Synthesis of Diethyl Carbonate from Carbon Dioxide, Propylene Oxide and Ethanol over Kno 3 -ceo 2 and Kbr-kno 3 -ceo 2 Catalysts

One-pot syntheses of diethyl carbonate (DEC) from CO 2 , propylene oxide and ethanol were carried out using different solid catalysts. The supercritical CO 2 extraction method was used to separate the liquid products and reactants from the catalysts after reaction. The KNO 3-CeO 2 and KBr-KNO 3-CeO 2 were found to be active for the reaction after calcinations. The catalyst was also reusable. The thermodynamic properties of the reaction were also evaluated. The effects of various conditions, such as reaction time, amount of catalysts, molar ratio of the reactants, the composition and calcination temperature of the catalysts on the conversion and yields, were investigated, and the yield of DEC was about 13.0% with a selectivity of 38.5% over KBr-KNO 3-CeO 2. The yield of DEC was improved about 10-fold by using KBr-KNO 3-CeO 2 catalyst compared to CeO 2 .


Introduction
The synthesis of chemicals using CO 2 as a raw material is characterized at present by increasing industrial and academic efforts to use this carbon renewable [1].The production of carbonates [2,3], carbamates [4], methanol [5], formic acid and its derivatives could be synthesized from CO 2 [6,7].Diethyl carbonate (DEC) is one of the most important green chemicals among carbonate esters.It is an excellent solvent and an intermediate for various pharmaceuticals, such as antibiotics and phenobarbital [8].DEC has also been proposed as a replacement for MTBE as an attractive oxygen-containing fuel additive for its high oxygen content (40.6 wt.%) compared to MTBE (18.2 wt.%) [9,10].
Since the conventional methodologies for the DEC synthesis, including ethanol phosgenation [11], ethanol oxidative carbonylation [12] and the reaction of ethanol with urea [13], have many problems, such as the toxicity of phosgene, corrosion and low production rates [14], the novel technology for DEC synthesis starting from CO 2 and ethanol is a promising route.However, the reaction hardly occurs spontaneously, even under harsh conditions, due to the thermodynamic limitations (yield of less than 0.5%) [8].To address this issue, chemical dehydration reagent was usually involved to shift the reaction forward to the carbonate side.In the similar reaction of the synthesis dimethyl carbonate (DMC), acetals [15] and orthoesters [16] were used as the organic dehydrants, respectively, and both the DMC yields could be effectively improved above 20-fold.However, the high cost of acetals and orthoesters makes them difficult for industrial production.Acetonitrile [17] and amines [18] were also reported as being used as the dehydrants for the DEC synthesis, but their co-products were complex.Recently, butylene oxide was also used as the dehydrant for the direct synthesis of the DEC over CeO 2 catalyst [8,19].According to the results, the yield of DEC had a nine-fold enhancement compared to that over CeO 2 without dehydrant, but it was still not high enough (only 1.5%) in this system and needed to be improved.Besides, CeO 2 with 2-cyanopyridine was also used as the carboxylation/hydration cascade catalyst by Tomishige group [20] for the propylene carbonate synthesis from CO 2 and 1,2-propanediol, and the yield was much higher (>99%), which might be a landmark in carbonate synthesis using CeO 2 catalysis.
In addition, the one-pot synthesis of DEC from carbon dioxide, ethylene oxide (EO) and ethanol on the KI and sodium ethoxide binary homogeneous catalyst was also researched by Wang et al. [21], and the yield of DEC was improved.However, KI and sodium ethoxide are dissolved in ethanol and cannot be separated easily.
On the other hand, the synthesis of cyclic carbonate from epoxide and CO 2 was well established in industrial manufacturing.Furthermore, the transesterification of cyclic carbonate with ethanol to produce DEC was also proven to be feasible [22] However, from views of energy consumption, productivity and investment, the one-pot reaction directly from CO 2 was undoubtedly superior to the two-step separate reaction.Thus, the development of a more effective one-pot reaction to improve the productivity of DEC directly from CO 2 is highly desired.

