Bis-ammonium immobilized polystyrenes with co-catalyzing functional end groups as efficient and reusable heterogeneous catalysts for synthesis of cyclic carbonate from CO₂ and epoxides

Abstract

A series of novel bis-ammonium ionic liquid immobilized polystyrene (BisAm-i-PS) heterogeneous catalysts with co-catalyzing functional end groups of –CH₂COOH, –CH₂CH₂OH, and –CH₂CH₂NH₂ were synthesized for the cycloaddition reaction of CO₂ and epoxides without any additional co-catalyst and organic solvent. All the ammonium immobilized catalysts showed high selectivity. Bis-ammonium immobilized catalysts were more efficient than single-ammonium ones. The yield was further increased by introducing the functional end groups. Both –CH₂COOH and –CH₂CH₂OH terminated BisAm-i-PS catalysts achieved high activities (yield: >99%), while the –NH₂ terminated one had a slightly lower activity (yield: 97.3%) under the same reaction conditions. The effects of different parameters, such as the halide anions of the catalysts, reaction temperature, initial pressure and reaction time were also investigated with –CH₂CH₂OH terminated BisAm-i-PS (BisAm-OH-i-PS) as the catalyst. Under optimal reaction conditions (1.2 MPa, 130 °C and 2.5 h), BisAm-OH-i-PSs with 2Br⁻ and 2I⁻ showed very high efficiencies. For each of them, propylene carbonate selectivity and yield were more than 99% and 99%, respectively. In addition, the catalyst possessed good stability, and both catalytic activity and selectivity were always more than 99% for 5 times of recycle tests.

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Introduction

Carbon dioxide has been considered as a renewable carbon resource, which has the advantages of abundant reserves, nontoxicity and low cost.¹ One of the most promising ways to utilize CO₂ is its fixation to generate commodity chemicals such as cyclic carbonates, which is a significant and good example of 100% atom economy.² At present, cyclic carbonates play an increasingly important role in our society. They are widely used as battery electrolytes, precursors for polycarbonate, chemical intermediates, engineering plastics and excellent aprotic polar solvents.³

Over the past few decades, a wide range of homogeneous and heterogeneous catalysts including alkali metal salts,⁴ organometallic complexes,^5–12 organic bases,^13,14 ionic liquids (ILs),^15–18 metal oxides,^19–21 modified molecular sieves,^22,23 supported ILs,^24–30 ion-exchange resins³¹ and calcined hydrotalcite³² have been developed for the coupling of epoxides and carbon dioxide. The catalytic activity is usually efficient with homogeneous catalysts system, however, the separation of the catalysts from products would be a difficult work, and what's more, co-solvents or co-catalysts were usually required. Apparently, the application of heterogeneous catalysts make the separation easier, but the stability and reusability, especially the poor catalytic activity is urgently need to be improved. Therefore, to develop a kind of catalyst with high activity, low-cost, well reusability, and easy separation is still a challenge.

Recently, ionic liquids including quaternary ammonium salts,^33–38 quaternary phosphonium salts,^39–41 imidazolium salts,^25,42–44 pyridinium salts,⁴⁵ guanidinium salts^46–48 and other types of ILs^47,49 attract researchers' much interest because of their high catalytic activities, especially the functionalized ionic liquids, which is fabricated with hydroxyl, amidogen, carboxyl and so on. Sun et al. reported that hydroxyl-functionalized quaternary ammonium salts and imidazolium salts shown high activity for the synthesis of cyclic carbonate.⁵⁰ And then they made immobilization of it with polystyrene resin to get a reusable and efficient catalyst. Furthermore, a series of functionalized ILs were got and supported with different carriers.^25,38,39

Compared to single cation ILs, dication ILs exert better activities in physical and chemical.^35,53 They are widely used in high temperature solvents, lubricants, and catalysts in transesterification and esterification.^54,55 However, to the best of our knowledge, there are few reports about bis-functionalized heterogeneous catalysts for cyclic carbonate synthesis from carbon dioxide and epoxides.^33,55 Here, a series of bis-ammonium immobilized polystyrene resins with the co-catalyzing end groups of hydroxyl, carboxyl and amino groups were synthesized (Scheme 1). Then, these novel dication functionalized catalysts were used for the synthesis of propylene carbonate from CO₂ and epoxide without any solvent and co-catalyst (Scheme 2). The influence of catalyst structure, the types and amount of nucleophilicity anions, and the reaction conditions of temperature, initial pressure and reaction time were also studied in detail.

Scheme 1 The synthesis of bis-ammonium immobilized polystyrene heterogeneous catalysts.

Scheme 2 Synthesis of cyclic carbonates from epoxides and carbon dioxide.

