Imidazolium-based polymeric ionic liquids for heterogeneous catalytic conversion of CO2 into cyclic carbonates

Microporous and Mesoporous Materials 292 (2020) 109751

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso

Imidazolium-based polymeric ionic liquids for heterogeneous catalytic conversion of CO2 into cyclic carbonates Yinpeng Wang, Junqi Nie, Cuifen Lu, Feiyi Wang, Chao Ma, Zuxing Chen, Guichun Yang * Hubei Collaborative Innovation Center for Advanced Organochemical Materials, Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, College of Chemistry & Chemical Engineering, Hubei University, Wuhan, 430062, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Crosslinked polymer Imidazolium salt CO2 Cyclic carbonate Heterogeneous catalysis

Polyimidazolium salts are a class of important catalysts for CO2 cycloaddition with epoxides. However, initiator like azoisobutyronitrile or metal catalyst is often required to promote their synthetic process. Therefore, developing a clean route to synthesize polyimidazolium salts is still highly desirable. In this study, three crosslinked polymers PIM1, PIM2 and PIM3 containing imidazolium unit were constructed via nucleophilic substitution reaction of 1,3,5,7-tetrakis (4-(imidazole-1-yl)phenyl)adamantane and benzyl bromides. The structure and composition of the polymers were characterized by CP/MAS 13C NMR, FT-IR and XPS. Different porosities were observed for PIMs, and PIM2 has a larger BET surface are than that of PIM1 and PIM3. Accordingly, the polymeric imidazolium salt PIM2 shows superior activity for catalyzing cyclic addition of CO2 with a variety of epoxides without any solvent or cocatalyst, giving the cyclic carbonates in excellent yields. In addition, PIM2 shows high stability and easy recyclability.

1. Introduction In the past many years, due to the rapid development of industry and human dependence on fossil fuels, a large number of greenhouse gases such as CO2 have been produced, which has exerted a profound impact on the development of human society and the improvement of people’s living standards [1]. How to effectively convert CO2 into high value-added compounds has gained wide concern in the world [2,3]. At present, the most successful CO2 conversion process is the catalytic production of cyclic carbonates from epoxides because of its high atomic efficiency and low by-product formation [4,5]. In addition, the formed cyclic carbonates are important chemicals and have been widely used in the fields of extraction and separation, textile, electrochemistry, organic synthesis and preparation of polymers [6]. Thus, much efforts have been made to develop effective catalytic system for CO2 cycloaddition. Imidazolium salts, generally produced by the reaction of Nsubstituted imidazoles and halides, have attracted much attention in recent years for their versatile application in chemistry and pharma­ cology. The electrophilic imidazolium cations, nucleophilic halogen anions and good affinity with CO2 make them excellent candidates for catalyzing CO2 cycloaddition with epoxides [7]. However, their appli­ cation towards catalytic CO2 fixation has been confronted with some

problems such as handling difficulty due to the viscous nature of these imidazolium salts, tedious separation of product from catalyst and catalyst recycling. In order to solve these issues, the imidazolium salts are often heterogenized by immobilizing them on solid supports, which could be silca, polymer, metal oxide, chitosan or cellulose [8–12]. Recently, imidazolium-based polymers have emerged as important heterogeneous catalytic materials in the CO2 cycloaddition. These pol­ yimidazolium salts (PIMs) are mainly constructed through the classical free radical polymerization [13–16], and other condensation/coupling processes [17–20]. In these cases, some additives are required to pro­ mote the synthetic process. For freeradical polymerization, an initiator (e.g., azoisobutyronitrile) is necessary to promote the reaction, and for condensation/coupling process, a metal catalyst is needed. Therefore, there is still impetus to develop an alternative and additive-free method for clean synthesis of polyimidazolium salts. More recently, a nucleophilic substitution route, which involves the reaction of multidentate imdazoles and alkyl halides, has been suc­ cessfully applied to prepare the linear PIMs [21]. The clean synthetic route and outstanding catalytic activities of these linear PIMs inspired us to seek crosslinked PIMs based on nucleophilic substitution reaction for CO2 cycloaddition. On the other hand, we reported the synthesis of adamantane-based bis–NHC–palladium polymer mediated by 1,3,5,

* Corresponding author. E-mail address: [email protected] (G. Yang). https://doi.org/10.1016/j.micromeso.2019.109751 Received 9 May 2019; Received in revised form 16 September 2019; Accepted 20 September 2019 Available online 21 September 2019 1387-1811/© 2019 Elsevier Inc. All rights reserved.

