Highly efficient fixation of carbon dioxide to cyclic carbonates with new multi-hydroxyl bis-(quaternary ammonium) ionic liquids as metal-free catalysts under mild conditions

Fuel 224 (2018) 481–488

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Full Length Article

Highly efficient fixation of carbon dioxide to cyclic carbonates with new multi-hydroxyl bis-(quaternary ammonium) ionic liquids as metal-free catalysts under mild conditions ⁎

T

⁎

Jing Penga,b, Sheng Wanga,1, Hai-Jian Yanga, , Binru Bana, Zidong Weib, , Lihua Wanga, Bo Leia a

Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, Key Laboratory of Analytical Chemistry of the State Ethnic Affairs Commission, National Demonstration Center for Experimental Ethnopharmacology Education, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, China b The State Key Laboratory of Power Transmission Equipment & System Security and New Technology, College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon dioxide fixation Multi-hydroxyl ionic liquids Atmospheric pressure Activation energy Selectivity

A series of multi-hydroxyl bis-(quaternary ammonium) ionic liquids were prepared by a simple method, and used as bifunctional catalysts for the fixation of CO2 through the cycloaddition with epoxides in the absence of cocatalyst and solvent. All these ionic liquid compounds were proved to be efficient catalysts for the synthesis of cyclic carbonates from CO2 and epoxides in excellent yield and selectivity. Thanks to synergistic effects of hydroxyl groups and halogen anion, the cycloaddition reaction proceeded smoothly even at atmospheric pressure. The influences of the type of catalyst, catalyst loading, CO2 pressure, reaction time and temperature on the yields have been investigated in detail and the optimal conditions were screened as (120 °C, 2 MPa, 3 h, IL loading 0.25 mol%). Meanwhile, preliminary kinetic investigations were in progress by using four typical catalysts and clarified the activation energies (Ea) of cyclic carbonate formation (37.2 kJ/mol for IL1, 38.1 kJ/mol for IL3, 39.1 kJ/mol for IL5, and 58.9 kJ/mol for IL6), which agrees well with the catalytic activity. The catalyst system could be reused at least five consecutive times successfully. Notably, a high turnover frequency (TOF)

⁎

1

Corresponding authors. E-mail address: [email protected] (H.-J. Yang). Co-first author.

https://doi.org/10.1016/j.fuel.2018.03.119 Received 18 January 2018; Received in revised form 15 March 2018; Accepted 16 March 2018 0016-2361/ © 2018 Published by Elsevier Ltd.

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value of (33,300 h−1) for IL1 was achieved with a high yield of 83.2% within 1 h via adjusting reaction variables. Finally, an inferred mechanism was presented according to the kinetic studies and experimental results.

1. Introduction

bifunctional multi-hydroxyl ILs were synthesized via simple procedure with commercially available 2,2′,2″,2″′-ethanedinitrilotetrakisethanol (THEED) and halogenated hydrocarbon as starting reagents. These easy-to-synthesize multi-hydroxyl bis-(quaternary ammonium) ILs (IL1IL6) could serve as efficient catalysts for the cycloaddition reactions of different epoxides with CO2 in this work (Scheme 1). The catalytic performance has been systematically investigated, and the effects of reaction parameters such as temperature, pressure, time and catalyst loading were studied. A large scope of cyclic carbonates could also be obtained under optimal condition. In addition, these ILs can be easily reused at five consecutive times successfully without significance loss of activity. Furthermore, kinetic studies gave out a possible mechanism along with the reaction activation energy (Ea).