Experimental

General information

The products were analyzed by gas chromatograph (Techcomp 7900) that equipped with a FID detector and a DB-wax capillary column (30 m × 0.25 mm × 0.25 μm). Nuclear magnetic resonance (NMR) spectra were performed on Bruker 500 AVANCE II, using CDCl₃ as a solvent. Fourier transformation infrared (FT-IR) spectra were obtained on a Nicolet iN10 of Thermo Fisher. Scanning electron microscopic (SEM) observations were carried out over a NOVA NanoSEM 450 microscope. Energy dispersive X-ray spectroscopy (EDS) was performed using accessory (INCA 250) of the NOVA NanoSEM 450 instrument. X-ray diffraction (XRD) patterns were obtained on a XRD-7000S operating at 40 kV and 40 mA using Ni-filtered Cu Kα radiation (λ = 1.542 Å). The XRD patterns were recorded in the 2θ range of 5° to 80°. Thermal gravimetric analysis (TGA) was performed on a TGA/DSC1 of Mettler Toledo. The content of active components was determined by Elementar Vario EL III element analysis.

Chemicals

Spherical chloromethylated polystyrene resin (CMPS, 1 mm) with 17% Cl content and 7% cross-linked with divinylbenzene was purchased from the Chemical Plant of NanKai University (China). CO₂ was commercially available with a purity of 99.99% and be further purified by a filter. Propylene oxide was purchased from Sinopharm Chemical Reagent Co., Ltd and purified by distillation before the reaction. Other epoxides were purchased from J&K Chemical. 2-Bromine ethanol, 2-bromoethylamine hydrobromide, 2-bromoacetic acid and 1,4-diazobicyclo[2.2.2]octane (DABCO) were purchased from Aladdin Industrial Corporation. The other chemical solvents were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. without any further treatment.

Catalysts preparation

First, CMPS (1 g) was swelled in 30 mL acetonitrile for 12 h, and then a solution of DABCO (10 mmol) in 20 mL acetonitrile was added to the reaction system and reacted for 24 h at 30 °C, after that, the product was filtered and washed three times with diethyl ether and ethyl acetate, then dried at 65 °C under vacuum for 12 h to get the product [Am-i-PS]Cl. As for this reaction, the product is almost [Am-i-PS]Cl because that the reaction sites are nearly on the surface of the carrier and DABCO is also far excess, besides, the further reaction can be hard when one end of DABCO is reacted. After the [Am-i-PS]Cl was again swelled for 12 h, 2-bromine ethanol (10 mmol) was added into the system and the reaction solution was refluxed for 24 h. The obtained product was [BisAm-OH-i-PS]ClBr, then the catalyst was immersed into saturated NaBr solution at room temperature for 12 h to perform ion-exchange. After that, the product was filtered and washed three times with deionized water, diethyl ether and ethyl acetate, then dried at 65 °C under vacuum for 12 h to get the product of [BisAm-OH-i-PS]Br₂. And the catalysts of [BisAm-COOH-i-PS]Br₂ and [BisAm-NH₂-i-PS]Br₂ were prepared in the same procedure. For the synthesis of catalyst [BisAm-OH-i-PS]I₂, [BisAm-OH-i-PS]ClBr was immersed into saturated NaI solution to perform ion-exchange at nitrogen atmosphere. And for the synthesis of catalyst [BisAm-i-PS]Cl₂, when the catalyst [Am-i-PS]Cl was swelled for 12 h, methyl iodide (10 mmol) was added into the system at 40 °C to react for 12 h, the obtained product was [BisAm-i-PS]ClI, then the catalyst was immersed into saturated NaCl solution to perform ion-exchange to get the product [BisAm-i-PS]Cl₂. Besides, the catalysts [BisAm-i-PS]Br₂ and [BisAm-i-PS]I₂ were prepared in the same way.

The synthesis process and chemical structure of bis-ammonium immobilized polystyrene heterogeneous catalysts are shown in Scheme 1. FT-IR, SEM, XRD and elemental analysis were used to confirm the synthesis of bis-ammonium immobilized polystyrene. The thermal stability of the catalyst was measured by thermo gravimetric analysis (TGA) at a heating rate of 10 °C min⁻¹ with temperature ranging from 25 °C to 600 °C under a nitrogen atmosphere.

General procedure for the cycloaddition reaction

The cycloaddition reactions were conducted in a 100 mL high-pressure stainless-steel with magnetic stirring. In the typical reaction procedure, 0.75 mol% (calculated according to the amount of ionic liquid) of the catalyst was added into the reactor, then pressurized to 5 MPa of CO₂ pressure and then released it, after repeating the above steps three times, epoxide (21.5 mmol) was injected in the reactor and we finally pressurized it to the desired pressure at a specified temperature. After reaction, the reactor was chilled to −10 °C in low-temperature pump and CO₂ was liberated slowly. The product was segregated from the mixture by paper-filter and analyzed by GC. The products were identified by ¹H NMR with the internal standard of TMS.

