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Article

Hypercrosslinked Ionic Polymers with High Ionic Content for Efficient Conversion of Carbon Dioxide into Cyclic Carbonates

by
Xu Liao
1,†,
Baoyou Pei
2,†,
Ruixun Ma
2,
Lingzheng Kong
2,
Xilin Gao
1,
Jiao He
1,
Xiaoyan Luo
1,* and
Jinqing Lin
1,*
1
College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, China
2
College of Chemical Engineering, Huaqiao University, Xiamen 361021, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(1), 62; https://doi.org/10.3390/catal12010062
Submission received: 28 November 2021 / Revised: 30 December 2021 / Accepted: 31 December 2021 / Published: 6 January 2022
(This article belongs to the Topic Catalysis for Sustainable Chemistry and Energy)
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Abstract

:
The effective conversion of carbon dioxide (CO2) into cyclic carbonates requires porous materials with high ionic content and large specific surface area. Herein, we developed a new systematic post-synthetic modification strategy for synthesizing imidazolium-based hypercrosslinked ionic polymers (HIPs) with high ionic content (up to 2.1 mmol g−1) and large specific surface area (385 m2 g−1) from porous hypercrosslinked polymers (HCPs) through addition reaction and quaternization. The obtained HIPs were efficient in CO2 capture and conversion. Under the synergistic effect of high ionic content, large specific surface area, and plentiful micro/mesoporosity, the metal-free catalyst [HCP-CH2-Im][Cl]-1 exhibited quantitative selectivities, high catalytic yields, and good substrate compatibility for the conversion of CO2 into cyclic carbonates at atmospheric pressure (0.1 MPa) in a shorter reaction time in the absence of cocatalysts, solvents, and additives. High catalytic yields (styrene oxide, 120 °C, 8 h, 94% yield; 100 °C, 20 h, 93% yield) can be achieved by appropriately extending the reaction times at low temperature, and the reaction times are shorter than other porous materials under the same conditions. This work provides a new strategy for synthesizing an efficient metal-free heterogeneous catalyst with high ionic content and a large specific surface area from HCPs for the conversion of CO2 into cyclic carbonates. It also demonstrates that the ionic content and specific surface area must be coordinated to obtain high catalytic activity for CO2 cycloaddition reaction.

