Efficient Synthesis of Diethyl Carbonate from ... - ACS Publications

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Kinetics, Catalysis, and Reaction Engineering

Efficient synthesis of diethyl carbonate from propylene carbonate and ethanol using Mg-La catalysts: Characterization, parametric and thermodynamic analysis Kartikeya Shukla, and Vimal Chandra Srivastava Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02080 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on September 2, 2018

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Industrial & Engineering Chemistry Research

Efficient synthesis of diethyl carbonate from propylene carbonate and ethanol using Mg-La catalysts: Characterization, parametric and thermodynamic analysis Kartikeya Shukla, Vimal Chandra Srivastava* Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India *Corresponding author: Phone: +91–1332–285889; fax: +91–1332–276535. Email: [email protected], [email protected] (VCS), [email protected] (KS).

Synthesis of organic carbonates through non-phosgene routes is a thrust in the area of research now days due to its future potential use of organic carbonates as fuels. Transesterification of propylene carbonate (PC) using alcohols is a green route for organic carbonate synthesis. Present study investigates use of PC along with ethanol for the catalytic synthesis of diethyl carbonate (DEC) using Mg-La catalysts. First, thermodynamic study has been performed for the synthesis of DEC from PC. Benson group contribution and Rozicka-Domalski model method were used to estimate standard heat of formation some components and coefficient of heat capacity (Cp) with temperature. The reaction was found to be mild endothermic. Mg-La has been synthesized using precipitation method using various Mg/La molar ratios (0.5, 1, 2, 2.5, and 4). Mg-La2 was found to the catalyst performing best among all the synthesized catalysts. The effect of precipitants on the physio-chemical properties of catalyst was also studied. Basicity of the catalysts was found to be in high correlation of PC conversion. 46% DEC yield was obtained with 63.6% PC conversion and 72.3% selectivity at 150 °C in 4 h using 1.3% catalyst. Equilibrium thermodynamics study of the reaction was also studied by calculating equilibrium constant both on the basis of moles of components as well as on the basis of activity coefficients. Keywords: Diethyl carbonate; propylene carbonate; thermodynamics; characterization.

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1. Introduction Organic carbonates are well known for their property as an fuel additive.1 Diethyl carbonate (DEC), an important member of organic carbonates, possess wide applications.2,3 Owing to its high oxygen content than MTBE (40.6% versus 18.2%), it is among the promising candidates for fuel additive. DEC, a linear organic carbonate is known fuel additive and is a prospective standalone fuel. More than 50% decrease in particulate matter emissions has been reported with the use of 5 (by wt.%) DEC in diesel.4 DEC has high oxygen content (40.6 wt. %) and more favorable gasoline/water distribution coefficient as compared to ethanol or dimethyl carbonate (DMC) which makes it a favorable fuel additive. In contact with soil, DEC decomposes slowly to ethanol and CO2, which have almost no environment detrimental effect. Oldest known route for the synthesis of DEC is the phosgenation of ethanol. Owing to toxicity of phosgene, many non-phosgene routes including oxy-carbonylation of ethanol, ethanolysis of urea, carbonylation of ethanol using CO2, via ethyl nitrite route, transesterification of ethanol using DMC or propylene carbonate (PC), ethylene carbonate (EC) are being focused and explored in past two decades.5-9 The authors have reviewed these methods exhaustively earlier.10-11 Carbonylation of ethanol using CO2 is thermodynamically limited, oxy carbonylation of ethanol uses poisonous CO as carbonylating agent and which suffers from expensive catalyst. Urea ethanolysis is also well known method, for synthesis of DEC, as it involves abundant raw materials but it suffers from evaporation of ethanol in removal of ammonia during the reaction.12,13,14,15 However, transesterification of ethanol with PC is the thermodynamically favorable route. This is a green route as it involves PC as its reactant, which initially requires CO2 for its synthesis. Moreover, since PC can be easily synthesized from propylene glycol (PG)

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owing to its favorable thermodynamics, this route can integrate the process and can form a step towards conversion of PG to DEC.16 Transesterification of ethanol with PC is represented by equation mentioned below: OH O

O O + 2

OH

O

O

PC

Ethanol

O

+

DEC

OH

(1)

PG

Very few studies have been reported on transesterification of ethanol with PC, and no heterogeneous catalyst has been studied for this route to the best knowledge of the authors. Present study involves the thermodynamic and catalytic study for the synthesis of DEC from transesterification of PC and ethanol. Mg-La based catalysts were used for the purpose as they are known for their activity for transesterification reactions.17 The catalysts were synthesized using different precipitants and characterized using various sophisticated techniques. The effect of pH, and aging time on physical and chemical properties of catalysts was also explored in details. The performances of catalysts were evaluated and compared on the basis of their properties. Thermodynamics was studied experimentally and the results were compared with theoretical values of thermodynamics. This study explores the route in detail considering the synthesis as well as the engineering aspects.

2. Experimental 2.1. Materials For quantitative analysis of DEC in a reaction mixture, standard DEC, of purity > 99%, was used for calibration purpose. Biphenyl, of purity > 99%, was used as the standard in the calibration as well as quantification purpose. Both the mentioned chemicals were purchased from 3



