Synthesis of Diethyl Carbonate from Carbon Dioxide, Propylene Oxide and Ethanol Over KNO3-Ceo2 and Kbr-KNO3-Ceo2 Catalysts

catalysts

Article Synthesis of Diethyl Carbonate from Carbon Dioxide, Propylene Oxide and Ethanol over KNO3-CeO2 and KBr-KNO3-CeO2 Catalysts

Yanlou Wang, Dongdong Jia *, Zhen Zhu and Yongyue Sun

Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China; [email protected] (Y.W.); [email protected] (Z.Z.); [email protected] (Y.S.) * Correspondence: [email protected]; Tel.: +86-22-60214259; Fax: +86-22-60214259

Academic Editor: Keith Hohn Received: 22 February 2016; Accepted: 23 March 2016; Published: 29 March 2016

Abstract: One-pot syntheses of diethyl carbonate (DEC) from CO2, propylene oxide and ethanol were carried out using different solid catalysts. The supercritical CO2 extraction method was used to separate the liquid products and reactants from the catalysts after reaction. The KNO3-CeO2 and KBr-KNO3-CeO2 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-KNO3-CeO2. The yield of DEC was improved about 10-fold by using KBr-KNO3-CeO2 catalyst compared to CeO2.

Keywords: diethyl carbonate; carbon dioxide; propylene oxide; ethanol; solid catalyst

1. Introduction

The synthesis of chemicals using CO2 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 CO2 [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 CO2 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 CeO2 catalyst [8,19]. According to the results, the yield of DEC had a nine-fold

Catalysts 2016, 6, 52; doi:10.3390/catal6040052 www.mdpi.com/journal/catalysts Catalysts 2016, 6, 52 2 of 11

Catalysts 2016, 6, 52 2 of 11

enhancement compared to that over CeO2 without dehydrant, but it was still not high enough (only compared to that over CeO2 without dehydrant, but it was still not high enough (only 1.5%) in this 1.5%) in this system and needed to be improved. Besides, CeO2 with 2-cyanopyridine was also used as systemthe carboxylation/hydration and needed to be improved. cascade Besides, catalyst byCeO Tomishige2 with 2‐ groupcyanopyridine [20] for the was propylene also used carbonate 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 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. landmarkIn addition, in carbonate the one-pot synthesis synthesis using CeO of DEC2 catalysis. from carbon dioxide, ethylene oxide (EO) and ethanol on theIn addition, KI and sodium the one ethoxide‐pot synthesis binary of homogeneous DEC from carbon catalyst dioxide, was also ethylene researched oxide by(EO) Wang and etethanol al. [21 ], onand the the KI yieldand sodium of DEC ethoxide was improved. binary However,homogeneous KI and catalyst sodium was ethoxide also researched are dissolved by Wang in ethanol et al. [21], and andcannot the yield be separated of DEC was easily. 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 industrialOn the other manufacturing. hand, the synthesis Furthermore, of cyclic the carbonate transesterification from epoxide of and cyclic CO carbonate2 was well with established ethanol into industrial produce DECmanufacturing. was also proven Furthermore, to be feasible the transesterification [22] However, fromof cyclic views carbonate of energy with consumption, 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 productivitytwo-step separate and investment, reaction. Thus, the one the‐ developmentpot reaction directly of a more from effective CO2 one-potwas undoubtedly reaction to superior improve to the the two‐step separate reaction. Thus, the development of a more effective one‐pot reaction to productivity of DEC directly from CO2 is highly desired. improve the productivity of DEC directly from CO2 is highly desired. 2. Results and Discussion 2. Results and Discussion The one-pot reaction in DEC synthesis might be composed of two steps, the cycloaddition reaction andThe subsequent one‐pot transesterificationreaction in DEC synthesis reaction. Themight mechanisms be composed of the of reaction two steps, have the been cycloaddition studied and reactionproven and by many subsequent researchers transesterification [21,23]. As analyzed reaction. by The the mechanisms GC-MS method of the in reaction this work, have the been main studiedproducts and in proven the one-pot by many reaction researchers from CO [21,23].2, ethanol As analyzed and PO wereby the 1,2-propanediol GC‐MS method (PG), in this DEC work, and thepropylene main products carbonate in (PC)the one with‐pot the reaction side-product from CO 1-ethoxy-2-propanol2, ethanol and PO (EP). were The 1,2‐ EPpropanediol might be formed (PG), DECfrom and propylene propylene oxide carbonate by ethanolysis (PC) with [23,24 the] in side the‐product basic catalytic 1‐ethoxy environment.‐2‐propanol The (EP). possible The EP equations might beof formed these reactions from propylene are presented oxide as by follows ethanolysis (Scheme [23,24]1). in the basic catalytic environment. The possible equations of these reactions are presented as follows (Scheme 1).

