Kinetics and Mechanistic Insight into Efficient Fixation of CO2 to

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Kinetics and Mechanistic Insight into Efficient Fixation of CO2 to Epoxides over N‑Heterocyclic Compound/ZnBr2 Catalysts Mengshuai Liu,† Bo Liu,† Shifa Zhong,† Lei Shi,† Lin Liang,‡ and Jianmin Sun*,§,† §

State Key Laboratory of Urban Water Resource and Environment, †The Academy of Fundamental and Interdisciplinary Science, and School of Life Science and Technology, Harbin Institute of Technology, Harbin 150080, P. R. China

‡

S Supporting Information *

ABSTRACT: In this contribution, a series of N-heterocyclic compounds cooperated with ZnBr2 catalyst for chemical fixation of CO2 to cyclic carbonates was developed without utilization of additional organic solvents. It was found that the catalytic activity of N-heterocyclic compounds could be obviously enhanced in the presence of ZnBr2, and the N-methylimidazole (Mim)/ZnBr2 catalytic system was the most efficient among the catalysts employed. Under the optimum reaction conditions, 99% yield of propylene carbonate was achieved with TOF of 474 h−1, and the catalysts were also versatile for CO2 cycloaddition with less active epoxides such as styrene oxide and cyclohexene oxide. Furthermore, a possible synergistic catalytic mechanism was proposed. Moreover, the rate constants were determined as a function of reaction temperature in the range of 130−160 °C, the activation energy was determined to be 41.1 kJ·mol−1, and the kinetic equation on the synthesis of propylene carbonate catalyzed by Mim/ZnBr2 was also obtained. alkali metal salts,3 metal oxides,4 quaternary ammonium salts,5 Schiff base,6 transition metal complex,7 ion-exchange resins,8 and active species supported on natural or synthesized polymers,9 silica,10 zeolite,11 and other materials.12 However, the homogeneous catalysts have problems such as rigorous separation and purification of the products, and the heterogeneous systems are restricted due to the formation of side products as well as harsh reaction conditions, cosolvent requirement, and expensive catalyst. Hence, it is still a challenge to develop efficient, highly selective, stable, and reusable heterogeneous catalysts for the cycloaddition of CO2 to epoxides.13 Since the 2000s, the use of ILs has attracted much interest. Peng and Deng14 first reported the quantitative conversion of propylene oxide (PO) to propylene carbonate (PC) catalyzed by ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4). Afterward, Ikushima et al.15 used 1-octyl-3-methylimidazolium tetrafluoroborate (OMImBF4) as catalyst with supercritical CO2 as solvent and reactant; nearly 100% yield and selectivity of PC were obtained at 140 bar CO2 and 100 °C after 5 min (TOF = 517 h−1). Moreover, theoretical study indicated the formation of hydrogen bonding between catalyst and epoxide could activate the epoxide, thus enhancing the catalytic activity significantly.

1. INTRODUCTION Carbon dioxide is one of the greenhouse effect gases, and it has been attracting much attention as an inexpensive, nontoxic, nonflammable, biorenewable, and highly abundant singlecarbon atom (C1 resource) building block for organic synthesis in recent years.1 Therefore, the chemical fixation of CO2 is of great significance from the viewpoint of better utilization of carbon resources and prevention of global warming. One of the most promising strategies in this area is the synthesis of cyclic carbonates via the cycloaddition of CO2 with epoxides (Scheme 1). Not only is the reaction green for 100% atom efficiency but Scheme 1. Cycloaddition Reaction of CO2 with Epoxides

also the product cyclic carbonates have found wide applications as polar aprotic solvent, high-permittivity component of electrolytes in lithium batteries and intermediate in adhesives, paint strippers, and some cosmetics. However, to date, only a few industrial processes utilize CO2 as raw material, because CO2 is the most oxidized state of carbon, and it has inherent thermodynamic stability and kinetic inertness.2 In past decades, a plethora of catalytic systems for producing cyclic carbonates have been successfully developed, including © XXXX American Chemical Society

Received: October 30, 2014 Revised: December 29, 2014 Accepted: December 30, 2014

A

DOI: 10.1021/ie5042879 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Scheme 3. Chemical Structures of N-heterocyclic Compounds Used in This Work and Their Designations