As an abundant, cheap, nontoxic and nonflammable C1 building block, carbon dioxide (CO2) shows potential capability of replacing fossil fuel sources [1–3]. The environmental-friendly and 100% atomeconomic conversion of epoxides and CO2 into cyclic carbonates is one of the most significant ways for CO2 fixation into value added chemicals [4,5]. The products cyclic carbonates of this reaction are among the most important chemical feedstocks that are widely used as polar aprotic solvents, lithium batteries electrolytes, fuel additives and chemical manufacture intermediates [6–9]. Recently many catalytic systems have been developed for the synthesis of cyclic carbonates, such as metal-containing compounds [10–15], organocatalysts, immobilized catalysts [16–19], ionic liquids, bio-mass, and metal–organic frameworks (MOFs) [20–27] etc. Among all reported catalysts for the fixation of CO2 via the coupling reaction with epoxides, ionic liquids (ILs) have attracted much attentions because of high thermal and chemical stability, low vapor pressure, high solubility, easy recyclability, and especially the tunable properties [20–23]. By functionalizing various cationic and anionic ions, a large number of suitable IL-catalysts could be designed and synthesized for different chemical reaction. As to the coupling reaction of CO2 and epoxides, the cation and anion of ILs aim for offering Lewis acid and base site as bifunctionality in view of mechanistic considerations, which would make the catalytic system co-catalyst and solvent-free. However, the activation of CO2 and epoxide is still an urgent issue for this reaction. Many studies have shown that hydrogen bond could activate the epoxides and dramatically enhance the cycloaddition process [24–27]. Kleij et al. developed a phenolic compounds/TBAI catalytic system for CO2 fixation and showed good performance [28]. Zhang et al. synthesized a series of hydroxyl-functionalized poly(ionic liquids) (PILs) as one-component and recyclable catalyst for the coupling reaction of CO2 and epoxides [29]. Then, a binary system pentaerythritol/TBAI was developed by Cokoja et al. and utilized for cycloaddition reaction under mild reaction conditions [30]. Thereafter, various hydroxyl-functionalized ionic liquids were developed as bifunctional catalysts for the reaction under mild and cocatalyst-free conditions [31–33]. Dinaphthyl silanediol/TBAI catalytic system was developed for this reaction even down to the atmospheric condition [34]. Eliminating the complicated synthesized procedure, He et al. developed a cheap and readily available EDTA (ethylenediaminetetraacetic acid)/TBAB system in 2016 and showed excellent catalytic activity [35], which inspired us to design a new type of bifunctional multi-hydroxyl ILs consisting of halide ion as Lewis base for nucleophilic attack and hydroxyl as Lewis acid for active the epoxide within one molecule. In present work, a series of

2. Experimental section 2.1. Chemicals and analytical methods Carbon dioxide was purchased from Wuhan Steel Co. (mass fraction purity of 99.9%). Propylene oxide was purchased from Sinopharm Chemical Reagent Wuhan Co. and distilled with CaH2. All the other reagents and solvents used in the experiments were purchased from J& K Chemical Tech. and used without further purification. 1H and 13C NMR spectroscopy was performed on a Bruker Al-400 MHz instrument using TMS as internal standard. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 6700 FT-IR spectrometer in the range of 600–4000 cm−1 with the samples pressed into KBr. All spectra were recorded at room temperature. All the known compounds were identified by comparison of their physical and spectral data with those in previous reports. 2.2. Preparation of multi-hydroxyl bis-(quaternary ammonium) ILs IL1–IL6 2,2′,2″,2″′-Ethanedinitrilotetrakisethanol (THEED) (5 g, 0.02 mol), iodomethane (MeI) (4 mL, 0.06 mol) and acetone (20 mL) were added to a 50 mL three-necked flask equipped with a magnetic stirrer under the atmosphere of nitrogen. After 12 h reflux, acetone was removed by rotary evaporation. Subsequently, the residual mixture was washed with methanol (3 × 30 mL). The creamy viscous liquid was collected as IL1 and dried under vacuum for 24 h. Yield: 85%. 1H NMR (400 MHz, DMSO) δ 5.38 (m, 4H), 3.96 (m, 4H), 3.90 (m, J = 4.6 Hz, 8H), 3.57 (m, 8H), 3.34 (s, 12H), 3.20 (m, 6H); 13C NMR (101 MHz, DMSO) δ 64.78, 55.25, 51.70, 50.20. Selected IR peaks (KBr, cm−1): ν 2958, 2346, 1618, 1448, 1224, 922; Anal. Calcd for C12H30I2N2O4: C, 27.71; H, 5.81; N, 5.39, Found: C, 27.75; H, 5.88; N, 5.80. ILs 2–6 were prepared using the same procedure employed for the

Scheme 1. Coupling reaction of CO2 and PO catalyzed by bifunctional multi-hydroxyl ILs.