Scheme 1. The reaction schemes. Scheme 1. The reaction schemes.

2.1. The One‐Pot Synthesis of DEC over KNO3‐CeO2 Catalyst 2.1. 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 As reviewed in the literature [8], the heterogeneous catalyst CeO has catalytic activity for the one‐pot reaction in DEC synthesis. In order to improve the catalytic activity2 in DEC synthesis, 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 strong base of KOH and several typical alkali and alkaline-earth metal salts, which might be necessary necessary to meet the requirement for catalyzing the transesterification reaction combined with to meet the requirement for catalyzing the transesterification reaction combined with CeO2, were CeO2, were researched. Several metal oxides, such as γ‐Al2O3, ZrO2, SiO2 and La2O3, were also tested. researched. Several metal oxides, such as γ-Al2O3, ZrO2, SiO2 and La2O3, were also tested. In the In the typical reaction, the molar ratio of CO2, ethanol and PO was fixed as 0.29:0.17:0.14; the reaction typical reaction, the molar ratio of CO , ethanol and PO was fixed as 0.29:0.17:0.14; the reaction temperature was 150 °C; the initial pressure2 was 5 MPa; and during the reaction, the pressure could 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 reach 9 MPa, which is higher than the critical pressure of CO . The results are summarized in Table1, Table 1, Runs 1–12. 2 Runs 1–12.

Catalysts 2016, 6, 52 3 of 11

Table 1. Effects of different catalysts on the one-pot reaction. DEC, diethyl carbonate; PG, 1,2-propanediol; EP, 1-ethoxy-2-propanol.

Selectivity/% Run Catalyst Specific surface area/m2/g Ethanol conversion/% DEC yield/% DEC PG EP

1 CeO2 - 1.5 1.3 84.0 - - 2 NaNO3-CeO2 - 12.6 3.6 28.3 24.1 19.3 3 Ba(NO3)2-CeO2 - 8.1 0.9 12.6 10.2 16.6 4 Mg(NO3)2-CeO2 - 15.9 2.2 13.0 15.9 17.4 5 Ca(NO3)2-CeO2 - 6.6 1.3 22.3 16.7 19.8 6 KOH-CeO2 - 13.4 3.6 28.0 20.1 33.6 7 K2CO3-CeO2 - 9.1 1.8 19.0 17.6 25.1 8 KNO3-CeO2 30.5 34.0 11.2 33.3 23.7 23.5 9 KNO3-γ-Al2O3 102 46.8 11.5 24.5 23.1 16.5 10 KNO3-ZrO2 22.9 23.7 6.3 27 24.4 22.9 11 KNO3-SiO2 78.1 12.3 3.1 25.3 26.1 18.8 12 KNO3-La2O3 31.8 0.6 0.4 69.1 20.7 10.9 A 13 KI-CeO2 - 18.0 0.9 6.3 6.4 10.3 B 14 KI-CeO2 - 9.0 1.3 14.4 17.4 12.8 KI-KNO3-CeO2 15 A - 17.1 0.9 5.8 4.9 7.7