Therefore, Sun et al.16 developed a series of hydroxylfunctionalized ionic liquids (HFILs) and found to be efficient and reusable catalysts for the synthesis of cyclic carbonates with high selectivity in the absence of any cocatalyst and cosolvent. Similar to hydroxyl groups, carboxylic acid groups as hydrogenbond donors could also accelerate the ring-opening reaction of the epoxide. Zhou et al.17 reported the synthesis of betainebased salts containing quaternary ammonium ion and carboxylic acid groups and detected that the carboxylic acid groups in the catalyst had synergetic effects with halide anion; thus, a high yield and excellent selectivity to cyclic carbonate could be achieved at optimized conditions. To the best of our knowledge, the preparation of ionic liquids is high cost and time consuming and most of the ionic liquids are water or air sensitive. Soon afterward, ceria- and lanthana-doped zirconia (Ce−La−Zr−O) catalyst,18 K2CO3-based binary salts,19 MCM41-immobilized imidazole bromide,20 ZnBr2−Ph4PI binary catalysts,21 and the COOH-functionalized imidazolium-based ionic liquid grafted onto cross-linked polymer22 were found to be excellent catalysts for the synthesis of cyclic carbonates from CO2 and terminal epoxides. Although the insertion of CO2 into epoxides to produce five-membered carbonates has been studied extensively, there is still constant motivation for developing efficient heterogeneous catalysts for the chemical fixation of CO2. Organic nitrogen bases represent another class of metal-free catalysts for the transformation of CO2.23 The most commonly used bases for this purpose are illustrated in Scheme 2.

2. EXPERIMENTAL SECTION 2.1. Materials. Carbon dioxide with purity of 99.99% was purchased from Harbin Qinghua Industrial Gases Co. Ltd. Propylene oxide (99%), 1,2-butylene oxide, styrene oxide, and cyclohexene oxide were purchased from Beijing InnoChem Science & Technology Co., Ltd. N-Methyl pyrrolidone (NMP), pyridine (Py), imidazole (Im), and N-methylimidazole (Mim) were purchased from Aladdin Chemical Co. ZnI2 was purchased from Adamas Reagent Co., Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd.; all chemical reagents were analytic grades and used without further purification. 2.2. Typical Procedure for PC Synthesis from PO and CO2. The cycloaddition reaction of PO and CO2 was conducted in a 50 mL high-pressure stainless-steel reactor equipped with a magnetic stirring bar. In a typical run, the reactor was purged with CO2 to evacuate air and charged with the catalysts and PO (34.5 mmol); then the reactor was heated using an oil bath. At fixed reaction temperature such as 130 °C, CO2 was introduced into the reactor and the pressure was adjusted to 3.0 MPa. The autoclave was heated at that temperature for a designated period of time. After the reaction was completed, the reactor was cooled to 0 °C in an ice−water bath and the remaining CO2 was released. The product was extracted by ethyl acetate and analyzed on a gas chromatograph (Agilent 7890A) packed with a capillary column (Agilent 19091J-413) using a flame ionization detector. After reaction, the catalysts could be separated by adding ethyl acetate. Through subsequent centrifugation, washing, and drying, the spent catalysts were reused for the next run. The interaction between N-heterocyclic compound and CO2 was characterized by FT-IR according to the literature method.25 A 1.0 mmol amount of N-heterocyclic compound was charged into the high-pressure stainless-steel reactor, and the reactor was heated to 130 °C. It was then pressurized with CO2 up to 3.0 MPa and kept for 1 h. Afterward, the reactor was cooled to room temperature and depressurized to atmospheric pressure; the resulting mixture was taken for FT-IR characterization on a PerkinElmer Spectrum 100 FT-IR Spectrometer.

