Highly cross-linked cationic polymer microspheres as an efficient catalyst for facile CO2 fixation

    Highly cross-linked cationic polymer microspheres as an efficient catalyst for facile CO2 fixation Yan Leng, Dan Lu, Pingping Jiang, Chenjun Zhang, Jiwei Zhao, Weijie Zhang PII: DOI: Reference:

S1566-7367(15)30139-4 doi: 10.1016/j.catcom.2015.11.006 CATCOM 4502

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

7 September 2015 15 October 2015 10 November 2015

Please cite this article as: Yan Leng, Dan Lu, Pingping Jiang, Chenjun Zhang, Jiwei Zhao, Weijie Zhang, Highly cross-linked cationic polymer microspheres as an efficient catalyst for facile CO2 fixation, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.11.006

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ACCEPTED MANUSCRIPT Highly Cross-Linked Cationic Polymer Microspheres as an Efficient Catalyst for Facile CO2 Fixation

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Yan Leng*, Dan Lu, Pingping Jiang, Chenjun Zhang, Jiwei Zhao, Weijie Zhang

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The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of

*

Corresponding

author,

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Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China. +86-510-885917090,

+86-510-85917763,

E-mail:

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[email protected]

Fax:

Abstract: In this study, a new type of highly cross-linked cationic polymer microspheres was

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synthesized by a simple procedure of combining 1,2,4,5-tetrakis(bromomethyl)benzene and 4,4’-bipyridine. The influence of solvent used on the material structure and morphology was

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investigated and the obtained cationic polymers were fully characterized by FT-IR, BET, SEM, EDS, and TGA. Catalytic tests in the synthesis of cyclic carbonate from carbon dioxide (CO2) and

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epoxides under mild conditions, along with comparisons to various counterparts, well demonstrate that the newly designed cross-linked cationic polymer with regular spherical structure exhibits

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high activity and selectivity, coupled with easy recovery and steadily reuse. Keywords: CO2 Fixation, Cationic polymer, Microspheres, Cyclic carbonates

1. Introduction Catalyst-mediated reactions of carbon dioxide (CO2) represent one potential positive contributor to climate-relevant carbon capture and storage/sequestration, albeit a far from sufficient one to satisfy this enormous challenge.1-3 Well-designed reactions that utilize CO2 in the production of commercially relevant chemicals are receiving increased attention, including the chemical fixation of CO2.4-9 The conversion of CO2 to cyclic carbonates via epoxide substrates is an 1

ACCEPTED MANUSCRIPT atom-economical reaction, and the products can serve as excellent aprotic polar solvents and as intermediates in the production of pharmaceuticals and fine chemicals.10-12

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In the past few decades, numerous homogeneous and heterogeneous catalysts have been

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developed for the synthesis of cyclic carbonates from epoxides and CO2, such as salen-metal compounds13, transition metal complexes14, alkali metal salts15, ionic liquids (ILs)16, metal oxides17, metal–organic frameworks18, and so on. Among them, ILs with halogen anions are

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especially highlighted because of that the ILs can be tailored by designing their cations or anions

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with various functionalities for different applications.19,20 However, ILs always cause homogeneous reactions because of their good solubility in polar media, resulting in the difficulty

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of ILs isolation. To recover the ILs, ILs usually be heterogenized by chemical immobilization onto

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solid supports, such as silica, activated carbon, and polymeric microsphere.21-24 Nevertheless, this

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method depends on the modified solid supports and specific functionalized ILs. Thus, developing a new strategy for synthesis of heterogeneous ILs catalysts with high catalytic activity is still

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highly desirable.

Recent years have witnessed considerable interest in narrow or monodispherse highly cross-linked spherical polymer particles in the micrometer-size range due to their great potential in a wide range of materials science applications.25,26 Meanwhile, IL is an ideal building block for constructing multi-functional cationic polymer materials owning to the features of negligible vapor

pressure,

ionicity

and

versatile

functional

groups.27-29

Herein,

1,2,4,5-tetrakis(bromomethyl)benzene (TBB) was selected as an ideal four-connected building unit to construct highly cross-linked cationic polymer microspheres by reacting with the 4,4’-bipyridine in various solvents (Figure 1), and the resulting samples can be sufficiently applied 2

ACCEPTED MANUSCRIPT for the production of cyclic carbonates from CO2 and epoxides, and then can be easily separated from the reaction mixture to reuse.