KI-KNO3-CeO2 16 B - 9.6 0.4 7.5 7.9 7.2

KCl-KNO3-CeO2 17 A - 14.4 2.7 17.4 14.4 5.7

KCl-KNO3-CeO2 18 B - 39.9 10.3 25.9 22.3 5.9

KBr-KNO3-CeO2 19 A - 23.1 9.4 40.8 30.5 7.6

KBr-KNO3-CeO2 20 B - 33.9 13.0 38.5 27.7 7.5 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 Table1, when KNO 3-CeO2 was used as the catalyst, the yield of DEC was effectively improved about 10-fold, compared to that using CeO2 catalyst. The KNO3-CeO2 had better catalytic activity than KOH and other alkali and alkaline-earth metal salts loading on CeO2, such as KOH-CeO2, K2CO3-CeO2, NaNO3-CeO2, Ba(NO3)2-CeO2, Mg(NO3)2-CeO2 and Ca(NO3)2-CeO2. Then, KNO3 with other metal oxides, including γ-Al2O3, ZrO2, La2O3 and SiO2, was evaluated. The results indicate that the oxides with acid-base properties, especially ZrO2, γ-Al2O3 and CeO2 combined with KNO3, are more active for DEC synthesis. The basicity of the catalysts KNO3-γ-Al2O3, KNO3-CeO2 and CeO2 ˝ ˝ were analyzed by the CO2-TPD method (Figure1). The desorption peaks at about 280 C–340 C could be observed for KNO3-γ-Al2O3 and KNO3-CeO2, 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 KNO3-CeO2 catalyst obtaining the better yield and selectivity for DEC, it was finally selected for the following reactions. The XRD spectrums of KNO3-CeO2 and CeO2 are shown in Figure2. By comparison, the characteristic peaks of KNO 3 cannot be found, and the diffraction peaks of KNO3-CeO2 are stronger than CeO2. It is indicated that the KNO3 might be well dispersed and caused no considerable distortion in the structure of CeO2 [25]. The preparation conditions for KNO3-CeO2 including the load of KNO3 and the calcination temperature were optimized. The results are shown in Figure3a,b. It is indicated that the KNO 3-CeO2 with n(Ce)/n(K) = 1:0.4 has better catalytic activity (Figure3a). And the DEC yield reaches higher ˝ level (Figure3b) at the calcination temperature of 500 C. According to the TG-DTG analysis of KNO3 ˝ (Figure4), the decomposition temperature of KNO 3 is 520 C. When the temperature is higher than ˝ 520 C, the KNO3 will be decomposed to K2O, 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 Figure3c–e. As seen in Figure3c, 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 KNO3-CeO2 is 0.3 g (Figure4c). Figure4d shows the dependence of DEC yield on reaction time. The reaction reaches equilibrium in 2 h. Figure3e shows that the DEC 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. Catalysts 2016, 6, 52 4 of 11

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 0.14yield mol, reaches respectively) the higher and level, the when CO2 amount both of isthe fixed volumes as 0.25 of mol. ethanol and PO are 10 mL (0.17 mol and 0.14 mol, respectively) and the CO2 amount is fixed as 0.25 mol.

Figure 1. CO2‐temperature‐programmed desorption (TPD) analysis for the catalysts.

Figure 1. CO2‐temperature‐programmed desorption (TPD) analysis for the catalysts. Figure 1. CO2-temperature-programmed desorption (TPD) analysis for the catalysts.

Figure 2. XRD patterns of CeO2, KNO3‐CeO2 and KBr‐KNO3‐CeO2 (KBr, CeO2). FigureFigure 2. 2.XRD XRD patterns patterns of of CeO CeO22,, KNOKNO33-CeO‐CeO22 and KBr KBr-KNO‐KNO3‐-CeOCeO22 (KBr,(KBr, CeO CeO22).).

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 FigureFigure 3. Optimization 3. Optimization of of the the conditions for for the the catalyst catalyst preparation preparation and the DEC and synthesis. the DEC The synthesis. ethanol The volume5 MPa. was fixed as 10 mL; the reaction temperature was 150 ˝C; and the initial pressure was 5 MPa. ethanol volume was fixed as 10 mL; the reaction temperature was 150 °C; and the initial pressure was 5 MPa. Catalysts 2016, 6, 52 5 of 11 Catalysts 2016, 6, 52 5 of 11

Catalysts 2016, 6, 52 5 of 11

Figure 4. TG‐DTG analysis for KNO3 and KBr. Figure 4. TG-DTG analysis for KNO3 and KBr.