Sigma Aldrich, Mumbai, India. For the synthesis of DEC, ethanol of purity > 99%, was purchased from Merck while PC, of 99% purity, was purchased from Alfa Aesar, Mumbai, India.KBr of FT-IR grade, ≥99% purity trace metals basis, used for making pellets in Fourier transform infrared (FTIR) spectroscopy, was also purchased from Sigma Aldrich, Mumbai, India. N2 cylinder (> 99.8% purity) was used for pressurizing the autoclave while N2, H2 and air cylinders (> 99.8% purity) were used for quantification of reaction mixture in gas chromatograph. He, N2 and CO2 gases (> 99.8% purity) were used in CO2 temperature programmed desorption, and N2-adsorption desorption analysis. All the cylinders were purchased from Sigma Aldrich, Mumbai, India. Magnesium nitrate hexahydrate, Mg(NO3)2•6H2O, of purity > 98% and lanthanum nitrate hexahydrate La(NO3)3•6H2O, of purity > 99% were purchased from Sigma Aldrich. Sodium carbonate, Na2CO3, urea, and sodium hydroxide, NaOH, all of 99.5% purity, were purchased from Hi media Laboratories, Mumbai. 2.2. Preparation of catalysts For the synthesis of Mg-La based catalysts, parallel precipitation method was been used. Magnesium nitrate hexahydrate and lanthanum nitrate hexahydrate in the required ratio of Mg/La were taken and mixed in 100 mL of de-ionized water. 8 g of NaOH and 5.3 g of Na2CO3 were dispersed in 100 ml de-ionized water. Both the solutions were mixed to the 100 mL de-ionized water and the pH was maintained at 10.5. The catalysts were then aged at 80 °C for the required time in a reflux mode. The catalysts were separated with the help of centrifuge, washed several times with de-ionized water and subsequently with ethanol until the solution pH of filtrate returns to 7.0. The catalysts were then dried at 110 °C for 36 h. Mg-La based catalysts were denoted by Mg-LaX where X (X=0.5, 1, 2, 2.5, 4) is the molar ratio of Mg/La.

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MgO and La2O3 were also prepared by precipitation method. For MgO synthesis, Mg(NO3)2.6H2O was used as the salt and the catalyst precursor was precipitated by using Na2CO3 and NaOH as precipitant. Similarly, La(NO3)3.6H2O were used as the individual raw salts for the purpose and Na2CO3 and NaOH as precipitants. The pH was maintained at 10.5 during precipitation. The precursors were separated using centrifuge, washed several times and finally calcined at 600 °C. To study the effect of precipitants, Mg-La2 based catalyst was synthesized using NaOH and urea. Magnesium nitrate hexahydrate and lanthanum nitrate hexahydrate in the molar ratio (Mg/La) of 2. Mg-La2 prepared from NaOH was indicated by Mg-La2N. Mg-La prepared from urea was indicated by Mg-La2U. Both the catalysts were precipitated at pH of 10.5. The catalysts were then aged at 80 °C for 24 h in a reflux mode. The catalysts were separated with the help of centrifuge, washed several times with de-ionized water and subsequently with ethanol until the solution of filtrate returns to 7.0. The catalysts were then dried at 110 °C for 36 h. 2.3. Catalysts characterization To determine the phases present in the catalysts, they were characterized by X-ray diffraction (XRD) using Bruker D8 ADVANCE operating at 40 kV using Cu Kα radiation with wavelength of 0.15406 nm. The data was recorded in the range of 2θ from 5 to 90° with step size of 0.02°. The pore size analysis of the catalysts was performed using nitrogen adsorptiondesorption method. Data was generated using Micromeritics ASAP 2020 instrument. The basic properties of the catalysts were analyzed by using CO2 temperature programmed desorption (TPD) technique with Micromeritics ChemiSorb 2720 instrument, USA coupled with. Quartz Utube reactor was used for the purpose and the catalyst was degassed at 200 °C under N2 flow (20

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cm3/min) for 2 h. The adsorption of CO2 was carried out for 0.5 h, at room temperature. The physically adsorbed CO2 was removed using He gas (20 cm3/min). Finally, the desorption of CO2 was carried out and recorded in the temperature range of 50-950 °C. Continuous flow of He at 20 cm3/min was maintained during desorption. To perform the thermal analysis of the catalysts synthesized, thermogravimetric analysis (TGA) was carried out in the nitrogen atmosphere using SII 6300 EXSTAR. The weightlessness of the catalysts along with the heat liberated was monitored along the process in the temperature range of 30-900 °C at a heating rat of 10 °C/minute. For the analysis of functional groups present on the catalysts, Fourier transform infrared (FTIR) spectroscopy technique was used. Catalysts were blended KBr to form pellet under high pressure. The transmittance spectra were recorded within the wavenumber range of 400-4000 cm-1 using the Thermo Nicolet, Model Magna 760 spectrophotometer. Scanning electron microscopy-Energy-dispersive X-ray spectroscopy (SEM-EDX) was studied using Quanta 200 FEG for determining the morphology of catalysts, and the elemental analysis of the catalysts prepared. 2.4. DEC synthesis from Ethanol and PC The synthesis of DEC from PC was carried out in a high pressure Teflon lined reactor with magnetic stirring. The reactor (with a vessel volume of 50 mL) was procured from Amar Equipments Pvt. Ltd, Mumbai, India. Initially 0.05 moles of PC, 0.5 moles ethanol along with 0.4 g of catalyst were charged into the vessel followed by tightening and sealing of the reactor. The vessel was freed from any presence of air by purging nitrogen (N2) at high flow rates through the vessel. First, N2 gas was used to pressurize the reactor up to 3 MPa and then heating of the reactor was switched on. Upon reaching the reaction temperature, the mixture was stirred

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at 800 rpm which was enough to nullify the external mass transfer resistance. The catalyst size was kept in the range of 100-200 µm so as to nullify the internal mass transfer resistance. After the reaction, the vessel was cooled to room temperature followed by separation of catalyst from the reaction solution using centrifugation process. The reaction mixture was analyzed quantitatively using gas chromatograph (NETEL Michro-9100) equipped with DB-5 capillary column. The column had length of 30 m with the internal diameter of 0.25 mm. Injector and detector temperature was maintained at 240 °C. The temperature of column was increased from 80 °C to 240 °C with a ramp rate of 15 °C/min. Nitrogen with a flow rate of 30 ml/min was used as carrier gas. DEC yield and PC conversion were calculated by equations 2 and 3.