Graphical Abstract

1. Introduction

The exploitation of effective strategies for CO2 capture, sequestration, and utilization is crucial for the sustainable development of human society. An attractive and promising method is the utilization of CO2 as an abundant, low-cost, and renewable C1 resource to produce high value-added chemicals [1,2,3,4,5]. The CO2 cycloaddition reaction has elicited widespread attention because the reaction is 100% atom-economic [6]. In addition, the produced cyclic carbonates have extensive potential applications as polar aprotic solvents, electrolytes for lithium-ion cells and batteries, and intermediates [7].
To date, a large number of homogeneous and heterogeneous catalysts have been used for the CO2 cycloaddition reaction, including ionic liquids (ILs) [8,9,10,11], metal complexes [12,13,14,15], metal-organic frameworks (MOFs) [16,17,18,19,20,21], and porous organic polymers (POPs) [1,2,3,6,22,23,24,25,26]. Some strategies have been reported for homogeneous catalysts that catalyze the CO2 cycloaddition reaction, but these materials still face the issues of effective adsorption and catalyst recycling [27]. Additionally, metal catalysts can significantly improve catalytic activity for the CO2 cycloaddition reaction. Jiang et al. [28] developed a strategy to confine imidazolium-based poly(ionic liquid)s (denoted as polyILs) in the MOF material MIL-101 via in situ polymerization of encapsulated monomers. The resultant composite polyILs@MIL-101 with a large specific surface area (2462 m2 g−1) exhibited good CO2 capture and conversion capability at atmospheric pressure. However, the metal leaching and leftovers cause potential environmental pollution. Thus, developing effective and eco-friendly heterogeneous catalysts is necessary. The specific surface area and ionic content are important factors for the catalytic activity of porous materials. The poly(ionic liquid)s typically synthesized by free radical polymerization can have high ionic content, but they are mostly non-porous or low specific surface area materials. For example, Gai et al. [29] synthesized a new series of PIL-based copolymers through the free radical polymerization of ionic monomer and ethylene glycol dimethacrylate. The ionic content of PIL-4 reached up to 1.25 mmol g−1 and the specific surface area was only 1.97 m2 g−1. PIL-4 catalyzed CO2 with epichlorohydrin into cyclic carbonate with 99% yield at 1 MPa CO2, 100 °C in 12 h. In addition, Yavuz et al. [30] reported an imidazolinium-based catalyst produced by a one-pot reaction of terephthalaldehyde and ammonium chloride; the obtained catalyst COP-222 had high ionic content (4.05 mmol g−1) and a small specific surface area (21 m2 g−1). COP-222 catalyzed epichlorohydrin with CO2 into cyclic carbonates with a 99% yield at 0.1 MPa CO2, 100 °C in 24 h. The high pressure and long reaction time were needed due to the small specific surface area of catalysts.
The low-cost hypercrosslinked polymers (HCPs) are promising candidate materials for CO2 capture and conversion due to their exceptional advantages of large specific surface area and mild synthesis conditions. However, nonionic HCPs have few active functionalized sites, limiting their further application to the conversion of CO2. Therefore, the ILs were inserted into the porous HCPs through post-functionalization strategies to obtain HIPs, which combine the advantages of HCPs and ILs. Therefore, the HIPs are widely used for CO2 and conversion, adsorption of organic pollutants [31], and separation of bioactive molecules [32], etc. The large specific surface area and high microporosity of HCPs are efficient for CO2 capture, and ILs can provide the catalytic active site for the conversion of CO2. HIPs are synthesized mainly through the crosslinking of ionic and neutral monomers, in situ generations of ionic sites in the hypercrosslinked process, and post-functionalization strategies. Gai et al. [33] reported a strategy for synthesizing a series of novel HIPs via the Friedel–Crafts reaction of 2-phenylimidazole with α,α′-dichloro-p-xylene (DCX). The best active catalyst HP-[BZPhIm]Cl-DCX-1 has a large specific surface area (763 m2 g−1) and moderate IL content (0.762 mmol g−1). It can convert epichlorohydrin into cyclic carbonates with 99% yield and 98% selectivity at 120 °C, 0.1 MPa CO2 in 11 h. At present, there are few reports on the strategy of synthesizing HIPs from HCPs. Zhang et al. [34] developed a series of imidazolium-salt-modified porous HIPs through Friedel–Crafts alkylation and quaternization. The dichloroethane was used as crosslinker and solvent to construct the skeleton of HCPs containing methyl chloride, which further reacted with N-methylimidazole to form HIPs; the specific surface area and ionic content of as-prepared POM3-IM were 575 m2 g−1 and 1.01 mmol g−1, respectively. The resultant catalyst exhibited an 89% yield for the conversion of styrene oxide into cyclic carbonates at 120 °C for 12 h under 1 MPa CO2. Although the aforementioned porous catalysts exhibited high catalytic yields for the cycloaddition of CO2 with epoxides, they require harsh reaction conditions (high pressure or longer reaction time at low pressure). The primary reason is that the specific surface area and ionic content of HIPs are contradictory during the preparation process, resulting in HIPs either with a large specific surface area but excessively low ionic content or high ionic content but excessively small specific surface area. Therefore, the development of a new strategy for synthesizing HIP catalysts with high ionic content and large specific surface area is imminent.
In the current work, we developed a new strategy for synthesizing HIPs with high ionic content and large specific surface area from HCPs through addition reaction and quaternization. The synthesis route of [HCP-CH2-Im][Cl]-X is shown in Scheme 1. In the first step, the HCP skeleton was constructed through the Friedel-Crafts alkylation reaction of diphenyl. In the second step, the prepared HCP reacts with allyl chloride to produce HCPs with chloromethyl groups, and the addition reaction conditions (mass ratio of allyl chloride to HCP, mass ratio of H2SO4/HCP, reaction time, and temperature) were changed to adjust the chloromethyl content and specific surface area of HCPs. Finally, a series of imidazolium-based HIPs was prepared through quaternization with methylimidazole. The obtained [HCP-CH2-Im][Cl]-X with high ionic content and large specific surface area catalyzed cycloaddition of CO2 with epoxides; the reaction conditions (dosage of catalyst, reaction temperature, and reaction time) were systematically studied to obtain the optimal reaction conditions. Additionally, a range of epoxides with important industrial applications was tested to determine the universality of [HCP-CH2-Im][Cl]-X for CO2 cycloaddition reaction. The [HCP-CH2-Im][Cl]-X was reused for several recycling to investigate the reusability of the catalyst. In conclusion, the HIPs with high ionic content and large specific surface area were synthesized for CO2 cycloaddition at atmospheric pressure in a short reaction. In addition, we found that the coordination of ionic content and specific surface area is a crucial factor to achieve high catalytic activity.

