gas. The flow rate of the helium gas was maintained at 0.95 mL min−1. Both the injection and detector temperatures were maintained isothermally at 523 K. A ramp method was used to distinguish all of the components present in the sample mixture. A spilt ratio of 100:1 and injection volume of 0.5 μL were used as part of the GC ramp method. The initial column temperature was maintained at 323 K, and the sample was injected by an autosampler. The column temperature was set to hold at 323 K for 5 min after sample injection. The column temperature was increased to 523 K at the rate of 25 °C min−1. The total run time for each sample was ∼14 min. n-Pentane was used as a solvent to rinse the injection needle after the sample injection. The subsequent sample runs were started when the column temperature was cooled back to 323 K. Internal standard method was used for quantification of all the components present in the sample mixture, and iso-octane was used as an internal standard. A typical chromatogram of the sample mixture analyzed using the Shimadzu GC-2014 GC revealed that the PO peak emerged at a residence time of ∼3.5 min followed by the iso-octane peak at ∼5 min, the 2-ethyl-4methyl-1,3-dioxolane (side product) peak at ∼7.2 min, and the PC peak at ∼11 min (see Figure 2). The experimental error for batch studies conducted in a high pressure reactor using GC technique was in the range of ±3%.

Figure 1. Experimental setup of organic carbonate synthesis using a Parr high pressure reactor.

During any given experiment, the reactor was charged with a specific amount of catalyst and the limiting reactant, i.e., PO. The mechanical stirrer was started at a known stirrer speed, and the reactor was heated to the desired temperature. Once the desired temperature was achieved, a known amount of liquid CO2 was fed to the high pressure reactor by an SCF pump. The time at which the liquid CO2 was charged into the reactor was taken as zero time, i.e., t = 0, and the reaction mixture was left for a desired period. Once the experiment was completed, stirring and heating of the reactor were switched off, the reactor was cooled in an ice bath, and the excess CO2 present in the reactor was vented out. The reaction mixture was filtered and the catalyst was separated, washed with acetone, and dried in a vacuum oven. The sample was collected from the filtered reaction mixture and analyzed by a Shimadzu GC-2014 gas chromatograph (GC). The effect of various parameters such as catalyst types, catalyst loading, CO2 pressure, reaction temperature, and reaction time was studied for the optimization of the reaction conditions. Catalyst reusability studies were also conducted to assess the stability of the catalyst for synthesis of PC. Mass transfer resistance was investigated by varying the stirring speed (350−550 rpm). Artificial Neural Network Model. An increased complexity and undesirable method of linear conventional statistical models such as simple differentiation and optimization has led to the development of the nonlinearity and nonparametric approach of the ANN model. ANN is a computational modeling tool widely employed in chemical engineering with several applications such as modeling, pattern recognition, fault detection, and classification. The predictive ability of ANN without insight in the physical process depends on the experimental design and validation by independent variables. The basic unit of ANN is the neuron which comprises connecting nodes present in the layers consisting of input and hidden and output layers. The input layers are represented by the design parameter. Hidden layers behave like feature detectors, and it is possible to have more than one hidden layer. However, a single hidden layer with large number of neurons could interpret the input−output data efficiently. An output layer is the function of input variables for ANN response structure or data. Therefore, simplicity and predictive accuracy of ANN is a fundamental solution approach to practical problems in chemical engineering, such as organizing and detecting the features of unpredictable and imprecise process variables.

Figure 2. Typical chromatogram of a reaction mixture analyzed by a Shimadzu GC-2014 gas chromatograph.

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RESULTS AND DISCUSSION Proposed Reaction Mechanism. The exothermic cycloaddition reaction of PO and CO2 in the presence of heterogeneous catalyst produces a value added product, i.e., PC along with other side products. The reaction scheme for the synthesis of PC from PO and CO2 is shown in Figure 3a. A proposed reaction mechanism is shown in Figure 3b. In the proposed mechanism, M represents a metal atom that consists of a Lewis acid site and O represents an oxygen atom that C



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Figure 3. Synthesis of propylene carbonate (PC) using heterogeneous catalyst: (a) reaction scheme and possible side products and (b) possible reaction mechanism.

Table 1. Physical and Chemical Properties of Ceria and Lanthana Doped Zirconia (Ce−La−Zr−O), Ceria Doped Zirconia (Ce−Zr−O), Lanthanum Oxide (La−O), Lanthana Doped Zirconia (La−Zr−O), and Zirconium Oxide (Zr−O) Catalysts catalysts catalyst properties physical form composition (%)

particle size (μm) BET surface area (m2 g−1) pore volume (cm3 g−1) avg pore diam (nm) operating temp (K)a a

Ce−La−Zr−O

Ce−Zr−O

La−O

La−Zr−O

Zr−O

pale yellow powder CeO2: 17 ± 2 La2O3: 5 ± 1 ZrO2: 78 ± 3 1.7 55 0.29 21.1 673

pale yellow powder CeO2: 18 ± 2 ZrO2: 82 ± 2

white powder La2O3: 100

white powder La2O3: 10 ± 1 ZrO2: 90 ± 1

white powder ZrO2: 95 ± 5

30 70 0.2 11.4 673

0.1 22 0.015 2.7 2578

5 75 0.22 11.7 673

5 310 0.45 5.8 673

Manufacturer data.