2.2. The One‐Pot Synthesis of DECFigure over 4. KBr TG‐DTGKNO analysis3‐CeO2 Catalystfor KNO 3 and KBr. 2.2. The One-Pot Synthesis of DEC over KBr-KNO3-CeO2 Catalyst 2.2.As The potassium One‐Pot Synthesis halides (KI, of DECKBr and over KCl) KBr ‐wereKNO proven3‐CeO2 Catalystto be conducive to the cycloaddition reaction [15]; theAs KI, potassium KBr and KCl halides were added (KI, in KBr the andKNO KCl)3‐CeO2 were catalyst proven for the toone be‐pot conducive synthesis of toDEC. the The cycloaddition addition As potassium halides (KI, KBr and KCl) were proven to be conducive to the cycloaddition reaction [15]; reactionmethods [15 ];for the potassium KI, KBr halides, and KCl including were added solid mixed in the and KNO impregnation3-CeO2 catalyst methods, for were the compared. one-pot synthesis of the KI, KBr and KCl were added in the KNO3‐CeO2 catalyst for the one‐pot synthesis of DEC. The addition DEC. TheThe addition results are methods shown for in potassiumTable 1, Runs halides, 13–20. includingAs seen in solid Table mixed 1, when and KI impregnation‐CeO2 is used as methods, the methods for potassium halides, including solid mixed and impregnation methods, were compared. werecatalyst, compared. the ethanol conversion increases, but the yield of DEC is not improved compared to CeO2 The results are shown in Table 1, Runs 13–20. As seen in Table 1, when KI‐CeO2 is used as the alone.The results In addition, are shown when KBr in Table or KCl1, Runsis added 13–20. to KNO As3 seen‐CeO2 in, the Table ethanol1, when conversion KI-CeO decreases,2 is used but as the catalyst, the ethanol conversion increases, but the yield of DEC is not improved compared to CeO2 catalyst,the selectivity the ethanol of DEC conversion is effectively increases, improved. but the Additionally, yield of DEC the is catalysts not improved prepared compared by the solid to CeO alone. In addition, when KBr or KCl is added to KNO3‐CeO2, the ethanol conversion decreases, but 2 mixed method give a higher DEC yield and selectivity. However, when KI‐KNO3‐CeO2 is used as the alone.the In selectivity addition, of when DEC KBr is effectively or KCl is addedimproved. to KNO Additionally,3-CeO2, thethe ethanolcatalysts conversionprepared by decreases, the solid but catalyst, the DEC yield seriously decreases. This might be because that KI, with a stronger reduction the selectivitymixed method of DEC give is a effectivelyhigher DEC improved. yield and selectivity. Additionally, However, the catalysts when KI‐ preparedKNO3‐CeO by2 is the used solid as the mixed property, is oxidized by KNO3, which causes deactivation. methodcatalyst, give athe higher DEC yield DEC seriously yield and decreases. selectivity. This However, might be whenbecause KI-KNO that KI,3 -CeOwith a2 strongeris used asreduction the catalyst, The molar ratio of KBr and KNO3 was evaluated with fixed n(CeO2)/n(KNO3) = 1:0.4. The results are the DECproperty, yield is seriously oxidized decreases.by KNO3, which This mightcauses deactivation. be because that KI, with a stronger reduction property, shown in Figure 5. The XRD patterns of KBr‐KNO3‐CeO2 are presented in Figure 4, and the characteristic The molar ratio of KBr and KNO3 was evaluated with fixed n(CeO2)/n(KNO3) = 1:0.4. The results are is oxidizedpeaks of byKBr KNO are labeled.3, which The causes yield and deactivation. the selectivity of DEC are improved with the addition of KBr into shown in Figure 5. The XRD patterns of KBr‐KNO3‐CeO2 are presented in Figure 4, and the characteristic theThe catalyst. molar When ratio the of molar KBr and ration KNO is n(KBr)/n(KNO3 was evaluated3) = 6:4, with that fixedis n(CeOn(CeO2)/n(KNO2)/n3(KNO)/n(KBr)3) = = 1:0.4:0.6, 1:0.4. The peaks of KBr are labeled. The yield and the selectivity of DEC are improved with the addition of KBr into resultsthe yield are shown of DEC in reaches Figure 13.0%5. The with XRD a selectivity patterns of of 38.5% KBr-KNO on the 3ethanol-CeO2 basis.are presented Both the yield in Figure and 4, 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, and the characteristic peaks of KBr are labeled. The yield and the selectivity of DEC are improved selectivitythe yield ofof DEC DEC are reaches much 13.0% higher with than a reported selectivity in ofLeino’s 38.5% research on the ethanol[8] by using basis. butylene Both the oxide yield as and the withdehydration theselectivity addition of agent DEC of and KBrare CeOmuch into2 as higher the the catalyst. catalyst. than reported In When their in results, the Leino’s molar the research highest ration [8] obtained is by n(KBr)/n(KNO using yield butylene of DEC oxide3) was = 6:4,as 1.5%, the that is and selectivity to DEC was 10% on the ethanol basis. n(CeOdehydration2)/n(KNO agent3)/n(KBr) and CeO =2 1:0.4:0.6, as the catalyst. the yield In their of results, DEC reaches the highest 13.0% obtained with yield a selectivity of DEC was of 1.5%, 38.5% on the ethanolandThe selectivity basis.formation to Both DEC kinetics the was yield 10% of DEC on and the were selectivityethanol also basis.studied, of DEC and arethe dependence much higher of thanDEC yield reported on reaction in Leino’s time at 150 °C in 180 min is shown in Figure 6. It is also indicated that the reaction can reach research [8The] by formation using butylene kinetics oxide of DEC as were the dehydrationalso studied, and agent the and dependence CeO2 as of the DEC catalyst. yield on In reaction their results, equilibrium in 100 min. The reaction time was selected as 120 min. the highesttime at obtained 150 °C in yield 180 min of DEC is shown was 1.5%, in Figure and selectivity6. It is also to indicated DEC was that 10% the on reaction the ethanol can reach basis. equilibrium in 100 min. The reaction time was selected as 120 min.