Scheme 2. Structures of Most Frequently Used Bases for CO2 Activation

To our knowledge, the first example for the nitrogen basemediated cycloaddition of CO2 to epoxides was reported by Sartorio et al. in 2003. The guanidine MTBD (7-methyl-1,5,7triazabicyclo [4.4.0]dec-5-ene) and the supported analog (MCM-41 as carrier material) were used as catalysts for the formation of a variety of carbonates including styrene carbonate. However, harsh reaction conditions (140 °C, 5.0 MPa CO2, 4 mol % catalyst) and long reaction time (20−40 h) were necessary to obtain full conversion of styrene oxide (95% yield).24 In our previous work, we confirmed that catalysts composed of N,N-dimethylformamide (DMF) and ZnBr2 were highly effective for the cycloaddition of CO2 to propylene oxide and supposed that DMF containing tertiary nitrogen activated CO2 for further conversion.25 As a continuing work, herein, we present a study on the combination of N-heterocyclic compounds (Scheme 3) and zinc halide for the synthesis of cyclic carbonates from CO2 and epoxides. Moreover, kinetic studies for fixation of CO2 to PO were also performed, and the activation energy of the reaction was estimated, which filled the gaps in the fields of PC synthesis.

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance of N-Heterocyclic Compound/ZnBr2 Catalysts. The activities of different catalysts were tested on the reaction of PO and CO2, and the corresponding results are summarized in Table 1. It was found that the yield of PC was negligible in the case of only ZnBr2 or N-heterocyclic compound, although the N-heterocyclic compound was an excellent Lewis base for activation of CO2 (entries 1−5). However, the combination of ZnBr2 with N-heterocyclic compound realized the obvious enhancements in catalytic activity compared with the separate N-heterocyclic compound, indicating the necessary synergistic effects between N-heterocyclic compound and ZnBr2 on the acceleration of the reaction. The PO conversion approached 94% with 100% PC selectivity under mild, solvent-free conditions of 130 °C and 3 B


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ZnBr2, it was seen that the activity order of the above bases was not in strict accordance with the established pKa, which was Mim ≈ Py > Im > NMP (entries 6−9), which indicated that the basicity of the N-heterocyclic compounds was not the sole important factor; other factors also affected the activity. The activity of Im was lower than Py, which could be conjectured that the formation of hydrogen bonds between N(1)−H in imidazole molecule and N(3) in another imidazole molecule resulted in the decrease of the electron cloud density in imidazole N(3). Besides that, the steric hindrance as well as the effect of the hydrogen bond formed in the structure were also influencing factors on the catalytic activity. In order to demonstrate the formation of carbamate salt between organic base and CO2, FT-IR spectra were employed to identify the possible interaction between N-heterocyclic compound and CO2. As shown in Figure 1, after the reaction of CO2 with N-heterocyclic compounds, there appeared new bands centered at 1796 (Figure 1A), 1800 (Figure 1B), and 1792 cm−1 (Figure 1D), which could correspond to the new asymmetric (CO) vibrations in NMP−CO2, Py−CO2, and Mim−CO2, respectively. However, it was strange that the FTIR spectra showed no difference for Im before and after reaction with CO2 (Figure 1C), from which was it inferred that the existence of a strong hydrogen-bond interaction between Im molecules possibly resulting in the Im−CO2 intermediate was not too stable at room temperature under atmospheric pressure and thus not detected at the present FT-IR conditions. 3.2. Activities of Various Metal Compounds for Cycloaddition of CO2 with PO. The synergistic effects of various metal compounds and Mim on the probe reaction of PO and CO2 were investigated, and the corresponding results are summarized in Table 2. Mim or ZnBr2 alone showed very

Table 1. Synthesis of PC from CO2 and PO over ZnBr2 and Different N-heterocyclic Compound Catalystsa entry

catalyst

conversionb (%)

yieldb (%)

TOFc (h−1)

1 2 3 4 5 6 7 8 9

ZnBr2 NMP Py Im Mim NMP/ZnBr2 Py/ZnBr2 Im/ZnBr2 Mim/ZnBr2

trace trace trace trace trace 46 93 79 94

trace trace trace trace trace 46 92 78 94

153 261 196 272

a Reaction conditions: PO 34.5 mmol, N-heterocyclic compound 0.36 mmol, ZnBr2 0.09 mmol, T = 130 °C, P(CO2) = 3.0 MPa, t = 1.0 h. b On the basis of GC analysis. cMoles of PC produced per mole of ZnBr2 per hour.