consists of a Lewis basic site. Cycloaddition reaction was initiated by adsorption of CO2 on the basic site of the metal oxide catalyst to form a carboxylate anion, and PO was activated by adsorption on the acidic site. The carbon atom of PO was attacked by a carboxylate anion that leads to the ring opening of PO to form an oxy-anion species. Dissociation of metal oxide (catalyst) from the oxy-anion species leads to a ring closure and desorption of PC as a product. The side products

associated with cycloaddition reaction of CO2 and PO includes isomers of PO such as acetone and propionaldehyde, and dimers of PO including 2-ethyl-4-methyl-1,3-dioxolane and their derivatives (see Figure 3a). However, in this study only 2ethyl-4-methyl-1,3-dioxolane was identified as a side product by GC analysis, as shown in Figure 2. Catalyst Characterization. The physical and chemical properties of heterogeneous catalysts have been presented in D



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Figure 4. Scanning electron microscopy image of (a) Ce−La−Zr−O), (b) Ce−Zr−O, (c) La−O, (d) La−Zr−O, and (e) Zr−O catalysts.

Table 1. Zr−O catalyst has the largest surface area of 310 m2 g−1. Ce−La−Zr−O, Ce−Zr−O, and La−Zr−O catalysts showed surface areas of 55, 70, and 75 m2 g−1, respectively, while La−O catalyst showed the least surface area of 22 m2 g−1. It can be seen from Table 1 that La−O catalyst has the smallest total pore volume of 0.015 cm3 g−1. Zr−O and Ce−La−Zr−O catalysts exhibited pore volumes of 0.45 and 0.29 cm3 g−1, while Ce−Zr−O and La−Zr−O catalysts showed similar total pore volumes of 0.2 and 0.22 cm3 g−1, respectively. The surface topographic images of Ce−La−Zr−O, Ce−Zr− O, La−O, La−Zr−O, and Zr−O catalysts are presented in Figure 4. Figure 4a depicts the Ce−La−Zr−O image with a well dispersed circular smooth surface, whereas Figure 4b represents the Ce−Zr−O image with a homogeneously aggregated irregular shape surface. Figure 4c shows La−O catalyst with a rectangular grainlike surface. The SEM image of La−Zr−O catalyst shown in Figure 4d has an aggregated circular smooth surface, while Figure 4e shows Zr−O catalyst with an irregular grainlike surface. Figure 4 clearly depicts that

the incorporation of the dopants (lanthana and ceria) have strong influence on the morphology of the catalyst. Raman spectra of catalysts are presented in Figure 5. The parameters that affect the Raman bands include crystal size, structure, and stresses of a catalyst.49 High intensity and broad peaks appeared at a wavelength (λ) of 620 and 1060 cm−1, signifying the presence of Zr−O and La−O as the major support of the catalysts. Ce−Zr−O catalyst showed higher intensity and broader peaks at a wavelength (λ) of 620 cm−1 as compared to Ce−La−Zr−O and La−Zr−O and Zr−O catalysts. However, La−O catalyst showed no peak at λ = 620 cm−1, and a higher intensity peak of La−O was observed at λ = 1060 cm−1. The broad peak at λ = 620 cm−1 was in accordance with Zr−O catalyst as reported in the literature.50 The stronger intense peaks may be due to an increase in the surface area of the Zr−O catalyst which could have been influenced by the incorporation of dopant into the Zr−O. The XRD patterns of the Ce−La−Zr−O, Ce−Zr−O, La−O, La−Zr−O, and Zr−O catalysts are presented in Figure 6. It is evident from Figure 6 that the dopants (ceria and lanthana) E



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thus increases the stability of the catalyst as illustrated in Figure 6. The strong peak intensity of Ce−La−Zr−O catalyst could be due to a decrease in crystallinity of the zirconia catalyst, and hence the presence of dopants increases the stability of the catalysts. Therefore, Ce−La−Zr−O catalyst exhibited higher stability as compared to the Ce−Zr−O and La−Zr−O catalysts. Artificial Neural Network Model Predicted Results. An ANN model has been employed to predict the effect of various design parameters on PC synthesis from the reaction of PO and CO2 using Ce−La−Zr−O catalyst. The results are presented in Figures 9−12. The results confirmed that ANN model predicted the experimental results obtained for PC synthesis at various reaction conditions which include catalyst loading, CO2 pressure, reaction temperature, and the reaction time. Effect of Mass Transfer in Heterogeneous Catalytic Processes. Important attributes of a heterogeneous catalyst include high activity, good stability, ability to be recovered and reused, and its selectivity toward a specific direction of the reaction. Mass transfer resistance reduces the activity of the catalyst toward the selectivity of the desired product. The activity and selectivity of the solid catalysts thus depend on the catalytic system (i.e., active site and molecular and chemical structures of the catalyst), boundaries (i.e., internal surface such as pore size and porosity, and external surface such as surface area and particle size), and conditions prevailing at the catalyst boundaries (i.e., temperature and pressure). Heterogeneous catalytic reaction is always connected with the mass transfer process. Therefore, understanding the heterogeneous catalytic process could eliminate the effect of mass transfer processes during a chemical reaction. The effect of mass transfer resistance on cycloaddition reaction of PO and CO2 using Ce−La−Zr−O catalyst to produce PC was investigated using a high pressure reactor. The cycloaddition reaction of PO and CO2 was carried out at different stirring speed of 350−550 rpm. The results are shown in Figure 7. It was found that there were no significant changes in the conversion of PO and the yield and selectivity of PC when the stirring speed was increased from 350 to 550 rpm considering the experimental error of ±3%. Since there exists no external mass transfer resistance, it could be concluded that good homogeneous distribution of catalyst particles was possible even at low stirrer speed of 350 rpm (energy efficient process). Therefore, all of the subsequent experiments were carried out at a stirring speed of 350 rpm. On the other hand, the catalysts used in this work are fairly uniform and porous, have a particle size range of 0.1−30 μm, and exhibit a pore diameter of 2.7−