Figure 5. Effects of the molar ratio of KBr and KNO3 on the products (both of the volumes of ethanol and PO Figure 5. Effects of the molar ratio of KBr and KNO3 on the products (both of the volumes of ethanol and PO Figurewere 5. 10Effects mL; the of reaction the molar temperature ratio of was KBr 150 and °C; KNO the initial3 on pressure the products was 5 MPa). (both PC, of propylene the volumes carbonate. of ethanol and POwere were 10 mL; 10 the mL; reaction the temperature reaction temperature was 150 °C; the was initial 150 pressure˝C; the was initial 5 MPa). pressure PC, propylene was carbonate. 5 MPa). PC, propylene carbonate. Catalysts 2016, 6, 52 6 of 11

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 Figure6. It is also indicated that the reaction can reach equilibrium inCatalysts 100 min. 2016, The6, 52 reaction time was selected as 120 min. 6 of 11 Catalysts 2016, 6, 52 6 of 11

Figure 6. The relationship between the reaction time and yield of DECDEC atat 150150 ˝°CC for 180 min. (both of theFigure volumes 6. The of relationship ethanol and between PO were the 10 reaction mL; thethe time initialinitial and pressurepressure yield of waswas DEC 55 MPa).MPa). at 150 °C for 180 min. (both of the volumes of ethanol and PO were 10 mL; the initial pressure was 5 MPa). 2.3. Recycling Experiments 2.3. Recycling Experiments The recycling experiment results of the KBr‐KNO3‐CeO2 catalyst are listed in Figure 7. After The recycling experiment results of the KBr-KNO3-CeO2 catalyst are listed in Figure7. After threeThe times recycling reuse, reuse, the theexperiment catalyst catalyst still stillresults keeps keeps of good the good KBr catalytic catalytic‐KNO activity,3‐CeO activity,2 catalyst and and the theare DEC DEClisted yield yield in is Figure above is above 7.90%, After 90%, as inthree the timesprimary reuse, reaction. the catalyst The XRD still patterns keeps good of fresh catalytic KBr‐KNO activity,3‐CeO and2 and the the DEC catalyst yield afteris above using 90%, three as as in the primary reaction. The XRD patterns of fresh KBr-KNO3-CeO2 and the catalyst after using timesin the primarywere also reaction. compared. The XRDThe calcinationpatterns of freshtemperature KBr‐KNO KBr3‐CeO‐KNO2 and3‐CeO the2 catalystwas also after set usingas 500 three °C.˝ three times were also compared. The calcination temperature KBr-KNO3-CeO2 was also set as 500 C. Additionally,times were also thethe compared. KBrKBr waswas notnot The decomposeddecomposed calcination accordingaccording temperature toto thethe KBr TG-DTGTG‐KNO‐DTG3‐ analysisCeOanalysis2 was (Figure(Figure also 4set 4).). As asAs seen 500seen °C. in in FigureAdditionally,7 7,, the the characteristic characteristic the KBr was peaks peaksnot decomposed do do not not change change according after after three three to the runs. runs. TG This This‐DTG indicates indicates analysis that that(Figure the the active active4). As species speciesseen in ofFigure KBr and7, the KNO characteristic3 do not leach peaks from do thenot catalyst.change after The reasonthree runs. might This be indicatesthat CO2 wasthat thethe mainactive reactant species of KBr and KNO3 do not leach from the catalyst. The reason might be that CO2 was the main reactant inof thisKBr reaction,and KNO and3 do the not reactants leach from and the products catalyst. were The extractedreason might by Sc be‐CO that2 after CO2 thewas reaction, the main while reactant the in this reaction, and the reactants and products were extracted by Sc-CO2 after the reaction, while the catalystin this reaction, was not and soluble the reactants in Sc‐CO and2 and products left in the were reactor, extracted which by avoided Sc‐CO2 after the loss the reaction,of catalyst. while This the is catalyst was not soluble in Sc-CO2 and left in the reactor, which avoided the loss of catalyst. This is onecatalyst of the was advantages not soluble of inseparating Sc‐CO2 and the leftproducts in the byreactor, Sc‐CO which2. avoided the loss of catalyst. This is one of the advantages of separating the products by Sc-CO2. one of the advantages of separating the products by Sc‐CO2.