Results and Discussion
The one-pot reaction in DEC synthesis might be composed of two steps, the cycloaddition reaction and subsequent transesterification reaction.The mechanisms of the reaction have been studied and proven by many researchers [21,23].As analyzed by the GC-MS method in this work, the main products in the one-pot reaction from CO 2 , ethanol and PO were 1,2-propanediol (PG), DEC and propylene carbonate (PC) with the side-product 1-ethoxy-2-propanol (EP).The EP might be formed from propylene oxide by ethanolysis [23,24] in the basic catalytic environment.The possible equations of these reactions are presented as follows (Scheme 1).compared to that over CeO2 without dehydrant, but it was still not high enough (only 1.5%) in this system and needed to be improved.Besides, CeO2 with 2-cyanopyridine was also used as the carboxylation/hydration cascade catalyst by Tomishige group [20] for the propylene carbonate synthesis from CO2 and 1,2-propanediol, and the yield was much higher (>99%), which might be a landmark in carbonate synthesis using CeO2 catalysis.
In addition, the one-pot synthesis of DEC from carbon dioxide, ethylene oxide (EO) and ethanol on the KI and sodium ethoxide binary homogeneous catalyst was also researched by Wang et al. [21], and the yield of DEC was improved.However, KI and sodium ethoxide are dissolved in ethanol and cannot be separated easily.
On the other hand, the synthesis of cyclic carbonate from epoxide and CO2 was well established in industrial manufacturing.Furthermore, the transesterification of cyclic carbonate with ethanol to produce DEC was also proven to be feasible [22] However, from views of energy consumption, productivity and investment, the one-pot reaction directly from CO2 was undoubtedly superior to the two-step separate reaction.Thus, the development of a more effective one-pot reaction to improve the productivity of DEC directly from CO2 is highly desired.

Results and Discussion
The one-pot reaction in DEC synthesis might be composed of two steps, the cycloaddition reaction and subsequent transesterification reaction.The mechanisms of the reaction have been studied and proven by many researchers [21,23].As analyzed by the GC-MS method in this work, the main products in the one-pot reaction from CO2, ethanol and PO were 1,2-propanediol (PG), DEC and propylene carbonate (PC) with the side-product 1-ethoxy-2-propanol (EP).The EP might be formed from propylene oxide by ethanolysis [23,24] in the basic catalytic environment.The possible equations of these reactions are presented as follows (Scheme 1).Scheme 1.The reaction schemes.

The One-Pot Synthesis of DEC over KNO3-CeO2 Catalyst
As reviewed in the literature [8], the heterogeneous catalyst CeO2 has catalytic activity for the one-pot reaction in DEC synthesis.In order to improve the catalytic activity in DEC synthesis, the strong base of KOH and several typical alkali and alkaline-earth metal salts, which might be necessary to meet the requirement for catalyzing the transesterification reaction combined with CeO2, were researched.Several metal oxides, such as γ-Al2O3, ZrO2, SiO2 and La2O3, were also tested.In the typical reaction, the molar ratio of CO2, ethanol and PO was fixed as 0.29:0.17:0.14; the reaction temperature was 150 °C; the initial pressure was 5 MPa; and during the reaction, the pressure could reach 9 MPa, which is higher than the critical pressure of CO2.The results are summarized in Table 1, Runs 1-12.
Scheme 1.The reaction schemes.