Figure 5. Raman spectra of Ce−La−Zr−O, Ce−Zr−O, La−O, La− Zr−O, and Zr−O catalysts.

Figure 6. X-ray diffraction patterns of Ce−La−Zr−O, Ce−Zr−O, La− O, La−Zr−O, and Zr−O catalysts.

have stronger interaction with zirconia. The La−O and Zr−O peaks show low intensity at 2θ = 23°, 28°, 30°, 31°, 40°, 44°, 50°, 60°, and 66° as compared to Ce−La−Zr−O catalyst that showed higher intensity peaks at 2θ = 32°, 34°, 39°, 54°, and 63°. Ce−Zr−O and La−Zr−O catalysts have high intensity peaks that appeared at 2θ = 29°, 32°, 34°, 50°, and 60°. These results are in agreement with that reported in the literature.51 Zirconia peaks constitute the forms of zirconia (monoclinic and tetragonal zirconia) catalyst. As a result, the peaks at 2θ = 23°, 28°, and 31° exhibit monoclinic zirconia and the peaks that appear at 2θ = 30°, 32°, 50°, and 60° represent tetragonal zirconia. Tetragonal zirconia showed higher thermal stability as compared to monoclinic zirconia. The influence of the dopants

Figure 7. Effect of mass transfer resistance on conversion of propylene oxide versus (a) yield and (b) selectivity of propylene carbonate. Experimental conditions: catalyst, Ce−La−Zr−O; catalyst loading, 10% (w/w); reaction temperature, 443 K; CO2 pressure, 70 bar; reaction time, 20 h. F



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21.1 nm. Clerici and Kholdeeva52 reported that if the pore diameter of the catalyst particles is in the mesoporous region, i.e., 2−50 nm, mass transfer limitations could be eliminated. Mass transfer experiments were carried out using different particle sizes of trimetallic catalyst for alcohol synthesis by Surisetty et al.,53 and they have reported that particle sizes below 254 μm have negligible external mass transfer resistance. Similarly, the absence of both internal and external mass transfer resistances was reported using different stirrer speed and various size fractions of ion-exchange resins as catalysts for synthesis of n-hexyl acetate.54 It was found that a decrease in catalyst particle size permits the entire surface area and pore surface area to participate in the chemical reaction. For small catalyst particle, reaction is chemically controlled and uninfluenced by mass transfer resistance. In addition, we have investigated the effect of mass transfer at different reaction temperature, i.e., 443, 458, and 473 K, and found that there was no effect of mass transfer resistances on PC synthesis. Therefore, it can be concluded that there was no effect of mass transfer resistances on the synthesis of PC using Ce−La− Zr−O catalyst. Effect of Different Heterogeneous Catalysts. Several heterogeneous catalysts were tested in order to study the catalytic activity and selectivity for the production of PC from the reaction of PO and CO2 (Figure 3a) using a high pressure reactor. Figure 8 shows the effect of different catalysts (metal

