Design of Novel Poly(ionic liquids) for the Conversion of CO2 to Cyclic

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Design of Novel Poly(ionic liquid)s for the Conversion of CO2 to Cyclic Carbonates under Mild Conditions without Solvent Hongbing Song, Yongjie Wang, Meng Xiao, Lei Liu, Yule Liu, Xiaofeng Liu, and Hengjun Gai ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00865 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Sustainable Chemistry & Engineering

Design of Novel Poly(ionic liquid)s for the Conversion of CO2 to Cyclic Carbonates under Mild Conditions without Solvent Hongbing Song, Yongjie Wang, Meng Xiao, Lei Liu, Yule Liu, Xiaofeng Liu, Hengjun Gai State Key Laboratory Base for Eco-Chemical Engineering in College of Chemical Engineering, Qingdao University of Science and Technology, Zhengzhou Road No.53, Qingdao 266042, China 

Corresponding author: H. J. Gai, E-mail: [email protected]

Abstract The production of carbonates from carbon dioxide (CO2) and epoxides is an atom-economical reaction. This study developed a series of poly(ionic liquid)s (PILs) by copolymerizing a novel functional ionic liquid (ethyl ether bis(1-vinylimidazolium) chloride) with ethylene glycol dimethacrylate (EGDMA). The structures and morphologies of PILs were investigated by adjusting the mole ratio between IL and EGDMA in the PILs. The prepared PILs were investigated as catalysts for the cycloaddition of CO2 with epoxides and they show improved catalytic efficiency and accelerated mass transfer rate due to their mesoporous structure with a large number of uniformly distributed active ion site. The PIL was stable, and the activity remained unchanged during the recycling of CO2 cycloaddition to epoxides under mild conditions without any solvent and co-catalyst. This study emphasizes the PIL as a versatile platform to obtain cyclic carbonates from CO2 under mild conditions. Keywords: Ionic Liquid, Poly(Ionic Liquid)s, Carbon Dioxide, Heterogeneous Catalysis

Introduction In recent years, carbon dioxide (CO2) emissions have annually increased owing to extensive use of fossil fuels, resulting in the greenhouse effect. CO2 is a kind of abundant C1 resource that is nontoxic, cheap, easy to obtain, and renewable.1-4 On this basis, CO2 is considered a resource rather than a waste product. The conversion of CO2 into usable chemical products through chemical methods has attracted widespread attention.5-7 Among these chemicals, the production of carbonates from CO2 and epoxides is an atom-economical reaction and is in line with the concept of “green chemistry”.8-11 Moreover, substrate materials are inexpensive, and no other by-products exist in the reaction process. Given that several cyclic carbonates are used on a large scale, catalysts such as metal oxides, metal complexes, supported catalysts,12 quaternary ammonium salts, and ionic liquids (ILs), have been widely utilized for the cycloaddition of CO2 to epoxides.13-17 However, most of the catalysts have poor catalytic performance, and require cumbersome

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preparation, harsh catalytic conditions, and the addition of co-catalysts. For instance, the addition of an alkali metal as a co-catalyst is usually required to achieve a good catalytic effect when ILs are used as a catalyst.14 In addition, the reaction conditions required for such catalysts are severe and difficult to separate from the products. A metal complex catalytic system possesses mild reaction conditions, high conversion rate, and high yield. However, its preparation is complicated and difficult. Most of the catalysts contain toxic metals, which cause environmental pollution, and their high cost and other factors remarkably limit the promotion and application of metal complexes.18-19 The catalytic activity of metal oxides is generally lower than that of the other types of catalysts, and the required reaction conditions are relatively severe. Hence, the research and application value are low. The commonly used carriers for supported catalysts are mainly zeolite, and metal organic framework (MOF), and the active components are ILs or porphyrins. These catalysts can achieve a good catalytic effect under the synergistic catalysis of organic solvents such as tetrabutylammonium bromide and N, N-dimethylformamide (DMF).19-20 However, the separation between the product and organic solvent requires a new separation process. ILs have been designed to improve the catalytic processes in the cycloaddition reaction of CO2 to epoxides. In particular, the emergence of poly(ionic liquid)s (PILs) have enhanced these reactions, by combining several properties of ILs with polymers, such as high ionic conductivity, tunable physical and chemical properties, and broad structural diversity.21-25 In addition, PILs form the structures and morphologies that are inaccessible using ILs, their intrinsic properties are affected by their molecular structures due to the designability of ILs and the selectivity of polymer segments. In this context, the insertion of a specially designed IL in the polymers can improve the catalytic activity and obtain good stability. Their properties can be easily tuned by magnifying the monomers in the molecular level to achieve the required host-guest interaction. In the cycloaddition reaction, the designed IL moieties can form strong van der Waals forces and electrostatic interactions with CO2,26-29 and polymer moieties can maintain the stability of the structure, which result in PILs with high catalytic activity and stability.14,30 In this study, a new series of PIL-based copolymers were developed by using ethyl ether bis(1vinylimidazolium) chloride ([EEBVIM]Cl2) and ethylene glycol dimethacrylate (EGDMA) as monomers. To obtain an ideal catalyst for the cycloaddition of CO2 with epoxides, [EEBVIM]Cl2 was successfully fabricated by using 1-vinylimidazole and bis(2-chloroethylether) as raw materials