Y. Wang et al.

Microporous and Mesoporous Materials 292 (2020) 109751

Scheme 1. Syntheses of PIM1, PIM2 and PIM3.

7-tetrakis (4-(imidazole-1-yl)phenyl)adamantane [22]. As a continua­ tion of efforts to explore the synthetic utility of this tetradentate imda­ zole, herein we describe a nucleophilic substitution route to produce a number of crosslinked PIMs under metal catalyst- and initiator-free conditions. The reactivities of the resulting polymeric imidazolium salts were then evaluated as heterogeneous catalysts in the synthesis of cyclic carbonates from CO2 and epoxides. Results of the study showed that cycloaddition experiments of CO2 proceed very well in the absence of solvent and cocatalyst.

analysis was operated on Vario Micro Cube Elemental analyzer. Powder X-ray diffraction (PXRD) measurements were taken with a Bruker D8 advance with Cu Kα radiation at a scan rate of 10� /min. Scanning electron microscopy (SEM) images were conducted on a JEOL JSM-6510 electron microscope. Transmission electron microscope (TEM) images were taken with Tecnai G2 F20. The N2 sorption isotherms were measured at 77 K using a Micromeritics ASAP 2460 instrument. The samples were degassed at 120 � C for 8 h before the measurements. The surface areas were calculated from the adsorption data by using Brunauer-Emmett-Teller (BET) methods and pore-size distributions were calculated by using the BJH method.

2. Experimental 2.1. Materials

2.3. Syntheses of polyimidazolium salts PIM1, PIM2 and PIM3

All chemicals and solvents are commercially available and used without further purification unless otherwise stated. Propylene oxide (PO) and all other epoxides were purchased from Aladdin. The CO2 (99.999% purity) was ordered from Guangdong Huate Gas Co., Ltd. Acetonitrile was of analytical grade (AR), and supplied by Chengdu Kelong Chemicals Co., Ltd. 1,4-bis(bromomethyl)benzene, 4,4-bis(bro­ momethyl)biphenyl and 2,6-bis(bromomethyl)naphthalene were pur­ chased from Ark Chemical Company. 1,3,5,7-tetrakis (4-(imidazole-1yl)phenyl)adamantane was synthesized via the reported method [23].

In a typical procedure for the synthesis of PIMs, 1,3,5,7-tetrakis (4(imidazole-1-yl)phenyl)adamantane (1 g, 1.42 mmol), 1,4-bis(bromo­ methyl)benzene (0.75 g, 2.84 mmol) and acetonitrile (75 mL) were added into a 150 mL flask. The mixture was stirred at reflux for 24 h and cooled to room temperature. The resulting precipitate was filtered and thoroughly washed with diethyl ether and ethyl acetate, and dried under vacuum to give PIM1. Following a similar procedure, PIM2 and PIM3 were also synthesized using the same amount of 1,3,5,7-tetrakis (4(imidazole-1-yl)phenyl)adamantane, except that the dibromo com­ pounds used were 4,4-bis(bromomethyl)biphenyl and 2,6-bis(bromo­ methyl)naphthalene, respectively, instead of 1,4-bis(bromomethyl) benzene.