The One-Pot Synthesis of DEC over KNO 3 -CeO 2 Catalyst
As reviewed in the literature [8], the heterogeneous catalyst CeO 2 has catalytic activity for the one-pot reaction in DEC synthesis.In order to improve the catalytic activity in DEC synthesis, the strong base of KOH and several typical alkali and alkaline-earth metal salts, which might be necessary to meet the requirement for catalyzing the transesterification reaction combined with CeO 2 , were researched.Several metal oxides, such as γ-Al 2 O 3 , ZrO 2 , SiO 2 and La 2 O 3 , were also tested.In the typical reaction, the molar ratio of CO 2 , ethanol and PO was fixed as 0.29:0.17:0.14; the reaction temperature was 150 ˝C; the initial pressure was 5 MPa; and during the reaction, the pressure could reach 9 MPa, which is higher than the critical pressure of CO 2 .The results are summarized in Table 1, Runs 1-12.Both of the volumes of ethanol and PO were 10 mL; the reaction temperature was 150 ˝C; the initial pressure was 5 MPa.A Catalysts prepared by the impregnation method; B catalysts prepared by the solid mixed method.
As seen in Table 1, when KNO 3 -CeO 2 was used as the catalyst, the yield of DEC was effectively improved about 10-fold, compared to that using CeO 2 catalyst.The KNO 3 -CeO 2 had better catalytic activity than KOH and other alkali and alkaline-earth metal salts loading on CeO 2 , such as KOH-CeO 2 , K 2 CO 3 -CeO 2 , NaNO 3 -CeO 2 , Ba(NO 3 ) 2 -CeO 2 , Mg(NO 3 ) 2 -CeO 2 and Ca(NO 3 ) 2 -CeO 2 .Then, KNO 3 with other metal oxides, including γ-Al 2 O 3 , ZrO 2 , La 2 O 3 and SiO 2 , was evaluated.The results indicate that the oxides with acid-base properties, especially ZrO 2 , γ-Al 2 O 3 and CeO 2 combined with KNO 3 , are more active for DEC synthesis.The basicity of the catalysts KNO 3 -γ-Al 2 O 3 , KNO 3 -CeO 2 and CeO 2 were analyzed by the CO 2 -TPD method (Figure 1).The desorption peaks at about 280 ˝C-340 ˝C could be observed for KNO 3 -γ-Al 2 O 3 and KNO 3 -CeO 2 , which had better catalytic activities.It is indicated that the addition of a small amount of a moderate base is more effective for enhancing the activity of the catalyst.Considering the KNO 3 -CeO 2 catalyst obtaining the better yield and selectivity for DEC, it was finally selected for the following reactions.The XRD spectrums of KNO 3 -CeO 2 and CeO 2 are shown in Figure 2. By comparison, the characteristic peaks of KNO 3 cannot be found, and the diffraction peaks of KNO 3 -CeO 2 are stronger than CeO 2 .It is indicated that the KNO 3 might be well dispersed and caused no considerable distortion in the structure of CeO 2 [25].
The preparation conditions for KNO 3 -CeO 2 including the load of KNO 3 and the calcination temperature were optimized.The results are shown in Figure 3a,b.It is indicated that the KNO 3 -CeO 2 with n(Ce)/n(K) = 1:0.4has better catalytic activity (Figure 3a).And the DEC yield reaches higher level (Figure 3b) at the calcination temperature of 500 ˝C.According to the TG-DTG analysis of KNO 3 (Figure 4), the decomposition temperature of KNO 3 is 520 ˝C.When the temperature is higher than 520 ˝C, the KNO 3 will be decomposed to K 2 O, and the catalytic activity will decrease.
Then, the reaction conditions, including the amount of catalyst, reaction time and volume ratio of ethanol and PO, were studied.The results are shown in Figure 3c-e.As seen in Figure 3c, the DEC yield first increases and then decreases with the increase of catalyst amounts.The DEC yield reaches the peak value when the amount of KNO 3 -CeO 2 is 0.3 g (Figure 4c). Figure 4d shows the dependence of DEC yield on reaction time.The reaction reaches equilibrium in 2 h. Figure 3e shows that the DEC yield reaches the higher level, when both of the volumes of ethanol and PO are 10 mL (0.17 mol and 0.14 mol, respectively) and the CO 2 amount is fixed as 0.25 mol.
Catalysts 2016, 6, 52 4 of 11 yield reaches the higher level, when both of the volumes of ethanol and PO are 10 mL (0.17 mol and 0.14 mol, respectively) and the CO2 amount is fixed as 0.25 mol.yield reaches the higher level, when both of the volumes of ethanol and PO are 10 mL (0.17 mol and 0.14 mol, respectively) and the CO2 amount is fixed as 0.25 mol.Catalysts 2016, 6, 52 4 of 11 yield reaches the higher level, when both of the volumes of ethanol and PO are 10 mL (0.17 mol and 0.14 mol, respectively) and the CO2 amount is fixed as 0.25 mol.