DEC Yield (%) =

Moles of DEC formed ×100 Initial moles of PC taken

(2)

 Initial PC moles - PC moles in the mixture after the reaction  PC conversion (%) =   × 100 Initial PC moles   (3)

3. Results and Discussion 3.1. Thermodynamics of the reaction The parameters used during thermodynamic analysis of the reaction are compiled in Table S1 (supporting information). The heat of formation and standard entropy change of all the compounds were taken from literature.18-21 However, entropy change of DEC was not available; hence it was estimated using Benson group contribution method. ∆Hr° and ∆Gr° of the reaction was estimated it to be 36.6 kJ/mol and 54.5 kJ/mol. Hence, the reaction is endothermic in nature. For studying the effect of temperature on equilibrium constant of the reaction, heat capacities of

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different compounds were estimated from method suggested in previous reported.22 The results are summarized in Table S2. The effect of temperature was studied and is summarized in Figure S1 (Supporting Information). The equilibrium constant of the reaction was found to increase with an increase in temperature clearly indicating that the high temperature is favorable for the reaction. 3.2. Characterization of catalyst The phases of the catalysts were studied using XRD technique and the patterns of different catalysts are shown in Figure 1a and 1b. Mg-La based catalysts showed existence of three phases namely lanthanum carbonate hydroxide (JCPDS file no. 49-0981), La(OH)3 (JCPDS file no. 06-0585), and brucite (Mg(OH)2) (JCPDS file no. 44–1482) in all the catalysts. The formation of lanthanum carbonate hydroxide is suggested to occur through equation 4. LaCO3OH + 3 Na+ + 3 NO3-

La(NO3)3 + Na2CO3 + NaOH

(4)

The crystalline sizes of lanthanum carbonate hydroxide are shown in Table 1. Figure 1b shows the XRD pattern of MgO, La2O3 and their precursor. MgO (JCPDS file number 76-1363) phase was found to be present in the calcined MgO catalyst whereas La2O2CO3 (JCPDS file number 83-1355) was found to be present in the calcined La2O3 catalysts. Lanthanum carbonate hydroxide breaks into La2O2CO3 when calcined around 700-840 K which can be represented by the following equation 5. 2 LaCO3OH

La2O2CO3 + H2O + CO2

(5)

The crystalline size were calculated using Scherrer equation and are tabulated in Table 1. The crystallite size of the Mg-La based catalysts were found to be increasing with an increase in the Mg/La ratio in the range of 0.5-2. On further increasing the ratio, the crystallite size was found to be decrease. Also the peaks of Mg-La4 were found to be shifted by some margin in

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comparison to Mg-La1 and Mg-La2. This could be attributed to the decrease the lattice parameter. From Figure 1a, it can be observed that catalysts Mg-La2, Mg-La2.5 and Mg-La4 are found to contain lanthanum hydroxide carbonate is in abundance with less impurity of brucite. The EDX analysis has been reported in Table 1. However, Mg-La0.5 and Mg-La1 contain more impurities of La(OH)3 and Mg(OH)2. Figure 2a shows the XRD patterns of the Mg-La based catalysts synthesized using different precipitants. Mg-La2N consisted of two phases La(OH)3 (JCPDS file no. 06-0585) and Brucite (Mg(OH)2) (JCPDS file no. 44–1482).23,24 On increasing the pH of the solution from 9 to 10.5 the XRD patterns of Mg-La2 was not found to change noticeably. However, on further increasing the pH of solution from 10.5 to 12, the peaks were observed to disappear. This can be attributed to the fact that on increasing the pH of solution the phases might have dispersed again in the solution. Mg-La2U catalyst was found to possess hexagonal La2O2CO3 phase in abundance.25 Figure 2b shows the effect of aging time on the synthesis of Mg-La2. The effect of aging time was found to be profound. Different peaks of LaOHCO3 were fully observed at the aging time of 48 h. However, the phases were observed to form at the aging time of 32 h. Figure 3 shows TG-DTA analysis of the Mg-La2 catalyst prepared. There are two weight losses observed in the whole range of temperature (30-700 °C) accompanied by two endothermic peaks observed by DTA. As it can be seen from Figure 2a, Mg(OH)2, LaCO3OH and La(OH)3 were the major phases observed. Hence the weight losses can be attributed to thermal disintegration of brucite (Mg(OH)2) to MgO at 350 °C. Second loss can be attributed to the loss due to transformation of LaCO3OH to La2O2CO3 as shown in equation 4.26 The pore size is characterized by the width of pore, which is defined as the distance between the two opposite walls. It symbolizes the geometrical shape of the pores created on the

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surface of catalysts. Porosity of the catalyst is fraction of volume of voids to the total volume occupied by total solid. The pore sizes are frequently characterized from the data derived from liquid N2 sorption data.27 N2 adsorption-desorption isotherm was used for the analysis of textural structures of the Mg-La catalysts synthesized. The surface areas and pore volumes of the synthesized catalysts were estimated using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) models. The profiles are shown in Figure 4 and the results are summarized in Table 1. All the catalysts were found to be mesoporous and followed types IV and V with a clear hysteresis loop (Figure 4a).28 All the catalysts were found to be type H3. The catalysts were hence found to possess slit shaped pores.29,30 The pore volumes of the catalysts are summarized in Table 1 and Figure 4b. The order of increasing pore volume was found to be: Mg-La2 > Mg-La4 > Mg-La0.5 > Mg-La1. This trend was also found to be the same when relative pressure was varied. The same trend of increasing BET surface area and pore volume was found to occur as shown in Table 1 and Figure 4c. However, cumulative pore volume of Mg-La4 was found to be more than Mg-La2 as shown in Table 1. Similarly, BET surface area of Mg-La4 was found to be better than Mg-La2. The distribution curve of Mg-La4 was broader than MgAlO which was in agreement with the increase in BET surface area than the former one. The distribution of pore volume with respect to pore diameter is shown in Figure 4b. As shown in Figure 4d, bimodal pores are observed on the Mg-La2 and Mg-La4, at around 34 Å and 93 Å. The area under the curve of Mg-La2 is greater than that of Mg-La4 in both the modes of pores. Thus, it can attribute that MgLa2 possessed larger pores sizes and larger pore volume as compared to Mg-La4. Figure S2 shows the SEM images of Mg-La4 catalysts. It can be observed that the powdered catalyst consists of agglomerated clustered particles irregular shape. However, the