2.2. Characterization IR spectra were recorded on an IR-spectrum one (PE) spectrometer. NMR spectra were recorded on Varian Unity Inova 500 spectrometer (1H NMR at 500 MHz and 13C NMR at 125 MHz). 13C cross-polarization magic-angle spinning (CP/MAS) NMR spectroscopy was recorded on Varian Infinity-plus 300 spectrometer. Thermogravimetric analysis (TGA) was carried out in a N2 atmosphere with a heating rate of 10 � C/ min on a Diamond TG/DTA thermal analyzer (PerkinElmer). Elemental

PIM1. Yield: 1.68 g, pale yellow solid, 96%. Elemental analysis calcd (%) for (C62H56Br2N8)n: C, 69.40, H, 5.26, N, 10.44; found: C, 68.13, H, 5.86, N, 10.05. PIM2. Yield: 1.91 g, pale yellow solid, 97%. Elemental analysis calcd (%) for (C74H64Br2N8)n: C, 72.54, H, 5.27, N, 9.15; found: C, 71.84, H, 5.63, N, 8.56. 2

Y. Wang et al.

Microporous and Mesoporous Materials 292 (2020) 109751

Fig. 1. (a) FT-IR spectra and (b) solid state

PIM3. Yield: 1.83 g, pale yellow solid, 97%. Elemental analysis calcd (%) for (C70H60Br2N8)n: C, 71.67, H, 5.16, N, 9.55; found: C, 70.76, H, 5.54, N, 9.27.

13

C NMR of PIMs.

thoroughly with dichloromethane, dried and then reused for the next run under the same reaction conditions. Each experiment was performed for three times and the average yield value was chosen with the exper­ imental error of about �1%.

2.4. General procedure for the cycloaddition reaction of CO2 and epoxides

3. Results and discussion 3.1. Synthesis and characterization of PIMs

In a typical reaction, epichlorohydrin (10 mmol) and PIM2 (6.5 mg, 0.2 mol%) were added to a stainless steel autoclave. The mixture was purged with CO2 three times and pressurized to 1 MPa with CO2. After stirring at 130 � C for 4 h, the autoclave was cooled to 0 � C in an ice bath and the reaction mixture was filtered to recover the catalyst. The filtrate was subjected to 1H NMR analysis to determine the conversion of the epoxide. For reuse of the catalyst, the recovered catalyst was washed

As depicted in Scheme 1, the crosslinked polymers PIM1, PIM2 and PIM3 were synthesized by the substitution reaction of 1,3,5,7-tetrakis (4-(imidazole-1-yl)phenyl)adamantane and benzyl bromides. The ob­ tained PIMs were all insoluble in water and common organic solvents. FT-IR spectra of the polymers were consistent with the expected network

Fig. 2. Br 3 d spectra of (a) PIM1, (b) PIM2 and (c) PIM3 and N 1s spectra of (a) PIM1, (b) PIM2 and (c) PIM3. 3

Y. Wang et al.

Microporous and Mesoporous Materials 292 (2020) 109751

Fig. 3. SEM images of (a) PIM1, (b) PIM2 and (c) PIM3 and TEM images of (a) PIM1, (b) PIM2 and (c) PIM3.

Fig. 4. (a) Nitrogen sorption isotherms at 77 K and (b) BJH-derived pore size distribution curves of PIMs.

structure. The presence of the imidazolium rings was verified by ring – Nþ), 1550 (C– – C), 1620 (C– – N) and stretching bands at about 1150 (C– 3030 (C–H) cm 1 (Fig. 1a) [21,24]. The chemical structure of the net­ works were further confirmed by solid state 13C NMR spectroscopy (Fig. 1b). The signal at 150 ppm can be attributed to the benzene carbon substituted by adamantane, and the overlapping peaks at around 110–140 ppm correspond to the other carbon of benzene ring and imi­ dazolium ring. The peak at 60–70 ppm is related to the methylene car­ bon linking the imidazolium and benzene ring, while the chemical shifts at 35 and 50 ppm are assigned to the methylene carbon and quaternary carbon of adamantane. XPS analysis was used to obtain the chemical composition of the PIMs as well. As shown in Fig. S3, the survey scans reveal the presence of C, N, Br on the PIMs. The Br 3 d spectra contain two strong peaks at around 68.3 eV (3d5/2) and 69.1 eV (3d3/2) for all samples (Fig. 2a–c), while the peaks for Br (C–Br) 3 d level were negli­ gible, indicating that most of the Br was in the state of Br for the crosslinked polymers [25,26]. The peak for N 1s at about 400.1 eV is observed in the N 1s XPS spectra (Fig. 2d–f), which is related to the