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3. Results and discussion

preparation of IL1 (Scheme 2), except replacing iodomethane with the appropriate alkyl halides (iodoethane for IL2, iodopropane for IL3, iodoheptane for IL4, bromopropane for IL5 and chloropropane for IL6). L2, yellow viscous liquid. Yield: 76%. 1H NMR (400 MHz, D2O) δ 3.80 (m, 8H), 3.60 (m, 4H), 3.46 (m, 8H), 3.04 (m, 4H), 2.91 (m, 8H), 1.23 (m, 6H); 13C NMR (101 MHz, D2O) δ 59.00, 58.19, 54.99, 50.76, 7.44. Selected IR peaks (KBr, cm−1): ν 3629, 2957, 2823, 2342, 1541, 1458, 1204; Anal. Calcd for C14H34I2N2O4, C, 30.67; H, 6.25; N, 5.11, Found: C, 30.64; H, 6.21; N, 5.12. IL3, yellow viscous liquid. Yield: 70%. 1H NMR (400 MHz, D2O) δ 3.97 (m, J = 6.4 Hz, 8H), 3.68 (m, 4H), 3.43 (m, 8H), 3.24 (m, J = 8.0 Hz, 4H), 2.26 (m, J = 6.4 Hz, 4H), 1.73 (m, J = 7.8 Hz, 4H), 1.38 (m, J = 8.6, 8.2 Hz, 4H), 0.91 (m, J = 8.1 Hz, 6H); 13C NMR (101 MHz, D2O) δ 59.13, 58.20, 55.30, 50.86, 23.51, 19.13, 12.99. Selected IR peaks (KBr, cm−1): ν 2949, 2826, 2346, 1654, 1458, 1363, 1036; Anal. Calcd for C16H38I2N2O4, C, 33.35; H, 6.65; N, 4.86, Found: C, 33.37; H, 6.69; N, 4.72. IL4, yellow viscous liquid. Yield: 47%. 1H NMR (400 MHz, D2O) δ 3.97 (m, J = 6.2 Hz, 8H), 3.68 (m, 4H), 3.43 (m, 8H), 3.22 (m, J = 7.9 Hz, 4H), 2.26 (m, 4H), 1.71 (m, J = 8.2 Hz, 4H), 1.29 (m, J = 18.0 Hz, 20H), 0.88 (m, J = 8.2 Hz, 6H); 13C NMR (101 MHz, D2O) δ 60.79, 59.13, 58.12, 55.27, 50.82, 31.01, 28.20, 25.45, 21.98, 21.42, 13.55. Selected IR peaks (KBr, cm−1): ν 2952, 2820, 2745, 1648, 1480, 1381, 1204, 1175; Anal. Calcd for C24H54I2N2O4, C, 41.87; H, 7.91; N, 4.07, Found: C, 41.92; H, 7.94; N, 4.01. IL5, brown viscous liquid. Yield: 69%. 1H NMR (400 MHz, D2O) δ 3.85 (m, J = 7.6 Hz, 8H), 3.57 (m, 4H), 3.27 (m, 8H), 2.90 (m, J = 7.9 Hz, 4H), 2.05 (m, J = 6.5 Hz, 4H), 1.57 (m, J = 8.3, 7.5 Hz, 4H), 1.23 (m, J = 8.7, 8.1 Hz, 4H), 0.79 (m, J = 8.1 Hz, 6H); 13C NMR (101 MHz, D2O) δ 60.75, 59.12, 58.38, 55.38, 23.45, 19.11, 12.96. Selected IR peaks (KBr, cm−1): ν 2924, 2828, 2345, 1698, 1480, 1360, 1298; Anal. Calcd for C16H38Br2N2O4, C, 39.85; H, 7.94; N, 5.81, Found: C, 39.87; H, 8.00; N, 5.78. IL6, brown viscous liquid. Yield: 25%. 1H NMR (400 MHz, D2O) δ 3.97 (m, J = 7.5 Hz, 8H), 3.68 m, 4H), 3.43 (m, 8H), 3.22 (m, J = 8.1 Hz, 4H), 2.26 (m, 4H), 1.75–1.67 (m, 4H), 1.35 (m, J = 7.8 Hz, 4H), 0.87 (m, J = 8.1 Hz, 6H); 13C NMR (101 MHz, D2O) δ 59.00, 57.73, 54.84, 50.49, 25.52, 19.95, 13.70. Selected IR peaks (KBr, cm−1): ν 2949, 2877, 2360, 1685, 1458, 1363, 1288, 1035; Anal. Calcd for C16H38Cl2N2O4, C, 48.85; H, 9.74; N, 7.12, Found: C, 48.83; H, 9.72; N, 7.08.