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; the3 initial2 pressure was 5 MPa). Figure 7.7. RecyclingRecycling experiment experiment results results of of the the KBr-KNO KBr‐KNO3-CeO‐CeO2 (both (both of of the the volumes volumes of ethanolof ethanol and and PO werePO were 10 mL; 10 mL; the reactionthe reaction temperature temperature was was 150 ˝150C; the°C; initialthe initial pressure pressure was was 5 MPa). 5 MPa). 2.4. Thermodynamic Evaluation of the One‐Pot Synthesis of DEC 2.4. ThermodynamicIn order to perform Evaluation the ofthermodynamics the One‐Pot Synthesis evaluations, of DEC which are important in seeking novel synthesisIn order ideas, to theperform thermodynamic the thermodynamics data of various evaluations, substances, which such are as importantethanol, CO in2 , seekingPO, DEC novel and PG,synthesis involved ideas, in thethe reactionthermodynamic are tabulated data ofin variousTable 2. substances, such as ethanol, CO2, PO, DEC and PG, involved in the reaction are tabulated in Table 2.

Catalysts 2016, 6, 52 7 of 11

2.4. 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 Table2.

Table 2. Thermodynamic data of various substances in the reaction [26].

θ ´1 θ ´1 ´1 id ´1 ´1 Substance ∆fH298k/kJ¨ mol S298k/J¨ mol ¨ K Cp 298k/J¨ mol ¨ K

C2H5OH ´235.10 282.70 112.6 CO2 ´393.509 213.74 37.13 DEC ´637.9 412.94 a 211 H2O ´241.82 188.82 33.58 PO ´94.68 281.15 72.55 PG ´421.29 288 a 189.9 a Calculated by the Constantinous-Gani ( CG) group contribution method [27]. Catalysts 2016, 6, 52 7 of 11

θ θ The enthalpyTable and the 2. Thermodynamic entropy of the reaction data of various at 298 Ksubstances estimated in from the reaction the ∆f H [26].298k and S298k values θ ´ 1 θ ´1 ´1 amounted to ∆r H298k “ ´101.4 KJ¨ mol and ∆rS298k “ ´357.22 J¨ mol ¨ K . The Gibbs energy at −1 Substance ∆/kJ∙mol /J∙mol−1K−1 / Jθ∙mol−1K−1 ´1 298 K and 100 KPa could be calculated by Equation (1) and has a value ∆rG298k “ 5.10 KJ¨ mol . C2H5OH −235.10 282.70 112.6 ∆ Gθ “ ∆ Hθ ` T∆ Sθ (1) CO2 −393.509r 298k r 298k213.74 r 298k 37.13

DEC −637.9 412.94 a 211 Based on the obtained values, it can be concluded that the reaction is exothermic θ H2O ´1 −241.82 188.82 33.58 (∆r H298k “ ´101.4KJ¨ mol ă 0) and does not occur spontaneously at room temperature θ PO´ −1 94.68 281.15 72.55 ∆rG298k “ 5.10 KJ¨ mol ą 0. The relative patternPG − of the reaction421.29 heat with the288 temperature a is expressed189.9 by Kirchhoff’s law (Equation (2)),a whereas Calculated the by Gibbs the Constantinous energy of the‐Gani reaction, ( CG) group at different contribution temperatures, method [27]. can be given by the Gibbs–Helmholtz equation (Equation (3)) [8]. Gibbs energy values of this reaction∆ at different The enthalpy and the entropy of the reaction at 298 K estimated fromθ the and temperatures were calculated. The results are shown in Figure8. The value of ∆rG increases with the S values amounted to ∆ 101.4 KJ ∙ mol and ∆ 357.22 JT ∙ mol ∙K . The reactionGibbs energy temperature, at 298 and K theand increase 100 KPa in the could temperature be calculated is disadvantageous by Equation to (1) the and formation has a of value DEC. ∆ 5.10 KJ ∙ mol . θ θ ∆r HT “ ∆r H298k ` ∆Cp pT ´ 298q (2) ∆ ∆ ∆ (1) ∆ Gθ ∆ Hθ Based on the obtained values, rit Tcan be concludedr T that the reaction is exothermic d “ ´ 2 dT (3) ˜ T ¸ ˜ T ¸ ( ∆ 101.4KJ ∙ mol 0) and does not occur spontaneously at room temperature ∆ 5.10 KJ ∙ mol 0.