imidazolinium cations [20,27]. Moreover, the characteristic peak of the chemisorbed water (532.7 eV) was observed, which is consistent with FT-IR studies [28]. The morphologies of the PIMs were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images (Fig. 3a–c and Fig. S4) show that the PIMs consist of rela­ tively uniform granules with diameters ranging from 1.5 to 2.5 μm, and similar observations are also evident from their TEM images (Fig. 3d–f). In addition, the TEM images show that these polymers adopt highly dense texture. The porous properties of the PIMs were analyzed by ni­ trogen sorption analysis (Fig. 4a). BET surface areas of PIM1, PIM2 and PIM3 were calculated to be 12.3, 24.2 and 11.9 m2 g 1, respectively, which disclose the poor porous nature of these PIMs. This can be ascribed to two main factors: (1) the flexibility of the benzyl linkage, which may cause pore collapse within the networks [29], and (2) the strong polarity of imidazolium salts, which leads to charge interactions and tendency of intermolecular stacking [14,30]. The difference of BET surface areas of PIMs indicates that the linkage moiety could affect the 4

Y. Wang et al.

Microporous and Mesoporous Materials 292 (2020) 109751

Table 1 Cycloaddition reaction of CO2 with epichlorohydrin.a.

Table 2 Reaction of CO2 with various epoxide substrates.a. Pressure (MPa)

Time(h)

Yield (%)b

1

1

4

92

Entry

Entry

Catalyst

1 2 3 4 5 6 7 8

PIM2 PIM2 PIM2 PIM2 PIM1 PIM3 PIM1 PIM3

Amount (mol %) 0.2 0.2 0.1 0.2 0.2 0.2 0.2 0.2

Substrate

T (oC)

CO2 (MPa)

t (h)

Yield (%)b

2

1

8

99

130 110 130 130 130 130 130 130

1 1 1 1.2 1 1 1 1

4 10 10 4 4 4 6 6

92 54 78 93 81 79 92 91

3

1

8

90

4

1

8

96

5

1

10

93

6

1

8

92

7

1

18

99

8

1

24

96

a

The amount of epichlorohydrin was 10 mmol, each experiment was per­ formed for three times and the average yield value was chosen with the exper­ imental error of about �1%. b Yield was determined by1 HNMR.

a Reaction conditions: epoxide (10 mmol), CO2 (1.0 MPa), PIM2 (0.2 mol%), 130 � C; each experiment was performed for three times and the average yield value was chosen with the experimental error of about �1%. b Yield was determined by1 HNMR.

improve the reaction yield (entry 4). Under the same optimal conditions for 4 h, PIM1 and PIM3 gave the products in only 81% and 79% yield, respectively (entries 5 and 6). In these cases, a 6 h reaction time is required to achieve the full conversion of epichlorohydrin (entries 7 and 8). A comparative experimental study for the CO2 cycloaddition (Fig. 5) shows that PIM2 exhibits a higher catalytic efficiency than PIM1 and PIM3, and the catalytic efficiency of PIM1 is comparable with that of PIM3. The larger BET surface area may be responsible for the higher catalytic efficiency of PIM2. Then the versatility of PIM2 for the cycloaddition of other epoxides was investigated. As shown in Table 2, the catalytic system was effective for a variety of epoxides containing different terminal groups (entries 1–8). It can be seen that the epoxides with electron-withdrawing groups (entry 1) have higher reactivity than those with electron-donating groups (entries 2–4). Propylene oxide and styrene oxide also reacted well with CO2, affording the desired cyclic carbonates in 93% and 92% yields (entries 5, 6), respectively. The steric hindrance of the terminal groups has some effect on the reaction, and in this case a prolonged reaction time is required to achieve the full conversion of substrate. For example, the reaction of 1,2-epoxybutane was complete in 18 h (entry 7), while for 1,2-epoxycyclohexane, 24 h reaction time was required (entry 8). Compared with previously reported ionic polymers (Table S1), PIM2 affords comparable result under identical or even milder condi­ tions, demonstrating that it is a promising candidate for CO2 cycload­ dition with epoxides. The good activity of this flexible polymer may be ascribed to its high swelling ability in epoxides [31], which makes substrates easily accessible to the catalytic sites within the polymer network.