The One-Pot Synthesis of DEC over KBr-KNO3-CeO2 Catalyst
As potassium halides (KI, KBr and KCl) were proven to be conducive to the cycloaddition reaction [15]; the KI, KBr and KCl were added in the KNO3-CeO2 catalyst for the one-pot synthesis of DEC.The addition methods for potassium halides, including solid mixed and impregnation methods, were compared.
The results are shown in Table 1, Runs 13-20.As seen in Table 1, when KI-CeO2 is used as the catalyst, the ethanol conversion increases, but the yield of DEC is not improved compared to CeO2 alone.In addition, when KBr or KCl is added to KNO3-CeO2, the ethanol conversion decreases, but the selectivity of DEC is effectively improved.Additionally, the catalysts prepared by the solid mixed method give a higher DEC yield and selectivity.However, when KI-KNO3-CeO2 is used as the catalyst, the DEC yield seriously decreases.This might be because that KI, with a stronger reduction property, is oxidized by KNO3, which causes deactivation.
The molar ratio of KBr and KNO3 was evaluated with fixed n(CeO2)/n(KNO3) = 1:0.4.The results are shown in Figure 5.The XRD patterns of KBr-KNO3-CeO2 are presented in Figure 4, and the characteristic peaks of KBr are labeled.The yield and the selectivity of DEC are improved with the addition of KBr into the catalyst.When the molar ration is n(KBr)/n(KNO3) = 6:4, that is n(CeO2)/n(KNO3)/n(KBr) = 1:0.4:0.6, the yield of DEC reaches 13.0% with a selectivity of 38.5% on the ethanol basis.Both the yield and selectivity of DEC are much higher than reported in Leino's research [8] by using butylene oxide as the dehydration agent and CeO2 as the catalyst.In their results, the highest obtained yield of DEC was 1.5%, and selectivity to DEC was 10% on the ethanol basis.
The formation kinetics of DEC were also studied, and the dependence of DEC yield on reaction time at 150 °C in 180 min is shown in Figure 6.It is also indicated that the reaction can reach equilibrium in 100 min.The reaction time was selected as 120 min.

The One-Pot Synthesis of DEC over KBr-KNO 3 -CeO 2 Catalyst
As potassium halides (KI, KBr and KCl) were proven to be conducive to the cycloaddition reaction [15]; the KI, KBr and KCl were added in the KNO 3 -CeO 2 catalyst for the one-pot synthesis of DEC.The addition methods for potassium halides, including solid mixed and impregnation methods, were compared.
The results are shown in Table 1, Runs 13-20.As seen in Table 1, when KI-CeO 2 is used as the catalyst, the ethanol conversion increases, but the yield of DEC is not improved compared to CeO 2 alone.In addition, when KBr or KCl is added to KNO 3 -CeO 2 , the ethanol conversion decreases, but the selectivity of DEC is effectively improved.Additionally, the catalysts prepared by the solid mixed method give a higher DEC yield and selectivity.However, when KI-KNO 3 -CeO 2 is used as the catalyst, the DEC yield seriously decreases.This might be because that KI, with a stronger reduction property, is oxidized by KNO 3 , which causes deactivation.
The molar ratio of KBr and KNO 3 was evaluated with fixed n(CeO 2 )/n(KNO 3 ) = 1:0.4.The results are shown in Figure 5.The XRD patterns of KBr-KNO 3 -CeO 2 are presented in Figure 4, and the characteristic peaks of KBr are labeled.The yield and the selectivity of DEC are improved with the addition of KBr into the catalyst.When the molar ration is n(KBr)/n(KNO 3 ) = 6:4, that is n(CeO 2 )/n(KNO 3 )/n(KBr) = 1:0.4:0.6, the yield of DEC reaches 13.0% with a selectivity of 38.5% on the ethanol basis.Both the yield and selectivity of DEC are much higher than reported in Leino's research [8] by using butylene oxide as the dehydration agent and CeO 2 as the catalyst.In their results, the highest obtained yield of DEC was 1.5%, and selectivity to DEC was 10% on the ethanol basis.