MPa CO2 for 1 h (entry 9), and the turnover frequency (TOF) was 272 h−1. It was reported that N-heterocyclic compounds played the role of activating CO2 to form organic base−CO2 carbamate salt with excess amounts of CO2.23,26 Heldebrant et al. reported that the organic base with a lower pKa value had a weaker reaction ability with CO2.27 As shown in Scheme 3, since Mim contains the electron-donating group −CH3, NMP contains the electron-withdrawing group CO, the existence of inductive effects in the molecular structures of N-heterocyclic compounds resulted in the ability of the N-heterocyclic compounds to form carbamate salt with CO2 might decrease in the order of Mim > Im > Py > NMP theoretically. However, via the effects of the employed different N-heterocyclic compounds on the conversion of PO with the presence of

Figure 1. FT-IR spectra of pure N-heterocyclic compound and organic base−CO2 carbamate salt: (A) pure NMP and NMP−CO2, (B) pure Py and Py−CO2, (C) pure Im and Im−CO2, and (D) pure Mim and Mim−CO2. C


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of concentrations led to the optimal yield at 2.5 MPa CO2. In addition, the reusability of Mim/ZnBr2 catalysts was also investigated. On the basis of the reported literature,28−30 Lewis base Mim and ZnBr2 could form the [(Mim)nZnBr2] complex as shown in Scheme 4. After each run, the formed solid

Table 2. Activity of Various Metal Compounds with Mim Catalyst for the Cycloaddition of PO and CO2a entry 1 2 3 4 5 6 7 8 9 10 11 12d e

13

14f

catalyst Mim ZnBr2 Mim/ ZnCl2 Mim/ ZnBr2 Mim/ ZnI2 Mim/ KBr Mim/ FeCl3 Mim/ ZnO Mim/ CaO Mim/ ZnBr2 Mim/ ZnBr2 Mim/ ZnBr2 Mim/ ZnBr2 Mim/ ZnBr2

time (h)

pressure (MPa)

conversionb (%)

yieldb (%)

TOFc (h−1)

2.0 2.0 1.5

2.5 2.5 2.5

trace trace 63

trace trace 61

156

1.0

2.5

96

95

364

1.0

2.5

98

96

368

2.0

2.5

trace

trace

2.0

2.5

17

15

2.0

2.5

trace

trace

2.0

2.5

trace

trace

1.0

2.0

92

92

353

1.0

3.0

97

95

364

2.0

2.5

95

94

180

4.0

2.5

93

91

87

5.0

2.5

88

87

67

Scheme 4. Chemical Structures of [(Mim)nZnBr2] Complex Formed

29

[(Mim)nZnBr2] complex was separated from reaction by adding ethyl acetate by centrifugation, subsequently washed and dried, and then reused for the next batch. It was noted that in the fourth reuse the Mim/ZnBr2 catalysts exhibited high PO conversion and PC yield but at the expense of longer reaction time (entries 12−14). The reduction of catalytic efficiency was partly attributed to the accumulation of Mim−CO2 carbamate salt that hindered further activation of CO2 and partly ascribed to the difficulty in incomplete removal of the cyclic carbonate from the catalysts.31 3.3. Effects of Reaction Parameters. The influence of the reaction parameters was examined in Table 3: the PC yields

a

Reaction conditions: PO 34.5 mmol, Mim 0.36 mmol, metal compound 0.09 mmol, T = 150 °C. bOn the basis of GC analysis. c Moles of PC produced per mole of metal compound per hour. d Second run. eThird run. fFourth run.