Fig. 5. Yield of cycloadduct as a function of time in the CO2 cycloaddition with epichlorohydrin. Reaction conditions: epichlorohydrin (10 mmol), CO2 (1.0 MPa), PIM (0.2 mol%), 130 � C.

porosity of the networks. Thermogravimetric analysis (TGA) suggests that the PIMs all have good thermal stability, which show stable mass up to 310 � C (Fig. S5). Powder X-ray diffraction (PXRD) data indicates that PIMs are amorphous, which is due to the irreversibility of the substitu­ tion reaction (Fig. S6). 3.2. Cycloaddition of CO2 with epoxides The as-prepared PIMs were next used as catalysts for the cycload­ dition of CO2 with epoxides. Their catalytic performances were initially evaluated by using the model reaction of CO2 and epichlorohydrin. The effects of parameters such as catalyst amount, temperature, time and CO2 pressure were investigated in order to optimize the reaction con­ ditions. As shown in Fig. S15, the PIMs have the identical optimal conditions, which are found to be 130 � C for temperature, 1 MPa for CO2 pressure and 0.1 mol% for catalyst amount. Some of the results are listed in Table 1, excellent yield was obtained as 92% when the reaction was carried out at 130 � C and 1 MPa CO2 pressure for 4 h in the presence of 0.1 mol% PIM2 (entry 1). The use of a less amount of catalyst (0.1 mol%) or a lower temperature (110 � C) can’t realize high conversion of epichlorohydrin even increasing the reaction time (entries 2 and 3), and enhancing the CO2 pressure from 1 MPa to 1.2 MPa didn’t obviously

3.3. Recyclability of PIM2 As recyclability is an important aspect for the practical application of a heterogeneous catalytic system, reuse performance of PIM2 was investigated in the cycloaddition of CO2 with 1,2-epoxyhexane. After each run of the reaction, the catalyst can be recovered simply by filtration. To our delight, the crosslinked material PIM2 was successfully reused for five times without significant loss of its catalytic activity (Fig. 6). FT-IR spectrum (Fig. S16) and SEM image (Fig. S17) of the recycled catalyst indicated that the structure network and morphology 5