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images clearly show two different morphologies of the catalysts. This can be due to the presence of multiple phases on the catalysts as predicted in the FTIR and XRD analysis. The catalysts clearly show nanoflakes and needles shaped morphologies. This can be attributed to the Mg(OH)2 and LaOHCO3 morphologies respectively as reported before.31 Figure S3 shows the morphology of Mg-La2 based catalysts at different aging time. Presence of nano-rods and nanodiscs was observed in the catalysts. The effect of aging time on the catalysts morphology was observed to be profound. Figure S3(b, c and d) show the morphology of catalysts with the aging time of 16 h, 32 h and 48 h. The nano-rods were found to be decreasing with an increase in the aging time. The analysis of functional groups among Mg-La based catalysts were analyzed using FTIR spectra as shown in Figure 5a. The band at 3700 cm-1 is attributed to the O–H band stretch in the catalysts, which may be attributed to the lanthanum hydroxyl carbonate. The broad band at 3400-3435 cm-1 corresponds to the bound OH group of the water or moisture in the catalyst.32,33 Other bands at 1456 cm-1 and 1380 cm-1 may be attributed to the ν3 mode of carbonate. Peaks at 1080, 872, 843, and 776-705 cm-1 are due to ν1, ν2, and ν4 modes of carbonates. As seen from Figure 5a,b, the splitting of the band around 1400-1500 cm-1 could be reasoned to the location of carbonate ions at a crystallographically non-equivalent site.34 FTIR spectra of the Mg-La2 catalyst prepared from different precipitants are summarized in Figure 5b. In all the catalysts, bands were observed at 3400, and 1750 cm-1. These were attributed to H2O bending band in the catalysts.24 The sharp peak observed ~3700 cm-1 is attributed to the presence of Mg(OH)2. In Mg-La2, weak band ~1085 cm-1 observed, due to CO32

. Small shoulder peaks observed between 2400-2700 cm-1 was attributed to hydrogen bonding of

water molecules and carbonate anions. The other noticeable bands among Mg-La2 and Mg-La2U

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catalysts at 1450 cm-1 and 1380 cm-1 may be attributed to the ν3 mode of carbonate. In Mg-La2U based catalyst, peaks around 850 and 1080 cm-1 are due to La2O2CO3 group.35 The basic properties of the catalysts were measured with the help of CO2-TPD. The sites on the catalysts surface are distributed as weak (desorption temperature < 250 °C), moderate (250 °C < desorption temperature < 450 °C) and strong (desorption temperature > 450 °C). Their basicities are represented by the amount of CO2 desorbed by the catalysts. Figure S4 shows the CO2-TPD profiles of the synthesized catalysts. Table 2 summarizes the TPD analysis of the catalysts prepared. CO2-TPD of La2O3 shows the presence of medium and strong sites. However, the strong sites have the maximum contribution towards the basicity of Mg-La2 which is consistent with the study reported before.36 Although MgO was reported to possess weak basic sites, however, in the present study the sites on the catalyst were found both in the weak and the strong zone. It is due to the change in textural properties of MgO with the change of precursor and the preparation procedures as reported previously.37,38 Mg-La based catalysts possessed only strong sites and hence had more basic strength than others. The presence of super basic sites among all the Mg-La based catalysts can be attributed to the very high desorption temperature.39 The quantitative analysis of the basicities of individual catalysts is summarized in Table 2. It can be noted that all Mg-La based catalysts show strong basic nature while MgO and La2O3 based catalysts showed a mixture of moderate and strong sites. The basicities of all the Mg-La based catalysts were found to be varying with Mg/La ratios. Mg-La2 possessed maximum basicity among all the Mg/La based catalysts. These basic sites could be attributed to the presence of lanthanum hydroxy carbonate species.40 Mg-La3 was observed to be most basic amongst all the Mg-La based mixed oxides in earlier studies also.41

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The CO2-TPD profile of the catalysts is shown in Figure S4 while the quantification analysis is summarized in Table 2. It can be seen that Mg-La2N and Mg-La2 possess only strong sites while Mg-La2U possess weak, moderate and strong sites on the catalyst. This difference can be due to different precipitant used for Mg-La2U synthesis. 3.3. Catalytic activity As can be seen from equation 1, single mole of PC and two moles of ethanol are required to synthesize single mole of DEC. It can be inferred that high molar ratio of ethanol/PC will drive the reaction toward the product side. All the catalysts were evaluated in the batch type reactor for the given reaction at the fixed time, temperature and other conditions. The results are summarized in Table 3. It can be seen that the Mg-La based catalysts gave better yield than the individual MgO and La2O3. Mg-La2 was found to be the best performing catalyst in comparison to other Mg-La based catalysts. The basicity of the catalysts facilitated the performance in the transesterification reaction of PC to DEC. The basicity of Mg/La based catalysts was correlated to the PC conversion. Figure S5 shows that PC conversion is strongly correlated with the basicities of the catalysts. The effect of pore size also seemed to have a profound effect on the activity of catalysts. It can be seen from Figure 4, the pore size and volume of Mg/La = 2 and 4, are much greater than Mg-La0.5 and Mg-La1, so is the yield of DEC and PC conversion. For Mg-La catalysts prepared from different precipitants, it can be seen from Table 3, the yield obtained by Mg-La2N and Mg-La2 were better than Mg-La2U. 46% DEC yield was obtained for Mg-La2, which was far greater than 28.7% obtained when Mg-La2N and 9.4% obtained when Mg-La2U were used. For catalysts prepared from different precipitants, the effect of total basicity cannot be thought as the responsible factor for yield of DEC since total basicity