¹H NMR data of the cyclic carbonates in Table 2 are as follows:

4-Methyl-1,3-dioxolan-2-one (Table 2 entry 1). ¹H NMR (500 MHz, TMS CDCl₃): δ = 1.50 (d, CH₃, 3H), 4.05 (t, CH, 1H), 4.57 (t, CH₂, H), 4.88 (m, CH₂, H);

4-Ethyl-1,3-dioxolan-2-one (Table 2 entry 2). ¹H NMR (500 MHz, TMS CDCl₃): δ = 0.99 (t, CH₃, 3H), 1.47 (m, CH₂, 2H), 4.10 (dd, CH, 1H), 4.48 (dd, CH₂, H), 4.65 (m, CH₂, H);

4-Chloromethyl-1,3-dioxolan-2-one (Table 2, entry 3). ¹H NMR (500 MHz, TMS CDCl₃): δ = 3.73–3.82 (dd, CH₂, 2H), 4.26 (dd, CH, 1H), 4.63 (dd, CH₂, 1H), 4.95 (m, CH₂, 1H);

4-Phenyl-1,3-dioxolan-2-one (Table 2, entry 4). ¹H NMR (500 MHz, TMS CDCl₃): δ = 4.37 (t, CH, 1H), 4.72 (t, CH₂, 1H), 5.83 (t, CH₂, 1H), 7.36–7.48 (m, Ar–H, 5H);

4, 5-Tetramethylene-1,3-dioxolan-2-one (Table 2 entry 5). ¹H NMR (500 MHz, TMS CDCl₃): δ = 1.65–1.72 (m, CH₂, 2H); 1.99–2.08 (m, CH₂, 2H); 2.21–2.27 (m, CH₂, 4H); 4.96 (m, CH, 2H).

Results and discussion

Catalyst characterization

FT-IR spectra of [BisAm-OH-i-PS]Br₂, [BisAm-COOH-i-PS]Br₂, [BisAm-NH₂-i-PS]Br₂ and CMPS are shown in Fig. 1. As seen in Fig. 1(a), a typical peak at 1265 cm⁻¹ is the stretching frequency of the functional group –CH₂Cl. It disappears in the spectra of [BisAm-COOH-i-PS]Br₂ (Fig. 1(b)), [BisAm-OH-i-PS]Br₂ (Fig. 1(c)) and [BisAm-NH₂-i-PS]Br₂, that indicates the introduction of functional group DABCO. And in comparison with chloromethylated polystyrene resin, the newly appeared peaks centered at 1460 cm⁻¹, 2920 cm⁻¹ are the typical peaks of C–H bonds of DACBO. These results suggest the complete conversion from chloromethyl groups to ammonium groups. In addition, as shown in Fig. 1(b), [BisAm-COOH-i-PS]Br₂ exhibits the characteristic bands of C=O at 1700 cm⁻¹ and C–O–C at 1220 cm⁻¹, and the [BisAm-OH-i-PS]Br₂ displays a typical strong peak corresponding to O–H stretching frequency centered at about 3401 cm⁻¹ and C–OH stretching frequency centered at 1052 cm⁻¹ in Fig. 1(c). The peak centered at 3441 cm⁻¹ corresponds to the antisymmetric amino, which indicated the successfully modified of –NH₂ functional group of the catalyst.⁵⁶ Based on these, it is indicated that [BisAm-COOH-i-PS]Br₂, [BisAm-OH-i-PS]Br₂ and [BisAm-NH₂-i-PS]Br₂ were successfully synthesized.

Fig. 1 FT-IR spectra of (a) CMPS, (b) [BisAm-COOH-i-PS]Br₂, (c) [BisAm-OH-i-PS]Br₂ and (d) [BisAm-NH₂-i-PS]Br₂.

The elemental analysis (C, H, and N) as shown in Table 1 was also employed to confirm the introduction of functional groups. The nitrogen contents of the [BisAm-OH-i-PS]Br₂, [BisAm-COOH-i-PS]Br₂ and [BisAm-NH₂-i-PS]Br₂ are lower than [Am-i-PS]Br, but the [BisAm-NH₂-i-PS]Br₂'s is higher than [BisAm-OH-i-PS]Br₂ and [BisAm-COOH-i-PS]Br₂, which suggests the successfully modified of functional groups.