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2. Experimental section

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2.1 Materials and characterization

All chemicals were commercially available and used as received. FT-IR spectra were recorded on a Nicolet 360 FT-IR instrument (KBr discs) in the 4,000-400 cm-1 region. Liquid-state 1

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H-NMR spectra were measured with a Bruker DPX400 spectrometer at ambient temperature

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using TMS as internal reference. Field emission scanning electron microscope (FESEM; Hitachi S-4800, accelerated voltage: 5 kV) accompanied by Energy dispersive X-ray spectrometry (EDS;

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accelerated voltage: 20 kV) was used to study the morphology and the elements distribution. TG

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analysis was performed with a STA409 instrument in dry air at a heating rate of 10 °C/min. CHN

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elemental analysis was carried out on an elemental analyzer Vario EL cube. 2.2 Synthesis of cross-linked cationic polymers

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The highly cross-linked cationic polymers were solvothermally synthesized from the reaction of TBB and 4,4’-bipyridine. As a typical example, TBB (2 mmol, 0.9 g) and 4,4’-bipyridine (4 mmol, 0.62g) were dissolved in tetrahydrofuran (THF) (20 mL). After stirring at room temperature for 2 h, the mixture was solvothermally treated at 100°C for 24 h. The yellow solid product was filtered and the washed with THF for three times. After drying at 50°C for 12 h, the product cross-linked cationic polymer was obtained, signed as TBB-Bpy-a. TBB-Bpy-b (obtained in toluene), TBB-Bpy-c (obtained in acetonitrile), and TBB-Bpy-d (obtained in ethyl acetate) were prepared in the same way with different solvents. 2.3 Synthesis of TBB-based ionic compounds 3

ACCEPTED MANUSCRIPT TBB-py: TBB (2 mmol, 0.9 g) and pyridine (8 mmol, 0.64 g) were dissolved in CH3CN (40 mL). After stirring for 24 h at 80°C, the white precipitate formed was filtered and washed with

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CH3CN for three times, and dried in vacuum at 80°C for 12 h. 1H NMR (400 MHz, D2O, TMS)

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δ(ppm) = 6.09 (s, 8H, CH2), 6.77 (s, 2H, CH), 8.10 (t, 8H, CH), 8.61 (t, 4H, CH), 8.86 (d, 8H, CH) (see Figure S1 in Supporting Information (SI)). CHN elemental analysis for TBB-py found (wt %): C 47.05, H 3.86, N 7.28.

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TBB-pyA: TBB (2 mmol, 0.9 g) and 3-aminopyridine (8 mmol, 0.76g) were dissolved in

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CH3CN (20 mL). After stirring for 24 h at 80°C, the yellow precipitate formed was filtered and washed with CH3CN for three times, and dried in vacuum at 80°C for 12 h. TBB-pyA was

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unsuitable to be subjected to a 1H NMR test because of that it is insoluble in commonly used

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deuterated solvents. Thus, CHN elemental analysis was carried out to determine its structure.

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CHN elemental analysis for TBB-pyA found (wt %): C 43.65, H 4.10, N 13.63. TBB-pyH: TBB (2 mmol, 0.9 g) and 3-hydroxypyridine (8 mmol, 0.72 g) were dissolved in