3.1. Effect of reaction parameters In order to evaluate the catalytic performance and acquire the optimal reaction conditions, various reaction parameters such as reaction temperature, CO2 pressure, reaction time and catalyst loading were systematically studied. As shown in Fig. 1a, the reaction temperature had a prominent positive effect on the PC yield, meanwhile the PC selectivity kept above 99%. According to the literatures [36–38], low temperature would favor for the production of polycarbonate, but it is gratifying that no by-product was detected in the low temperature region. Further increasing the reaction temperature above 120 °C did not lead to a significant improvement of PC yield, indicating that 120 °C was the optimized reaction temperature. CO2 pressure is another crucial parameter to affect the PC yield of this coupling reaction. The PC yield climbed rapidly as pressure increased, reached a maximum in the low pressure region (1 to 2 MPa), yet in the high pressure region above 2 MPa, the PC yield dramatically declined (Fig. 1b). This result may be due to the reduction of CO2 polarity when increasing the CO2 pressure, which would lower the solubility of reactants in CO2 [39,40]. Such phenomenon was also observed in other catalytic systems [41–43]. Thus, 2 MPa was chosen for the optimal CO2 pressure. Fig. 1c shows the effect of PC yield on catalyst loading. The results indicated that the increase of the catalyst loading under a low catalyst amount level 0.1–0.25 mol% resulted in an increase in PC yield, and the yield almost stayed steady when further increase of the catalyst loading to 0.5 mol%. Therefore, 0.25 mol% was considered as the optimal catalyst loading for the reaction. The dependence of PC yield on reaction time was also investigated and clarified in Fig. 1d. The PC yield increased with reaction time in 3 h, and no further increase was observed with prolonged reaction time. Therefore, a reaction time of 3 h was obtained as the optimal choice for the following study. 3.2. Catalytic activity of various catalysts With an optimized reaction condition of (120 °C, 2 MPa, 3 h, IL loading 0.25 mol%) in hand, various catalytic systems were utilized for the coupling reaction between CO2 and PO under identically solventand additive-free conditions. As shown in Table 1, all the bifunctional multi-hydroxyl ILs 1–6 showed excellent yield and selectivity of PC, and catalytic activities of various catalysts were related to their structures (entries 1–6). The order of the activity of halogen anions was found to be I− > Br− > Cl− (entries 3, 5 and 6), which indicated that the ratelimiting step in the catalytic reaction is the PO CeO bond cleavage of epoxide initiated by the attack of halogen anion [44–46]. The PC yield had no big difference with IL1-IL4 (entries 1–4) as catalyst under the same condition. It is suggested that the alkyl chain length of haloalkane has little effect on the reaction. It is also found that catalytic performances of these bifunctional multi-hydroxyl ILs were comparable to the nonfunctional two-component catalytic systems under the identical

2.3. General procedure for the coupling reaction of CO2 and epoxides The coupling reaction of CO2 and epoxides at both atmospheric and high pressure were carried out by employing the identical procedure reported in our published papers [11,14,15], and please refer to the supporting informations for the detail method.