Figure 8.8.Dependence Dependence of of temperature temperature on theon Gibbsthe Gibbs energy energy of DEC of synthesis DEC synthesis from ethanol, from ethanol, CO2 and CO PO.2 and PO.

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 the reaction temperature, and the increase in the temperature is disadvantageous to the formation of DEC.

∆ ∆ ∆(T−298) (2)

∆ ∆ dT (3) When the temperature is fixed at 423 K, the Gibbs energy of the reaction is a function of pressure and can be written as follows [8]: ∆ ∆ ln (4) The equilibrium constant K was determined from the Gibbs energy according to Equation (5): Catalysts 2016, 6, 52 8 of 11

When the temperature is fixed at 423 K, the Gibbs energy of the reaction is a function of pressure and can be written as follows [8]:

p ∆ G “ ∆ Gθ ´ RTln (4) r p r pθ ˆ ˙ The equilibrium constant K was determined from the Gibbs energy according to Equation (5):

∆rGT “ ´RTlnK (5)

rDECs rPGs K “ 2 (6) rC2H5OHs rPOs rCO2s

In this work, the reaction temperature and pressure are 423 K and 9 MPa; the ∆rGp calculated by Equation (4) is 36.07 kJ¨ mol´1 > 0; and the equilibrium constant K calculated by Equation (5) is ´5 3.5 ˆ 10 . When the amounts of CO2, ethanol and PO are 0.29, 0.17 and 0.14 mol, respectively, and assuming that the reaction is in the gas phase, the equilibrium yield of DEC at 423 K and 9 MPa calculated by Equation (6) is about 14.2% based on ethanol. It is indicated that the yield of DEC in the experiment (13.0%) is close to the equilibrium yield calculated.

3. Experimental Section

3.1. Material Ethanol (99.8%, analytical grade) was dehydrated by adding spherical 3A molecular sieves before the reactions. Propylene oxide (PO), n-propanol, Ce(NO3)3¨ 6H2O, KNO3, KOH, NaNO3, Mg(NO3)2¨ 6H2O, Ba(NO3)2, Ca(NO3)2¨ 4H2O, KBr, KCl, KI, ammonium hydroxide (NH3¨ H2O, 28%) and spherical 3A molecular sieves were purchased from Jiangtian Chemical Reagent Co., Ltd., Tianjin, China. All of the chemicals were of analytical grade and used without further purification. CO2 (99.99%) was purchased from Lianbo Gas Co., Ltd., Tianjin, China.

3.2. Catalyst Preparation and Characterization

CeO2 was prepared by the precipitation method. The 6.5 g of cerium (III) nitrate hexahydrate (Ce(NO3)3¨ 6H2O) were firstly dissolved in 50 mL water. Then, the precipitating solution of ammonium hydroxide of 50% (v/v) was slowly added into the well-stirred cerium nitrate aqueous solution. The pH of the solution was controlled at 10 throughout the synthesis process. The resulting precipitate was filtrated, washed with deionized water and then dried overnight at 100 ˝C and calcined at 500 ˝C for 2 h in air. KNO3-CeO2 was prepared by the incipient impregnation method. The slurry (n(Ce)/n(K) = 1:0.2, 1:0.4, 1:0.6, 1:0.8 or 1:1) was kept in a static state for 24 h and dried at 110 ˝C for 5 h. The dried samples ˝ were calcined at 500 C for 3 h. Similar methods were used for the preparation of NaNO3-CeO2, KOH-CeO2, Ca(NO3)2-CeO2, Mg(NO3)2-CeO2 and Ba(NO3)2-CeO2. KBr-KNO3-CeO2 was prepared by both the impregnation method and solid mixed method. The impregnation method was performed as in the KNO3-CeO2 preparation. For the solid mixed method, ˝ the dried solids of KBr and KNO3-CeO2 were mixed evenly and then calcined at 500 C for 3 h. The specific surface area was measured by using nitrogen adsorption (Sorptometer 1900, Carlo Erba Instruments, Milan, Italy). The sample was out gassed at 150 ˝C for 3 h prior to the measurement of the surface area. For calculating the surface area, the BET equation was used. The structural properties of the catalysts were investigated by X-ray diffraction (XRD) (X’Pert Pro MPD, PANalytical, Lelyweg, The Netherlands) using CuKα (40 kV, 50 mA) radiation with 2θ ranging from 0˝ to 80˝ at a scanning speed 0.04˝/3 s. Catalysts 2016, 6, 52 9 of 11

