Unusual Regularities of Propylene Carbonate Obtained by Propylene Oxide Carboxylation in the + − Presence of Znbr2/Et4n Br System

chemengineering

Article Unusual Regularities of Propylene Carbonate Obtained by Propylene Oxide Carboxylation in the + − Presence of ZnBr2/Et4N Br System

Alexander R. Elman * , Sergei A. Zharkov and Liudmila V. Ovsyannikova Rostkhim Ltd., 123001 Moscow, Russia; [email protected] (S.A.Z.); [email protected] (L.V.O.) * Correspondence: [email protected]; Tel.: +7-499-254-4438

Received: 11 February 2019; Accepted: 28 April 2019; Published: 4 May 2019

Abstract: The aim of the work was to study the unusual regularities of propylene carbonate prepared + by carboxylation of propylene oxide in the presence of the catalytic system ZnBr2/Et4N Br−. Using the kinetic method, a long induction period was detected, followed by the rapid formation of propylene carbonate in a quantitative yield, where the maximum turnover frequency (TOF) values reached 1 21,658 h− . The regularities of the inﬂuence of the main process parameters on the induction period duration and the reaction rate were established. Based on the results obtained and considering the literature, assumptions about the mechanism of the process were proposed and ways for its further study were outlined.

+ Keywords: propylene oxide carboxylation to propylene carbonate; catalytic system ZnBr2/Et4N Br−; unusual regularities of the process

1. Introduction The catalytic carboxylation of α-oxides is one of the most effective methods for producing cyclic carbonates, which are widely used as electrolytes for Li-batteries, solvents, monomers, etc. [1,2]. Amongst the numerous catalysts used for carrying out these processes, from a practical point of view, + zinc compounds in combination with quaternary ammonium salts ZnX2/R4N X− (X = Hal, R = Alk) are very promising [3,4]. It is known that each of the components of these systems, as a rule, is a low-active catalyst in this reaction, but their combined use significantly increases the rate and ensures mild processing conditions. It should be noted that these reactions are typically studied by determining the yield of the final product, whereas the observation of changes in processing parameters (pressure and temperature) over time are usually not carried out. In this work, the preparation of propylene carbonate (PC) by the carboxylation of propylene oxide (PO) was carried out together with the continuous automatic recording of the pressure and temperature using electronic sensors. Since the studied reaction was used to obtain the PC labeled with a stable carbon isotope 13C (an intermediate product in the synthesis of the diagnostic drug 13C-urea [5]) [6–8] as shown in Equation (1), the process was carried out under conditions of CO2 deficiency (relative to PO)ChemEngineering for the economy 2019, 3,of x FOR the labeledPEER REVIEW raw materials. This technique allowed us to reveal several unusual 2 of 10 features of the reaction, the study of which was the purpose of this work.

O 13 O C 13CO O O + 2 (1)

CH3 CH 3 (1)

ChemEngineering 2019, 3, 46; doi:10.3390/chemengineering3020046 www.mdpi.com/journal/chemengineering

2. Materials and Methods

2.1. Chemicals Propylene oxide, zinc bromide, tetrabutylammonium bromide, tetrabutylammonium iodide, and butyltriphenylphosphonium bromide (all 99% purity) were obtained from Acros Organics. Zinc chloride, aluminum bromide (anhydrous), aluminum chloride hexahydrate, potassium iodide, catechol, tetraethylammonium bromide, and triethanolamine were commercial chemicals (reagent grade) and were used without purification. Carbon dioxide 99.99% was obtained from NII KM (Moscow, Russia).

[9]) and a purity of more than 99% on the 1H NMR data in DMSO-d6 (δ, ppm): 1.37 d (3H, CH3, 3JCH– 2 3 2 CH3 6.1 Hz), 4.06 m (1H, not equiv. CH2, JCH2 ≈ JCH–CH2 7.3–8.6 Hz), 4.57 t (1H, not equiv. CH2, JCH2 ≈ 3 JCH–CH2 7.9–8.6 Hz), 4.89 m (1H, CH). To determine the yield of the propylene carbonate, based on the

CO2 loaded, the gas was loaded into the autoclave from a small cylinder and weighed with an accuracy of 0.01 g before and after the process. In a standard experiment, ZnBr2 (33.4 mg, 0.148 mmol) and Et4NBr (122.5 mg, 0.583 mmol) were loaded into the autoclave, and after sealing and evacuating the autoclave, propylene oxide (20 mL, 16.6 g, 285.8 mmol) was added using the siphon. Then, CO2 was supplied from the weighted balloon at a pressure of 15.6 bar such that after the second weighing of the balloon, the CO2 loading was determined, which amounted to 11.92 g (6068.4 mL, 270.9 mmol). Stirring and heating to a temperature of 103 °C commenced, and during the process, changes in pressure and temperature in the autoclave were observed. After 150 min the pressure drop ceased, indicating that the reaction was complete. Next, the autoclave was cooled, and the product was isolated according to the above procedure for analysis. The rest of the experiments were carried out using the same method, the loads and the conditions of which are presented below. ChemEngineering 2019, 3, 46 2 of 11

2. Materials and Methods

2.1. Chemicals Propylene oxide, zinc bromide, tetrabutylammonium bromide, tetrabutylammonium iodide, and butyltriphenylphosphonium bromide (all 99% purity) were obtained from Acros Organics. Zinc chloride, aluminum bromide (anhydrous), aluminum chloride hexahydrate, potassium iodide, catechol, tetraethylammonium bromide, and triethanolamine were commercial chemicals (reagent grade) and were used without puriﬁcation. Carbon dioxide 99.99% was obtained from NII KM (Moscow, Russia).