Table 1 Elemental analysis (CHN) of catalysts

Entry	Catalyst	C (wt%)	H (wt%)	N (wt%)
1	CMPS	74.06	6.46	0.067
2	[Am-i-PS]Br	70.22	7.90	8.69
3	[BisAm-OH-i-PS]Br2	51.26	6.66	5.54
4	[BisAm-COOH-i-PS]Br2	52.9	6.71	5.73
5	[BisAm-NH2-i-PS]Br2	59.60	7.27	8.49

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The SEM images of the [BisAm-OH-i-PS]Br₂ and CMPS were shown in Fig. 2. The surface of resin is roughened with the grafting of [BisAm-OH]Br₂, which is ascribed to the loss of crystallinity. Fig. S1 (ESI†) shows the XRD curves of CMPS and [BisAm-OH-i-PS]Br₂. CMPS exhibits a peak at 2θ = 20.1° ascribed to diffraction at the crystalline region of the polymer structure. With the grafting of functional group, the catalyst [BisAm-OH-i-PS]Br₂ shows a broadened peak centered at 2θ = 22.0°, suggesting an amorphous structure.⁵⁵ This phenomenon may be caused by the collapse of regular carrier and also can be an evidence for the successful synthesis of the catalyst. The EDS spectra of [BisAm-OH-i-PS]Cl₂, [BisAm-OH-i-PS]Br₂ and [BisAm-OH-i-PS]I₂ were observed to confirm the successfully progress of ion-exchange.^52,57 To make sure the accuracy of the test, the measurement area of the catalysts are more than 200 × 200 μm². As shown in Fig. S3 (ESI†) the percentage of Cl/Br, Br/Cl and I/Cl are all above 99%.

Fig. 2 SEM images of CMPS and [BisAm-OH-i-PS]Br₂.

TGA and DTG curves of [BisAm-OH-i-PS]Br₂ are shown in Fig. 3. [BisAm-OH-i-PS]Br₂ has two weight loss steps characterized by the onset decomposition temperature, T_OD, and the fastest decomposition temperature, T_FD. The first step (T_OD: 215 °C, T_FD1: 251 °C) is ascribed to the degradation of the side chain ammonium groups. The second one (T_FD2: 307 °C) is attributed to the decomposition of the polystyrene resin. The thermal stability of bis-ammonium immobilized polystyrene heterogeneous catalyst is high enough for the application in the cycloaddition reaction.

Fig. 3 TGA and DTG curve of [BisAm-OH-i-PS]Br₂.

Catalytic performance

The synthesis of propylene carbonate (PC) from CO₂ and propylene epoxide (PO) according to cycloaddition reaction was chosen as the model to text the catalytic activity of different catalyst systems. All of the reactions were taken under 130 °C, 1.2 MPa and 2.5 h, and corresponding results were listed in Table 2. As shown in Table 2, CMPS, 2-bromopropane–ethanol and DABCO have poor activities and selectivities for the reaction. All the ammonium immobilized heterogeneous catalysts have high selectivity of above 99%. The two single-ammonium immobilized PS heterogeneous catalysts of [Am-i-PS]Cl and [Am-i-PS]Br show the yields of 40.5% and 60.3%, respectively. It indicates that DABCO-based ammonium could activate CO₂ in the reaction as a kind of organic base and the halide ion can promote the ring-opening reaction of PO. Besides, the catalyst [Am-i-PS]Br which contains bromide ion behaves better than that of [Am-i-PS]Cl, indicating the importance of nucleophilic attack ability of anion. [BisAm-i-PS]Br₂ shows higher yield than [Am-i-PS]Br does. It is because that the ammonium content is two times for [BisAm-i-PS]Br₂ than [Am-i-PS]Br. By using [BisAm-OH-i-PS]Br₂ as the catalyst, the PC yield comes up to 98.7%, which is ascribed to a great promotion of the hydroxyl functional group of [BisAm-OH-i-PS]Br₂ to the ring-opening of epoxide.⁵⁸ [BisAm-COOH-i-PS]Br₂ and [BisAm-NH₂-i-PS]Br₂ also exhibit high catalytic activities. It might be because of the hydrogen bonding between these three functioned groups and epoxides. [BisAm-OH-i-PS]Br₂ and [BisAm-COOH-i-PS]Br₂ show similarly high activities. The yield for each of them is above 97%. With different halide anions, the activity order of the catalysts is I + I > Br + Br > Br + Cl > Cl + Cl, which is accordance with the nucleophilicity order of halides. It indicates that the stronger the nucleophilicity is, the easier the attack at the β-carbon atom of epoxide is. All these results suggest that the synergistic effect of polarization caused by hydrogen bonding and the more nucleophilic attacks by the double halide anions accounts for the excellent activity.