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acetone (20 mL). After stirring for 24 h at 60°C, the white precipitate formed was filtered and washed with acetone for three times, and dried in vacuum at 50°C for 12 h. 1H NMR (400 MHz, D2O, TMS) δ(ppm) = 5.90 (s, 12H, OH, CH2), 6.47 (s, 2H, CH), 7.84 (t, 8H, CH), 7.95 (d, 4H, CH), 8.25 (s, 4H, CH) (Figure S2 in SI). CHN elemental analysis for TBB-pyH found (wt %): C 43.44, H 3.58, N 6.70. TBB-pyC: TBB (2 mmol, 0.9 g) and 3-carboxylicpyridine (8 mmol, 0.98 g) were dissolved in toluene (15 mL) and ethyl alcohol (30 mL), respectively. The mixture of the above two solutions was stirred at 80°C for 24 h. After reaction, the white precipitate formed was filtered and washed with toluene and ethyl alcohol for many times, and dried in vacuum at 60°C for 12 h. 1H NMR 4

ACCEPTED MANUSCRIPT (400 MHz, D2O, TMS) δ(ppm) = 6.07 (s, 8H, CH2), 6.70(s, 2H, CH), 8.12 (t, 4H, CH), 8.91 (d, 12H, CH), 9.19 (s, 4H, COOH) (Figure S3 in SI). CHN elemental analysis for TBB-pyC found (wt

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%): C 43.27, H 3.22, N 5.93.

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2.4 General procedures for the preparation of cyclic carbonates

Epoxide derivatives (20 mmol), such as propylene oxide (PO) and catalyst TBB-Bpy (0.08 g) were charged into the reactor vessel without using any co-solvent. The reaction vessel was placed

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under a constant pressure of CO2 and then heated to 120°C for 4 h (Scheme 1). After the reaction,

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the reactor was cooled to ambient temperature, and the resulting mixture was filtered. The liquid mixture was analyzed by gas chromatography (GC) using biphenyl as an internal standard (Figure

run.

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O

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S4 in SI). The solid catalyst was washed with ethyl alcohol, dried, and directly used for the next

+

CO2

O TBB-Bpy-a

O

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120°C, 1 MPa, 4 h Con: 99%; Sel: 100%

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Scheme 1. Synthesis of propylene carbonate from propylene oxide and CO2.

3. Results and discussion 3.1 Preparation and Characterization of TBB-Bpy The cationic polymers are solvothermally synthesized by reacting of TBB with 4,4’-bipyridine in a stainless steel reaction still as shown in Figure 1. The solvents used in the synthetic media are considered to be an important factor in the formation of microsphere structures. Thus, various solvents, including THF, toluene, acetonitrile, and ethyl acetate, are applied in the preparation of organic polymers. The Figure 2 illustrates the SEM images of the obtained samples TBB-Bpy-a, 5

ACCEPTED MANUSCRIPT TBB-Bpy-b, TBB-Bpy-c, and TBB-Bpy-d. The primary particles of the TBB-Bpy-a sample are relatively uniform microspheres with smooth surface and sizes of approximately 2-3 μm (Figure

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2A). The TBB-Bpy-b sample is agglomerated spherical particles with irregular blocks on the

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surface (Figure 2B). The TBB-Bpy-c sample is also spherical structure (Figure 2C), whereas the TBB-Bpy-d is amorphous stripe structure (Figure 2D). Moreover, the surface of TBB-Bpy-c and TBB-Bpy-d became rough with some agglomeration, which is different with those of TBB-Bpy-a

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and TBB-Bpy-b. Due to the different surface tension of solvents, the interaction between

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solvent media and nucleation particles is different, which strongly affect the morphology of the nucleation particles. The above phenomenon suggests that the spherical particles formed in

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THF solvent are the most stable. The EDS structural characterization validates the uniform

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distribution of Br- on the surface of TBB-Bpy-a microspheres (Figures 2E and 2F). The pore

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structure and the surface area for these polymers were characterized by nitrogen sorption experiments at 77 K. The results show that they are almost nonporous with BET surface areas less

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than 10 m2g-1.