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of Mg-La2U and Mg-La2 are close to each other while their performances are quiet apart. Hence the basicity due to strong basic sites could be the reason for the better yield obtained when MgLa2N and Mg-La2 were used. 3.4. Effect of reaction conditions The effect of temperature was studied for Mg-La2 catalyst with 0.4 g of catalyst, 5 h of reaction time and ethanol/PC ratio of 10. As mentioned in the thermodynamic section of the study, the reaction is endothermic. This is also evident experimentally as the conversion increased with an increase in temperature (Figure 6a). DEC yield was found to be optimum at 150 °C, after that the yield of DEC decreased slightly. This can be attributed to the polymerization of PC at higher reaction temperatures. Since, reaction rate increases with rise in temperature, therefore, it seems that the reaction rate of side reactions also increase which in turn increases the PC conversion but not the DEC yield. The effect of time was studied for Mg-La2 catalyst is shown in Figure 6b. DEC yield and PC conversion increased with time and it almost became constant after 4 h. The rate of increase of PC conversion and DEC yield was initially very fast which is not observed after 4 h. This can be attributed to the high concentration of PC initially which decreased with the course of reaction. High ratio of ethanol/PC favors the synthesis of DEC. This was observed in the experimental study also (Figure 6c). The optimum ethanol/PC ratio was found to be 10. Increasing the ethanol/PC beyond this decreased the DEC yield. This is due to coverage of catalytic sites by ethanol which disallowed the contact of PC and ethanol on the surface of catalyst. 3.5. Reusability of Mg-La2

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Mg-La2 catalyst was the best performing catalysts amongst all. Hence, the reusability of catalysts was studied for Mg-La2. The catalysts were centrifuged out of the reaction solution. It was washed several times with de-ionized water and ethanol before drying it for 48 h at 110 C. The catalyst was reused four times and its performance was found to be almost same with marginal decrease in the activity as can be seen from Figure 7. The average value of the conversion and yield decrease each time by 2-3%. 3.6. Equilibrium thermodynamics study This section deals with the experimental thermodynamics for the transesterification reaction of PC with ethanol. Experimental equilibrium constants were calculated by determining activity coefficients of each component to account for the non-ideal behavior in the liquid-phase. As well known, for liquid phase reactions, experimental equilibrium constant (Keq) can be written as equation: ν

ν

Keq = ∏ ( ai ) i = ∏ ( xiγ i ) i i

(6)

i

where, ai is the activity, γ i is the activity coefficient, xi is the mole fraction, and υi is the stoichiometric coefficient of various species at equilibrium. The relationship between γ i , ai , and x is shown in equation 7. The molar equilibrium constant (Kx) can be expressed on the basis of moles of components as represented by equation 8, whereas equilibrium constant based on the activity coefficients ( K γ ) can be expressed by equation 9. Hence the expression for equilibrium constant based on activities (Keq) can be written in form of x and γ i as shown in equation 10.

a i =γx i

(7)

 x PG x DEC   2  x PC x Ethanol 

Kx= 

(8)

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 γ

Kγ= 

γ

PG DΕC  γ γ2  PC Ethanol

   

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(9)

 x PG x DEC   γ PG γ DΕC    2 2  x PC x Ethanol   γ PC γ Ethanol 

K eq = 

(10)

The activities of various components were calculated by UNIFAC.41 The activity coefficient ( γ i ) can be calculated using the (UNIversal QUAsi-Chemical) UNIQUAC equation gE-model. The parameters used for calculations of activity coefficients are summarized in Table S3.42 The values of mole fraction, activity coefficients, molar-based equilibrium constants and activity-based equilibrium constants at different temperatures are summarized in Table 4. It is seen that the values of activity-based chemical equilibrium constants are slightly lower than the molar-based chemical equilibrium constant. This is due to the low activity coefficients of the products in comparison with that of the reactant PC.43 The values of ∆ r H m0 and ∆ r G m0 were calculated van’t Hoff equation:

ln Keq(T)=

∆ r Hmο -∆r Gοm ∆r Hοm  1    RTo R T

(11)

The graphs were plotted for both ln Keq versus 1/T (Figure 8a) and ln Keq versus 1/T (Figure 8b) to calculate the values of ∆ r H m0 and ∆ r G m0 with respect to both ideal and non-ideal conditions in the solution. The values of ∆ r H m0 and ∆ r G m0 were found to be 61.9 kJ/mol and 38.5 kJ/mol when correlated with Keq. The values of ∆ r H m0 and ∆ r G m0 obtained from experimental equilibrium constants are having same trend as those calculated earlier from the standard thermodynamic data ( ∆ r H m0 =36.6 kJ/mol and ∆ r G m0 =54.5 kJ/mol).

4. Conclusions 16


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All the Mg-La based catalysts were found to have mixed phases as confirmed by the physiochemical characterization. Mg-La2 catalyst was found to be most active for the endothermic reaction leading to synthesis of DEC from PC. Lanthanum hydroxyl carbonate was found to be the reason behind the high basic strength and basicity of the catalysts. The yield of DEC was found to be best in the Mg-La2 owing to its high surface area and basicity. High correlation was found to occur of conversion of PC with the basicity of catalysts. The temperature of 150 °C, time of 5 h at a high molar ratio of ethanol/PC of 10 was found to be optimum in the synthesis of DEC from PC. Mg-La2 catalyst was regenerated and reused found to be reusable without any significant loss in activity till the 4th run of the reaction. The values of thermodynamic parameters, ∆ r H m0 and ∆ r G m0 , were found to be 61.9 kJ/mol and 38.5 kJ/mol.