2.2. Catalytic Experiments The reaction was carried out in a 200 mL stainless steel thermostatted autoclave equipped with sensors for automatic recording of temperature and pressure in 1-min increments. Automatic recording of pressure was carried out using a PD100-DI6.0-111-0.5 sensor, and the temperature was measured with a chromel-copel thermocouple. The signals of the pressure sensor and thermocouple were transmitted to a TRM 200 pressure meter and then through an AC4 automatic converter (all equipment was purchased from “Oven”, Moscow, Russia)—and finally—to the computer. The components of the catalytic system were placed into the autoclave, and after sealing, it was evacuated and propylene oxide was loaded using a siphon from a measuring vessel, and CO2 was supplied at the required pressure, followed by the closure of the gas supply valve. Thereafter, the stirring and heating were turned on, and changes in temperature and pressure during the reaction were registered. Upon process completion (pressure drop stopping), the autoclave was cooled, the unreacted propylene oxide was removed using a water jet vacuum-pump, and the resulting crude product was unloaded. Propylene carbonate was distilled off on a rotary evaporator under vacuum at 82–84 ◦C/3 mm Hg (lit. m.p. 241.7 ◦C/760 mm Hg [9]). 20 The finished product had a nD = 1.4209 (lit. nD = 1.4209 [9]) and a purity of more than 99% on the 1 3 H NMR data in DMSO-d6 (δ, ppm): 1.37 d (3H, CH3, JCH–CH3 6.1 Hz), 4.06 m (1H, not equiv. CH2, 2J 3J 7.3–8.6 Hz), 4.57 t (1H, not equiv. CH , 2J 3J 7.9–8.6 Hz), 4.89 m (1H, CH). CH2 ≈ CH–CH2 2 CH2 ≈ CH–CH2 To determine the yield of the propylene carbonate, based on the CO2 loaded, the gas was loaded into the autoclave from a small cylinder and weighed with an accuracy of 0.01 g before and after the process. In a standard experiment, ZnBr2 (33.4 mg, 0.148 mmol) and Et4NBr (122.5 mg, 0.583 mmol) were loaded into the autoclave, and after sealing and evacuating the autoclave, propylene oxide (20 mL, 16.6 g, 285.8 mmol) was added using the siphon. Then, CO2 was supplied from the weighted balloon at a pressure of 15.6 bar such that after the second weighing of the balloon, the CO2 loading was determined, which amounted to 11.92 g (6068.4 mL, 270.9 mmol). Stirring and heating to a temperature of 103 ◦C commenced, and during the process, changes in pressure and temperature in the autoclave were observed. After 150 min the pressure drop ceased, indicating that the reaction was complete. Next, the autoclave was cooled, and the product was isolated according to the above procedure for analysis. The rest of the experiments were carried out using the same method, the loads and the conditions of which are presented below.

2.3. Product Analysis Analysis of the impurities content in the obtained propylene carbonate was carried out using a Crystallux-4000M gas chromatograph (from “Meta-chrom” company, Yoshkar-Ola, Russia) with a ﬂame ionization detector and a capillary column (25 m 0.2 mm) with a stationary liquid phase SP-1000 × (layer thickness 0.25 µm). Helium was used as the carrier gas (80 mL/min), the column temperature was 200 ◦C, and temperatures of the evaporator and detector were 230 and 200 ◦C, respectively. In addition, 2-methoxyacetophenone was used as the internal standard. PC analysis using NMR was performed on a Bruker AVANCE 600 MHz spectrometer using a WinNMR data processing program from Bruker. ChemEngineering 2019, 3, 46 3 of 11

2.4. Calculations The reaction rate was determined as the maximum gas absorption rate at the inflection point on the graph of the dependence of pressure in the reactor (p), at time (t), using the TableCurve 2D program v5.01.05 (by SYSTAT Software Inc. as in Reference [10]). This was based on the best mathematical description of the indicated dependence p = f (t) and the determination of the first derivative maximum value rmax = dp/dt . − max 3. Results Table1 shows a comparison of the various catalytic systems activity during the synthesis of propylene carbonate under the conditions of obtaining a 13C-labeled product. Even though Table1 shows the inﬂuence of the processing conditions, the nature and composition of the tested catalytic systems on their activity indicators (i.e., turnover number (TON) and turnover frequency (TOF)), as well as the yield of the PC, the nature of the processes taking place over time did not reveal. Therefore, to develop the most reasonable choice for the catalytic system, the reaction was carried out while automatically recording the pressure and temperature in the reactor in 1-min increments. Figure1 shows the dependence of pressure and temperature on time in the presence of various catalyticChemEngineering systems. 2019, 3 Despite, x FOR PEER the REVIEW diﬀerent catalytic activities of these systems (see the TON and 4TOF of 10 values in Table1), the common feature between them was the pressure drop that began even during the heating of the reaction mass (10–20 min after the start of heating). The The reaction reaction completion completion almost lasted for an hour, which was accompanied byby the complete absorption ofof COCO22 and volatile volatile propylene oxide with a yield of high boiling propylene carbonatecarbonate nearnear 100%,100%, whichwhich waswas notnot unusual.unusual.