and mixed metal oxides) on the conversion of PO and on the yield and selectivity of PC. It can be seen from Figure 8 that Ce−La−Zr−O catalyst gave the highest conversion of PO (∼92%) and highest PC yield (66%) and selectivity (72%). The presence of La2O3 has a significant effect on the catalytic activity of Ce−La−Zr−O catalyst for the cycloaddition reaction of PO and CO2. Similarly, La−O catalyst exhibits PO conversion of ∼86% and PC yield and selectivity of ∼55% and ∼64%, respectively. The reason for La−O catalyst to perform better than other mixed oxide catalysts could be due to its lowest particle size (100 nm) and highest pore volume (15 cm3 g−1) that could have helped reactant molecules (PO and CO2) to access the available active catalytic sites with ease. On the other hand, Zr−O catalyst showed the least yield and selectivity of PC. The addition of cerium oxide (Ce2O3) to Zr− O catalyst gave similar PO conversion (∼43%), but exhibited an increase in the yield and selectivity of PC when compared with the experiment conducted using only Zr−O catalyst. Therefore, it could be concluded that the addition of Ce2O3 to Zr−O (i.e., Ce−Zr−O) catalyst was found to be selective in the formation of propylene carbonate as compared to Zr−O catalyst. During the batch experiments, dimers of PO i.e., 2ethyl-4-methyl-1,3-dioxolane was detected as a byproduct and confirmed by GC analysis. Based on this study, Ce−La−Zr−O catalyst was found to be the best performing catalyst for synthesis of PC and all further studies were conducted using Ce−La−Zr−O catalyst. Effect of Reaction Time. A series of experiments were carried out by varying the reaction time to determine the optimum reaction time for synthesis of PC using Ce−La−Zr− O catalyst. All experiments for this study were conducted at 443 K and 70 bar CO2 pressure. Figure 9 illustrates that an increase in reaction time increases PO conversion as well as the yield and selectivity of PC. PO conversion of ∼63%, ∼38% yield, and ∼61% selectivity of PC was observed for reaction time of 8 h, whereas ∼93% conversion of PO and ∼66% yield and ∼72% selectivity of PC were obtained for reaction time of 20 h. However, when reaction was carried out beyond 20 h, i.e., 24 h, the conversion of PO as well as yield and selectivity of PC were similar to that obtained at 20 h. It can be concluded that the reaction reaches equilibrium at 20 h and the reaction time beyond 20 h would not be beneficial for this reactive system. Based on this study, the reaction time of 20 h was considered to be the optimum, and all further investigations were carried out at a reaction time of 20 h.

Figure 8. Effect of different catalysts on conversion of propylene oxide and the selectivity, and yield of propylene carbonate. Experimental conditions: catalyst loading, −10% (w/w); reaction temperature, 443 K; CO2 pressure, 70 bar; reaction time, 20 h; stirring speed, 350 rpm.

Figure 9. Time dependence and prediction by an artificial neural network model on conversion of propylene oxide versus (a) yield and (b) selectivity of propylene carbonate. Experimental conditions: catalyst, Ce−La−Zr−O; catalyst loading, 10% (w/w); reaction temperature, 443 K; CO2 pressure, 70 bar; stirring speed, 350 rpm. G



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Figure 10. Catalyst loading dependence and prediction by an artificial neural network model on conversion of propylene oxide versus (a) yield and (b) selectivity of propylene carbonate. Experimental conditions: catalyst, Ce−La−Zr−O; reaction temperature, 443 K; CO2 pressure, 70 bar; reaction time, 20 h; stirring speed, 350 rpm.

Figure 11. Temperature dependence and prediction by an artificial neural network model on conversion of propylene oxide versus (a) yield and (b) selectivity of propylene carbonate. Experimental conditions: catalyst, Ce−La−Zr−O; catalyst loading, 10% (w/w); CO2 pressure, 70 bar; reaction time, 20 h; stirring speed, 350 rpm.

Figure 12. Pressure dependence and prediction by an artificial neural network model on conversion of propylene oxide versus (a) yield and (b) selectivity of propylene carbonate. Experimental conditions: catalyst, Ce−La−Zr−O; catalyst loading, 10% (w/w); reaction temperature, 443 K; reaction time, 20 h; stirring speed, 350 rpm.

Effect of Catalyst Loading. The catalyst loading for this work is defined as the percentage ratio of the mass of catalyst to the mass of limiting reactant (PO). The synthesis of PC was studied with different amounts of Ce−La−Zr−O catalyst at 443 K and 70 bar CO2 pressure. The results are presented in Figure 10. It can be seen from Figure 10 that an increase in catalyst loading increases PO conversion and the yield and selectivity of PC. For experiment conducted at 10% (w/w) catalyst loading, PO conversion and the yield and selectivity of PC were ∼93%, ∼66%, and ∼72%, respectively. However, for experiment carried out at 15% (w/w) catalyst loading, PO conversion and the yield and selectivity of PC were ∼93%, ∼65%, and 70%, respectively. In view of the experimental error of ±3%, it seems that the number of active sites for PO and CO2 to react and produce PC was large enough at 10% (w/w) catalyst

loading. Therefore, it was not necessary to increase the catalyst loading above 10% (w/w). Based on this study, catalyst loading of 10% (w/w) was chosen as an optimum, and all further experiments were conducted at 10% (w/w) catalyst loading. Effect of Reaction Temperature. Cycloaddition reaction of PO and CO2 was carried out at different reaction temperatures to study their effect on PO conversion and PC yield and selectivity. Figure 11 shows the temperature dependence on the yield and selectivity of PC and on the PO conversion. For this study, all experiments were carried out at 10% catalyst loading and 70 bar CO2 pressure. The experimental results of this study are plotted in Figure 11. It can be observed from Figure 11 that an increase in reaction temperature increases PO conversion and PC yield and selectivity. At 443 K, PO conversion and the yield and H



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oven for 12 h at 353 K and reused for run 2. The same procedure was repeated for subsequent runs. From Figure 13, it is evident that there was no appreciable decrease in the PO conversion and the yield and selectivity of PC after several runs. It can be concluded that Ce−La−Zr−O catalyst exhibits an excellent reusability and stability for the synthesis of PC and the catalyst can be reused several times without any significant loss in its catalytic activity.

