1 Introduction

Carbon dioxide is the most abundant waste produced by human activities and is one of the most important greenhouse gases. As a C1 resource, however, CO2 is recognized to be naturally abundant, inexpensive, recyclable, and nontoxic source [1–3]. Under these circumstances, chemical fixation of CO2 has become more important for both the ecological and economic advantage. The reaction of coupling carbon dioxide and epoxides to generate five-membered cyclic carbonates (Scheme 1) is one of the most promising route for the utilization of CO2 as a C1 building block in organic synthesis [4, 5]. Significantly, cyclic carbonates can be used in many application areas such as solvents, monomers, valuable raw materials and intermediates in the production of pharmaceuticals and fine chemicals [6–8].

Scheme 1
scheme 1

The synthesis of cyclic carbonates from epoxides and carbon dioxide

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In the last decades, numerous homogeneous and heterogeneous catalysts have been developed for the insertion of carbon dioxide into epoxides to form cyclic carbonates [9]. Such examples include alkali metal salts [10, 11], metal oxides [12], Schiff bases [13, 14], transition metal complexes [15–19], ion-exchange resins [20], quaternary ammonium and phosphonium salts [21–26], gold nanoparticles supported on resins [27], cross-linked polymeric nanoparticles [28] and ionic liquids [29–37]. Although the insertion of CO2 into epoxides to produce five-membered carbonates has been studied extensively, there is still constant motivation for developing efficient catalysts for the chemical fixation of CO2.

Among the catalysts mentioned above, ionic liquid is one of the most important catalyst for the cycloaddition of CO2 with epoxides. Peng and Deng [29] reported that ionic liquids could be used as catalyst for the synthesis of cyclic carbonates without using solvent. Although the reaction could be conducted at low temperature (100 °C), the catalytic activity was low. Recently, Xia et al. [32] described that this reaction rate was significantly accelerated when the Lewis acidic metal salt was added into the BMImBr ionic liquid.

The acid-functionalized ionic liquids have received much attention for their potential application in replacing conventional acidic catalysts [38]. In 2002, Cole et al. published an article about the synthesis of sulfonic acid group functionalized ionic liquids with strong Brønsted acidity [39]. Until now, the acid-functionalized ionic liquids have been successfully used as catalyst and/or solvent for the protection of carbonyl groups [40], dimerization of α-methylstyrene [41], the Prins reaction [42], reductive amination of carbonyl compounds [43], selective oxidation of alcohols [44] and hydroamination of alkenes [45]. In previous literature, Shi et al. [46] found that phenol could act as a Brønsted acid to accelerate the ring opening reaction of epoxides by the hydrogen bond in the reaction of carbon dioxide and epoxides to form cyclic carbonates. In the present work, the COOH- and SO3H-functionalized Brønsted acidic ionic liquids were synthesized (Fig. 1) and used as catalysts. Mediating with metal salts, their catalytic performances were investigated in the coupling of carbon dioxide and epoxides to form cyclic carbonates without using co-solvent. The reaction conditions were investigated and a possible mechanism was proposed.

Fig. 1
figure 1

The acid-functionalized ionic liquids

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2 Experimental

2.1 Materials and Instruments

All the chemicals were commercially available and were used without further purification. 1H NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer with TMS as the internal standard. The products were analyzed by a HP 6890/5973 GC-MS and a gas chromatography (GC, Agilent 6820) equipped with a flame-ionized detector (FID).

2.2 Synthesis of the Brønsted Acidic Task-Specific Ionic Liquids

The ionic liquids used in this article (Fig. 1) were synthesized according to previous methods [39, 47].

2.2.1 Preparations of COOH-Functionalized Ionic Liquids

Brønsted acidic ionic liquid (V) was prepared as follows: 1-methylimidazole (7.9 mL, 0.1 mol), 2-chloroacetic acid (9.45 g, 0.1 mol) and toluene (30 mL) were charged into a 100 mL round bottom flask. Then the mixtures were refluxed for 24 h. Then the mixture was cooled down to room temperature, and the toluene was dumped. The ionic liquid (V) was washed repeatedly with ether and dried in vacuum. The 1H NMR characterizations of Brønsted acidic ionic liquid (V) were provided as follows: 1H NMR (DMSO-d6, δ/ppm): 3.92 (s, 3H), 5.20 (s, 2H), 7.75 (s, 1H), 7.77(s, 1H), 9.24 (s, 1H), 13.77 (s, 1H); 13C NMR (DMSO-d6, δ/ppm): 168.39, 138.15, 124.15, 123.60, 50.20, 36.35; FT-IR (cm−1): 3163, 2580, 2490, 1735, 1573, 1169, 774, 681, 639.