Figure 1. Pressure (solid lines) and temperature (dashed(dashed lines) changes during the carboxylation of propylene oxide. PO quantity—20 mL; for other conditions see Table1 1 (Runs (Runs 1–4). 1–4).

+ It should be noted that the ZnBr2/Bu4N Br− system was used to develop the industrial process It should be noted that the ZnBr2/Bu4N+Br− system was used to develop the industrial process of of producing 13C-propylene carbonate [7]. Figure1 shows the typical curves of the pressure and producing 13C-propylene carbonate [7]. Figure 1 shows the typical curves of the pressure and temperature changes over time for the chosen processing mode of this system. temperature changes over time for the chosen processing mode of this system. However, using the ZnBr2/Et4N+Br− catalytic system under the same conditions, these dependencies acquired completely different characteristics (Figure 2). These include the appearance of a long induction period (over 60 min), followed by rapid gas absorption, and the release of a noticeable amount of heat (peak on the temperature curve). In this case, the reaction was completed at about the same time, starting from the moment the pressure drop began. Under comparable conditions, the absorption rates of the gases (see the experiment section) for the catalytic systems ZnBr2/Bu4N+Br− and ZnBr2/Et4N+Br− were 3.5 and 4.4 bar/min, respectively.

ChemEngineering 2019, 3, 46 4 of 11

Table 1. Carboxylation of propylene oxide under the conditions of the synthesis of 13C-propylene carbonate.

1 2 3 1 Run Catalyst (mol%) Cocatalyst (mol%) CO2/PO, mol/mol p0 , bar T, ◦C Time, min Yield PC ,% TON TOF , h− + 1 ZnBr2 (0.05) Bu4N Br− (0.30) 0.74 12.0 114 80 97.0 1417 1214 + 2 ZnBr2 (0.05) Ph3BuP Br− (0.28) 0.83 13.7 114 120 98.8 3217 958 + 3 o-C6H4(OH)2 (1.0) Bu4N I− (1.0) 0.89 14.9 104 40 95.7 85 231 4 KI (1.0) (HOCH2CH2)3N (1.0) 0.99 17.0 120 100 97.3 96 115 + 5 ZnBr2 (0.05) Et4N Br− (0.20) 0.95 15.6 103 150 93.8 1713 1285 + 6 ZnBr2 (0.05) Et4N Br− (0.20) 0.37 7.0 103 70 98.5 717 2152 + 7 ZnBr2 (0.05) Et4N Br− (0.20) 1.26 20.0 104 180 74.4 1839 1839 + 4 8 ZnBr2 (0.05) Et4N Br− (0.20) 0.97 15.9 86 >300 94.7 1816 908 + 9 ZnBr2 (0.025) Et4N Br− (0.20) 0.99 16.7 102 180 93.1 3603 1802 + 10 ZnBr2 (0.008) Et4N Br− (0.20) 0.93 15.8 103 300 92.9 11,102 2961 + 11 ZnCl2 (0.06) Et4N Br− (0.20) 0.89 15.4 100 210 99.3 1547 1237 5 12 ZnBr2 (0.05) – 0.93 15.2 102 360 No prod. –– + 13 – Et4N Br− (0.20) 0.93 15.3 104 360 14.5 69 11 + 4 14 AlBr3 (0.07) Et4N Br− (0.20) 0.91 15.4 103 >300 88.3 1211 242 15 AlCl 6H O (0.05) Et N+Br (0.20) 0.90 15.4 104 >300 4 64.5 1129 226 3 × 2 4 − 1 2 3 The volume of PO 20 mL. The initial pressure of CO2 in the autoclave at room temperature. The yield of the isolated product based on the loaded CO2 (see the experiment section). To eliminate the eﬀect of the induction period, the TOF (turnover frequency) was counted from the moment gas began to be absorbed. 4 The process has been interrupted, but the reaction has not ended. 5 No products. ChemEngineering 2019, 3, x FOR PEER REVIEW 4 of 10

the heating of the reaction mass (10–20 min after the start of heating). The reaction completion almost lasted for an hour, which was accompanied by the complete absorption of CO2 and volatile propylene oxide with a yield of high boiling propylene carbonate near 100%, which was not unusual.

Figure 1. Pressure (solid lines) and temperature (dashed lines) changes during the carboxylation of propylene oxide. PO quantity—20 mL; for other conditions see Table 1 (Runs 1–4).

It should be noted that the ZnBr2/Bu4N+Br− system was used to develop the industrial process of ChemEngineering 2019, 3, 46 5 of 11 producing 13C-propylene carbonate [7]. Figure 1 shows the typical curves of the pressure and temperature changes over time for the chosen processing mode of this system.