Table 2 Catalytic performance of various catalysts^a

Entry	Catalyst	Catalytic results
		Yield (%)	Selectivity (%)
a Reaction conditions: PO 21.5 mmol, catalyst 0.75 mol%, initial CO2 pressure 1.2 MPa, temp. 130 °C, time 2.5 h.b CMPS 0.1 g.c Equal catalyst amount (0.75 mol%).
1	CMPS^b	1.7	98.2
2	BrC2H4OH^c	5.3	97.5
3	DABCO^c	6.1	95.7
4	[Am-i-PS]Cl	40.5	99.5
5	[Am-i-PS]Br	60.3	99.5
6	[BisAm-i-PS]Br2	71.4	99.2
7	[BisAm-i-PS]I2	83.8	99.2
8	[BisAm-OH-i-PS]Cl2	81.6	99.3
9	[BisAm-OH-i-PS]ClBr	92.5	99.2
10	[BisAm-OH-i-PS]Br2	99.1	99.2
11	[BisAm-OH-i-PS]I2	99.3	99.3
12	[BisAm-COOH-i-PS]Br2	99.3	99.3
13	[BisAm-NH2-i-PS]Br2	97.3	99.4

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Effect of reaction parameters

Fig. 4 shows the catalytic activity of [BisAm-OH-i-PS]Br₂ as a function of temperature. Apparently, the performance of [BisAm-OH-i-PS]Br₂ is closely bound with temperature. The yield of PC sharply increases from 21.0 to 96.9% when the temperature rises from 90 to 120 °C, and with further rising to 140 °C, there is a slight increase in the yield of PC. However, the selectivity of PC is not correlated with temperature. The selectivity is always above 99% at 90–140 °C. It indicates that this type of ammonium salt has a good selectivity of the cycloaddition reaction.

Fig. 4 PC yield and selectivity as a function of reaction temperature (reaction conditions: PO 21.5 mmol, [BisAm-OH-i-PS]Br₂ 0.75 mol%, initial CO₂ pressure 1.2 MPa, 2.5 h).

Generally, the CO₂ pressure is one of the significant factors impacting on the cycloaddition reaction.^30,41,57,59 The influence of initial CO₂ pressure was studied under the condition of 130 °C with [BisAm-OH-i-PS]Br₂ as the catalyst and the reaction time of 2.5 h. Fig. 5 shows the PC yield and selectivity as a function of initial pressure. The yield increases from 92.6 to 99% with increasing initial pressure from 0.3 to 1.2 MPa. However, when the initial pressure was further improved from 1.2 MPa to 3.0 MPa, the yield of PC decreases. A possible explanation for this phenomenon is: in the liquid phase of the reaction system, the CO₂ concentration increases with adding initial system pressure, however, too high pressure would reduce the PO concentration around the catalyst, causing a decline of catalytic activity. The selectivity is almost irrelevant to the initial pressure of CO₂.

Fig. 5 PC yield and selectivity as a function of reaction pressure (reaction conditions: PO 21.5 mmol, [BisAm-OH-i-PS]Br₂ 0.75 mol%, 130 °C, 2.5 h).

Fig. 6 represents the influence of reaction time on PC yield and selectivity at 130 °C and initial CO₂ pressure of 1.2 MPa. It is found that the yield increases with reaction time from 0 to 2.5 h, and then there is almost no change in the yield after 2.5 h. It indicates that a reaction time of 2.5 h is required for the complete conversion from PO to PC. The selectivity is always above 99%.

Fig. 6 PC yield and selectivity as a function of reaction time (reaction conditions: PO 21.5 mmol, [BisAm-OH-i-PS]Br₂ 0.75 mol%, 130 °C, initial CO₂ pressure 1.2 MPa).

Fig. 7 shows the influence of catalyst dosage on the catalytic activity. It was found that the PC yield increased with catalyst dosage from 0.3 mol% to 0.75 mol%, however, there is a slight increase in the yield of PC when the dosage of catalyst is further improved. It is indicated that the catalyst dosage of 0.75 mol% is suitable for the reaction in this condition. Besides, the selectivity of PC is always more than 99%.

Fig. 7 PC yield and selectivity as a function of reaction dosage of catalyst (reaction conditions: PO 21.5 mmol, [BisAm-OH-i-PS]Br₂, 130 °C, 2.5 h, and initial CO₂ pressure 1.2 MPa).

Catalyst recovery

With PO as the substrate, the recyclability of [BisAm-OH-i-PS]Br₂ were carried out at 130 °C, initial CO₂ pressure 1.2 MPa for 2.5 h. In each run, the catalyst was recovered by filtration. As shown in Fig. 8, the [BisAm-OH-i-PS]Br₂ catalyst exhibited excellent efficiency for 5 consecutive cycles. Both the yield and selectivity are always above 99%. The results show that the [BisAm-OH-i-PS]Br₂ catalyst is highly stable and very promising for commercial application. Besides, the characterization of FT-IR and TGA were employed as shown in Fig. S4 and S5 (ESI†) to test the [BisAm-OH-i-PS]Br₂ after five times recycle. The recovered catalyst showed the similar character to the fresh one, which also suggests a good reusability of the catalyst [BisAm-OH-i-PS]Br₂.

Fig. 8 Recyclability of [BisAm-OH-i-PS]Br₂ catalyst (reaction conditions: PO 21.5 mmol, [BisAm-OH-i-PS]Br₂ 0.75 mol%, 130 °C, 2.5 h, and initial CO₂ pressure 1.2 MPa).