Y. Wang et al.

Microporous and Mesoporous Materials 292 (2020) 109751 [7] D. Kerl� e, R. Ludwig, A. Geiger, D. Paschek, Temperature dependence of the solubility of carbon dioxide in imidazolium-based ionic liquids, J. Phys. Chem. B 113 (38) (2009) 12727–12735. [8] L. Han, S.W. Park, D.W. Park, Silica grafted imidazolium-based ionic liquids: efficient heterogeneous catalysts for chemical fixation of CO2 to a cyclic carbonate, Energy Environ. Sci. 2 (12) (2009) 1286–1292. [9] W.L. Dai, J. Bi, S.L. Luo, X.B. Luo, X.M. Tu, C.T. Au, Polymers anchored with carboxyl-functionalized di-cation ionic liquids as efficient catalysts for the fixation of CO2 into cyclic carbonates, Catal. Sci. Technol. 4 (2) (2014) 556–562. [10] X.X. Zheng, S.Z. Luo, L. Zhang, J.P. Cheng, Magnetic nanoparticle supported ionic liquid catalysts for CO2 cycloaddition reactions, Green Chem. 11 (4) (2009) 455–458. [11] J. Sun, J.Q. Wang, W.G. Cheng, J.X. Zhang, X.H. Li, S.J. Zhang, Y.B. She, Chitosan functionalized ionic liquid as a recyclable biopolymer-supported catalyst for cycloaddition of CO2, Green Chem. 14 (3) (2012) 654–660. [12] K.R. Roshan, G. Mathai, J. Kim, J. Tharun, G.A. Park, D.W. Park, A biopolymer mediated efficient synthesis of cyclic carbonates from epoxides and carbon dioxide, Green Chem. 14 (10) (2012) 2933–2940. [13] Y. Xie, Z.F. Zhang, T. Jiang, J.L. He, B.X. Han, T.B. Wu, K.L. Ding, CO2 cycloaddition reactions catalyzed by an ionic liquid grafted onto a highly crosslinked polymer matrix, Angew. Chem. Int. Ed. 46 (38) (2007) 7255–7258. [14] S. Ghazali-Esfahani, H.B. Song, E. P� aunescu, F.D. Bobbink, H.Z. Liu, Z.F. Fei, G. Laurenczy, M. Bagherzadeh, N. Yana, P.J. Dyson, Cycloaddition of CO2 to epoxides catalyzed by imidazolium-based polymeric ionic liquids, Green Chem. 15 (6) (2013) 1584–1589. [15] A. Dani, E. Groppo, C. Barolo, J.G. Vitillo, S. Bordiga, Design of high surface area poly(ionic liquid)s to convert carbon dioxide into ethylene carbonate, J. Mater. Chem. 3 (16) (2015) 8508–8518. [16] Y.Q. Xie, J. Liang, Y.W. Fu, M.T. Huang, X. Xu, H.T. Wang, S. Tu, J. Li, Hypercrosslinked mesoporous poly(ionic liquid)s with high ionic density for efficient CO2 capture and conversion into cyclic carbonates, J. Mater. Chem. 6 (15) (2018) 6660–6666. [17] H.C. Cho, H.S. Lee, J. Chun, S.M. Lee, H.J. Kimb, S.U. Son, Tubular microporous organic networks bearing imidazolium salts and their catalytic CO2 conversion to cyclic carbonates, Chem. Commun. 47 (3) (2011) 917–919. [18] J.Q. Wang, W.H. Sng, G.S. Yi, Y.G. Zhang, Imidazolium salt-modified porous hypercrosslinked polymers for synergistic CO2 capture and conversion, Chem. Commun. 51 (60) (2015) 12076–12079. [19] H. Zhong, Y.Q. Su, X.W. Chen, X.J. Li, R.H. Wang, Imidazolium- and triazine-based porous organic polymers for heterogeneous catalytic conversion of CO2 into cyclic carbonates, ChemSusChem 10 (24) (2017) 4855–4863. [20] T.T. Liu, R. Xu, J.D. Yi, J. Liang, X.S. Wang, P.C. Shi, Y.B. Huang, R. Cao, Imidazolium-based cationic covalent triazine frameworks for highly efficient cycloaddition of carbon dioxide, ChemCatChem 10 (9) (2018) 2036–2040. [21] Z.Z. Yang, Y.F. Zhao, G.P. Ji, H.Y. Zhang, B. Yu, X. Gao, Z.M. Liu, Fluorofunctionalized polymeric ionic liquids: highly efficient catalysts for CO2 cycloaddition to cyclic carbonates under mild conditions, Green Chem. 16 (8) (2014) 3724–3728. [22] W. Xia, L.S. Huang, X. Huang, Y.M. Wang, C.F. Lu, G.C. Yang, Z.X. Chen, J.Q. Nie, Main-chain NHC-palladium polymers based on adamantane: synthesis and application in Suzuki-Miyaura reactions, J. Mol. Catal. A Chem. 412 (2016) 93–100. [23] M. Tominaga, A. Iekushi, K. Katagiri, K. Ohara, K. Yamaguchi, I. Azumaya, Tetrahedron Lett. 55 (2014) 5789–5792. [24] Z.J. Guo, X.C. Cai, J.Y. Xie, X.C. Wang, Y. Zhou, J. Wang, Hydroxyl-exchanged nanoporous ionic copolymer toward low temperature cycloaddition of atmospheric carbon dioxide into carbonates, ACS Appl. Mater. Interfaces 8 (20) (2016) 12812–12821. [25] S. Jayakumar, H. Li, J. Chen, Q.H. Yang, Cationic Zn-porphyrin polymer coated onto CNTs as a cooperative catalyst for the synthesis of cyclic carbonates, ACS Appl. Mater. Interfaces 10 (3) (2018) 2546–2555. [26] Y. Chen, R. Luo, Q. Xu, J. Jiang, X. Zhou, H.B. Ji, Metalloporphyrin polymers with intercalated ionic liquids for synergistic CO2 fixation via cyclic carbonate production, ACS Sustain. Chem. Eng. 6 (1) (2018) 1074–1082. [27] J. Li, D.G. Jia, Z.J. Guo, Y.Q. Liu, Y.N. Lyu, Y. Zhou, J. Wang, Imidazolinium based porous hypercrosslinked ionic polymers for efficient CO2 capture and fixation with epoxides, Green Chem. 19 (11) (2017) 2675–2686. [28] S.S. Cui, M.M. Qian, X. Liu, Z.J. Sun, P.W. Du, A copper porphyrin-based conjugated mesoporous polymer-derived bifunctional electrocatalyst for hydrogen and oxygen evolution, ChemSusChem 9 (17) (2016) 2365–2373. [29] L.Y. Li, C.S. Zhou, H.X. Zhao, R.H. Wang, Spatial control of palladium nanoparticles in flexible click-based porous organic polymers for hydrogenation of olefins and nitrobenzene, Nano Res 8 (3) (2015) 709–721. [30] Y.J. Chen, R.C. Luo, J.H. Bao, Q.H. Xu, J. Jiang, X.T. Zhou, H.B. Ji, Functionoriented ionic polymers having high density active sites for sustainable carbon dioxide conversion, J. Mater. Chem. 6 (19) (2018) 9172–9182. [31] Z.J. Huang, J.G. Uranga, S.L. Zhou, H.Y. Jia, Z.F. Fei, Y.F. Wang, F.D. Bobbink, Q. H. Lu, P.J. Dyson, Ionic liquid containing electron-rich, porous polyphosphazene nanoreactors catalyze the transformation of CO2 to carbonates, J. Mater. Chem. 6 (42) (2018) 20916–20925.