Table 3. Effect of Reaction Conditions on the Cycloaddition of PO and CO2a

poor activity in the reaction (entries 1 and 2), and the behavior of Mim with different metal compounds varied considerably. The reaction almost did not take place when Mim combined with KBr, ZnO, and CaO (entries 6, 8, and 9) under the employed reaction conditions. The yield was unsatisfactory over Mim/FeCl3 (entry 7). However, Mim combined with zinc halide showed higher activities than with other metallic halides in accordance with the stronger Lewis acidity of Zn2+ (entries 3−5). In addition, the activity order of zinc halide was found to be ZnI2 ≈ ZnBr2 > ZnCl2, which was consistent with the order of the nucleophilicity of the anion. As there is no significant difference in the catalytic performance between Mim/ZnBr2 and Mim/ZnI2, thereafter, ZnBr2 was selected combined with Mim for further investigation. Due to the necessary synergistic effects of Mim and zinc iodide, the reaction approached 98% PO conversion with 96% PC yield under solvent-free condition at 150 °C and 2.5 MPa CO2 in 1 h. Over Mim/ZnBr2 catalysts, the influence of CO2 pressure on the reaction was investigated under identical reaction conditions (entries 4, 10, and 11); the yield of PC increased with CO2 pressure to 2.5 MPa. In addition, further increase of the CO2 pressure led to an almost unchanged PC yield. The reason for this phenomenon was ascribed to the phase behavior involving CO2-rich gas phase and PO-rich liquid phase in the system. The initial increase of CO2 pressure resulted in the enhanced concentration of CO2 in the liquid phase, thus improving PC yield remarkably. However, the yield kept unchanged with pressure beyond 2.5 MPa up to 3.0 MPa, which was possibly explained that the higher pressure extracted a certain amount of PO into the gas phase, causing the reduction of PO concentration in the vicinity of the catalyst in the liquid phase.14,15 The contradicted effects

entry

molar ratio ZnBr2/ Mim

temp. (°C)

time (h)

yield (%)b

TOF (h−1)c

1 2 3 4 5 6 7 8 9 10 11 12d 13e 14f 15 16 17

1:2 1:3 1:4 1:5 1:6 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:4 1:3 1:4 1:6

120 120 120 120 120 60 90 150 160 150 150 150 150 150 120 120 120

3.0 3.0 3.0 3.0 3.0 3.0 3.0 1.0 0.8 0.8 2.0 1.0 1.0 1.0 6.0 6.0 6.0

68 72 88 89 90 18 54 96 99 92 94 97 93 81 87 91 93

87 92 112 114 115 23 69 368 474 441 180 279 535 621 56 58 59

a Reaction conditions: PO 34.5 mmol, ZnBr2 0.09 mmol, P(CO2) = 2.5 MPa. bOn the basis of GC analysis. cMoles of PC produced per mole of ZnBr2 per hour. dn (ZnBr2) = 0.12 mmol. en (ZnBr2) = 0.06 mmol. f n (ZnBr2) = 0.045 mmol.

increased with the molar ratio of ZnBr2/Mim up to 1:4 at 120 °C, 2.5 MPa CO2, and 3 h, whereas further increase resulted in a slight change in the product yield (entries 1−5). Thus, the optimum molar ratio was 1:4 for ZnBr2/Mim catalysts. At the ZnBr2/Mim molar ratio of 1:4, the effects of reaction temperature, reaction time, and catalyst loadings on PC synthesis were investigated. It was clear that temperature had a notable effect on this transformation. At 60 °C an almost D


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With the increase of the steric hindrance from side chain substituents of epoxides, the catalytic activity was impeded, thus needing higher temperature or longer reaction time to achieve excellent yields. Especially for cyclohexene oxide, both ends of the epoxy ring were connected to the six-membered ring, which hindered the nucleophilic attack from catalysts, causing a decrease of the ring-opening rate. Nevertheless, the product yield of cyclohexene carbonate still reached 90% at 140 °C and 2.5 MPa CO2 pressure for 6.0 h (entry 5). Thus, the extension to various epoxides reflected the outstanding efficiency of the Mim/ZnBr2 catalyst system. 3.5. Possible Reaction Mechanism. It has been proved that epoxides could be activated by zinc halides as the Lewis acid; ZnBr2 and epoxides could form the adduct of the zinc− epoxide complex.25 The N(3) atom of N-methylimidazole (Mim) has a lone pair electron and is also known to activate CO2 involving a zwitterionic adduct between the base and CO2.24 In our catalyst system, Mim and ZnBr2 showed the synergetic effects on the enhanced reaction. The tentative mechanism was suggested in Scheme 5. The C(2)−H groups in