Catalytic activity towards different epoxides

The catalytic performance of the catalyst [BisAm-OH-i-PS]Br₂ on the cycloaddition of CO₂ with varieties of epoxides were studied, and the results were listed in Table 3. It is found that the catalyst works on diversified terminal epoxides. And the propylene epoxide shows the best reactivity and selectivity. The selectivity order is propylene epoxide = epoxybutane > epichlorohydrin > styrene oxide > cyclohexene. The reactivity order is propylene epoxide > epoxybutane > epichlorohydrin > styrene oxide > cyclohexene. The lower reactivity of styrene oxide and cyclohexene is possibly due to the larger sterically hindered of β-C.

Table 3 Cycloaddition of CO₂ with different epoxides catalyzed by [BisAm-OH-i-PS]Br₂^a

Entry	Epoxide	Products	t/h	Selectivity (%)	Yield (%)
a Reaction conditions: PO 21.5 mmol, [BisAm-OH-i-PS]Br2 0.75 mol%, 130 °C, 2.5 h and initial CO2 pressure 1.2 MPa.
1			2.5	99.2	99.1
2			2.5	99.2	98.6
3			5	98.8	98.3
4			5	96.1	94.4
5			5	95.2	64.7

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Possible reaction mechanism

As shown in Table 2, the nucleophilic attack of halogen anion on epoxide is essential for the cycloaddition reaction, and the stronger the nucleophilicity is, the easier the attack at the β-carbon atom of epoxide is. On the other hand, the BisAm-i-PS heterogeneous catalysts with functional end groups of –CH₂COOH, –CH₂CH₂OH, and –CH₂CH₂NH₂ show better catalytic activities compared with those without functional groups, which is accordant with the results of other researchers reported.^41,55,57,60 It indicates that these functional groups play an important role in the cycloaddition reaction. According to the present work and the previous literatures,^26,30,41,51 a possible mechanism was put forwarded as shown in Scheme 3. The hydrogen bonds between O atom in propylene epoxide and H atom in OH group of the catalyst would give rise to the polarization of C–O bonds. As a result, the PO can be activated to ring-opening. At the same time, halide ions make the nucleophilic attack on β-C of PO which has the minimal steric hindrance. Bis-ammonium immobilized catalysts prepared here have two halide anions, which makes the attack more efficient compared to those with one halide anion. It further promotes the ring-opening reaction of PO. After that, the anion shifts to O atom of PO to attack the C atom of CO₂ to form C–O bond because of the acidity of CO₂. At last, the intermediate is converted into propylene carbonate by the intramolecular substitution of halide anion.

Scheme 3 Proposed mechanism for the cycloaddition reaction.

Conclusions

A series of novel BisAm-i-PS heterogeneous catalysts with the functional end groups of –COOH, –OH, and –NH₂ were synthesized in the present work for the cycloaddition reaction of CO₂ with epoxides without any additional co-catalyst and organic solvent. The series of functionalized catalysts showed excellent performance under relatively mild conditions, which is ascribed to the nucleophilic attack of halide anion to the epoxide and the synergetic effect of hydrogen bonding formed between functional group and epoxide. Bis-ammonium immobilized catalysts were more efficient than single-ammonium one did because of the doubled nucleophilic anion concentration in one unit function group. The effects of parameters, such as halide anions of the catalysts, reaction temperature, initial pressure, reaction time and dosage of catalyst were also investigated with –OH ended BisAm-i-PS as the catalyst. BisAm-i-PS with Br⁻ anion I⁻ anion showed higher efficiency than those with Cl⁻. Under an optimal reaction condition (1.2 MPa, 130 °C and 2.5 h), propylene carbonate selectivity and yield were above 99% and 99%, respectively. In addition, the catalyst possessed good stability for 5 times recycle tests, which is in conformity with the development of green chemistry and can be used as a potential catalyst for large-scale production of cyclic carbonates.

Acknowledgements

The authors thank the support of National Science Fund for Distinguished Young Scholars of China (Grant no. 21125628), National Natural Science Foundation of China (Grant no. 21406031 and 21404018), the Fundamental Research Funds for the Central Universities (Grant no. DUTPJ14RC(3)003, DUT14RC(3)018, and DUT14QY50), and State Key Laboratory of fine chemicals (Panjin) project (Grant no. JH2014009).