2. Results and Discussion

2.1. Influences of Addition Reaction Conditions on Chloromethyl Content and Specific Surface Area

The HCP skeleton was constructed through the Friedel–Crafts alkylation reaction of diphenyl and FDA; the specific surface area is 1153 m2 g−1. Although the specific surface areas will gradually decrease during the addition reaction and quaternization processes, the high specific surface area of HCP will provide a favorable guarantee for obtaining HIPs with high specific surface area. The addition reaction was carried out with active hydrogen on the benzene ring in the HCP skeleton by using allyl chloride as the chlorine source to produce a series of HCPs containing chloromethyl group, denoted as HCP-CH2-Cl-X.
To obtain HCP-CH2-Cl-X with high chloromethyl content and larger specific surface area, the influences of addition reaction conditions, including the mass ratio of allyl chloride to HCP, the mass ratio of H2SO4 to HCP, reaction time and temperature on chloromethyl content, and specific surface area were investigated. As shown in Figure 1a and Table S1, the effect of the mass ratio of allyl chloride to HCP was discussed. The results show that the chloromethyl content reaches the maximum (3.1 mmol g−1) when the mass ratio of allyl chloride to HCP was increased to 5. However, when the mass ratio of allyl chloride to HCP increased from 5 to 9, the chloromethyl content decreased to 2.3 mmol g−1 because the excess allyl chloride causes the dilution of catalyst (H2SO4). Furthermore, with the increase of the mass ratio of allyl chloride to HCP, the specific surface areas constantly decreased due to the pore being occupied by the grafted chloromethyl. Then, on this basis, other addition reaction conditions were further studied.
The experimental results (Figure 1b and Table S2) show that when increasing the mass ratio of H2SO4/HCP from 0.5 to 1.5, a positive effect was achieved on the chloromethyl content of HCP-CH2-Cl-X (2.0–3.1 mmol g−1). However, when the mass ratio of H2SO4/HCP increased to 2.5, the chloromethyl contents decreased to 1.7 mmol g−1 because the excessive sulfuric acid generated more sulfonation reactions of the benzene ring of HCP, resulting in the passivation of the benzene ring and decreasing the chloromethyl content [35]. In addition, the specific surface area of HCP-CH2-Cl-X decreased from 783 m2 g−1 to 530 m2 g−1 when the mass ratio of H2SO4/HCP increased from 0.5 to 1.5, which was attributed to the numerous pores that were occupied by the grafted chloromethyl group. With the increase of the mass ratio of H2SO4/HCP 2.5, the specific surface area was further decreased due to the pores that were occupied after sulfonation.
The results presented in Figure 1c and Table S3 show that a reaction time of 24 h is appropriate, and the highest chloromethyl content (3.1 mmol g−1) was obtained. However, due to the side reaction of allyl chloride carbonization, the specific surface area was significantly reduced from 755 m2 g−1 to 250 m2 g−1 when the reaction time was prolonged. The effect of the reaction temperatures on the chloromethyl content and specific surface area of HCP-CH2-Cl-X are shown in Figure 1d and Table S4. When the reaction temperature increased from 30 °C to 50 °C, the chloromethyl content gradually increased. However, the grafting amount of chloromethyl exhibited a downward trend when the temperature was higher than 50 °C because the addition reaction is exothermic. Furthermore, the specific surface area of HCP-CH2-Cl-X was reduced when increasing the reaction temperature. The reason for this phenomenon is similar to that for the change in reaction time. The optimal addition reaction conditions were identified as follows: the mass ratio of allyl chloride to HCP (5), the mass ratio of H2SO4/HCP (1.5), reaction time (24 h), and temperature (50 °C); the chloromethyl content and specific surface area of HCP-CH2-Cl-1 can reach 2.1 mmol g−1 and 385 m2 g−1, respectively. The HCP-CH2-Cl-X with high chloromethyl content was obtained for the following reasons: (1) the benzene ring in the HCP framework contains abundant active sites for addition reaction; (2) the active hydrogens have been fully utilized under the optimal reaction conditions. Additionally, the HCP-CH2-Cl-X with a large specific surface area or high chloromethyl content was selected for further quaternization reaction, denoted by HCP-CH2-Cl-1–7. The synthesis conditions of HCP-CH2-Cl-1–7 are shown in Table S5.