Fig. 6. The reusability of PIM2 for six consecutive runs of reaction.

were still maintained, which account for the stable reusability. 4. Conclusion In summary, we have synthesized three crosslinked ionic polymers containing imidazolium unit via nucleophilic substitution reactions of 1,3,5,7-tetrakis (4-(imidazole-1-yl)phenyl)adamantane with benzyl bromides. Different porosities were observed for PIMs, and PIM2 has a higher BET surface are than that of PIM1 and PIM3. The polymeric imidazolium salt PIM2 shows superior activity in the catalyzed cyclic addition of CO2 with epoxides without any solvent or cocatalyst. A va­ riety of epoxides can be converted to the corresponding cyclic carbon­ ates with excellent yields in the presence of PIM2 under mild conditions. Furthermore, PIM2 can be reused five times without an appreciable loss of activity. We believe that this work could provide a new way for designing more sustainable, non-toxic catalysts for the transformation of CO2 and other substrates. Acknowledgements This work was supported by the National Natural Science Foundation of China (51603064, 21702053) and the Natural Sciences Foundation of Hubei province in China (2016CFB126). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.micromeso.2019.109751. References [1] Z.H. Yuan, M.R. Eden, Toward the development and deployment of large-scale carbon dioxide capture and conversion processes, Ind. Eng. Chem. Res. 55 (12) (2016) 3383–3419. [2] I. Omae, Recent developments in carbon dioxide utilization for the production of organic chemicals, Coord. Chem. Rev. 256 (13–14) (2012) 1384–1405. [3] C. Maeda, Y. Miyazaki, T. Ema, Recent progress in catalytic conversions of carbon dioxide, Catal. Sci. Technol. 4 (6) (2014) 1482–1497. [4] M. North, R. Pasquale, C. Young, Synthesis of cyclic carbonates from epoxides and CO2, Green Chem. 12 (9) (2010) 1514–1539. [5] M. Cokoja, M.E. Wilhelm, M.H. Anthofer, W.A. Herrmann, F.E. Kìhn, Synthesis of cyclic carbonates from epoxides and carbon dioxide by using organocatalysts, ChemSusChem 8 (15) (2015) 2436–2454. [6] M. Mikkelsen, M. Jørgensen, F.C. Krebs, The Teraton Challenge. A review of fixation and transformation of carbon dioxide, Energy Environ. Sci. 3 (1) (2010) 43–81.

6