The cationic polymers are insoluble in water or common organic solvents, such as acetone, ethanol, chloroform, hexanes, N,N-dimethylformamide (DMF), and THF. The IR spectra of TBB-Bpy-a in Figure 3 exhibits the characteristic peaks at 1635 and 1216 cm-1 assigned to stretching vibrations of C=N and C-N in dipyridyl, respectively, the bands between 1552~1445 cm-1 are assigned to stretching vibrations of the aromatic ring, and the peak at 810 cm-1 represents the stretching vibrations of C-H in aromatic rings. These vibrational signatures confirm the successful combination of TBB and 4,4’-bipyridine by covalent interaction. The cationic polymer microspheres TBB-Bpy-a exhibits high thermal stability as evidenced by thermogravimetric 6

ACCEPTED MANUSCRIPT analysis (Figure 4), they are stable up to 280 oC, which is higher than the non cross-linked ionic compound TBB-py synthesized by combining TBB and pyridine (Tdec ≈ 250oC).

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3.2 Catalytic activity

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The catalytic activities were first tested on PO as the model substrate under solvent free conditions, and the results are summarized in Table 1. In the absence of catalyst, almost no product was detected (entry 1). Cetyl trimethyl ammonium bromide (CTAB) has been

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demonstrated to be a highly efficient catalyst for the synthesis of cyclic carbonates from epoxides

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and CO2.30 Herein, when CTAB was used as the catalyst at 120°C with 1 MPa CO2 for 4 h, it leads to a homogeneous catalysis, and offers 88% conversion and 98% selectivity (entry 2).

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Interestingly, the prepared cationic polymers are all insoluble in the reaction system, and thus

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result in heterogeneous catalysis, and the TBB-Bpy-a microspheres exhibit the conversion of 99%

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with 100% selectivity (entry 3), that is higher than those of TBB-Bpy-b, TBB-Bpy-c, and TBB-Bpy-d with a relative amorphous structure, which show 95%, 92%, and 95% conversions

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(entries 4-6), respectively. This is mostly due to the good dispersion and high surface area of TBB-Bpy-a that allow the Br anions on the catalyst surface giving full play as active centres

and to contact the reaction substrate more easily. It was reported that functional groups, such as hydroxyl and carboxyl are favorable for the cycloaddition reactions. For comparison, the non cross-linked ionic compounds TBB-py, TBB-pyC, TBB-pyA, and TBB-pyH were prepared as the control catalysts by reacting of tetrakisbenzene with pyridine or functionalized pyridine, as verified by 1H NMR spectroscopy (see Figures in Supporting Information). All ionic compounds were obtained as white powders and led to similar solid-liquid heterogeneous catalysis. The catalytic activities of functional ionic 7

ACCEPTED MANUSCRIPT compounds (entries 3-6) are obviously higher than that of the functional group-free ionic compound TBB-py that shows only a 89% conversion with 98% selectivity (entry 7). Among

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which, TBB-pyH offers the highest conversion of 97% (entry 10). However, it is still less active

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than cross-linked cationic microspheres TBB-Bpy-a. Besides, these ionic compounds are easy to absorb water and aggregate to a thick solid during the reaction, which cause great difficulties in the catalyst recycling and reuse. The above results suggest that the cationic polymer structure

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plays an important role for the catalytic efficiency of the catalysts.

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Compared with the previously reported homogeneous ILs and heterogeneous catalysts (Table S1 in SI), such as salen-metal compounds31, fluorous polymer immobilized phosphonium

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chlorides32, porous metal-metalloporphyrin framework33, polymer anchored ILs34, and supported

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phosphotungstate17, TBB-Bpy-a clearly stands out in terms of yield and milder conditions. It also

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should be mentioned that the hafnium-based metal-organic framework18 and organic sulphonate salts tethered to mesoporous silicas34 are good heterogeneous catalysts for fixation of CO2 with

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yields above 98%, comparable to our catalyst, but additives (KI, tetrabutylammonium bromide, etc.) are usually added into reaction media aiming to get high conversion and selectivity. In contrast, our new catalyst provides high conversion and selectivity without the aid of any additives. We consider that the electronic interaction because of the pyridine cations and the nucleophilic attack by the Br anion, accounts for the excellent acitivity. A possible mechanism is suggested and shown in Scheme S1 of SI, which is in accordance with that of the previous literatures8,11,20, 35. The crucial roles, such as the temperature, CO2 pressure, catalyst amount, and reaction time were also investigated in the fixation of CO2 with PO in the presence of TBB-Bpy-a as a catalyst. 8