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(Scheme 1A) to form a porous structure, and EGDMA was selected as a crosslinker and structural stabilizer. Meanwhile, the IL moieties in the PIL backbone provided the required active sites for the catalytic reaction. Although the obtained PILs from the copolymerization of [EEBVIM]Cl2 and EGDMA exhibited low surface area, their meso/macroporous structures are suitable for mass transfer. The results of cycloaddition catalytic experiments suggested that PILs exhibit high catalytic activity, selectivity, and cycle stability on various epoxides without any co-catalyst, solvent, and additives. The effects of different chemical compositions and pore structures on the CO2 cycloaddition reaction were systematically discussed through various well-designed comparative catalytic experiments.

Experimental Materials 1-Vinylimidazole (VIM, 99%), bis(2-chloroethyl ether) (BCEE, 99%), ethylene glycol dimethacrylate (EGDMA, 98%), propylene oxide (PO, 98%), epichlorohydrin (EO, 98%), styrene oxide (SO, 98%), N, N-dimethylformamide (DMF, 99%) were purchased from Aladdin Internet Reagent Database Inc. (Shanghai, China). Other solvents, such as acetonitrile, diethyl ether, ethanol, ethyl acetate were purchased from Sinopharm Chemical Reagent Co. Ltd. All reagents were used without further purification. Synthesis of bis-imidazolium-based IL ([EEBVIM]Cl2) First, VIM (0.125 mol, 11.76 g) was added to a Schlenk tube and dissolved in 20 mL of acetonitrile, then BCEE (0.05 mol, 7.15 g) was slowly added dropwise thereto at 40 °C. At the end of dropwise addition, the mixture was stirred at 75 °C for 48 h under nitrogen atmosphere. After reaction, the IL was precipitated by adding diethyl ether to the reaction liquid. The IL of [EEBVIM]Cl2 was collected and washed with ethyl acetate until unreacted substrates were removed. [EEBVIM]Cl2. 1H NMR (400 MHz, DMSO, TMS) δ (ppm) = 9.44 (s, 2H), 8.21 (s, 2H), 7.88 (s, 2H), 6.89 (q, 2H), 5.98 (d, 2H), 5.45 (d, 2H), 4.42 (t, 4H), 3.85 (t, 4H). Copolymerization of IL with EGDMA The PIL is obtained by copolymerization of [EEBVIM]Cl2 with EGDMA. In a typical synthesis of PIL-1, [EEBVIM]Cl2 (2 g, 6.04 mmol) and EGDMA (1.2 g, 6.05 mmol) were dissolve in 15 mL ethanol, followed by the addition of AIBN (39 mg, 0.24 mmol) at room temperature. The mixture