Notes references

1. T. Sakakura, J.-C. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365–2387.
2. L. Xiao, D. Su, C. Yue and W. Wu, J. CO₂ Util., 2014, 6, 1–6.
3. A. Decortes, A. M. Castilla and A. W. Kleij, Angew. Chem., Int. Ed., 2010, 49, 9822–9837.
4. V. Calo, A. Nacci, A. Monopoli and A. Fanizzi, Org. Lett., 2002, 4, 2561–2563.
5. J. Peng, H.-J. Yang, Z. Wei and C.-Y. Guo, RSC Adv., 2015, 5, 53063–53072.
6. X.-B. Lu and D. J. Darensbourg, Chem. Soc. Rev., 2012, 41, 1462–1484.
7. M. North, B. Wang and C. Young, Energy Environ. Sci., 2011, 4, 4163–4170.
8. I. S. Metcalfe, M. North and P. Villuendas, J. CO₂ Util., 2013, 2, 24–28.
9. C.-Y. Li, C.-R. Wu, Y.-C. Liu and B.-T. Ko, Chem. Commun., 2012, 48, 9628–9630.
10. O. V. Zalomaeva, A. M. Chibiryaev, K. A. Kovalenko, O. A. Kholdeeva, B. S. Balzhinimaev and V. P. Fedin, J. Catal., 2013, 298, 179–185.
11. S. Verma, R. I. Kureshy, T. Roy, M. Kumar, A. Das, N. H. Khan, S. H. R. Abdi and H. C. Bajaj, Catal. Commun., 2015, 61, 78–82.
12. L. Cuesta-Aluja, M. Djoufak, A. Aghmiz, R. Rivas, L. Christ and A. M. Masdeu-Bultó, J. Mol. Catal. A: Chem., 2014, 381, 161–170.
13. H. Kawanami and Y. Ikushima, Chem. Commun., 2000, 2089–2090.
14. A. Barbarini, R. Maggi, A. Mazzacani, G. Mori, G. Sartori and R. Sartorio, Tetrahedron Lett., 2003, 44, 2931–2934.
15. J. Xu, M. Xu, J. Wu, H. Wu, W.-H. Zhang and Y.-X. Li, RSC Adv., 2015, 5, 72361–72368.
16. Y.-L. Hu, M. Lu and X.-L. Yang, RSC Adv., 2015, 5, 67886–67891.
17. J. K. Lee, Y. J. Kim, Y.-S. Choi, H. Lee, J. S. Lee, J. Hong, E.-K. Jeong, H. S. Kim and M. Cheong, Appl. Catal., B, 2012, 111–112, 621–627.
18. B. Wang, X. Feng, L. Zhang, S. Yang, X. Jiang, J. Zhou and G. Gao, J. CO₂ Util., 2013, 1, 88–91.
19. Z. Huo, J. Guo, J. Lu, Q. Xu, P. Ma, J. Zhao, D. Zhang, J. Niu and J. Wang, RSC Adv., 2015, 5, 69006–69009.
20. W.-L. Dai, S.-F. Yin, R. Guo, S.-L. Luo, X. Du and C.-T. Au, Catal. Lett., 2009, 136, 35–44.
21. B. M. Bhanage, S. Fujita, Y. Ikushima and M. Arai, Appl. Catal., A, 2001, 219, 259–266.
22. R. Srivastava, D. Srinivas and P. Ratnasamy, Catal. Lett., 2003, 91, 133–139.
23. E. J. Doskocil, J. Phys. Chem. B, 2005, 109, 2315–2320.
24. A. S. Aquino, F. L. Bernard, J. V. Borges, L. Mafra, F. Dalla Vecchia, M. O. Vieira, R. Ligabue, M. Seferin, V. V. Chaban, E. J. Cabrita and S. Einloft, RSC Adv., 2015, 5, 64220–64227.
25. Y. Zhang, S. Yin, S. Luo and C. T. Au, Ind. Eng. Chem. Res., 2012, 51, 3951–3957.
26. L. Han, H.-J. Choi, S.-J. Choi, B. Liu and D.-W. Park, Green Chem., 2011, 13, 1023.
27. K. R. Roshan, G. Mathai, J. Kim, J. Tharun, G.-A. Park and D.-W. Park, Green Chem., 2012, 14, 2933–2940.
28. W. Cheng, X. Chen, J. Sun, J. Wang and S. Zhang, Catal. Today, 2013, 200, 117–124.
29. R. A. Watile, K. M. Deshmukh, K. P. Dhake and B. M. Bhanage, Catal. Sci. Technol., 2012, 2, 1051–1055.
30. J. Sun, W. Cheng, W. Fan, Y. Wang, Z. Meng and S. Zhang, Catal. Today, 2009, 148, 361–367.
31. Y. Xiong, H. Wang, R. Wang, Y. Yan, B. Zheng and Y. Wang, Chem. Commun., 2010, 46, 3399–3401.