The synthetic procedure of ionic liquids I–IV was similar to that of ionic liquid V. The main difference was the corresponding halide substituted carboxylic acid was used when other ionic liquids were prepared.

2.2.2 Preparation of SO3H-Functionalized Ionic Liquids

1-Methylimidazole (7.9 mL, 0.1 mol) and 1,4-butane sulfone (10.3 mL, 0.1 mol) were charged into a 100 mL round bottom flask. Then the mixtures were stirred at 40 °C for 10 h. The white solid zwitterion was washed repeatedly with ether to remove non-ionic residues and dried in vacuum. Then, a stoichiometric amount of H2SO4 (5.5 mL, 0.1 mol) was dropped and the mixture stirred for 6 h at 80 °C. The obtained viscous liquid was washed with ether for three times and dried in vacuum to form ionic liquid. The 1H NMR characterizations of Brønsted acidic ionic liquid (VI) were provided as follows: 1H NMR (D2O, δ/ppm): 1.46–1.54 (m, 2H), 1.75–1.82 (m, 2H), 2.71 (t, 2H), 3.65 (s, 3H), 4.01 (t, 2H), 7.20(s, 1H), 7.25 (s, 1H), 8.49(s, 1H); 13C NMR (D2O, δ/ppm): 20.43, 27.58, 35.24, 48.38,49.62, 121.70,123.21, 135.36; FT-IR (cm−1): 3157, 2960, 1576, 1450, 1194, 1171, 1044, 883, 748.

2.3 Coupling Reactions of Carbon Dioxide and Epoxides

The coupling reaction was carried out in a 50 mL stainless steel autoclave equipped with a magnetic stirrer. For each typical reaction process: ionic liquid (0.67 mmol), metal salts (0.048 mmol) and propylene oxide (1a) (10 mL) were charged into the reactor without using any co-solvent. The reactor vessel was placed under a constant pressure of carbon dioxide and then heated to 110 °C for 1 h. Then the reactor was cooled to ambient temperature, and the resulting mixture was transferred to a 50 mL round bottom flask. By distillation under vacuum, the product propylene carbonate (1b) was obtained as a colorless liquid. The cyclic carbonates were identified on a GC-MS (HP6890/5973). The catalyst was separated from the resulting mixture by distillation under vacuum and reused directly.

3 Results and Discussion

3.1 Effects of Varying the Catalytic System

In previous research, we found that the poor catalytic activity was shown when ionic liquid or ZnX2(X = Cl, Br) was used as the sole catalyst in the coupling of carbon dioxide and propylene oxide [32]. In the reaction of carbon dioxide and epoxide, the Zn tetrahalides was formed from ionic liquid and ZnX2(X = Cl, Br) which act as the catalyst [31], and Xie et al. [48] found that the molar ratio of ionic liquid to metal salt affected the catalytic activity of catalytic system and the yield of cyclic carbonates, so the influence of the molar ratio on the synthesis of propylene carbonate was investigated and the results were listed in Table 1. When the ionic liquid (I) was used as the sole catalyst in the coupling reaction of carbon dioxide and propylene oxide, the yield of propylene carbonate was achieved 33.6%, it was because the propylene oxide was activated by the hydrogen bond with carboxyl of ionic liquid (I) (Table 1, entry 17); but the yield was low because the Lewis acid was absence which could active propylene oxide effectively. And the yield of propylene carbonate was trace with using ZnBr2 as the sole catalyst (Table 1, entry 18). Fixing the molar amount of ZnCl2 at 0.048 mmol and varying the amount of ionic liquid (I) from 0.29 to 0.77 mmol (Table 1, entries 1–5), the yield of propylene carbonate was increased when the molar ratio of ionic liquid (I) to ZnCl2 was enhanced and the turnover frequency (TOF) of the catalyst based on Zn2+ was achieved at 3,270 h−1 while increasing the amount of ionic liquid (I) to 0.67 mmol (Table 1, entry 5). When the amount of ionic liquid (I) was increased from 0.67 to 0.77 mmol, the yield of propylene carbonate was improved slightly. Therefore, the optimum amount of ionic liquid (I) was 0.67 mmol in this catalytic system when the amount of ZnCl2 was fixed at 0.48 mmol.