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was stirred at 80 °C for 24 h under a nitrogen (N2) atmosphere. The precipitate was collected by filtration and washed with ethanol until the filtrate was clear. The solid was dried in a vacuum at 40 °C for 24 h. PIL-1 was obtained as a white powder solid. PILs with various IL content were synthesized via varying the initial chemical composition of the raw materials. The samples PIL-1/2, PIL-2, PIL-4 were synthesized by regulating the initial molar ratio composition of VIM (2 g, 6.04 mmol) and EGDMA (2.4 g, 12.10 mmol) for PIL-1/2, VIM (2 g, 6.04 mmol) and EGDMA (0.6 g, 3.25 mmol) for PIL-2 and VIM (2 g, 6.04 mmol) and EGDMA (0.3 g, 1.51 mmol)for for PIL-4. The structural formulas of [EEBVIM]Cl2 are shown in Scheme 1A. For comparison of subsequent catalytic tests, polyethylene glycol dimethacrylate without integrating IL monomer was synthesized, named PIL-0. Characterization Transmission electron microcopy (TEM) images were performed on a Tecnai F30 electron microscope (Phillips Analytical) operated at an acceleration voltage of 300 kV. Scanning electron microscopy (SEM) images were observed by scanning electron microscope (Sirion200, Philips, Netherlands. X-ray photoelectron spectra (XPS) were conducted on a Quantum 2000 Scanning ESCA Microprobe (Physical Electronics) using Al Kα radiation (1846.6 eV) as the X-ray source. The C 1s peak at 284.5 eV was used as the reference for the binding energies. Fourier transform infrared (FTIR) spectra were recorded with a Bruker Tensor 27 FTIR spectrometer. 1H and

13C

nuclear magnetic resonance (NMR) spectroscopy of the samples were analyzed with a Bruker AVANCE III 400 MHz at ambient temperature using DMSO as deuterium reagents. Thermogravimetric (TG) analysis was performed with a SDT Q600 instrument in a dry air atmosphere at the rate of 10 °C·min-1. The CHN elemental analysis was performed on an elemental analyzer Vario MACRO cube. Nitrogen adsorption–desorption isotherms were measured using an ASAP 2020 volumetric adsorption analyzer with the 99.998% purity of N2. Before adsorption measurement, the samples were degassed for 10 h at 120 °C. The specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) equation in the relative pressure (P/P0) range from 0.05 to 0.2. The pore–size distribution curves were obtained via the Barrett– Joyner–Halenda (BJH) model. The total pore volumes were obtained via BJH Desorption model between 1.7000 nm and 300.0000 nm diameter.


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Synthesis of cyclic carbonates from epoxide and CO2 Taking epichlorohydrin (EO) as an example, the cycloaddition reaction was carried out in a 25 mL stainless autoclave reactor equipped with a magnetic stirrer at the set temperature and pressure. In the typical procedure, EO and catalyst were placed in the reactor and purged with CO2 three times, and then the reactor was sealed. CO2 was injected into the reactor to reach the required pressure. Subsequently, the reactor was heated to the desired temperature by put into a preheated oil bath and the mixture was stirred at the set temperature. After the reaction, the reactor was placed in an ice-water mixture and cooled to room temperature, excess CO2 was vented slowly. Ethyl acetate was added to the reaction mixture, the solid catalyst was separated by filtration. The filtrate was analyzed using a gas chromatograph (Beijing Beifen-Ruili Analytical Instrument) equipped with a FID detector and a capillary column (DB-5, 30 m × 0.25 mm × 0.25 μm) to determine the product yield. The injection temperature was 250 °C and the detector temperature was 270 °C, while the column temperature was maintained for 1 min at 70 °C and then up to 110 °C at a rate of 5 °C·min-1, where this temperature was maintained for 1 min. The reusability of the catalyst was tested in six-run cycling experiments. The solid catalyst was collected by centrifuged, washed with ethanol, dried in the vacuum and then charged into the next run.