Scheme 2. Synthesis of bifunctional multi-hydroxyl ILs 1–6.

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Fig. 1. Effects of different reaction parameters on PC yield. (a) Effects of reaction temperature, conditions: PO (5 mL, 71.5 mmol), IL1 (0.25 mol%), CO2 pressure 2 MPa, reaction time 3 h. (b) Effects of CO2 pressure, conditions: PO (5 mL, 71.5 mmol), IL1 (0.25 mol%), reaction time 120 °C, reaction time 2 h. (c) Effects of catalyst loading, conditions: PO (5 mL, 71.5 mmol), reaction time 120 °C, CO2 pressure 2 MPa, reaction time 3 h. (d) Effects of reaction time, conditions: PO (5 mL, 71.5 mmol), IL1 (0.25 mol%), reaction time 120 °C, CO2 pressure 2 MPa.

3.3. Cycloaddition of various epoxides and CO2

Table 1 Catalyst screening for the synthesis of propylene carbonate.a Entry

1 2 3 4 5 6 7d 8d 9d 10e 11e

Catalyst

IL1 IL2 IL3 IL4 IL5 IL6 IL1 EDTA/TBAB THEED/TBAB IL-1/TBAI TBAI

To survey the potential and general applicability of these bifunctional multi-hydroxyl ILs, the cycloaddition reactions of CO2 with various epoxides were investigated. As shown in Table 2, the bifunctional multi-hydroxyl IL1 was found to be workable with various epoxides, which have both electron-withdrawing and electron-donating substituents, to give the corresponding cyclic carbonates in excellent yields and selectivity. Due to the steric hindrance, the cyclic carbonate yields of internal epoxide, cyclohexene oxide and isobutylene oxide, were very low (entries 4 and 5), and similar results were also found in other reported literatures [47–50]. As raw materials to form non-isocyanate polyurethanes (NIPUs), bicyclic carbonates are good alternatives of toxic phosgene or isocyanates via reaction with polyfunctional primary amines [51,52]. Present results exhibited that the bifunctional multi-hydroxyl IL1 could also catalyze the cycloaddition of CO2 and diepoxides in excellent yields and selectivities (entries 6 and 7), which demonstrated its potential application value in PU industry. In addition, the coupling reaction between CO2 and a series of glycidyl ethers was also studied under our optimal condition. All the glycidyl ethers could be smoothly converted to their corresponding cyclic carbonates with high yield and selectivity, even for the very challenging unsaturated substrate allyl glycidyl ether (entry 11).

TOF (h−1)c

Catalytic activity Yield (%)b

Sel. (%)b

99.4 99.0 98.8 97.3 97.0 84.1 98.2 94.0 97.3 83.2 18.2

> 99 > 99 > 99 > 99 > 99 > 99 > 99 > 99 > 99 > 99 > 99

133 131 130 130 132 112 – – – 33,300 –

a Reaction conditions: PO (5 mL, 71.5 mmol), catalyst 0.25 mol%, CO2 pressure 2 MPa, reaction temperature 120 °C, reaction time 3 h. b Determined by 1H NMR spectra analysis using TMS as an internal standard. c Turnover frequency for PC calculated as mole of PC produced per mole of catalyst per hour. d Reaction conditions: PO 5 mL, catalyst 5 mol%, co-catalyst 5 mol%, CO2 pressure 0.5 MPa, time 18 h, temperature 70 °C. e Reaction conditions: PO 5 mL, catalyst 0.0025 mol%, co-catalyst 0.5 mol%, CO2 pressure 2 MPa, time 1 h, temperature 120 °C.

condition (entries 7 vs 8 and 9), and our catalyst loading could be even lower after optimization (entries 1 vs 8 and 9). Besides, no additive or solvent was needed herein. To our delight, the maximum TOF of bifunctional catalyst IL-1/TBAI could reach up to 33,300 h−1 (entry 10), which was also comparable with other efficient metal-free catalysts [35].