Table 1 The effects of catalytic system on chemical fixation of carbon dioxide to form propylene carbonate
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The effects of different metallic salts on the chemical fixation of carbon dioxide to form propylene carbonate were also investigated and the results were summarized in Table 1. From Table 1, it can be seen that the yield of cyclic carbonate was greatly influenced by the different metallic ions (Table 1, entries 5–9) and hardly affected by the valency of the same metallic element (Table 1, entries 6 and 7). The results showed the order of activity to be Zn2+ > Fe3+ > Cu2+ ~ Cu+. Among various metallic cations which were used in the reaction, the reactivity of Zn2+ showed the highest catalytic activity. This is probably due to Zn2+ being a strong Lewis acid [49], which is beneficial to the ring-opening of propylene oxide [50]. In the meantime, the activity of different anions in zinc salts were tested in the synthesis of cyclic carbonates and the activity of anions decreased in the following order: Br− > I− > Cl− > SO4 2−. Among the examined anions, Br− was shown to give the highest TOF. It was probably because of Br− having a moderate nucleophilicity and the fact that it is a good leaving group; these could be a good compromise the nucleophilicity and leaving ability of the anion [48]. As such, ZnBr2 was chosen as the catalyst for the coupling of carbon dioxide and epoxides to form cyclic carbonates.

In order to investigate the influence of the ionic liquid on the catalytic activity, several Brønsted acidic ionic liquids were prepared (Fig. 1). The catalytic activity of zinc bromide combined with various Brønsted acidic ionic liquids were evaluated by the coupling reaction of carbon dioxide and propylene oxide, and the results were listed in Table 1. From Table 1, it can be seen that the catalytic systems, which used halides as anions, exhibited good catalytic activities (entries 11–16). The results indicated that the activity of halides were higher than HSO4 − and the activity decreased in the order Br− > Cl− > HSO4 − (Table 1, entries 5, 9, 11). Among these ionic liquids, ionic liquid (I) was the most effective co-catalyst in this study. Furthermore, the catalytic activities of ionic liquids were sensitive to their bulkiness of cations and the order of activity was (I) > (II) ~ (III) > (IV) ~ (V). This was because of the different strength of hydrogen bond between the proton of carboxyl group in the ionic liquid and the oxygen of the propylene oxide. So, in order to obtain a higher activity, the condign acidity was necessary.

3.2 Optimum Reaction Condition

A significant drawback associated with the use of carbon dioxide as the reagent in organic synthesis is the potential danger associated with operating under high temperature and pressure. So the effects of temperature and pressure of the catalytic system were investigated and the results were shown in Figs. 2 and 3.

Fig. 2
figure 2

The effects of reaction temperature on the synthesis of propylene carbonate: ionic liquid 0.67 mmol, ZnBr2 0.048 mmol, propylene oxide 10 mL, reaction pressure 1.5 MPa, reaction time 1 h

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Fig. 3
figure 3

The effects of reaction pressure on the synthesis of propylene carbonate: ionic liquid 0.67 mmol, ZnBr2 0.048 mmol, propylene oxide 10 mL, reaction temperature 110 °C, reaction time 1 h

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The coupling reaction of propylene oxide and carbon dioxide was performed under different reaction temperatures. From Fig. 2, we found that the activity of this catalytic system was sensitive to reaction temperatures. When the reaction temperature was lower than 110 °C, the TOF of catalytic system and the yield of propylene carbonate were significantly improved with increasing reaction temperature. However, increasing the reaction temperature from 120 to 140 °C resulted in only a slight increase in the yield of propylene carbonate and TOF. From the results mentioned above, increasing the reaction temperature within a certain range is propitious for accelerating the reaction rate of chemical fixation of carbon dioxide to form propylene carbonate.

Under the varied reaction pressure, the reaction was carried out and the results were shown in Fig. 3. When the reaction was carried out at 0.5 MPa, the yield of propylene carbonate was 14.5%. With the increase in the carbon dioxide pressure, the yield of propylene carbonate and TOF were greatly improved, and the highest yield of propylene carbonate was found between 1.5 and 2.0 MPa. Increasing the pressure beyond 2.0 MPa resulted in a slight decrease in activity and an optimum CO2 pressure was shown to exist in this reaction [4, 51]. It can be inferred that the introduced CO2 dissolves in propylene oxide or “liquefies” through the formation of a CO2–propylene oxide complex. Hence, excessively high CO2 pressure could have retarded the interaction between propylene oxide and the catalyst, thus resulting in low catalytic activity [52]. It was noteworthy that the synthesis of propylene carbonate was carried out under ambient temperature and CO2 atmosphere, but the reaction rate was very slow [14, 53].