Results and discussion Structure of polymer materials. Scheme 1A shows the synthetic procedure, which involves the synthesis of bis-vinylimidizolium, salt monomer ([EEBVIM]Cl2) and the dissolution of [EEBVIM]Cl2, EGDMA, and initiator AIBN (azodiisobutyronitrile) in the ethanol solvent, followed by copolymerization at 80 °C for 24 h, as shown in Scheme 1B. PILs were copolymerized from EGDMA and vinylimidazolium salt monomers, which are typically [EEBVIM]Cl2 with ethyl ether bridging two vinylimidazolium rings that form double cationic structure. This structure provides more active sites than that of monovinylimidazolium salt for the conversion of CO2 into cyclic carbonate. At the same time, a mesoporous structure is formed during polymerization due to the flexible structure of the bridge chain ethyl ether, which provides a large pore structure for mass transfer. Here, [EEBVIM]Cl2 is the new imidazolium salt compound that is specifically designed, prepared, and characterized (Fig. S1). PIL-1 was synthesized with the initial composition of 2 g [EEBVIM]Cl2, 1.2 g EGDMA, 0.062



g AIBN, and 15 mL ethanol solvent. A white powdery solid was obtained after the polymerization of the mixture under certain conditions (Scheme 1B).

Scheme 1. (A) Synthesis of ionic liquid [EEBVIM]Cl2; (B) Possible mechanism diagram of the formation of mesoporous structure by copolymerization of [EEBVIM]Cl2 and EGDMA monomers. Copolymers with a molar ratio of [EEBVIM]Cl2 to EGDMA of 1:2 (PIL-1/2), 2:1 (PIL-2), and 4:1 (PIL-4) were synthesized to determine the morphological structure and catalytic effect of copolymer materials formed under different molar ratios of monomers. For comparison, PIL-a derived from [EEBVIM]Cl2 and PIL-0 derived from EGDMA were synthesized through selfpolymerization. PIL-a sample was unstable in air and did not meet the conditions for investigating heterogeneous catalysts due to its strong water absorption. Thus, we did not evaluate its catalytic performance. All the samples, except PIL-a, were insoluble in common organic solvents, such as DMF, N, N-dimethylacetamide, ethanol, toluene, and dimethyl sulfoxide. As shown in Fig. 1, all the samples exhibit a broad and strong band at 3000-2850 cm-1 due to the stretching vibrations of the C-H group in the FTIR spectra.31 The bands at 1647 cm-1 and 1568 cm1

are attributable to the stretching vibrations of imidazole ring.32-33 The FTIR spectra of PILs show


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two absorption bands of imidazole ring compared with PIL-0 without IL. The absorption peak of imidazole ring increases with the increase on the proportion of ILs. In addition, the bands at 963 cm-1 attributed to the unsaturated C-H vibrations of vinyl groups can be observed on the spectra of [EEBVIM]Cl2 but disappear on the spectra of PILs,34 which suggest the occurrence of polymerization. Their chemical compositions are verified through the analysis of CHN elements (Table 1). The nitrogen (N) contents of PILs range between 1.71–6.99 wt%, which reflect the contents of [EEBVIM]Cl2 between 0.306–1.249 mmol·g-1. Not all bis-vinylimidazolium salts are involved in the polymerization, and the amount of IL in the polymer increases with the increase on the proportion of [EEBVIM]Cl2. All of these observations confirm the presence of [EEBVIM]Cl2 in the polymeric structure.

Fig. 1 FT-IR spectra of samples. Table 1 Elemental analysis for the PILs with varied the molar ratio of [EEBVIM]Cl2 to EGDMA. Entry

Sample

IL content (mmol·g-1)

1 2 3 4

PIL-1/2 PIL-1 PIL-2 PIL-4

0.306 0.435 0.892 1.249

Found C[%]

N[%]

H[%]

55.2 52.7 50.5 47.8

1.71 2.44 4.99 6.99

7.05 6.73 6.94 6.89

SEM images (Fig. 2) indicate that they are composed of irregular particles on the micrometer level. The primary spherical particles of PIL-1/2 samples are cross-linked with each other to form a



cluster-like morphology. The pore size of the polymer increases with the increase of IL content in the polymer due to the flexible structure of IL. This conditions is also evidenced by nitrogen adsorption desorption (Table 2). For example, the pore size of PIL-4 is larger than that of the pore size of PIL-1. Elemental mapping images (Figs. 3A and B) present the homogeneous dispersions of C, N, O, and Cl throughout the entire skeleton, which suggest the homogeneous dispersion of IL units in the polymeric framework. TEM images (Figs. 3C and D) show that the surface of spherical particles have randomly oriented micropores. Images of SEM and TEM for other samples is shown in the supporting information (Figs. S2-S4). TG analyses show that these polymers have high thermal stability with starting decomposition temperature greater than 260 °C (Fig. S5).