ACCEPTED MANUSCRIPT The results (Table S2 in SI) clearly indicate that these conditions have a remarkable influence on the yield of PC. The best temperature and reaction time for this reaction is 120oC and 4 h,

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respectively. Notably, the catalyst mixture reaches a 93% conversion with 100% selectivity even

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at 0.5 MPa CO2 pressure for 8 h, demonstrating the high efficiency of this novel catalyst. Consequently, we examined the performances of TBB-Bpy-a in chemical fixation of CO2 with different functional group substituted epoxides. The results are summarized in Table 2. It is

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observed that the terminal epoxides can be transformed to the corresponding cyclic carbonates in

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high yield and selectivity (entries 1-3). Due to the larger hindrance as a result of the rings, the internal epoxide cyclohexane oxide requires a much longer reaction time 12 h to reach 76%

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conversion and 61% selectivity (entry 5).

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After the reaction, the cross-linked cationic polymer microspheres can be easily recycled by

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filtration or centrifugation and reused in the next run. The catalytic reusability of TBB-Bpy-a for the fixation of CO2 with PO is shown in Figure 5. The catalyst was reused six successive times

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without a significant decrease in the efficiency of the catalyst or structural deterioration as determined by FT-IR analysis (Figure 3, curve b). To confirm the heterogeneous nature of the reaction, under the same conditions as in Table 1, entry 3, 1 h after the outset of the reaction, the catalyst was removed and the reaction was allowed to continue. As expected, no increase in the formation of carbonate was detected, indicating that the catalytic reaction is indeed heterogeneous in nature.

4. Conclusions In conclusion, we have developed a novel type of cross-linked cationic polymer microspheres by

reacting

four-connected

building

unit 9

1,2,4,5-tetrakis(bromomethyl)benzene

and

ACCEPTED MANUSCRIPT 4,4’-bipyridine. The solvents used in the synthetic media are demonstrated to be an important factor for the formation of microspheres structure, and the spherical particles TBB-Bpy-a formed

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in THF solvent are the most stable. The obtained spherical particles are proved to be highly

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efficient heterogeneous catalyst for the transformation of CO2 and epoxides into cyclic carbonates under metal-free and solvent-free conditions. In addition, the TBB-Bpy-a had good recyclability, which was mainly attributed to the featured cross-linked covalent cationic structure. This synthetic

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method provides a green way to effectively prepare low-cost IL-based solid catalysts and is

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promising for the development of other useful materials.

Acknowledgments: The authors thank the National Natural Science Foundation of China (No

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21206052) and MOE & SAFEA for the 111 Project (No. B13025).

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References

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1. E. K. Noh, S. J. Na, S. S, S. W. Kim, B. Y. Lee, J Am Chem Soc 129 (2007) 8082. 2. J. A. Kozak, J. Wu, X. Su, Fritz Simeon, T. Alan Hatton, Timothy F. Jamison, J Am Chem

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Soc 135 (2013) 18497.

3. C. E. Anderson, S. I. Vagin, W. Xia, H. Jin, B. Rieger, Macromolecules 45 (2012) 6840. 4. W. M. Ren, Y. Liu, X. Lu, J Org Chem 79 (2014) 9771. 5. R. Luo, X. Zhou, S. Chen, Y. Li, L. Zhou, H. Ji, Green Chem 16 (2014) 1496. 6. J. Wang, Curr Green Chem 2 (2015) 3. 7. Q. He, J. W. O’Brien, K. A. Kitselman, L. E. Tompkins, G. C. T. Curtis, F. M. Kerton, Catal Sci Technol 4 (2014) 1513. 8. B. Xu, J. Wang, J Sun, Y. Huang, J. Zhang, X. Zhang, S. Zhang, Green Chem 17 (2015) 108.

10

ACCEPTED MANUSCRIPT 9. W. N. R. Wan Isahak, Z. A. Che Ramli, M. W. Mohamed Hisham, M. A. Yarmo, Renew Sust Eener Revi 47 (2015) 93.