The One-Pot Synthesis of DEC over KBr-KNO3-CeO2 Catalyst
As potassium halides (KI, KBr and KCl) were proven to be conducive to the cycloaddition reaction [15]; the KI, KBr and KCl were added in the KNO3-CeO2 catalyst for the one-pot synthesis of DEC.The addition methods for potassium halides, including solid mixed and impregnation methods, were compared.
The results are shown in Table 1, Runs 13-20.As seen in Table 1, when KI-CeO2 is used as the catalyst, the ethanol conversion increases, but the yield of DEC is not improved compared to CeO2 alone.In addition, when KBr or KCl is added to KNO3-CeO2, the ethanol conversion decreases, but the selectivity of DEC is effectively improved.Additionally, the catalysts prepared by the solid mixed method give a higher DEC yield and selectivity.However, when KI-KNO3-CeO2 is used as the catalyst, the DEC yield seriously decreases.This might be because that KI, with a stronger reduction property, is oxidized by KNO3, which causes deactivation.
The molar ratio of KBr and KNO3 was evaluated with fixed n(CeO2)/n(KNO3) = 1:0.4.The results are shown in Figure 5.The XRD patterns of KBr-KNO3-CeO2 are presented in Figure 4, and the characteristic peaks of KBr are labeled.The yield and the selectivity of DEC are improved with the addition of KBr into the catalyst.When the molar ration is n(KBr)/n(KNO3) = 6:4, that is n(CeO2)/n(KNO3)/n(KBr) = 1:0.4:0.6, the yield of DEC reaches 13.0% with a selectivity of 38.5% on the ethanol basis.Both the yield and selectivity of DEC are much higher than reported in Leino's research [8] by using butylene oxide as the dehydration agent and CeO2 as the catalyst.In their results, the highest obtained yield of DEC was 1.5%, and selectivity to DEC was 10% on the ethanol basis.
The formation kinetics of DEC were also studied, and the dependence of DEC yield on reaction time at 150 °C in 180 min is shown in Figure 6.It is also indicated that the reaction can reach equilibrium in 100 min.The reaction time was selected as 120 min.The formation kinetics of DEC were also studied, and the dependence of DEC yield on reaction time at 150 ˝C in 180 min is shown in Figure 6.It is also indicated that the reaction can reach equilibrium in 100 min.The reaction time was selected as 120 min.

Recycling Experiments
The recycling experiment results of the KBr-KNO3-CeO2 catalyst are listed in Figure 7.After three times reuse, the catalyst still keeps good catalytic activity, and the DEC yield is above 90%, as in the primary reaction.The XRD patterns of fresh KBr-KNO3-CeO2 and the catalyst after using three times were also compared.The calcination temperature KBr-KNO3-CeO2 was also set as 500 °C.Additionally, the KBr was not decomposed according to the TG-DTG analysis (Figure 4).As seen in Figure 7, the characteristic peaks do not change after three runs.This indicates that the active species of KBr and KNO3 do not leach from the catalyst.The reason might be that CO2 was the main reactant in this reaction, and the reactants and products were extracted by Sc-CO2 after the reaction, while the catalyst was not soluble in Sc-CO2 and left in the reactor, which avoided the loss of catalyst.This is one of the advantages of separating the products by Sc-CO2.

Thermodynamic Evaluation of the One-Pot Synthesis of DEC
In order to perform the thermodynamics evaluations, which are important in seeking novel synthesis ideas, the thermodynamic data of various substances, such as ethanol, CO2, PO, DEC and PG, involved in the reaction are tabulated in Table 2.