2 4 ++ − However,However, usingusing thethe ZnBrZnBr2//EtEt4NN BrBr− catalyticcatalytic system system under under the same conditions,conditions, thesethese dependenciesdependencies acquiredacquired completely completely di differentﬀerent characteristics characteristics (Figure (Figure2). 2). These These include include the the appearance appearance of aof long a long induction induction period period (over (over 60 min), 60 followedmin), follow by rapided by gas rapid absorption, gas absorpti and theon, releaseand the of arelease noticeable of a amountnoticeable of amount heat (peak of heat on the (peak temperature on the temperatur curve). Ine curve). this case, In thethis reactioncase, the was reaction completed was completed at about theat about same time,the same starting time, from starting the moment from the the pressuremoment dropthe pressure began. Under drop comparablebegan. Under conditions, comparable the + absorptionconditions, ratesthe absorption of the gases rates (see of the the experiment gases (see section) the experiment for the catalytic section) systems for the ZnBrcatalytic2/Bu 4systemsN Br− 2 4 + − + 2 4 + − andZnBr ZnBr/Bu2N/EtBr4N andBr− ZnBrwere/Et 3.5N andBr 4.4 were bar /3.5min, and respectively. 4.4 bar/min, respectively.

+ Figure 2. Comparison of the catalytic systems ZnX2/Et4N Br− (X = Cl, Br) in the synthesis of PC. Solid lines–pressure; dashed lines–temperature; PO quantity–20 mL; molar ratio CO /PO 0.9 (Table1, 2 ≈ Runs 5, 11).

+ Moreover, the features of the kinetics process in the system with Et4N Br− appeared more clearly when using ZnCl2 (instead of zinc bromide) (Figure2). Under comparable conditions, the induction period increased significantly (from 70 to 135 min) and the gas absorption rate increased by about + three times (from 2.5 to 7.6 bar/min). Thus, by using Et4N Br− in the considered catalytic system + based on ZnX2 (X–halogen), the process ran significantly different to that in the presence of Bu4N Br−. + 1 Interestingly, the maximum TOF value for the ZnCl2/Et4N Br− system was 21,658 h− in terms of the highest gas absorption rate (Figure2, Run 11 in Table1). This was quite large since the greatest activity 1 of the known catalytic system of this process based on porphyrin metal complexes is 46,000 h− [11]. + In the case of the ZnBr2/Et4N Br− system, a change in the initial loading of CO2 and temperature significantly affected the duration of the induction period and the rate of the process. Figure3a shows that with an increasing p0 CO2 from 7 bar to 20 bar, the duration of the induction period is increased 2.5 times reaching about 100 min. At the same time, the process rate varied according to the extremal curve reaching a maximum in the region of the stoichiometric ratio CO /PO 1:1. A temperature 2 ≈ increase (Figure3b) from 86 ◦C to 117 ◦C caused a significant decrease in the induction period (from 185 to 40 min); whilst the temperature dependence of the process rate did not obey the Arrhenius law, which may have indicated a change in the rate-limiting stage at a temperature near 100 ◦C. Plots of these experiments (dependences of pressure and temperature versus time) are shown in Figure A1. To clarify the nature of the induction period and the possibility of generating the active form of the catalyst, an experiment was conducted involving the addition of the reaction mass, formed in the other run during the gas uptake, into the initial mixture of the reaction. For this purpose, the standard experiment was stopped at the start of gas absorption (i.e., at a 10–15% pressure drop level), and the resulting reaction mass at an amount of 7% was added to the initial mixture of the subsequent experiment, where the experiment was carried out under the same conditions. It turned out that such ChemEngineering 2019, 3, x FOR PEER REVIEW 5 of 10

Figure 2. Comparison of the catalytic systems ZnX2/Et4N+Br− (X = Cl, Br) in the synthesis of PC. Solid

lines–pressure; dashed lines–temperature; PO quantity–20 mL; molar ratio CO2/PO ≈ 0.9 (Table 1, Runs 5, 11).

Moreover, the features of the kinetics process in the system with Et4N+Br− appeared more clearly when using ZnCl2 (instead of zinc bromide) (Figure 2). Under comparable conditions, the induction period increased significantly (from 70 to 135 min) and the gas absorption rate increased by about three times (from 2.5 to 7.6 bar/min). Thus, by using Et4N+Br− in the considered catalytic system based on ZnX2 (X–halogen), the process ran significantly different to that in the presence of Bu4N+Br−. Interestingly, the maximum TOF value for the ZnCl2/Et4N+Br− system was 21,658 h−1 in terms of the highest gas absorption rate (Figure 2, Run 11 in Table 1). This was quite large since the greatest activity of the known catalytic system of this process based on porphyrin metal complexes is 46,000 h−1 [11]. In the case of the ZnBr2/Et4N+Br− system, a change in the initial loading of CO2 and temperature significantly affected the duration of the induction period and the rate of the process. Figure 3a shows

that with an increasing p0 CO2 from 7 bar to 20 bar, the duration of the induction period is increased 2.5 times reaching about 100 min. At the same time, the process rate varied according to the extremal ChemEngineering 2019, 3, 46 6 of 11 curve reaching a maximum in the region of the stoichiometric ratio CO2/PO ≈ 1:1. A temperature increase (Figure 3b) from 86 °C to 117 °C caused a significant decrease in the induction period (from an185 additive to 40 min); shortened whilst thethe induction temperature period dependence by almost of three the timesprocess from rate 72 did to 25 not min. obey It wasthe alsoArrhenius found thatlaw, thewhich loading may ofhave the indicated CO2 into thea change mixture in the of the rate-limiting catalyst and stage PO, at heated a temperature to a given near temperature, 100 °C. Plots led toof thethese full experiments disappearance (dependences of the induction of pressure period an (Figured temperature4). versus time) are shown in Figure A1.