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10. B. Chatelet, L. Joucla, J. P. Dutasta, A. Martinez, K. C. Szeto, V. Dufaud, J Am Chem Soc

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135 (2013) 5348.

11. W. Dai, B. Jin, S. Luo, X. Luo, X. Tu, Au Chak-Tong, Catal Sci Technol 4 (2014) 556. 12. T. Roy, R. I. Kureshy, N. H. Khan, S. H. R. Abdi, H. C. Bajaj, Catal Sci Technol 3 (2013)

NU

2661.

MA

13. D. Adhikari, S. T. Nguyen, M. Baik, Chem Commun 50 (2014) 2676. 14. T. Ema, Y. Miyazaki, J. Shimonishi, C. Maeda, J. Hasegawa, J Am Chem Soc 136 (2014)

D

15270.

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15. M. A. Fuchs, C. Altesleben, S. C. Staudt, O. Walter, T. A. Zevaco, E. Dinjus, Catal Sci

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Technol 4 (2014) 1658.

16. J. Wang, J. Leong, Y. Zhang, Green Chem 16 (2014) 4515.

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17. S. M. Sadeghzadeh, Green Chem 17 (2015) 3059. 18. M. H. Beyzavi, R. C. Klet, S. Tussupbayev, J. Borycz, N. A. Vermeulen, C. J. Cramer, J. F. Stoddart, J. T. Hupp, O. K. Farha, J Am Chem Soc 136 (2014) 15861. 19. B. Xu, J. Wang, J. Sun, Y. Huang, J. Zhang, X. Zhang, S. Zhang, Green Chem 17 (2015) 108. 20. M. Liu, B. Liu, S. Zhong, L. Shi, L. Liang, J. Sun, Ind Eng Chem Res 54 (2015) 633. 21. Y. Zhang, S. Yin, S. Luo, C. T. Au, Ind Eng Chem Res 51 (2012) 3951. 22. F. Adam, J. N. Appaturi, E. Ng, J Mol Catal A 386 (2014) 42. 23. A. Pourjavadi, S. H. Hosseini, M. Doulabi, S. M. Fakoorpoor, F. Seidi, ACS Catal 2 (2012) 1259. 11

ACCEPTED MANUSCRIPT 24. W. Zhang, Q. Wang, H. Wu, P. Wu. M. He, Green Chem 16 (2014) 4767. 25. J. Xu, G. Chen, R. Yan, D. Wang, M. Zhang, W. Zhang, P. Sun, Macromolecules 44 (2011)

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3730.

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26. H. Li, P. Zhang, L. Zhang, T. Zhou, D. Hu, J Mater Chem 19 (2009) 4575. 27. J. L. Bideau, L. Viau, A. Vioux, Ionogels, Chem Soc Rev 40 (2011) 907. 28. J. P. Hallett, T. Welton, Chem Rev 111 (2011) 3508.

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29. S. Soll, Q. Zhao, J. Weber, J. Yuan Chem Mater 25 (2013) 3003.

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30. J. Tian, J. Wang, J. Chen, J. Fan, F. Cai, L. He, Appl Catal A 301 (2006), 215. 31. Q. An, Z. Li, R. Graff, J. Guo, H. Gao, C. Wang, ACS Appl Mater Interfaces 7 (2015) 4969.

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32. Q. Song, L. He, J. Wang, H. Yasuda, T. Sakakura, Green Chem 15 (2013) 110.

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33. W. Gao, L. Wojtas, S. Ma, Chem Commun 50 (2014) 5316.

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34. S. Ravi, D. H. Kang, R. Roshan, J. Tharun, A. C. Kathalikkattil, D. W. Park, Catal Sci Technol 5 (2015) 1580.

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35. J. Tian, J. Wang, J. Chen, J. Fan, F. Cai, L. He, Appl Catal A 301 (2006) 215.

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Figure captions

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Figure 1. Synthesis of cross-linked cationic polymer microspheres TBB-Bpy-a and ionic compounds TBB-pyR.