32. B. M. Bhanage, S. Fujita, Y. Ikushima, K. Torii and M. Arai, Green Chem., 2003, 5, 71–75.
33. A. R. Hajipour, Y. Heidari and G. Kozehgary, RSC Adv., 2015, 5, 22373–22379.
34. Y. Du, J.-Q. Wang, J.-Y. Chen, F. Cai, J.-S. Tian, D.-L. Kong and L.-N. He, Tetrahedron Lett., 2006, 47, 1271–1275.
35. J. Li, L. Wang, F. Shi, S. Liu, Y. He, L. Lu, X. Ma and Y. Deng, Catal. Lett., 2011, 141, 339–346.
36. Y. Zhao, J.-S. Tian, X.-H. Qi, Z.-N. Han, Y.-Y. Zhuang and L.-N. He, J. Mol. Catal. A: Chem., 2007, 271, 284–289.
37. J.-Q. Wang, D.-L. Kong, J.-Y. Chen, F. Cai and L.-N. He, J. Mol. Catal. A: Chem., 2006, 249, 143–148.
38. B. Song, L. Guo, R. Zhang, X. Zhao, H. Gan, C. Chen, J. Chen, W. Zhu and Z. Hou, J. CO₂ Util., 2014, 6, 62–68.
39. J. Sun, L. Wang, S. Zhang, Z. Li, X. Zhang, W. Dai and R. Mori, J. Mol. Catal. A: Chem., 2006, 256, 295–300.
40. S.-S. Wu, X.-W. Zhang, W.-L. Dai, S.-F. Yin, W.-S. Li, Y.-Q. Ren and C.-T. Au, Appl. Catal., A, 2008, 341, 106–111.
41. D. Wei-Li, J. Bi, L. Sheng-Lian, L. Xu-Biao, T. Xin-Man and A. Chak-Tong, Appl. Catal., A, 2014, 470, 183–188.
42. W. Zhang, Q. Wang, H. Wu, P. Wu and M. He, Green Chem., 2014, 16, 4767–4774.
43. S. Ghazali-Esfahani, H. Song, E. Păunescu, F. D. Bobbink, H. Liu, Z. Fei, G. Laurenczy, M. Bagherzadeh, N. Yan and P. J. Dyson, Green Chem., 2013, 15, 1584.
44. J. Wang, J. Leong and Y. Zhang, Green Chem., 2014, 16, 4515–4519.
45. H.-Y. Ju, J.-Y. Ahn, M.-D. Manju, K.-H. Kim and D.-W. Park, Korean J. Chem. Eng., 2008, 25, 471–473.
46. H. Xie, S. Li and S. Zhang, J. Mol. Catal. A: Chem., 2006, 250, 30–34.
47. B. Ochiai, T. Iwamoto and T. Endo, Green Chem., 2006, 8, 138–140.
48. H. Xie, H. Duan, S. Li and S. Zhang, New J. Chem., 2005, 29, 1199–1203.
49. Z.-Z. Yang, L.-N. He, C.-X. Miao and S. Chanfreau, Adv. Synth. Catal., 2010, 352, 2233–2240.
50. J. Sun, S. Zhang, W. Cheng and J. Ren, Tetrahedron Lett., 2008, 49, 3588–3591.
51. L.-F. Xiao, D.-W. Lv, D. Su, W. Wu and H.-F. Li, J. Cleaner Prod., 2014, 67, 285–290.
52. L. Han, H.-J. Choi, D.-K. Kim, S.-W. Park, B. Liu and D.-W. Park, J. Mol. Catal. A: Chem., 2011, 338, 58–64.
53. H. Zhao and G. A. Baker, J. Chem. Technol. Biotechnol., 2013, 88, 3–12.
54. T. Payagala, J. Huang, Z. S. Breitbach, P. S. Sharma and D. W. Armstrong, Chem. Mater., 2007, 19, 5848–5850.
55. D. Wei-Li, J. Bi, L. Sheng-Lian, L. Xu-Biao, T. Xin-Man and A. Chak-Tong, Catal. Sci. Technol., 2014, 4, 556–562.
56. Z. Feng, W. Wei, L. Wang and R. Hong, Appl. Surf. Sci., 2015, 357, 759–765.
57. C. Jing-Xian, J. Bi, D. Wei-Li, D. Sen-Lin, C. Liu-Ren, C. Zong-Jie, L. Sheng-Lian, L. Xu-Biao, T. Xin-Man and A. Chak-Tong, Appl. Catal., A, 2014, 484, 26–32.
58. D.-W. Kim, R. Roshan, J. Tharun, A. Cherian and D.-W. Park, Korean J. Chem. Eng., 2013, 30, 1973–1984.
59. L.-F. Xiao, F.-W. Li, J.-J. Peng and C.-G. Xia, J. Mol. Catal. A: Chem., 2006, 253, 265–269.
60. D. Wei-Li, J. Bi, L. Sheng-Lian, L. Xu-Biao, T. Xin-Man and A. Chak-Tong, J. Mol. Catal. A: Chem., 2013, 378, 326–332.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23808k

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