selectivity of PC were 93%, 66%, and 72%, respectively. However, further increase in the reaction temperature from 443 to 458 K resulted in the formation of an increased amount of 2ethyl-4-methyl-1,3-dioxolane. At 458 K, even though the conversion of PO was increased to 97%, the yield and selectivity of PC were decreased to 61% and 63%, respectively, when compared to the experiment conducted at 443 K. Yasuda et al.55 reported that, from an industrial point of view, a temperature range of 423−473 K is preferable for cycloaddition reaction of epoxides and CO2 in the presence of heterogeneous catalysts because cycloaddition reaction of epoxide and CO2 is an exothermic reaction. All of the subsequent experiments for PC synthesis were carried out at 443 K. Effect of CO2 Pressure. The reaction of PO and CO2 to produce PC was investigated at different CO2 pressures. For this study, the experiments were carried out at 443 K, 10% (w/ w) catalyst loading and various supercritical CO2 (scCO2), where scCO2 acts both as a reagent and as a solvent. Figure 12 shows the effect of CO2 pressure on conversion of PO and on the yield and selectivity of PC formation. It can be observed from Figure 12 that an increase in CO2 pressure increases PO conversion and the yield and selectivity of PC. At a CO2 pressure of 70 bar, the conversion of PO and the yield and selectivity of PC were ∼93%, ∼66%. and ∼72%, respectively. However, at a CO2 pressure of 80 bar, the PO conversion and the yield and selectivity of PC decreased to 85%, ∼59%, and ∼70%, respectively. The reason for such outcomes from this study are inconclusive; however, similar CO2 pressure effects are often reported for various homogeneous catalyzed systems using supercritical CO2.52 Based on the experimental results, it can be concluded that 70 bar CO2 pressure was the optimum CO2 pressure and all the subsequent experiments for the PC synthesis were performed at a CO2 pressure of 70 bar. Catalyst Reusability Studies. The catalyst reusability experiments were carried out to investigate the stability of the best performing heterogeneous catalyst, i.e., Ce−La−Zr−O. The experiments were carried out in a high pressure reactor using a fresh Ce−La−Zr−O catalyst (10% (w/w) catalyst loading) at a reaction temperature of 443 K, CO2 pressure of 70 bar, and reaction time of 20 h and plotted as run 1 as shown in Figure 13. After run 1, the catalyst was separated from the reaction mixture by filtration, washed with acetone, and recovered by centrifugation. The catalyst was dried in an

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CONCLUSION The cycloaddition reaction of carbon dioxide (CO2) and propylene oxide (PO) for the synthesis of propylene carbonate (PC) was successfully conducted in a high pressure reactor in the presence of various heterogeneous catalysts and without the use of a solvent. Catalyst characterization was systematically investigated using Raman spectroscopy, scanning electron microscopy and X-ray diffraction. It was found that an increase in reaction time increases PO conversion and PC yield and selectivity. Experimental results showed that an increase in reaction temperature above 443 K and CO2 pressure beyond 70 bar decreases PC yield. Ceria and lanthana doped zirconia (Ce−La−Zr−-O) catalyst was found to be the best performing catalyst for PC synthesis as compared to other heterogeneous catalysts. The optimum reaction condition was found at 443 K, 70 bar CO2 pressure, 10% (w/w) catalyst loading, and 20 h reaction time using Ce−La−Zr−O catalyst. The reusability studies of Ce−La−Zr−O catalyst were conducted to investigate the long-term stability of the best performing heterogeneous catalyst for PC synthesis. It was found that Ce−La−Zr−O catalyst could be easily separated from the reaction mixture and reused several times without any loss in the catalytic activity for cycloaddition reaction of PO and CO2. An ANN model successfully predicted the experimental results obtained for PC synthesis at various reaction conditions with a certain probability.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +44 (0)20 7815 7190. Fax: +44 (0)20 7815 7699. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank MEL Chemicals, U.K. for supplying the catalysts for this work. The authors are grateful to Dr. Suela Kellici of London South Bank University (LSBU), U.K. and Dr. Zofia Luklinska and Dr. Rory Wilson of Queen Mary University of London, U.K., for catalyst characterization. Special thanks to Professor Hari Reehal and Dr. Jeremy Ball of LSBU, U.K., for giving access to Raman spectroscopy. A.I.A. is grateful to the Faculty of Engineering, Science and The Built Environment (FESBE) of LSBU, U.K. for partial financial assistance.