Figure 3. Eﬀect of the (a) initial pressure of CO 2 at 103 C and (b) temperature at p CO 2 15 bar on the ChemEngineeringFigure 3. 2019 Effect, 3, xof FOR the PEER(a) initial REVIEW pressure of CO2 at 103 ◦°C and (b) temperature at p00 CO2 15 bar on the 6 of 10 ChemEngineering 2019, 3, x FOR PEER REVIEW + 6 of 10 duration of the induction period and the rate of the process in the presence of ZnBr2/Et4N +Br−− = 1:4. duration of the induction period and the rate of the process in the presence of ZnBr2/Et4N Br = 1:4. PO quantity—20 mL; ZnBr2—0.05 mol% (also see Figure A1). PO quantity—20 mL; ZnBr2—0.05 mol% (also see Figure A1).

To clarify the nature of the induction period and the possibility of generating the active form of the catalyst, an experiment was conducted involving the addition of the reaction mass, formed in the other run during the gas uptake, into the initial mixture of the reaction. For this purpose, the standard experiment was stopped at the start of gas absorption (i.e., at a 10–15% pressure drop level), and the resulting reaction mass at an amount of 7% was added to the initial mixture of the subsequent experiment, where the experiment was carried out under the same conditions. It turned out that such an additive shortened the induction period by almost three times from 72 to 25 min. It was also found that the loading of the CO2 into the mixture of the catalyst and PO, heated to a given temperature, led to the full disappearance of the induction period (Figure 4).

Figure 4. The effect of the PC synthesis method on the kinetics of the process: (1) under standard Figure 4. TheThe effect eﬀect of the PC synthesis method + − on the kinetics of the process: (1) under standard conditions (Run 5 in Table 1, i.e., ZnBr2/Et4N +Br = 1:4. PO quantity—20 mL, ZnBr2—0.05 mol%); (2) conditions (Run 5 inin TableTable1 ,1, i.e., i.e., ZnBr ZnBr/2Et/Et4NN+Br− == 1:4.1:4. PO PO quantity—20 quantity—20 mL, mL, ZnBr ZnBr2—0.05—0.05 mol%); mol%); (2) (2) with the addition of 7% of the reaction 2mass4 formed− at the beginning of gas absorption2 to the initial with the addition of 7% of the reaction mass formed at the beginning of gas absorption to the initial mixture in Run 5 under standard conditions; and (3) CO2 is introduced into the heated mixture of the mixture in Run 5 under standard conditions; and (3) CO 2 isis introduced introduced into into the the heated heated mixture mixture of of the the catalyst and PO (loadings as in Run 5). 2 catalyst and PO (loadings as in Run 5).

4.4. Discussion Discussion 4. Discussion FigureFigure 44 showsshows that that during during the the induction induction period, period, an an active active complex complex of of PO PO with with a a catalyst catalyst is is Figure 4 shows that during the induction period, an active complex of PO with a catalyst is formed.formed. The The subsequent subsequent addition addition of of CO CO2 led2 led to to a arapid rapid drop drop in in the the total total pressure pressure in in the the reactor reactor with with formed. The subsequent addition of CO2 led to a rapid drop in the total pressure in the reactor with thethe formation formation of of the the desired desired propylene propylene carbonate. carbonate. The The results results obtained obtained and and the the data data on on the the influence inﬂuence the formation of the desired propylene carbonate. The results obtained and the data on the influence ofof the the initialinitial pressurepressure of of CO CO2 (Figure2 (Figure3a) indicated3a) indicated that the that active the complexactive complex was formed, wasand formed, apparently and of the initial pressure of CO2 (Figure 3a) indicated that the active complex was formed, and 2 2 apparentlywithout the without participation the participation of CO2. In of this CO case,. In CO this2 couldcase, CO have could a deactivating have a deactivating eﬀect on the effect initial on formthe apparently without the participation of CO2. In this case, CO2 could have a deactivating effect on the initialof the form catalyst, of the for catalyst, example, for by example, reversibly by bindingreversibly zinc binding complexes zinc complexes in a manner in like a manner Equation like (2) Scheme [12,13 ], initial form of the catalyst, for example, by reversibly binding zinc complexes in a manner like Scheme 2 (2)which [12,13], causes which an increasecauses an in increase the induction in the periodinduction duration period with duration the increase with the in increasep0 CO2. Duringin p0 CO the. (2) [12,13], which causes an increase in the induction period duration with the increase in p0 CO2. Duringprocess, the the process, catalytically the catalytically active centers active are centers released, are andreleased, their and concentration their concentration increases increases and, therefore, and, During the process, the catalytically active centers are released, and their concentration increases and, therefore,the reaction the ratereaction increases rate increases as well. It as should well. beIt sh notedould thatbe noted in all that the experimentsin all the experiments presented presented in Figure4 , therefore, the reaction rate increases as well. It should be noted that in all the experiments presented inthe Figure PC yield 4, the was PC nearyield quantitative. was near quantitative. in Figure 4, the PC yield was near quantitative.