Thermogravimetric (TG) analysis was carried out on a thermogravimetric analyzer (TG209, ´1 Netzsch Co., Wittelsbacherstr, Germany) under a N2 stream of 40 mL min . The temperature was raised at a heating rate of 10 ˝C min´1. Temperature-programmed desorption (TPD) of CO2 was carried out using the Micromeritics Instrument (Auto Chem 2910, Micromeritics Instrument Co., Norcross, GA, USA). Zero-point-one gram of catalyst was placed in a quartz-made U-shaped tube. The catalyst was firstly treated in a flow of helium (50 mL¨ min´1) at 200 ˝C for 2 h to remove any physisorbed organic molecules. The sample ´1 ˝ was then saturated by adsorption of CO2 (50 mL¨ min ) at 60 C for 30 min. Physisorbed CO2 was flushed at 150 ˝C for 1 h. After the sample was cooled, the furnace temperature was increased from 60 ˝C to 800 ˝C at a heating rate of 5 ˝C¨ min´1 under a flow of helium (30 mL¨ min´1). The desorbed CO2 was detected by using a TCD (thermal conductivity detector).

3.3. Catalytic Test All experiments were carried out in a laboratory-scale stainless steel autoclave (Weihai Chemical Machinery Co., Ltd., Weihai, China) with an inner volume of 100 mL equipped with a stirrer and an electric heater. In a standard procedure, 0.3–0.5 g of catalyst, 170 mmol (10 mL) of ethanol and 140 mmol (10 mL) propylene oxide were introduced into the autoclave. The reactor was purged with CO2 at room temperature. Then, the reaction system was pressurized to the predetermined initial pressure with CO2 by a pump (Model SFC-24, SSI/LabAlliance, State College, PA, USA). The amount of CO2 was calculated from measuring the weight difference of the CO2 cylinder before and after ˝ charging the CO2. After the reaction, the reactor was cooled to about 50 C. The CO2 was released slowly through a receiving flask with methanol as the absorbent. Then, the liquid chemicals in the ˝ reactor were extracted in situ by supercritical CO2 at 12 MPa and 50 C. All of the reactants and products were collected using methanol in a flask. The catalyst was left in the reactor. About 240 g CO2 were required to extract all liquid chemicals. For the recycling experiments, the catalyst was used directly after the extraction process by repeating the procedures above.

mole of product i Yield of product Y p%q “ ˆ 100% i 0.5 ˆ mole of ethanol charged

mole of product i Selectivity S p%q “ ˆ 100% i 0.5 ˆ mole of ethanol convered

3.4. Analysis Method The products were analyzed by gas chromatography (GC) with an FID detector (6890N, Agilent Co., Santa Clara, CA, USA) and GC–MS (HP 6890 gas chromatograph coupled with mass detector HP 5973N, Agilent Co., Santa Clara, CA, USA) using an HP-5MS (30 m ˆ 0.25 mm ˆ 0.25 µm, Agilent Co., Santa Clara, CA, USA) fused silica capillary column. n-Propanol was used as the internal standard. The chromatography conditions were the same as our previous studies [14].

4. Conclusions

The direct synthesis of DEC from carbon dioxide, ethanol and PO was developed over KNO3-CeO2 and KBr-KNO3-CeO2 catalyst. Supercritical CO2 was used to extract the products and reactants in site, and the catalysts could be reused. In comparison with the direct synthesis of DEC from CO2 and ethanol, the involvement of PO could improve the formation of DEC, simultaneously with the important chemicals of glycol coproduced. The yield of DEC was improved about 10-fold compared to that using single CeO2 as the catalyst. The yield of DEC was about 13.0% with a selectivity of 38.5% under the optimized conditions with KBr-KNO3-CeO2 as the catalyst, which was close to the equilibrium yield calculated in this work. Catalysts 2016, 6, 52 10 of 11

Acknowledgments: This study is supported by the Youth Fund of the National Natural Science Foundation of China (No. 21106107 and No. 21206077). Author Contributions: Zhen Zhu synthesized the catalysts and performed most of the catalytic reactions, while Yongyue Sun performed some of the charaterisation. The initial draft of the paper was written by Yanlou Wang; while the final copy and corrections made at various stages of this paper was done by the corresponding author, Dongdong Jia. Conflicts of Interest: The authors declare no conflict of interest.

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