■ Figure 13. Catalyst reusability studies on conversion of propylene oxide and the selectivity and yield of propylene carbonate. Experimental conditions: catalyst, Ce−La−Zr−O; catalyst loading, 10% (w/w); reaction temperature, 443 K; CO2 pressure, 70 bar; reaction time, 20 h; stirring speed, 350 rpm. I

NOMENCLATURE ANN = artificial neural network BET = Brunauer−Emmett−Teller Ce−Zr−O = ceria doped zirconia Ce−La−Zr−O = ceria and lanthana doped zirconia CO2 = carbon dioxide DMC = dimethyl carbonate EC-5 = Econo-Cap-5 dx.doi.org/10.1021/ie500345z | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX


Article

FID = flame ionization detector GC = gas chromatograph La−O = lanthanum oxide La−Zr−O = lanthana doped zirconia MEL Chemicals = Magnesium Elektron Limited Chemicals PC = propylene carbonate PO = propylene oxide t = time (h) scCO2 = supercritical CO2 SCF = supercritical fluid SEM = scanning electron microscope XRD = X-ray diffraction

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Complexes as Efficient Catalysts in Transformation of Carbon Dioxide (CO2) to Cyclic Carbonates. J. Organomet. Chem. 2011, 696, 1372. (17) Shiels, R. A.; Jones, C. W. Homogeneous and Heterogeneous 4(N,N-dialkylamino)pyridines as Effective Single Component Catalysts in the Synthesis of Propylene Carbonate. J. Mol. Catal. A: Chem. 2007, 261, 160. (18) Wang, J.-Q.; Yue, X.-D.; Cai, F.; He, L.-N. Solventless Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides Catalyzed by Silica-Supported Ionic Liquids under Supercritical Conditions. Catal. Commun. 2007, 8, 167. (19) Bhanage, B. M.; Fujita, S.-I.; Ikushima, Y.; Arai, M. Synthesis of Dimethyl Carbonate and Glycols from Carbon Dioxide, Epoxides, and Methanol using Heterogeneous Basic Metal Oxide Catalysts with High Activity and Selectivity. Appl. Catal., A 2001, 219, 259. (20) Sakakura, T.; Choi, J.-C.; Yasuda, H. Transformation of Carbon Dioxide. Chem. Rev. (Washington, DC, U. S.) 2007, 107, 2365. (21) Dai, W. L.; Chen, L.; Yin, S.-F.; Luo, S.-L.; Au, C.-T. 3-(2Hydroxyl-Ethyl)-1-Propylimidazolium Bromide Immobilized on SBA15 as Efficient Catalyst for the Synthesis of Cyclic Carbonates via the Coupling of Carbon Dioxide with Epoxides. Catal. Lett. 2010, 135, 295. (22) Sun, J.; Fujita, S.-I.; Arai, M. Development in the Green Synthesis of Cyclic Carbonate from Carbon Dioxide using Ionic Liquids. J. Organomet. Chem. 2005, 690, 3490. (23) Eghbali, N.; Li, C.-J. Conversion of Carbon Dioxide and Olefins into Cyclic Carbonates in Water. Green Chem. 2007, 9, 213. (24) Shaikh, A.-A. G.; Sivaram, S. Organic Carbonates. Chem. Rev. (Washington, DC, U. S.) 1996, 96, 951. (25) Calo, V.; Nacci, A.; Monopoli, A.; Fanizzi, A. Cyclic Carbonate Formation from Carbon Dioxide and Oxiranes in Tetrabutylammonium Halides as Solvents and Catalysts. Org. Lett. 2002, 4, 2561. (26) Kawanami, H.; Sasaki, A.; Matsui, K.; Ikushima, Y. A Rapid and Effective Synthesis of Propylene Carbonate using a Supercritical CO2Ionic Liquid System. Chem. Commun. (Cambridge, U. K.) 2003, 896. (27) Kim, H. S.; Kim, J. J.; Kim, H.; Jang, H. G. Imidazolium Zinc Tetrahalide-Catalyzed Coupling Reaction of CO2 and Ethylene Oxide or Propylene Oxide. J. Catal. 2003, 220, 44. (28) Sun, J.; Fujita, S.-I.; Bhanage, B. M.; Arai, M. One-Pot Synthesis of Styrene Carbonate from Styrene in Tetrabutylammonium Bromide. Catal. Today 2004, 93−95, 383. (29) Ra, C. S.; Hwang, J.-C.; Lee, H. B.; Shim, J.-J. Solvent-Free Carboxylation of Styrene Oxide: Enhanced Reactivity of Quaternary Onium Salts in Ionic Liquid-CO2 System. Bull. Korean Chem. Soc. 2007, 28, 1060. (30) Fujita, S.-I.; Nishiura, M.; Arai, M. Synthesis of Styrene Carbonate from Carbon Dioxide and Styrene Oxide with Various Zinc Halide-Based Ionic Liquids. Catal. Lett. 2010, 135, 263. (31) Lu, X.-B.; Feng, X.-J.; He, R. Catalytic Formation of Ethylene Carbonate from Supercritical Carbon Dioxide/Ethylene Oxide Mixture with Tetradentate Schiff-Base Complexes as Catalyst. Appl. Catal., A 2002, 234, 25. (32) Shen, Y.-M.; Duan, W.-L.; Shi, M. Chemical Fixation of Carbon Dioxide Catalyzed by Binaphthyldiamino Zn, Cu, and Co Salen-Type Complexes. J. Org. Chem. 2002, 68, 1559. (33) Paddock, R. L.; Nguyen, S.-B. T. Chiral (salen)CoIII Catalyst for the Synthesis of Cyclic Carbonates. Chem. Commun. (Cambridge, U. K.) 2004, 1622. (34) He, L.-N.; Wang, J.-Q.; Wang, J.-L. Carbon Dioxide Chemistry: Examples and Challenges in Chemical Utilization of Carbon Dioxide. Pure Appl. Chem. 2009, 81, 2069. (35) Peng, J.; Deng, Y. Cycloaddition of Carbon Dioxide to Propylene Oxide Catalyzed by Ionic Liquids. New J. Chem. 2001, 25, 639. (36) Sun, J.; Fujita, S.-I.; Zhao, F.; Arai, M. A Highly Efficient Catalyst System of ZnBr2/n-Bu4NI for the Synthesis of Styrene Carbonate from Styrene Oxide and Supercritical Carbon Dioxide. Appl. Catal., A 2005, 287, 221.