+ CO2, + ROH Zn(L) + CO 2 , + ROH Zn(L) O COR Zn(L) OR + CO (2) Zn(L) (base) Zn(L) O2 COR Zn(L) OR + CO2 (base) 2 2 (2)(2)

It was noteworthy that in the experiments performed, the induction period reached 70% of the It was noteworthy that in the experiments performed, the induction period reached 70% of the total duration of the synthesis. Such phenomena are characteristic of autocatalytic processes [14], total duration of the synthesis. Such phenomena are characteristic of autocatalytic processes [14], especially for processes with a “high degree” of autocatalysis and in particular for cubic autocatalysis, especially for processes with a “high degree” of autocatalysis and in particular for cubic autocatalysis, for example, the reaction +2→3+ [15,16] (Figure 5). for example, the reaction +2→3+ [15,16] (Figure 5).

Figure 5. The variation of the reactant concentration, a, and reaction rate, −/, with time for the Figure 5. The variation of the reactant concentration, a, and reaction rate, −/, with time for the cubic autocatalysis according to the equation = described in Reference [16]. cubic autocatalysis according to the equation = described in Reference [16]. ChemEngineering 2019, 3, x FOR PEER REVIEW 6 of 10

Figure 4. The effect of the PC synthesis method on the kinetics of the process: (1) under standard

conditions (Run 5 in Table 1, i.e., ZnBr2/Et4N+Br− = 1:4. PO quantity—20 mL, ZnBr2—0.05 mol%); (2) with the addition of 7% of the reaction mass formed at the beginning of gas absorption to the initial

mixture in Run 5 under standard conditions; and (3) CO2 is introduced into the heated mixture of the catalyst and PO (loadings as in Run 5).

4. Discussion Figure 4 shows that during the induction period, an active complex of PO with a catalyst is formed. The subsequent addition of CO2 led to a rapid drop in the total pressure in the reactor with the formation of the desired propylene carbonate. The results obtained and the data on the influence of the initial pressure of CO2 (Figure 3a) indicated that the active complex was formed, and apparently without the participation of CO2. In this case, CO2 could have a deactivating effect on the initial form of the catalyst, for example, by reversibly binding zinc complexes in a manner like Scheme

(2) [12,13], which causes an increase in the induction period duration with the increase in p0 CO2. During the process, the catalytically active centers are released, and their concentration increases and, therefore, the reaction rate increases as well. It should be noted that in all the experiments presented in Figure 4, the PC yield was near quantitative.

ChemEngineering 2019, 3, 46 + CO2, + ROH 7 of 11 Zn(L) (base) Zn(L) O2COR Zn(L) OR + CO2 (2)

It wasIt noteworthywas noteworthy that inthat the in experiments the experiments performed, performed, the induction the induction period period reached reached 70% of70% the of total the totalduration duration of the of synthesis. the synthesis. Such phenomena Such phenomena are characteristic are characteristic of autocatalytic of autocatalytic processes processes [14], especially [14], especiallyfor processes for withprocesses a “high with degree” a “high of autocatalysisdegree” of autocatalysis and in particular and in for particular cubic autocatalysis, for cubic autocatalysis, for example, forthe example, reaction Athe+ reaction2X 3X +2→3++ C [15,16] (Figure [15,16]5). (Figure 5). →

Figure 5. The variation of the reactant concentration, a, and reaction rate, da/dt, with time for the Figure 5. The variation of the reactant concentration, a, and reaction rate, −/− , with time for the ChemEngineeringcubic autocatalysis 2019, 3, x FOR according PEER REVIEW to the equation r = kCACX described in Reference [16]. 7 of 10 cubic autocatalysis according to the equation = described in Reference [16].

AmongAmong the the known known models models of of autocatalytic autocatalytic processes, processes, a amodel model for for the the mechanism mechanism called called the the “Brusselator“Brusselator model” model” described described in in Reference Reference [15] [15 ](S (Equationcheme 3) (3))corresponds corresponds very very closely closely to this to thiscase: case:

A →X → B↔Y ↔ (3) Y + 2X 3X + C (3) +2→3+→ C + X P, → +→, where A, B, and P mean PO, CO2, and PC, respectively, while X, Y, and C are the intermediate products whereof the A, interaction B, and P betweenmean PO, PO CO and2, and CO PC,with respectively, a catalyst.In while this case,X, Y, theand process C are ratethe intermediatedP/dt follows 2 productsthe equation of ther interaction= f C2 , i.e., between sharp increasesPO and CO in2 the with rate a occurcatalyst. as theIn this X concentration case, the process increases, rate / then X X ( ) followstransforms the equation into the ﬁnal= product , i.e., P. sharp increases in the rate occur as the X concentration increases, then XTo transforms determine into the the nature final of product the intermediate P. products, the various means of α-oxide carboxylation duringTo determine the induction the periodnature were of considered.the intermediate It is knownproducts, that the these various processes means can proceedof α-oxide with carboxylationthe formation during of not the only induction cyclic carbonates, period were but alsoconsidered. polyethers It is and known polyesters, that these i.e., polyglycolsprocesses can and proceedpolycarbonates with the according formation to of Equation not only (4) cyclic [17,18 carb]. onates, but also polyethers and polyesters, i.e., polyglycols and polycarbonates according to Scheme (4) [17,18]. O