3.4. Conversion of CO2 under atmospheric pressure to cyclic carbonate Until now, despite many catalytic systems have been developed for the cycloaddition of CO2 and epoxides, it is still a great challenge to run the reaction under atmospheric pressure (0.1 MPa) [53–58]. In order to develop an easy-to-handle and cost-cutting catalytic system, we 484

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Table 2 Various cyclic carbonates synthesis catalyzed by IL1.a Yield (%)b

Selectivity (%)b

1

99

99

2

97

99

3

67

98

4c

94

98

5d

10

99

6

91

98

7d

94

99

8

99

98

9

99

98

10d

98

99

11

98

98

Entry

a b c d

Epoxide

Product

Reaction conditions: epoxide (71.5 mmol), IL1 0.25 mol%, CO2 pressure 2 MPa, reaction temperature 120 °C, reaction time 3 h. Determined by 1H NMR spectra analysis using TMS as an internal standard. Reaction time 20 h. Reaction time 10 h.

Fig. 2. Cycloaddition between CO2 and various epoxides catalyzed by IL1 at atmospheric pressure. Reaction conditions: 5 mL epoxide, 0.5 mol% catalyst loading, reaction temperature 120 °C. The selectivity for products are all > 99%. ●: tert-Butyl glycidyl ether; ▲: phenyl glycidyl ether; ▾: isopropyl glycidyl ether; ◄: allyl glycidyl ether; ■: butyl glycidyl ether; ▾: 1,2-ethanediol diglycidyl ether; ♦: epichlorohydrin.

Fig. 3. The stability of IL1 in the cycloaddition of CO2 to BGE. Reaction conditions: BGE (5 mL), IL1 (0.5 mol%), reaction temperature 120 °C, pressure 0.1 MPa, (> 99% PC selectivity is maintained). *: 5 mL BGE was re-added into the reaction mixture.

atmospheric pressure within 12 h. Satisfyingly, 1,2-ethanediol diglycidyl ether could also produce its bicyclic carbonate efficiently under as mild as atmospheric CO2 pressure. By contrast, epichlorohydrin showed a slightly slower conversion under identical atmospheric pressure, probably because of the electron withdrawing CH2Cl group, which could result in the reduced electron density of the oxygen atom in the epoxide ring. Some other literatures also reported similar results [59,60,39].

evaluated the catalytic performance of bifunctional multi-hydroxyl IL1 under 0.1 MPa CO2 using various epoxides as the benchmark substrates. As depicted in Fig. 2, most epoxides exhibited good conversion to their corresponding cyclic carbonates under atmospheric pressure. In particular, all the glycidyl ethers, i.e., tert-butyl glycidyl ether (BGE), phenyl glycidyl ether, isopropyl glycidyl ether, allyl glycidyl ether and butyl glycidyl ether, could convert to cyclic carbonates in good yield under 485

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3.5. The stability of the catalytic system It is vital to measure the catalyst stability and longevity because they have much applications for industrial processes. It can be seen from Fig. 3 that the complete conversion of BGE under atmospheric pressure could be accomplished within 6 h. It’s not feasible to separate the IL catalyst directly by standard method because the catalytic system is homogenous. Thus, the re-employment of bifunctional multi-hydroxyl IL1 was operated by adding another prescribed amount of BGE after each run rather than removing IL1 or cyclic carbonate from the reaction mixture according to our reported method [61]. As shown in Fig. 3, the catalytic activity of IL1 could be maintained after relatively as long as 6 × 10 h. The loss of activity was probably ascribed to the following reasons: (1) reverse shift of chemical equilibrium and the corresponsive decrease of active sites with the accumulation of generated cyclic carbonate, (2) the loss of the catalysts resulted from sampling process, and (3) the dequaternization at high temperature [62–64]. This activity loss could be compensated by adding a small amount of catalyst (about 30% of the initial amount of IL1).

Fig. 4. Arrhenius plots for the formation of cyclic carbonate from the cycloaddition of CO2 and BGE using various catalysts.