Fig. 2 SEM images of samples (A-D represents the samples PIL-1, PIL-1/2, PIL-2, PIL-4, respectively).


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Fig. 3 (A) SEM and (B) elemental (C, N, O and Cl) mapping images of PIL-4. TEM images of (C) PIL-2 and (D) PIL-4. The structure of PIL-4 sample was confirmed though XPS analysis. Peaks corresponding to C 1s, N 1s, O 1s, and Cl 2p were clearly observed in the full spectrum of XPS (Fig. 4A), which indicate that the chemical components of the sample are carbon, nitrogen, oxygen, and chlorine. Notably, the presence of a peak at 401.0 eV in response to the N 1s of imidazolium-based cations (Fig. 4B) indicated that [EEBVIM]Cl2 is successfully implanted in the backbone of PIL-4 through polymerization.35 The chemical state of each element of the sample containing different amounts of [EEBVIM]Cl2 did not change based on the XPS analysis of other samples in the supporting information (Figs. S6-8). However, the peaks of N 1s and Cl 2p corresponding to samples containing considerable [EEBVIM]Cl2 were observed to be higher than others in the full spectrum of XPS. The porous characteristics of the samples were obtained at 77 K based on N2 adsorptiondesorption analysis (Fig. 5A). The N2 adsorption isotherm indicates a slow absorption at low relative pressure (P/P0 < 0.03) and a continuous increasing adsorption at high relative pressure (0.3 < P/P0 < 0.9). They show a type IV isotherm that reflects the mesoporous structure. This finding is indicated by the pore size distribution curves in Fig. 5B. All samples showed abundant meso-macroporous structures with a porous diameter of approximately 15-65 nm. Their structural characteristics are summarized in Table 2. PIL-1, PIL-1/2, PIL-2, and PIL-4 (BET) specific surface areas are 9.03, 10.40, 8.55 and 1.97 m2·g-1, respectively. PIL-0 without IL is as high as 22.8 m2·g-1. The surface



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area and pore volume of the polymer gradually decrease with the increase of IL content in the polymer. This experimental condition is because the abundant halogen anions in the nanopore structure lead to a decrease in BET surface area.36-37 Table 2 The results of Nitrogen adsorption–desorption

a

Entry

Catalyst

SBETa (m2·g-1)

Vpb (cm3·g-1)

Davec (nm)

1 2 3 4 5

PIL-0 PIL-1 PIL-1/2 PIL-2 PIL-4

22.80 9.03 10.40 8.55 1.97

0.039 0.037 0.033 0.044 0.005

25.93 21.67 19.45 20.23 63.97

BET surface area. b Total pore volume. c Average pore size.

Fig. 4 (A) Full XPS spectrum and (B) the high resolution of N 1s XPS spectrum for PIL-4.


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Fig. 5 (A) N2 sorption isotherms and (B) pore size distribution curves. The sorption isotherms of PIL-0, PIL-1, PIL-1/2 and PIL-2 are shifted by 20, 15, 10 and 5 cm3·g−1. The pore size distribution curves of PIL-0, PIL-1 and PIL-1/2 are shifted by 0.006, 0.004 and 0.002 cm3·g−1·nm−1. Catalyst evaluation The cycloaddition reaction of CO2 with epoxide is a promising strategy for the effective chemical fixation of CO2, and the obtained product cyclic carbonates have important applications as valuable precursors for polymers, such as polycarbonates and polyurethanes, and as raw materials for a wide range of reactions.38-39 PILs were investigated as heterogeneous catalysts in this reaction. The investigation started with the synthesis of (chloromethyl) ethylene carbonate (EC) from epichlorohydrin and CO2 without any co-catalysts, solvents, and other additives. We used the