negligible yield of PC was observed, the catalytic activity then increased exponentially from 90 to 150 °C (entries 3, 6−8), and PC yield reached 96% with TOF of 368 h−1. These results were attributed to the increased homogeneity of the catalyst system in the reaction mixture at elevated temperature, thereby furnishing more effective collisions of substrates with the active catlsysts. When the reaction temperature was higher than 150 °C, the PC yield increased slightly (entry 9). Conclusively, 150 °C was the optimal temperature for PC synthesis. Subsequently, by evaluating the dependences of PC yield on the catalyst loadings and reaction time, it was found that the PC yields increased with improving ZnBr2 loadings from 0.045 to 0.09 mmol; with further increased ZnBr2 loadings higher than 0.09 mmol there was no significant increase in the PC yield (entries 8, 12−14). For this catalytic process, an increase of catalyst concentrations did not produce any significant increase in the product yield (entry 8 vs entry 12), probably because of the mass transfer resistances between the active site and reagent caused by the low dispersity of the excess catalysts in the reaction mixture.32 At a fixed molar ratio of ZnBr2/Mim of 1:4, the yields of PC kept unobvious changes among 92−97% with the reaction time extended from 0.8 to 2.0 h (entries 8, 10, 11), which suggested that the thermodynamic factors had a more significant impact than dynamic factors. Moreover, by increasing the molar concentration of catalysts or reaction time, higher PC yields could be achieved at a relatively low temperature of 120 °C (entries 1−5 vs entries 15−17). Therefore, Mim/ZnBr2 catalysts displayed promising application prospect. 3.4. Catalytic Activity toward CO2 Cycloaddition to Other Terminal Epoxides. In order to show the potential and general applicability of the novel Mim/ZnBr2 catalysts, the coupling reactions of CO2 with various epoxides were investigated under the adopted conditions in Table 4. Mim/ ZnBr2 catalysts were found to be effectively versatile for CO2 cycloaddition with different epoxides, yielding the corresponding cyclic carbonates with high yields and selectivity (≥98%).

Scheme 5. Proposed Mechanism for Mim/ZnBr2-Catalyzed Cycloaddition Reaction of CO2 and Epoxides

Table 4. Coupling Reaction of CO2 with Various Epoxides Catalyzed by Mim/ZnBr2a

Mim coordinated with the oxygen of epoxide through hydrogen bonds;33 moreover, ZnBr2 also interacted with the oxygen of the epoxide to form the of zinc−epoxide adduct, resulting in activation of the epoxide molecule. In addition, at the same time, the N(3) atom of Mim and the bromine anion of ZnBr2 as Lewis bases nucleophilicly attacked the less sterically hindered β-carbon atom of the epoxide,34,35 finishing the ring opening of the epoxide easily to produce intermediate 1. In parallel, another Mim might coordinate with CO2, affording a carbamate salt 2 as the activated species of CO2.24,36 Thereafter, intermediate 1 made a nucleophilic attack on the carbamate salt 2 to produce the new alkyl carbonate compound 3. By subsequent intramolecular ring closure, the cyclic carbonate product could be formed and the catalysts were regenerated simultaneously. The synergetic catalytic roles played by Mim and ZnBr2 made the reaction proceed smoothly. By the proposed catalytic cycle, the activity of Mim coupled with

a

Reaction conditions: Epoxide 34.5 mmol, ZnBr2 0.09 mmol, Mim 0.36 mmol, t = 2.0 h. bYPC: PC yield; SPC: PC selectivity; all based on GC analysis. ct = 6.0 h. E


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coefficient R′ were all approximately equal to 1, which indicated that the rate constant had a linear relationship with PO concentration, and the reaction was first order here. Using the Arrhenius equation, the activation energy for the reaction can be calculated from the relationship between the observed rate constant and the temperature, as shown in eq 6

ZnBr2 was higher than ZnCl2, well explained by the stronger nucleophilicity of the anion. 3.6. Activation Energy and Kinetic Equation for PC Synthesis. A dynamic model was developed to investigate the kinetics for the cycloaddition reaction of PO and CO2 over Mim/ZnBr2 catalysts. There were two steps in this process essentially, and the following elementary reaction rate equations can be proposed.37

k = Ae−Ea /(RT )

where A and Ea are the pre-exponential factor (min ) and the activation energy (kJ·mol−1), respectively, R is the universal gas constant (8.314 J·mol−1·K−1), and T is the absolute temperature (K). Taking the natural logarithm of eq 6 gave eq 7

k1

S + IZ ⇀ SIZ ξ ↽⎯⎯⎯

(1)

k−1

k2

SIZ ξ + CO2 → P + IZ

(2)