When the catalyst of Brønsted acidic ionic liquids-ZnBr2 is compared with other catalyst system in the literature [31, 46, 57] for the coupling of carbon dioxide and epoxides, these reported values of TOF are lower than that obtained in the present work. The reaction temperature for the PhOH-organic base catalyst [46], but the reaction pressure is higher by 3.57 MPa than the pressure in the present work and the organic solvent of 1,2-dichloroethane was needed. A high yield of propylene carbonate is obtained with tetrahaloindate(III)-based ionic liquids catalyst [57] at low carbon dioxide pressure (100 psi); however, the high loading of ionic liquid was necessary. Compared to the catalyst of ZnX2/hexabutylguanidinium salts [48], the similar TOF for propylene carbonate synthesis was obtained.

3.3 Coupling Carbon Dioxide and Other Epoxides

Under the optimum reaction conditions, a series of epoxides were used as substrates for the synthesis of corresponding cyclic carbonates. From the results, which were summarized in Table 2, the catalytic system was efficient for the mono-substituted terminal epoxides (Table 2, entries 1, 2) and shown higher than 80% yield and TOF more than 2,400 h−1. When styrene oxide (4a) was used as the substrate, 59.3% yield of the corresponding cyclic carbonate (4b) and TOF of 1,779 h−1 were achieved (Table 2, entry 3). In general, the styrene oxide was a bulky epoxide and its conversion was low compared with that of propylene oxide, it was probably due to the low reactivity of the β-carbon atom of styrene oxide. Additionally, we also examined cyclohexene oxide (5a) as the substrate for cycloaddition under the same reaction conditions; the corresponding cyclic carbonate (5b) was obtained with a 207 h−1 TOF (Table 2, entry 4).

Table 2 The effects of catalytic system on chemical fixation of carbon dioxide to form propylene carbonate
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3.4 The Reusability of the Catalyst

After the reaction, propylene carbonate and propylene oxide were distilled off from the product mixture, the remaining catalyst was reused for further reactions directly and the results were listed in Table 2. When the catalyst was used as third time, the yield of propylene carbonate was obtained at 94.2% and TOF was achieved at 2,826 h−1. It was shown that the catalytic system could be reusable up to three times and no significant drop in the yield of propylene carbonate was observed (Table 2, entries 5 and 6).

3.5 Possible Mechanism for the Coupling of Carbon Dioxide and Epoxides

Based on the above results and previous literature[11, 29, 54–57], we propose the plausible mechanism for the chemical fixation of CO2 in the presence of the Brønsted acidic ionic liquids mediated metallic salts catalytic system (Scheme 2). It is widely recognized that at first the epoxide is coordinated to the Lewis acidic Zn2+ to form the metal-epoxide adduct complex; at the same time, the epoxide was activated by the hydrogen bond with carboxylic acid group of ionic liquid in the presence of the Brønsted acidic ionic liquids mediated metallic salts catalytic system; followed by the X− anion (Lewis base) of the catalyst opening the epoxy ring, which was stable by the Brønsted acidic ionic liquid cation through hydrogen bonding with its carboxylic acid group and electronic interaction with the ionic liquid’s cation and Zn2+ to give the intermediate (3). Next, the insertion of CO2 into the intermediate (3) would give a carbonate active species (4), which eventually afforded the cyclic carbonate and regenerated the catalyst. Hence in this catalytic system, a synergetic effect of hydrogen bond donors (–COOH), ionic liquid cation, Zn2+ and halide anion was shown which could be the key to promote the reaction.

Scheme 2
scheme 2

The possible mechanism for the coupling reaction of carbon dioxide and epoxides

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4 Conclusions

In summary, the Brønsted acidic ionic liquids mediated metallic salts catalytic system was developed and it was found to be an effective catalyst for the synthesis of cyclic carbonates from carbon dioxide and epoxides under mild conditions without any co-solvent. Due to the synergetic effect of carboxylic acid group in ionic liquid, Zn2+ and halide anions, excellent yield of cyclic carbonates and high TOF were achieved in the presence of catalyst 1-(1-ethyl)carboxylethyl-3-methylimidazolium bromine/ZnBr2. The catalytic system can be reused at least three times without noticeable decrease in activity and selectivity.