Recycling Experiments
The recycling experiment results of the KBr-KNO 3 -CeO 2 catalyst are listed in Figure 7.After three times reuse, the catalyst still keeps good catalytic activity, and the DEC yield is above 90%, as in the primary reaction.The XRD patterns of fresh KBr-KNO 3 -CeO 2 and the catalyst after using three times were also compared.The calcination temperature KBr-KNO 3 -CeO 2 was also set as 500 ˝C.Additionally, the KBr was not decomposed according to the TG-DTG analysis (Figure 4).As seen in Figure 7, the characteristic peaks do not change after three runs.This indicates that the active species of KBr and KNO 3 do not leach from the catalyst.The reason might be that CO 2 was the main reactant in this reaction, and the reactants and products were extracted by Sc-CO 2 after the reaction, while the catalyst was not soluble in Sc-CO 2 and left in the reactor, which avoided the loss of catalyst.This is one of the advantages of separating the products by Sc-CO 2 .

Recycling Experiments
The recycling experiment results of the KBr-KNO3-CeO2 catalyst are listed in Figure 7.After three times reuse, the catalyst still keeps good catalytic activity, and the DEC yield is above 90%, as in the primary reaction.The XRD patterns of fresh KBr-KNO3-CeO2 and the catalyst after using three times were also compared.The calcination temperature KBr-KNO3-CeO2 was also set as 500 °C.Additionally, the KBr was not decomposed according to the TG-DTG analysis (Figure 4).As seen in Figure 7, the characteristic peaks do not change after three runs.This indicates that the active species of KBr and KNO3 do not leach from the catalyst.The reason might be that CO2 was the main reactant in this reaction, and the reactants and products were extracted by Sc-CO2 after the reaction, while the catalyst was not soluble in Sc-CO2 and left in the reactor, which avoided the loss of catalyst.This is one of the advantages of separating the products by Sc-CO2.

Thermodynamic Evaluation of the One-Pot Synthesis of DEC
In order to perform the thermodynamics evaluations, which are important in seeking novel synthesis ideas, the thermodynamic data of various substances, such as ethanol, CO2, PO, DEC and PG, involved in the reaction are tabulated in Table 2.

Thermodynamic Evaluation of the One-Pot Synthesis of DEC
In order to perform the thermodynamics evaluations, which are important in seeking novel synthesis ideas, the thermodynamic data of various substances, such as ethanol, CO 2 , PO, DEC and PG, involved in the reaction are tabulated in Table 2.
Based on the obtained values, it can be concluded that the reaction is exothermic (∆ r H θ 298k " ´101.4KJ¨mol´1 ă 0) and does not occur spontaneously at room temperature ∆ r G θ 298k " 5.10 KJ¨mol ´1 ą 0. The relative pattern of the reaction heat with the temperature is expressed by Kirchhoff's law (Equation ( 2)), whereas the Gibbs energy of the reaction, at different temperatures, can be given by the Gibbs-Helmholtz equation (Equation ( 3)) [8].Gibbs energy values of this reaction at different temperatures were calculated.The results are shown in Figure 8.The value of ∆ r G θ T increases with the reaction temperature, and the increase in the temperature is disadvantageous to the formation of DEC.The relative pattern of the reaction heat with the temperature is expressed by Kirchhoff's law (Equation ( 2)), whereas the Gibbs energy of the reaction, at different temperatures, can be given by the Gibbs-Helmholtz equation (Equation ( 3)) [8].Gibbs energy values of this reaction at different temperatures were calculated.The results are shown in Figure 8.The value of ∆ increases with

Figure 3 .
Figure 3. Optimization of the conditions for the catalyst preparation and the DEC synthesis.The ethanol volume was fixed as 10 mL; the reaction temperature was 150 °C; and the initial pressure was 5 MPa.

Figure 3 .
Figure 3. Optimization of the conditions for the catalyst preparation and the DEC synthesis.The ethanol volume was fixed as 10 mL; the reaction temperature was 150 °C; and the initial pressure was 5 MPa.