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catalyst PIL-1 to investigate the effects of time, pressure, dosage and reaction time on catalysis to determine a suitable catalytic condition. The results are listed in Table 3. Temperature played an important role in the reaction. The reaction yield rapidly increased from 4.2% to 58.8% when the temperature increased from 60 °C to 100 °C (entries 2-4). The temperature remained constant because catalysis was conduced in a mild environment. The reaction yield increased from 35% to 58% with the increase of pressure 0.5 MPa to 1 MPa. The yield slightly reduced with the increase of pressure. Therefore, 1 MPa was considered to be the most suitable reaction pressure (entries 3, 7, and 8). The reaction yield increased from 35.6% to 58.8% when the reaction time increased from 6 h to 12 h. Long reaction time led to a slight increase in EC yield (entries 3, 5, 6, and 7). The effect of catalyst dosage on reaction yield was evaluated. The reaction yield increased from 58.8% to 71.1% when the catalyst increased from 10 mg to 20 mg. The increase of dosage to 30 mg led to a slight increase of reaction yield. On the basis of the above experimental results, we selected this catalytic condition: epichlorohydrin (5 mmol), catalyst (20 mg), CO2 (1 MPa), 100 °C, and 12 h as the suitable condition. For comparison, approximately no EC conversion was observed in the control experiment due to the absence of any catalyst in this condition. Table 3. Results of cycloaddition reaction of CO2 with EOa Entry

Catalyst

T [°C]

t [h]

P [MPa]

Dosage [mg]

Yieldb [﹪]

Sel.c [﹪]

1 2 3 4 5 6 7 8 9 10 11

blank PIL-1 PIL-1 PIL-1 PIL-1 PIL-1 PIL-1 PIL-1 PIL-1 PIL-1 PIL-1

100 60 80 100 100 100 100 100 100 100 100

12 12 12 12 6 9 15 12 12 12 12

1 1 1 1 1 1 1 0.5 1.5 1 1

10 10 10 10 10 10 10 10 10 20 30

3.6 4.2 16.5 58.8 35.6 47.5 61.2 53.7 57.7 71.7 74.2

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

aReaction

conditions: epichlorohydrin (5 mmol). bYield and cSelectivity were determined by GC using n-dodecane as the internal standard. The experiments were conducted on PILs to investigate the catalytic activity of different samples under appropriate catalytic conditions selected for the above studies. The results are listed in Table 4. As expected, approximately no product was detected in PIL-0 (entry 1) due to the lack of active sites of halogen anions for this reaction. All PILs showed catalytic activity for the reaction (entries 2-5). High surface area and abundant active sites are important conditions to catalyze the reaction.


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The CHN elemental analysis indicated that PIL-4 contains the most active sites compared with other samples. Thus, PIL-4 exhibited the highest yield of 99.7% (selectivity >99%). Although the reaction time was shortened from 12 h to 6 h, PIL-4 showed a high yield of 82.3% with a TON of 13.7. The BET data showed that PIL-1/2 has the largest surface area and the least active site compared with other samples and shows the lowest yield of 57.8% (selectivity >99%) but with a high TON of 19.6. This condition is because PIL-1/2 contains a larger specific surface area, which has a better enrichment effect on CO2. The experimental results showed that high surface area and abundant active sites play an equally important role in this reaction. Table 4. Cycloaddition of CO2 with EOa O Cl

+ CO2

O

catalyst

O

Cl

O

Entry

Catalyst

Yieldc [%]

Sel.d [%]

TOFe [h-1]

1 2 3 4 5 6b

PIL-0 PIL-1 PIL-1/2 PIL-2 PIL-4 PIL-4

3.2 71.7 57.8 85.1 99.7 82.3

98 >99 >99 >99 >99 >99

0 17.2 19.6 9.9 8.3 13.7

aReaction

conditions: epichlorohydrin (5 mmol), catalyst (20 mg), CO2 (1 MPa), 100 °C, 12 h. bConditions: catalyst (20 mg), CO (1 MPa), 100 °C, 6 h. cYield and dSelectivity were determined 2 by GC using n-dodecane as the internal standard. eTurnover frequency (TOF) = [mmol (product)] / [mmol (ionic sites) × (reaction time)]. The cycloaddition reactions of CO2 with different epoxide substrates were conducted on PIL-4. The yield and selectivity of target products determined by GC using n-dodecane as the internal standard are summarized in Fig. 6. On the basis of the above experiment, the reaction can be completed in 12 h at 100 °C and 1 MPa when the R group is a chloromethyl group (Table 3, entry 5) with a weak electron attracted effect. The reaction shows a low activity (2a-2c) when R has a strong conjugation effect. However, high yield of 95% can be obtained when the reaction time is increased to 96 h. The reaction smoothly proceeds at a high yield of 90% at 100 °C unless the reaction time rises to 96 h when R is a weak electron-catalyzed methyl group. The reaction exhibits high selectivity greater than 98% whether the R group is an electron withdrawing or electron donating substituent, especially in the case of low yields.