The first step described the formation of ring-opened intermediate 1, and the second step described formation of cyclic carbonate PC and regeneration of catalyst through intermediate 1 nucleophilic attack to activated CO2, where S, P, IZ and SIZξ represent PO, PC, Mim/ZnBr2 catalyst, and intermediate 1, respectively. k1, k−1, and k2 represent reaction rate constants. To simplify, assuming the reaction is at pseudosteady state, the rate of PC formation was described as given in eq 338 d[P] d[S] − = k[PO] dt dt

(6) −1

ln k = ln A − Ea /(RT )

(7)

The activation energy for the CO2 insertion reaction into PO catalyzed by Mim/ZnBr2 binary catalysts was determined over the range 130−160 °C by fitting the data from a plot of the natural logarithm of the observed pseudo-first-order rate constant (ln k) against the reciprocal absolute temperature (K/T), and the results are shown in Figure 3. Thus, the values

(3)

where d[P] is the PC concentration at a particular reaction time and k is the observed pseudo-first-order rate constant. Integrating eq 3 yielded eq 4 ln([PO]0 /[PO]) = kt

(4)

Defining [PO] = [PO]0(1 − α), α representing the PO conversion, then gave eq 5, and the remaining PO concentration−time profile at different temperatures is shown in Figure S1, Supporting Information.

ln[1/(1 − α)] = kt

(5)

The plot of ln[1/(1 − α)] versus time was linear, as shown in Figure 2. The kinetic equations, correlation coefficient R′, and reaction rate constant k are listed in the Supporting Information (Table S1). It can be seen that the fitted values of the correlation

Figure 3. Arrheniul plot for the determination of activation energy for Mim/ZnBr2 catalysts. Reaction conditions: PO 34.5 mmol, P(CO2) = 2.5 MPa, ZnBr2 0.09 mmol, Mim 0.36 mmol.

of the pre-exponential factor A and the activation energy Ea derived from the studies were 5.768 × 103 min−1 and 41.1 kJ· mol−1, respectively. The kinetic equation on the synthesis of PC catalyzed by Mim/ZnBr2 was obtained at r = −dCPO/dt = 5.768 × 103e−41.1/(RT)CPO.

4. CONCLUSIONS The novel simple and efficient binary systems consisting of Nheterocyclic compounds and ZnBr 2 were employed as effectively versatile catalysts for the synthesis of cyclic carbonates from CO2 and epoxides. Due to the synergistic effects of the acid−base binary system, the excellent yield and selectivity to cyclic carbonates were obtained and the possible synergistic catalytic mechanism was proposed. The catalytic activity order of the employed bases was Mim ≈ Py > Im > NMP, which was not in strict accordance with the established pKa, but the catalytic activities were deeply influenced by the steric hindrance as well as the effect of the hydrogen bond in the structure of N-heterocyclic compounds. The reaction rate

Figure 2. Relationship of ln[1/(1 − α)] with reaction time at different temperatures over Mim/ZnBr2 catalysts. Reaction conditions: PO 34.5 mmol, P(CO2) = 2.5 MPa, ZnBr2 0.09 mmol, Mim 0.36 mmol. F


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constant, activation energy, as well as the kinetic equation on the synthesis of propylene carbonate catalyzed by Mim/ZnBr2 were obtained, and the essential data had a guiding significance for industrial synthesis of cyclic carbonates from CO2 and epoxides.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the kinetic formula deduction procedures for PC formation rate and the remaining PO concentration−time profile at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-451-86403715. Fax: +86-451-86403715. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely acknowledge financial support from the National Natural Science Foundation of China (21373069), Science Foundation of Harbin City (NJ20140037), State Key Lab of Urban Water Resource and Environment of Harbin Institute of Technology (HIT2013TS01), and the Fundamental Research Funds for the Central Universities (HIT, IBRSEM, 201327).

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REFERENCES

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Kinetics and Mechanistic Insight into Efficient Fixation of CO2 to

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