REFERENCES

(1) Paddock, R. L.; Nguyen, S. T. Chemical CO2 Fixation: Cr(III) Salen Complexes as Highly Efficient Catalysts for the Coupling of CO2 and Epoxides. J. Am. Chem. Soc. 2001, 123, 11498. (2) Bu, Z.; Qin, G.; Cao, S. A Ruthenium Complex Exhibiting High Catalytic Efficiency for the Formation of Propylene Carbonate from Carbon Dioxide. J. Mol. Catal. A: Chem. 2007, 277, 35. (3) Dai, W.-L.; Luo, S.-L.; Yin, S.-F.; Au, C.-T. The Direct Transformation of Carbon Dioxide to Organic Carbonates over Heterogeneous Catalysts. Appl. Catal., A 2009, 366, 2. (4) Aresta, M.; Dibenedetto, A.; Gianfrate, L.; Pastore, C. Nb(V) Compounds as Epoxides Carboxylation Catalysts: The Role of the Solvent. J. Mol. Catal. A: Chem. 2003, 204 − 205, 245. (5) Song, C. Global Challenges and Strategies for Control, Conversion and Utilization of CO2 for Sustainable Development Involving Energy, Catalysis, Adsorption and Chemical Processing. Catal. Today 2006, 115, 2. (6) Bai, D.; Wang, Q.; Song, Y.; Li, B.; Jing, H. Synthesis of Cyclic Carbonate from Epoxide and CO2 Catalyzed by Magnetic Nanoparticle-Supported Porphyrin. Catal. Commun. 2011, 12, 684. (7) Klaus, S.; Lehenmeier, M. W.; Anderson, C. E.; Rieger, B. Recent Advances in CO2/Epoxide CopolymerizationNew Strategies and Cooperative Mechanisms. Coord. Chem. Rev. 2011, 255, 1460. (8) Wang, J. Q.; Kong, D. L.; Chen, J. Y.; Cai, F.; He, L. N. Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide over SilicaSupported Quaternary Ammonium Salts under Supercritical Conditions. J. Mol. Catal. A: Chem. 2006, 249, 143. (9) Darensbourg, D. J. Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2. Chem. Rev. (Washington, DC, U. S.) 2007, 107, 2388. (10) Wu, S.-S.; Zhang, X.-W.; Dai, W.-L.; Yin, S.-F.; Li, W.-S.; Ren, Y.-Q.; Au, C.-T. ZnBr2-Ph4PI as Highly Efficient Catalyst for Cyclic Carbonates Synthesis from Terminal Epoxides and Carbon Dioxide. Appl. Catal., A 2008, 341, 106. (11) Ion, A.; Parvulescu, V.; Jacobs, P.; de Vos, D. Sc and ZnCatalyzed Synthesis of Cyclic Carbonates from CO2 and Epoxides. Appl. Catal., A 2009, 363, 40. (12) Castro-Osma, J. A.; Lara-Sánchez, A.; North, M.; Otero, A.; Villuendas, P. Synthesis of Cyclic Carbonates using Monometallic, and Helical Bimetallic, Aluminium Complexes. Catal. Sci. Technol. 2012, 2, 1021. (13) Pescarmona, P. P.; Taherimehr, M. Challenges in the Catalytic Synthesis of Cyclic and Polymeric Carbonates from Epoxides and CO2. Catal. Sci. Technol. 2012, 2, 2169. (14) Tomishige, K.; Yasuda, H.; Yoshida, Y.; Nurunnabi, M.; Li, B. T.; Kunimori, K. Catalytic Performance and Properties of Ceria Based Catalysts for Cyclic Carbonate Synthesis from Glycol and Carbon Dioxide. Green Chem. 2004, 6, 206. (15) Verevkin, S. P.; Emel’yanenko, V. N.; Toktonov, A. V.; Chernyak, Y.; Schäffner, B.; Börner, A. Cyclic Alkylene Carbonates. Experiment and First Principle Calculations for Prediction of Thermochemical Properties. J. Chem. Thermodyn. 2008, 40, 1428. (16) Ulusoy, M.; Kilic, A.; Durgun, M.; Tasci, Z.; Cetinkaya, B. Silicon Containing New Salicylaldimine Pd(II) and Co(II) Metal J