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Fig. 6 Substrate scope in the synthesis of various cyclic carbonates from epoxides 1−2 using PIL-4 as catalyst (epoxide 5 mmol, catalyst (20 mg), CO2 (1 MPa), 100 °C, a: 12 h, b: 48 h, c: 96 h). Recycling experiments were conducted to investigate the reusability of PIL-4 in the cycloaddition of CO2 with epichlorohydrin under 100 °C and 1 MPa (Fig. 7). After six runs, the catalyst still keeps its initial reactivity. The yield and selectivity of cycloaddition remained constant in the recycling tests, which prove the stable reusability of PIL-4.

99

99

Yield (%) selectivity (%) 99 99 99 99 99 99 98 98

99

2

6

99

100

50

0 1

3

4

5

Fig. 7 Reusability of PIL-4 in the cycloaddition of CO2 with epichlorohydrin (EO). Reaction conditions: epichlorohydrin (5 mmol), catalyst (20 mg), CO2 (1 MPa), 100 °C, 12 h. Reaction Mechanism We present a possible catalytic mechanism for the cycloaddition of CO2 with EO in the presence


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of PILs based on our current observations and previous literature,36,40-42 as shown in Fig. 8. The porosity and distributed active sites of catalytic material are the important factors for heterogeneous catalysis. PIL-4 has abundant pores, surface area, and uniform distribution of active ion sites for the cycloaddition reaction of CO2 with epoxide. The abundant porosity and mesoporous structure provide the catalyst material with high CO2 absorption and mass transfer rate, and the distributed active sites provide power for the CO2 cycloaddition reaction. The catalytic cycle begins with a synergistically activated transition state within the molecule of the epoxide. Moreover, the epoxide binds with hydrogen on C2 on the imidazole ring in the PIL-4 network by forming an H bond. At the same time, the nucleophilic attack of chloride ions on the side where the epoxide is slightly hindered causes the ring opening of the activated epoxide. Subsequently, CO2 enriched in PIL-4 with abundant channel material is rapidly inserted in the open epoxide. After the insertion of CO2 in the opened epoxide, intramolecular cyclization forms the corresponding cyclic carbonate. The reactions of catalytic cycle stop and regenerate the monolith catalyst. The above-mentioned advantages may be because that the PIL-4 catalyst exhibits high catalytic activity for the cycloaddition reaction of CO2.

Fig. 8 A possible mechanism for the synthesis of cyclic carbonate over PILs.



Conclusions A novel bis-vinylimidazolium-based chloride salt was copolymerized with EGDMA to form stable porous PILs by using simple and green synthetic methods. Different copolymers were synthesized by adjusting the molar ratio of the monomeric IL and EGDMA, and their changes in morphology and chemical composition were evaluated. They exhibited excellent catalytic activity and high selectivity for the cycloaddition of CO2 with epoxides under mild conditions. In addition, they show excellent recyclability with little loss in activity even after six catalytic runs.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. This file contains 1H NMR spectra of [EEBVIM]Cl2, SEM images of PILs, Elemental (C, N, O and Cl) mapping images, TG and XPS spectrum.

Acknowledgments We thank the support provided by the National Key R&D Program of China (No. 2017YFB0602804), the National Natural Science Foundation of China (No. 21878164), the Key Scientific and Technological Project of Shanxi Province (No. MH2014–10) and the National Key Technology Support Program of China (No. 2014BAC10B01).

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Table of Contents Graphic

A novel PIL was developed as a versatile platform to converse CO2 into usable chemical products under mild conditions


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Design of Novel Poly(ionic liquids) for the Conversion of CO2 to Cyclic

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