Acknowledgment Authors are thankful to Coal India Limited (CIL), Ranchi, India, for providing financial help for carrying out this work.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website http://pubs.acs.org. Thermodynamic parameters of the DEC synthesis reaction; Coefficients of specific heat capacity (Cp/R) varying with temperature calculated by Rozicka-Domalski functional group method; Groups and group parameter in UNIFAC model; variation of theoretical

17



equilibrium constant with temperature; SEM morphology and CO2-TPD of the catalysts; correlation of basicity of catalysts with the PC conversion.

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16. Yasir, A.; Shukla, K.; Srivastava, V. C. Synthesis of Propylene Carbonate from Propane-1, 2diol and Urea Using Hydrotalcite-Derived Catalysts. Energ Fuel. 2017, 31, 9890-9897. 17. Babu, N. S.; Sree, R.; Prasad, P. S.; Lingaiah, N. Room-temperature transesterification of edible and nonedible oils using a heterogeneous strong basic Mg/La catalyst. Energ Fuel. 2008, 22, 1965-1971. 18. Green J. H. S. Thermodynamic properties of organic oxygen compounds. Part 5. Ethyl alcohol. Trans. Faraday Soc. 1961, 57, 2132-2137. 19. Choi, J. K.; Joncich, M. J. Heats of combustion, heats of formation and vapor pressures of some organic carbonates. Estimation of carbonate group contribution to heat of formation. J. Chem. Eng. Data 1971, 16, 87-90. 20. Mansson, M., Enthalpies of combustion and formation of ethyl propionate and diethyl carbonate. J. Chem. Thermodyn. 1972, 4, 865-871. 21. Zhao, X.; Zhang, Y.; Wang, Y. Synthesis of propylene carbonate from urea and 1, 2propylene glycol over a zinc acetate catalyst. Ind. Eng. Chem. Res. 2004, 43(15), 4038-4042. 22. Zábranský, M.; Růžička Jr, V. Estimation of the heat capacities of organic liquids as a function of temperature using group additivity: an amendment. J. Phys. Chem. Ref. Data 2004, 33, 1071-1081. 23. Wang, X.; Li, Y. Synthesis and Characterization of Lanthanide Hydroxide SingleÅCrystal Nanowires. Angew Chem. Int. Edit. 2002, 41, 4790-4793. 24. Qiu, L.; Xie, R.; Ding, P.; Qu, B. Preparation and characterization of Mg(OH)2 nanoparticles and flame-retardant property of its nanocomposites with EVA. Compos. Struct. 2003, 62, 391395.

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25. Bakiz, B.; Guinnetion, F.; Arab, M.; Benlhachemi A.; Gavarria, J. R. Elaboration. Characterization of LaOHCO3, La2O2CO3 And La2O3 Phases And Their Gas Solid Interactions With CH4 And CO Gases. MJ Conden. Ma, 2010, 12, 60-67. 26. Lee, M. H.; Jung, W. S. Hydrothermal Synthesis of LaCO3OH and Ln3+-doped LaCO3OH Powders under Ambient Pressure and Their Transformation to La2OCO3 and La2O3. B. Korean Chem. Soc. 2013, 34, 3609-3614. 27. Sing, K. S. W.; Everett, D.H.; Haul, R.A.W.; Moscou, L.; Pierotti, R.A.; Rouquerol, J.; Siemieniewska, T. Physical and biophysical chemistry division commission on colloid and surface chemistry including catalysis. Pure Appl. Chem 1985, 57, 603-619. 28. ALOthman, Z. A. A review: fundamental aspects of silicate mesoporous materials. Materials 2012, 5, 2874-2902. 29. Kumar, P.; Srivastava, V. C.; Mishra, I. M. Dimethyl Carbonate Synthesis from Propylene Carbonate with Methanol Using Cu−Zn−Al Catalyst. Energ. Fuel. 2015, 29, 2664−2675. 30. Kumar, P.; Srivastava, V. C.; Mishra, I. M. Dimethyl carbonate synthesis by transesterification of propylene carbonate with methanol: Comparative assessment of Ce-M (M=Co, Fe, Cu and Zn) catalysts. Renewable Energy 2016, 88, 457-464. 31. Sierra-Fernandez, A.; Gomez-Villalba, L. S.; Muñoz, L.; Flores, G.; González R. F.; Jiménez, M. R. Effect of temperature and reaction time on the synthesis of nanocrystalline brucite. Int. J. Mod. Manu. Technol., 2014, 6, 50-54. 32. Zeng, H. Y.; Deng, X.; Wang, Y. J.; Liao, K. B. Preparation of MgÅAl hydrotalcite by urea method and its catalytic activity for transesterification. AIChE J. 2009, 55, 1229-1235. 33. Murteja, V.; Singh, S.; Ali, A. Potassium impregnated nanocrystalline mixed oxides of La and Mg as heterogeneous catalysts for transesterification. Renewable Energy 2014, 62, 226-233.