3.6. Comparison of various IL catalytic systems The performance of various IL catalytic systems for coupling reaction of PO and CO2 is depicted in Table S1. Other ILs have to be performed in either high catalyst loading or high CO2 pressure and temperature. By contrast, bifunctional multi-hydroxyl ILs in this work need no co-catalyst, small catalyst loading and considerable mild reaction conditions. Moreover, this easy-to-synthesize IL has a long catalyst lifetime. To our delight, high TOF value of 33,300 h−1 could also be achieved by this type of IL catalyst. The good catalytic performance of these bifunctional multi-hydroxyl ILs is strongly dependent on the synergistic effect between the multi-hydroxyl group as a hydrogen bond donor and halide anion as a nucleophile, which provided more ideas for the design of catalysts in future work. Fig. 5. The activation energies (Ea) for the cycloaddition of CO2 and BGE catalyzed by IL1, IL3, IL5 and IL6.

Scheme 3. Proposed mechanism for the coupling reaction of CO2 with epoxide catalyzed by IL1.

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3.7. Kinetic studies on the cycloaddition of CO2 to BGE Following our previous work about the kinetic studies on this cycloaddition reaction [15,61], the activation energies Ea of four catalysts (IL1, IL3, IL5 and IL6) have been calculated according to the Arrhenius rule of thumb from the apparent rate constants at various temperatures and the results were illustrated in Fig. 4 (please refer to the supporting information for details). The apparent activation energy Ea for IL1 is only 37.2 kJ/mol, while the Ea (IL3) is 38.1 kJ/mol, Ea (IL5) is 39.1 kJ/ mol, and Ea (IL6) is 58.9 kJ/mol (Fig. 5). The sequence of Ea agrees well with the catalytic activities in Table 1. The best catalytic activity obtained by IL1 could probably attribute to the higher Lewis acidity of tertiary amine and stronger leaving ability of I− [65,66].

[3]

[4] [5]

[6] [7]

[8]

3.8. Reaction mechanism [9]

According to the reported literatures [67–74] and kinetic studies, an anion-cation synergetic catalysis mechanism for the bifunctional multihydroxyl ILs was shown in Scheme 3. Firstly, the epoxide was activated by hydrogen bond interaction between hydroxyl group of IL and oxygen of epoxide, which would accelerate the ring opening step [75–80]. Next, the nucleophilic I- attacked the less hindered carbon atom of epoxide followed by ring opening, and then CO2 inserted, following by an intramolecular ring-closure step to product cyclic carbonate and simultaneously regenerate the catalyst IL. The hydrogen bond in this process helped to stabilize the intermediates, which would facilitate the conversion.

[10]

[11]

[12]

[13]

[14]

4. Conclusions

[15]

In summary, a series of bifunctional cheap multi-hydroxyl ILs were designed and synthesized via simple procedure to be organocatalysts for the cycloaddition of CO2 and epoxides without any co-catalyst or solvent. The effects of catalyst structure and reaction parameters on the catalytic activity were investigated systematically. Above 99% PC yield and nearly 100% selectivity could be obtained using IL1 as catalyst under optimal conditions (120 °C, 2 MPa, 3 h, IL loading 0.25 mol%). Additionally, the ILs could also be employed to produce various cyclic carbonates with excellent yield and selectivity under atmospheric pressure. This series of catalyst can effectively activate epoxides through a synergistic effect of multi-hydroxyl groups and the halide anion. The development of this type of ILs as organocatalysts represents an easy-to-handle, low-cost, long-term continuous use and metal-free process of CO2 conversion to high value-added products.

[16]

[17]

[18]

[19]

[20] [21]

Acknowledgements

[22]

We acknowledge gratefully the financial support from National Natural Science Foundation of China (No. 51073175), Natural Science Foundation of Hubei Province, P. R China (No. 2016CKB704) and the Fundamental Research Funds for the Central Universities, SouthCentral University for Nationalities (CZP17066). Authors are also grateful to Prof. Buxing Han for his valuable help and the financial support of Beijing National Laboratory for Molecular Sciences (BNLMS).

[23]

[24]

[25]

[26] [27]

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2018.03.119.

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