Article

(37) Sankar, M.; Tarte, N. H.; Manikandan, P. Effective Catalytic System of Zinc-Substituted Polyoxometalate for Cycloaddition of CO2 to Epoxides. Appl. Catal., A 2004, 276, 217. (38) Dai, W.-L.; Yin, S.-F.; Guo, R.; Luo, S.-L.; Du, X.; Au, C.-T. Synthesis of Propylene Carbonate from Carbon Dioxide and Propylene Oxide Using Zn-Mg-Al Composite Oxide as HighEfficiency Catalyst. Catal. Lett. 2010, 136, 35. (39) Xiao, L.-F.; Li, F.-W.; Peng, J.-J.; Xia, C.-G. Immobilized Ionic Liquid/Zinc Chloride: Heterogeneous Catalyst for Synthesis of Cyclic Carbonates from Carbon Dioxide and Epoxides. J. Mol. Catal. A: Chem. 2006, 253, 265. (40) Anastas, P. T.; Kirchhoff, M. M.; Williamson, T. C. Catalysis as a Foundational Pillar of Green Chemistry. Appl. Catal., A 2001, 221, 3. (41) Mohammed, M. L.; Patel, D.; Mbeleck, R.; Niyogi, D.; Sherrington, D. C.; Saha, B. Optimisation of Alkene Epoxidation Catalysed by Polymer Supported Mo(VI) Complexes and Application of Artificial Neural Network for the Prediction of Catalytic Performances. Appl. Catal., A 2013, 466, 142. (42) Huang, K.; Chen, F.-Q.; Lü, D.-W. Artificial Neural NetworkAided Design of a Multi-Component Catalyst for Methane Oxidative Coupling. Appl. Catal., A 2001, 219, 61. (43) Corma, A.; Serra, J. M.; Argente, E.; Botti, V.; Valero, S. Application of Artificial Neural Networks to Combinatorial Catalysis: Modeling and Predicting ODHE Catalysts. ChemPhysChem 2002, 3, 939. (44) Jodaei, A.; Salari, D.; Niaei, A.; Khatamian, M.; Hosseini, S. A. Oxidation of Ethyl Acetate by a High Performance Nanostructure (Ni, Mn)-Ag/ZSM-5 Bimetallic Catalysts and Development of an Artificial Neural Networks Predictive Modelling. J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 2011, 46, 50. (45) Basheera, I. A.; Hajmeerb, M. Artificial Neural Networks: Fundamentals, Computing, Design, and Application. J. Microbiol. Methods 2000, 43, 3. (46) Serra, J. M.; Corma, A.; Chica, A.; Argente, E.; Botti, V. Can Artificial Neural Networks Help the Experimentation in Catalysis? Catal. Today 2003, 81, 393. (47) Zhao, Y.; Li, W.; Zhang, M.; Tao, K. A Comparison of Surface Acidic Features Between Tetragonal and Monoclinic Nanostructured Zirconia. Catal. Commun. 2002, 3, 239. (48) Niaei, A.; Salari, D.; Aghazadeh, F.; Caylak, N.; Sepehrianazar, A. Catalytic Oxidation of 2-Propanol over (Cr,Mn,Fe)-Pt/γ-Al2O3 Bimetallic Catalysts and Modeling of Experimental Results by Artificial Neural Networks. J. Environ. Sci. Health, Part A: Toxic/Hazard. Subst. Environ. Eng. 2010, 45, 454. (49) Barberis, P.; Méjean, T.-M.; Quintard, P. On Raman Spectroscopy of Zirconium Oxide Films. J. Nucl. Mater. 1997, 246, 232. (50) Liu, L.; Yao, Z.; Liu, B.; Dong, L. Correlation of Structural Characteristics with Catalytic Performance of CuO/CexZr1‑xO2 Catalysts for NO Reduction by CO. J. Catal. 2010, 275, 45. (51) Hu, C.; Liu, H.; Dong, W.; Zhang, Y.; Bao, G.; Lao, C.; Wang, Z. L. La(OH)3 and La2O3 NanobeltsSynthesis and Physical Properties. Adv. Mater. (Weinheim, Ger.) 2007, 19, 470. (52) Clerici, M. G.; Kholdeeva, O. A. Liquid Phase Oxidation via Heterogeneous Catalysis: Organic Synthesis and Industrial Applications; John Wiley & Sons: Hoboken, NJ, USA, 2013. (53) Surisetty, V. R.; Dalai, A. K.; Kozinski, J. Intrinsic Reaction Kinetics of Higher Alcohol Synthesis from Synthesis Gas over a Sulfided Alkali-Promoted Co−Rh−Mo Trimetallic Catalyst Supported on Multiwalled Carbon Nanotubes (MWCNTs). Energy Fuels 2010, 24, 4130. (54) Patel, D.; Saha, B. Heterogeneous Kinetics and Residue Curve Map (RCM) Determination for Synthesis of n-Hexyl Acetate using Ion-Exchange Resins as Catalysts. Ind. Eng. Chem. Res. 2007, 46, 3157. (55) Yasuda, H.; He, L. N.; Takahashi, T.; Sakakura, T. Non-Halogen Catalysts for Propylene Carbonate Synthesis from CO2 Under Supercritical Conditions. Appl. Catal., A 2006, 298, 177.

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Efficient and Greener Synthesis of Propylene Carbonate from Carbon

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