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34. Tok, A. I. Y.; Su, L. T.; Boey F. Y. C.; Ng, S. H. Homogeneous precipitation of Dy2O3 nanoparticles-effects of synthesis parameters. J. Nanosci. Nanotechnol., 2007, 7, 1–90. 35. Hou, Y. H.; Han, W. C.; Xia, W. S.;Wan, H. L. Structure sensitivity of La2O2CO3 catalysts in the oxidative coupling of methane. ACS Catal. 2015, 5, 1663-1674. 36. Murugan C.; Bajaj, H. C. Effect of temperature and reaction time on the synthesis of nanocrystalline brucite. IJCA, 2013, 52A, 459-466. 37. Bancquart, S.; Vanhove, C.; Pouilloux, Y.; Barrault, J. Glycerol transesterification with methyl stearate over solid basic catalysts: I. Relationship between activity and basicity. Appl. Catal., A. 2001, 218, 1-11. 38. Martra, G.; Cacciatori, T.; Marchese, L.; Hargreaves, J. S. J.; Mellor, I. M.; Joyner, R. W.; Coluccia, S. Surface morphology and reactivity of microcrystalline MgO: Single and multiple acid–base pairs in low coordination revealed by FTIR spectroscopy of adsorbed CO, CD3CN and D2. Catal. Today 2001, 70, 121-130. 39. Di Serio, M.; Ledda, M.; Cozzolino, M.; Minutillo, G.; Tesser, R.; Santacesaria, E. Transesterification of soybean oil to biodiesel by using heterogeneous basic catalysts. Ind. Eng. Chem. Res. 2006, 45, 3009-3014. 40. Babu, N. S.; Pasha, N.; Rao, K. V.; Prasad, P. S.; Lingaiah, N. A heterogeneous strong basic Mg/La mixed oxide catalyst for efficient synthesis of polyfunctionalized pyrans. Tetrahedron Lett. 2008, 49, 2730-2733. 41. Fredenslund, A.; Gmehling, J.; Rasmussen, P. Vapor–Liquid Equilibria using UNIFAC: A Group Contribution Method, Elsevier Scientific Pub. Co., New York, 1977. 42. Stolzenburg, P.; Capdevielle, A.; Teychené, S.; Biscans, B. Struvite precipitation with MgO as a precursor: Application to wastewater treatment. Chem.Eng. Sci. 2015, 133, 9-15.

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Page 22 of 32

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Textural Properties of the Mg-La Catalysts Catalysts

Crystalline BET size (nm)

Pore

BJH

BJH

Elemental

surface volume Adsorption Desorption

Composition

area

(cm3/g) average

average

(wt.%)

(m2/g)

(×10-1)

pore

pore

Mg

diameter

diameter

(4V/A)

in (4V/A)

nm

nm

La

O

in

Mg-La0.5

30.1

24.7

0.6

9.2

8.2

10.2 79.2

10.6

Mg-La1

45.3

20.8

0.2

15.6

15.8

18.2 64.2

17.4

Mg-La2

46.8

39.2

2.2

20.5

18.1

22.1 49.26 28.6

Mg-La4

22.2

49

1.7

12.5

10.0

25.6 33

Table 2. Basicity of Catalysts Used for DEC Synthesis Basicity or amount desorbed CO2 (mmol g−1) Catalysts

Weak sites -1

(×10 )

Medium sites -1

Strong sites -1

(×10 )

(×10 )

Total basicity (×10-1)

1.9 (631.2 °C)

2.6

4.1 (863.9 °C)

4.9

Mg-La0.5

1 (517.2 °C & 824.3 °C)

1

Mg-La1

5.9 (503.9 °C & 843.3 °C)

5.9

Mg-La2

13.6 (527.9 °C & 859.6 °C)

13.6

Mg-La2.5

12.2

12.2

Mg-La4

7.3 (478.1 °C & 823.2 °C)

7.3

MgO

0.7 (209.3 °C) 0.8 (312.6°C)

La2O3

Mg-La-2N

-

-

13.6

13.6

Mg-La-2U

1.6

1.2

5.4

8.2

23


41.2


Page 24 of 32

Table 3. Evaluation of Catalysts for the Synthesis of DEC from PC. Reaction conditions: catalyst mass=0.4 g, ethanol volume=30 ml, PC volume=4.36 ml, ethanol/PC molar ratio=10, reaction time=5 h, and reaction temperature=423 K. Catalyst

Yield

PC

DEC

TOF

Crystallite Crystallite

DEC

Conversion selectivity (h-1)

size (nm) size (nm)

(%)

(%)

(%)

at 38.05

at 33.51

Mg-La0.5

15.4

19.7

78.1

0.5

19.5

-

Mg-La1

20.3

36.1

56.2

0.7

24.3

-

Mg-La2

46

63.6

72.3

1.6

20.4

-

Mg-La2.5

28.8

40.9

70.4

-

-

-

Mg-La4

23.9

48.8

48.9

0.8

18.6

-

MgO

28.7

42.7

67.2

-

-

10.8

La2O3

10.2

18.2

55.7

-

-

34.3

Mg-La-2N

28.7

-

-

-

-

-

Mg-La-2U

9.4

-

-

-

-

-

24


Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4. Parameters Used for Calculation of Activity Coefficients of Various Components at Various Temperatures. Temperature=373 K x

γi

x× γi

PC

0.087

1.062

0.092

ET

0.904

0.926

0.838

DEC

0.003

0.846

0.003

PG

0.003

0.817

0.003

Kx (×10-3)

Keq (×10-3)

0.22

0.17

Kx (×10-3)

Keq (×10-3)

0.25

0.19

Kx (×10-3)

Keq (×10-3)

1.69

1.28

Kx (×10-3)

Keq (×10-3)

6.01

4.49

Temperature=393 K 393 K

x

γi

x× γi

PC

0.087

1.065

0.092

ET

0.904

0.928

0.839

DEC

0.004

0.848

0.003

PG

0.004

0.820

0.003


x

γi

x× γi

PC

0.081

1.14

0.092

ET

0.897

0.975

0.875

DEC

0.01

0.921

0.009

PG

0.01

0.888

0.009


x

γi

x× γi

PC

0.073

1.246

0.092

ET

0.888

1.039

0.923

DEC

0.018

1.024

0.019

PG

0.018

0.984

0.018

25



Mg-La4

Intensity (A.U)

Mg-La2.5 Mg-La2 Mg-La1 Mg-La0.5 I

I

I I

I

I

I I 15

I I II

I I

I 20

25

30

I

I II

I

I

LaOHCO3

I

I

I

35

40

I

I 45

II

I Mg(OH)2

I

La(OH)3

60

65

50

55

^

La2O3 calcined

70

2θ (a) ^ ^ ^^ ^

^ ^ ^ ^

La2O3 precursor Intensity (A.U)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

* * MgO calcined

*

* #

# 10

15

20

25

30

35

# MgO precursor # #

# 40

45 2θ (b)

50

*

55

60

65

70

75

80

Figure 1. XRD pattern of (a) Mg-La based catalysts, (b) MgO, La2O3 and their precursors (#Mg(OH)2, *-MgO, ^-La2O2CO3).