OO O CO + 2 R O R (4)

O O

R O R (4) In the absence of water (as in the experiments described above), the synthesis of polyglycols in In the absence of water (as in the experiments described above), the synthesis of polyglycols in solutions of metal salts or quaternary ammonium leads to the formation of crown ethers [19]. On the solutions of metal salts or quaternary ammonium leads to the formation of crown ethers [19]. On the other other hand, polycarbonates formed during the carboxylation of α-oxides can be destructed with the hand, polycarbonates formed during the carboxylation of α-oxides can be destructed with the formation formation of monomeric cyclic carbonates (the “backbiting” mechanism) according to Scheme (5) of monomeric cyclic carbonates (the “backbiting” mechanism) according to Equation (5) [12,20]. [12,20]. O O catalyst CO catalyst O O * + 2 O O

n0 O

backbiting (5) polyether O

Therefore, it could be expected that during the induction period, some of the PO oligomerization products are formed, which then rapidly turn into a target carbonate. In this case, in Scheme (3), X, Y, and C are intermediate structures (or complexes) based on polypropylene glycol, CO2, and polypropylene carbonate, respectively. However, attempts to detect such an oligomeric product for a long induction period (by stopping the synthesis, removal of volatile PO under vacuum, and analysis of the residual mass) have proved to be unsuccessful. Apparently, in our case, during the formation of an active complex of the catalyst and PO, no significant number of high-molecular products were formed, which could be formed in catalytic amounts (see Table 1). This hypothesis is supported by the literature on the catalytic activity of metal complexes with crown ethers [21–23] and polyglycols [24] in α-oxide carboxylation with the formation of cyclic carbonates. The catalytic activity of hydroxyl derivatives of polyglycols has also been reported, even in the absence of metals, due to the hydroxyl protons which form a hydrogen ChemEngineering 2019, 3, x FOR PEER REVIEW 7 of 10

Among the known models of autocatalytic processes, a model for the mechanism called the “Brusselator model” described in Reference [15] (Scheme 3) corresponds very closely to this case: → ↔ (3) +2→3+ +→, where A, B, and P mean PO, CO2, and PC, respectively, while X, Y, and C are the intermediate products of the interaction between PO and CO2 with a catalyst. In this case, the process rate / follows the equation =( ), i.e., sharp increases in the rate occur as the X concentration increases, then X transforms into the final product P. To determine the nature of the intermediate products, the various means of α-oxide carboxylation during the induction period were considered. It is known that these processes can proceed with the formation of not only cyclic carbonates, but also polyethers and polyesters, i.e., polyglycols and polycarbonates according to Scheme (4) [17,18]. O

OO O CO + 2 R O R (4)

O O

R O R In the absence of water (as in the experiments described above), the synthesis of polyglycols in solutions of metal salts or quaternary ammonium leads to the formation of crown ethers [19]. On the otherChemEngineering hand, polycarbonates2019, 3, 46 formed during the carboxylation of α-oxides can be destructed with8 the of 11 formation of monomeric cyclic carbonates (the “backbiting” mechanism) according to Scheme (5) [12,20]. O O catalyst CO catalyst O O * + 2 O O