Figure 3 .
Figure 3. Optimization of the conditions for the catalyst preparation and the DEC synthesis.The ethanol volume was fixed as 10 mL; the reaction temperature was 150 °C; and the initial pressure was 5 MPa.

Figure 3 .
Figure 3. Optimization of the conditions for the catalyst preparation and the DEC synthesis.The ethanol volume was fixed as 10 mL; the reaction temperature was 150 ˝C; and the initial pressure was 5 MPa.

Figure 5 .
Figure 5. Effects of the molar ratio of KBr and KNO3 on the products (both of the volumes of ethanol and PO were 10 mL; the reaction temperature was 150 °C; the initial pressure was 5 MPa).PC, propylene carbonate.

Figure 5 .
Figure 5. Effects of the molar ratio of KBr and KNO3 on the products (both of the volumes of ethanol and PO were 10 mL; the reaction temperature was 150 °C; the initial pressure was 5 MPa).PC, propylene carbonate.

Figure 5 .
Figure 5. Effects of the molar ratio of KBr and KNO 3 on the products (both of the volumes of ethanol and PO were 10 mL; the reaction temperature was 150 ˝C; the initial pressure was 5 MPa).PC, propylene carbonate.

Figure 6 .
Figure 6.The relationship between the reaction time and yield of DEC at 150 °C for 180 min.(both of the volumes of ethanol and PO were 10 mL; the initial pressure was 5 MPa).

Figure 7 .
Figure 7. Recycling experiment results of the KBr-KNO3-CeO2 (both of the volumes of ethanol and PO were 10 mL; the reaction temperature was 150 °C; the initial pressure was 5 MPa).

Figure 6 .
Figure 6.The relationship between the reaction time and yield of DEC at 150 ˝C for 180 min.(both of the volumes of ethanol and PO were 10 mL; the initial pressure was 5 MPa).

Figure 6 .
Figure 6.The relationship between the reaction time and yield of DEC at 150 °C for 180 min.(both of the volumes of ethanol and PO were 10 mL; the initial pressure was 5 MPa).

Figure 7 .
Figure 7. Recycling experiment results of the KBr-KNO3-CeO2 (both of the volumes of ethanol and PO were 10 mL; the reaction temperature was 150 °C; the initial pressure was 5 MPa).

Figure 7 .
Figure 7. Recycling experiment results of the KBr-KNO 3 -CeO 2 (both of the volumes of ethanol and PO were 10 mL; the reaction temperature was 150 ˝C; the initial pressure was 5 MPa).

∆
r H θ T " ∆ r H θ 298k `∆C p pT ´298q ( The enthalpy and the entropy of the reaction at 298 K estimated from the ∆ and S values amounted to ∆ 101.4 KJ • mol and ∆ 357.22 J • mol • K .The Gibbs energy at 298 K and 100 KPa could be calculated by Equation (1) and has a value ∆ 5.10 KJ • mol .on the obtained values, it can be concluded that the reaction is exothermic ( ∆ 101.4KJ • mol 0 ) and does not occur spontaneously at room temperature ∆ 5.10 KJ • mol 0.

Figure 8 .
Figure 8. Dependence of temperature on the Gibbs energy of DEC synthesis from ethanol, CO2 and PO.

Figure 8 .
Figure 8. Dependence of temperature on the Gibbs energy of DEC synthesis from ethanol, CO 2 and PO.

Table 2 .
[27]modynamic data of various substances in the reaction[26].Calculated by the Constantinous-Gani ( CG) group contribution method[27].The enthalpy and the entropy of the reaction at 298 K estimated from the ∆ f H θ 298k and S θ 298k values amounted to ∆ r H θ 298k " ´101.4KJ¨mol´1 and ∆ r S θ 298k " ´357.22J¨mol´1¨K´1.The Gibbs energy at 298 K and 100 KPa could be calculated by Equation (1) and has a value ∆ r G θ 298k " 5.10 KJ¨mol ´1.