26


Page 27 of 32

$

$

$

$

$

Mg-La2U

$

$ $

$

Intensity (A.U)

^ ^

^

^

^

^

^

^

Mg-La2

**

**

^

^

*

* *

*

#

^ 15

^^ 20

^

^ 25

#

*

*

^

^

^ 35

#

#

Mg-La2 pH=12 Mg/La2 pH=9 ^

^

40 2θ

*

#

*

*

30

Mg-La2N

*

45

50

55

60

65

(a)

Mg-La2 48 h Intensity (A.U)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Mg-La2 32 h

Mg-La2 16 h

15

20

25

30

35

2θ

40

45

50

55

60

(b) Figure 2. (a) XRD patterns of Mg-La based catalysts prepared using various precipitants at different pH. (*-La(OH)3, #-Mg(OH)2, ^- LaOHCO3, $-hexagonal La2O2CO3 phase), (b) Effect of aging time on XRD pattern of Mg-La2.

27


120

50 45 40 35 30 25 20 15 10 5 0

TG (Weight loss (%))

TG 100

DTA

80 60 40 20 0 0

100 200 300 400 500 600 700 Temperature (°C)

Figure 3. TG-DTA of Mg-La2 catalyst.

0.25

160

Mg-La0.5 Mg-La1 Mg-La2 Mg-La4

0.23

140

Cumulative Pore Volume (cm³/g)

Cumulative Pore Volume (cm³/g)

150 130 120 110 100 90


80 70 60 50 40

0.20 0.18 0.15 0.13 0.10 0.08 0.05

30

0.03

20 10

0.00

0

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

50

100 150 200 250 300 350 400 450 500 550 600

1.0

Average Diameter (Å)

Relative Pressure (P/Po)

(a)

(b) 0.25

60 55

0.23

50


45 40 35

dV/dlog(D) Pore Volume (cm³/g·Å)

Cumulative Pore Area (m²/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

DTA (Heat flow (uV))


30 25 20 15 10


0.20 0.18 0.15 0.13 0.10 0.08 0.05 0.03

5 0.00

0 0

50

0

100 150 200 250 300 350 400 450 500 550 600 650

50

100

150

200

250 300

350

400 450



(c) Figure 4. N2 adsorption-desorption of various Mg-La catalysts.

28


(d)

500

550

600

Page 29 of 32

Mg-La2 Reg

Mg-La4

Intensity (A.U)

Mg-La2.5

Mg-La2 857 cm-1 Mg-La1 2941 cm-1 Mg-La0.5 -1

3690 cm 3442 cm-1

1784 cm-11455 cm-1 1068 cm-1 709 cm-1

2496 cm-1

4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 Wavenumber (cm-1)

(a) -1 860 cm-1 729 cm

Mg/La-N

% Transmittance (A.U)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3620 cm-1

Mg/La-U 1418 cm-1 -1

2527 cm

-1

Mg/La-NC

3452 cm 3689 cm-1

-1

-1

2916 cm

1507 cm

1092 cm-1

592 cm-1

4000 3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750

Wavenumber (cm-1)

500

(b) Figure 5. FTIR spectra of Mg-La catalysts prepared from (a) NaOH and Na2CO3, (b) other precipitants.

29



100 90 80 70 60 50 40 30 20 10 0

100 90 80 70 60 50 40 30 20 10 0

DEC yield (%) PC conversion (%)

410

420

430 440 Temperature (K)

450

Page 30 of 32

DEC yield (%) PC conversion (%)

0

1

2

3 4 time (h)

(a)

5

6

7

(b) 100 90 80 70 60 50 40 30 20 10 0

DEC Yield (%) PC Conversion (%)

0

5

10 15 20 Ethanol/PC molar ratio

25

(c) Figure 6. Effect of reaction conditions on synthesis of DEC using Mg-La2 as the catalyst. Reaction conditions: (a) catalyst mass=0.4 g, ethanol volume=30 ml, PC volume=4.36 ml, ethanol/PC molar ratio=10, reaction time=5 h, and reaction temperature=413-443 K; (b) catalyst mass=0.4 g, ethanol volume=30 ml, PC volume=4.36 ml, ethanol/PC molar ratio=10, reaction temperature=423 K, and reaction time=1-6 h; (c) catalyst mass=0.4 g, ethanol volume=30 ml, reaction time=4 h, reaction temperature=423 K, and ethanol/PC molar ratio=5-20.

30


Page 31 of 32

100 90

DEC yield (%)

80 70 60

PC conversion (%)

50 40 30 20 10 0 1st run

2nd run

3rd run

4th run

Figure 7. Reusability of Mg-La2 for the synthesis of DEC from PC. Reaction conditions catalyst mass=0.4 g, ethanol volume=30 ml, PC volume=4.36 ml, ethanol/PC molar ratio=10, reaction time=5 h, and reaction temperature=423 K.

-5

-5.5

-5.5

-6

-6

-6.5

-7

ln Keq

-6.5

ln Kx

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


R² = 0.8984

-7.5 -8

R² = 0.8917

-8 -8.5

-8.5 -9 0.0023

-7 -7.5

-9 0.0025

-9.5 0.0023

0.0027

1/T (K-1 )

(a) Figure 8. Correlation of ln Kx and ln Kγ with 1/T.

31


0.0025 1/T (K-1 )

(b)

0.0027


Table of Content figure 216x155mm (96 x 96 DPI)


Page 32 of 32

Efficient Synthesis of Diethyl Carbonate from ... - ACS Publications

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