n0 O

backbiting (5) polyether O

OO (5) Therefore, it could be expected that during the induction period, some of the PO oligomerization Therefore, it could be expected that during the induction period, some of the PO oligomerization products are formed, which then rapidly turn into a target carbonate. In this case, in Scheme (3), X, products are formed, which then rapidly turn into a target carbonate. In this case, in Equation (3), Y, and C are intermediate structures (or complexes) based on polypropylene glycol, CO2, and X, Y, and C are intermediate structures (or complexes) based on polypropylene glycol, CO2, and polypropylene carbonate, respectively. However, attempts to detect such an oligomeric product for polypropylene carbonate, respectively. However, attempts to detect such an oligomeric product for a a long induction period (by stopping the synthesis, removal of volatile PO under vacuum, and long induction period (by stopping the synthesis, removal of volatile PO under vacuum, and analysis analysis of the residual mass) have proved to be unsuccessful. of the residual mass) have proved to be unsuccessful. Apparently, in our case, during the formation of an active complex of the catalyst and PO, no Apparently, in our case, during the formation of an active complex of the catalyst and PO, no significant number of high-molecular products were formed, which could be formed in catalytic signiﬁcant number of high-molecular products were formed, which could be formed in catalytic amounts (see Table 1). This hypothesis is supported by the literature on the catalytic activity of metal amounts (see Table1). This hypothesis is supported by the literature on the catalytic activity of complexes with crown ethers [21–23] and polyglycols [24] in α-oxide carboxylation with the metal complexes with crown ethers [21–23] and polyglycols [24] in α-oxide carboxylation with the formation of cyclic carbonates. The catalytic activity of hydroxyl derivatives of polyglycols has also formation of cyclic carbonates. The catalytic activity of hydroxyl derivatives of polyglycols has also been reported, even in the absence of metals, due to the hydroxyl protons which form a hydrogen been reported, even in the absence of metals, due to the hydroxyl protons which form a hydrogen bond with oxygen in the initial α-oxide [25]. In addition, several studies [2,26] have suggested the activation of CO2 under the action of quaternary ammonium or phosphonium salts, including the action of liberated free amines or phosphines [21,27]. Therefore, in the processes of α-oxide carboxylation in + the presence of ZnX2/R4N X−, small amounts of polyglycols with terminal ammonium groups can be formed [28]. Such products can form active complexes that are capable of activating CO2 using terminal ammonium groups and can also coordinate zinc (similar to crown ethers [21,22]) as a Lewis center for α-oxide activation. Taking into consideration the above, the induction period may be due to the formation of propylene oxide oligomers of a certain size, which are able to coordinate zinc and which can possibly have terminal ammonium groups. This process can proceed together with the gradual release of zinc centers initially deactivated by CO2 (see the dependence of the induction period on p0 CO2). Having reached a required concentration, such complexes catalyze (possibly at a high rate) the process of propylene oxide carboxylation to produce propylene carbonate. In addition, a certain contribution to the formation of the active forms of the catalyst during the induction period can be achieved through the depolymerization of the initial ZnX2 salt and the formation of complexes, for example, the composition + ZnX2(R4N X−)2 [29]. The duration of this process may depend on the nature of the halogen and the quaternary ammonium salt.

5. Conclusions Therefore, unlike many well-known catalytic systems for producing propylene carbonate through propylene oxide carboxylation (including metal salts or organic catalysts in the presence of various + halides), the use of the ZnX2/Et4N Br− system (X = Br, Cl) leads to a longer induction period after which the reaction accelerates with the formation of propylene carbonate. In this case, substantial heat 1 release occurs, and very high catalyst productivity is observed (TOF reaches 21,658 h− in the region of the maximum process rate values). Under selected conditions (closed system, mole loads of CO2 and PO are comparable), the increase in the initial pressure of the CO2 leads to a noticeable increase in the duration of the induction period. Moreover, at a molar ratio of CO /PO 1:1, the rate reaches a 2 ≈ maximum and with a further rise in the CO2/PO, the rate decreases. In addition, with an increase in ChemEngineering 2019, 3, 46 9 of 11

temperature, the induction period is rapidly shortened. The introduction of part of the reaction mass from the gas uptake stage into the initial mixture significantly reduces the induction period which completely disappears when the CO2 is supplied to the heated mixture of the catalyst with PO. Unfortunately, the data obtained has yet to allow us to understand the reasons for such evident + + differences in the behavior of ZnBr2/Et4N Br− from ZnBr2/Bu4N Br−, as well as many other catalytic systems of propylene oxide carboxylation to propylene carbonate [30]. The clarification of the mechanism requires further study using the method of stoichiometric interaction of propylene oxide with catalytic system components, or by using spectral studies of this reaction in situ. One cannot exclude that regularities like the ones found in this work may be inherent to many other catalytic systems that are used in obtaining cyclic carbonates. Therefore, the study of such features would significantly improve the efficiency of these processes by reducing the induction period while maintaining a high rate of the target product formation using simple and cheap catalytic systems.

Author Contributions: A.R.E. conceived and designed the experiments; S.A.Z. and L.V.O. performed the experiments; S.A.Z. and A.R.E. performed the data analysis; and A.R.E. wrote the paper. Funding: This research received no external funding. Acknowledgments: The authors are grateful to Y.N. Belokon, O.N. Temkin, V.N. Sapunov, and Yu.G. Noskov for the useful discussion of this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

ChemEngineering 2019, 3, x FOR PEER REVIEW 9 of 10 Appendix A

Figure A1. Dependences of pressure (solid lines) and temperature (dashed lines) versus time for the Figure A1. Dependences of pressure (solid lines) and temperature (dashed lines) versus time for the data shown in Figure3: effect of the ( a) initial pressure of CO2 at 103 ◦C and (b) temperature at the p0 data shown in Figure 3: effect of the (a) initial pressure of CO2 at 103 °C and (b) temperature at the p + 0 CO2 15 bar on the induction period duration and the process rate in the presence of ZnBr2/Et4N Br− = CO2 15 bar on the induction period duration and the process rate in the presence of ZnBr2/Et4N+Br− = 1:4. PO quantity—20 mL, ZnBr2—0.05 mol%. Peaks on the temperature curves appear when the reaction 1:4. PO quantity—20 mL, ZnBr2—0.05 mol%. Peaks on the temperature curves appear when the rate is maximal (areas of the calculated rmax values are indicated by circles on the gas absorption curves). reaction rate is maximal (areas of the calculated rmax values are indicated by circles on the gas Referencesabsorption curves).

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