Figure 2. SEM images of TBB-Bpy obtained in different solvents, (A) and (E) TBB-Bpy-a (in

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THF), (B) TBB-Bpy-b (in toluene), (C) TBB-Bpy-c (in acetonitrile), (D) TBB-Bpy-d (in ethyl

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acetate), and (F) EDS elemental mapping of Br element.

Figure 3. FT-IR spectra of (a) fresh TBB-Bpy-a and (b) reused TBB-Bpy-a.

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Figure 4. TG curves of (a) TBB-Bpy-a and (b) TBB-py.

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Figure 5. Catalytic reusability of TBB-Bpy-a for the fixation of CO2 with PO.

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Reaction conditions: PO 20 mmol, catalyst 0.08g, temperature 120°C, initial CO2 pressure 1.0

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MPa, reaction time 4 h.

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compounds TBB-pyR.

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Figure 1. Synthesis of cross-linked cationic polymer microspheres TBB-Bpy-a and ionic

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Figure 2. SEM images of TBB-Bpy obtained in different solvents, (A) and (E) TBB-Bpy-a (in THF), (B) TBB-Bpy-b (in toluene), (C) TBB-Bpy-c (in acetonitrile), (D) TBB-Bpy-d (in ethyl

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acetate), and (F) EDS elemental mapping of Br element.

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(b)

1552

4000

1216

810

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1635

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3000

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Transmittance (%)

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2500

2000

1500

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500

-1

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Wavenumbers (cm )

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Figure 3. FT-IR spectra of (a) fresh TBB-Bpy-a and (b) reused TBB-Bpy-a.

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Weight mass (%)

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60

(a)

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(b)

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20 200

300

400

500

600

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Temperature( C)

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Figure 4. TG curves of (a) TBB-Bpy-a and (b) TBB-py.

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99 100

94

100

Selectivity

95

100

100

92

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98

99 100

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Conversion 100

60 40 20 0 1

2

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PC yield and selectivity (%)

120

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4

5

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Figure 5. Catalytic reusability of TBB-Bpy-a for the fixation of CO2 with PO.

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MPa, reaction time 4 h.

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Reaction conditions: PO 20 mmol, catalyst 0.08g, temperature 120°C, initial CO2 pressure 1.0

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Catalyst

Conversion (%)

1

Non

—

2

CTAB

88

3

TBB-Bpy-a

4

TBB-Bpy-b

5

TBB-Bpy-c

6

TBB-Bpy-d

7

TBB-py

8

TBB-pyC

a

—

95

100

92

100

95

100

89

98

93

100

TBB-pyA

94

100

TBB-pyH

97

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Selectivity (%)

99

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Entry

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Table 1. Effect of various catalysts on the synthesis of propylene carbonate (PC).a

Reaction conditions: PO 20 mmol, catalyst 0.08g, temperature 120°C, initial CO2 pressure 1.0

MPa, time 4 h. Selectivity was calculated as npc / (npc + nbyproduct) × 100%; Conversion was calculated as (npc + nbyproduct ) / (npo + npc + nbyproduct ) × 100%.

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Epoxide

Product

Time (h)

O

O

1

4

O

O

100%

O

Cl

96%

92%

4

98%

98%

4

71%

100%

12

76%

61%

4

O

Cl O

O

3

O

O

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O O

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Sel (%)

99%

O

O

Con (%)

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Entry

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Table 2. Cycloaddition of CO2 to different epoxides catalyzed by TBB-Bpy-a.

O

O

O

O

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4

O

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O

5

O O

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1.0 MPa.

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Reaction conditions: epoxides 20 mmol, catalyst 0.08g, temperature 120 °C, initial CO2 pressure

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Graphical Abstract

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ACCEPTED MANUSCRIPT

T

Highlights

IP

> Highly cross-linked cationic polymer microspheres were synthesized.

SC R

> The catalyst leads to the heterogeneous transformation of CO2 and epoxides into cyclic carbonates.

NU

> The catalyst gives high conversion and selectivity.

AC

CE P

TE

D

MA

> The catalyst can be easily recovered and steadily reused.

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