2.2. The Effects of Ionic Content and Specific Surface Area on Catalytic Activity

The corresponding HCP-CH2-Cl-X reacted with methylimidazole to produce various HIPs ([HCP-CH2-Im][Cl]-X) with different ionic content and specific surface area. The catalyst [HCP-CH2-Im][Cl]-X was used to catalyze the cycloaddition reaction of CO2 with styrene epoxide (SO) at 140 °C, 0.1 MPa in 4 h. As indicated in Table 1, [HCP-CH2-Im][Cl]-X can effectively catalyze the cycloaddition of CO2 with SO, and the selectivity of all the catalysts is beyond 99%. The production of styrene carbonate was negligible by using the nonionic HCP (entry 1, 5%) and HCP-CH2-Cl-1 (entry 2, 7%) as catalysts. The catalysts with low ionic content and small specific surface area exhibited worse yields (entries 5, 7, and 9: 74%, 72%, and 72% yields, respectively). The higher yields were achieved for catalysts with low ionic content and large specific surface area under identical reaction conditions (entries 4, 6, and 8: 89%, 88%, and 84% yields, respectively). The comparison of entry 8 with entry 7 shows that the larger specific surface area (406 m2 g−1 versus 183 m2 g−1) presented a higher yield (84% versus 72%) when the catalysts with the same ionic content because of the larger specific surface area can concentrate more CO2 molecule. Entry 3 and entry 5 show that when the specific surface area of the catalysts is nearly the same, the yield of catalysts with higher ionic content is higher (95% versus 74%). In addition, the catalyst with higher ionic content (entry 7, 1.65 mmol g−1) but a tiny specific surface area (183 m2 g−1) exhibited lower yield (72%) than the catalysts with lower ionic content (entries 4, 5, and 6: 0.99, 0.48, and 0.85 mmol g−1, respectively) but a larger specific surface area (510, 386, and 503 m2 g−1, respectively) under identical reaction conditions. Notably, the catalyst [HCP-CH2-Im][Cl]-1 with the highest ionic content and higher specific surface area has the highest yield of 95% (entry 3). The experimental results indicated that catalysts with excessively low ionic content or small specific surface area are not conducive to the CO2 cycloaddition reaction; the high catalytic activity requires high ionic content, large specific surface area, and the coordination of them to achieve efficient catalysis. The catalytic activity and characterization of the catalyst [HCP-CH2-Im][Cl]-1 were further studied.

2.3. Characterization of Polymers

The Fourier transform infrared (FTIR) spectra of HCP (a), HCP-CH2-Cl-1 (b), and [HCP-CH2-Im][Cl]-1 (c) are shown in Figure 2. The (b) exhibited a characteristic peak at 617 cm−1, which belonged to the vibrative absorption of C-Cl [36]. The absorption of C-Cl weakened and the characteristic peak of the imidazolium ring appeared at 1598 cm−1 and 1167 cm−1 after quaternization, which belonged to the imidazolium ring stretching of C=N and C-N, respectively [1,6,37]. The preceding results indicated that imidazolium-based HIPs were successfully prepared. The thermogravimetric analysis (TGA) curves of HCP, HCP-CH2-Cl-1, and [HCP-CH2-Im][Cl]-1 in Figure 3a illustrate that the catalysts exhibited favorable thermal stability. In particular, the weight loss of HCP, HCP-CH2-Cl-1, and [HCP-CH2-Im][Cl]-1 started at 324 °C, 360 °C, and 236 °C, respectively, which are considerably higher than the reaction temperature.
The [HCP-CH2-Im][Cl]-1 sample was further characterized via solid-state 13C NMR and X-ray photoelectron spectroscopy (XPS). As shown in Figure 3b, the signals at 134 ppm and 123 ppm belonged to the imidazole ring and aromatic carbons of the benzene ring, respectively. Meanwhile, the peak at 33 ppm originated from the methylene carbon formed via Friedel–Crafts reaction [34,38,39]. The survey scan XPS spectrum produced signals at 532.3, 401.6, 284.0, 270.4, and 199.8 eV, which belonged to the element species of C 1s, O 1s, N 1s, Br 3s, and Cl 2p (Figure 3c), respectively. The XPS of the N 1s of [HCP-CH2-Im][Cl]-1 (Figure 3d) shows two peaks at 399.5 eV and 401.5 eV, which are assigned to the nonionic N atoms and imidazolinium cations, respectively [40,41]. The preceding results indicate that imidazolium was successfully grafted onto the backbone of the HIP.
Scanning electron microscopy (SEM) images (Figure 4a,b,e,f,i,j) showed that the polymers were composed of anomalistic nanoparticles at the micrometer level. Elemental mapping images (Figure 4c,d,g,h,k,l) showed that Cl and N were dispersed throughout the polymer skeleton, indicating that ionic sites were uniformly dispersed in the polymer skeleton. The Cl content was significantly increased after the addition reaction. Meanwhile, N content was significantly increased and Cl content remained unchanged after quaternization. These results further prove that the successful preparation of HIPs occurred. In addition, the aforementioned morphology of [HCP-CH2-Im][Cl]-1 was illustrated through transmission electron microscopy images (Figure 5). X-ray diffraction patterns indicate that HCPs and HIPs were amorphous (Figure S1).
The porosity of [HCP-CH2-Im][Cl]-X was studied through N2 adsorption experiments. As shown in Figure 6a, the prepared [HCP-CH2-Im][Cl]-X presents typical Type IV isotherms [42] and the adsorption curve rose faster in the low-pressure area, which indicates that the material contained a certain number of microporous structures, which was conducive to CO2 concentration [43]. Furthermore, the pore size distribution diagram was calculated using the nonlocal density function in Figure 6b; the pores of the polymers were mostly concentrated at approximately 1 nm to 6 nm and are microporous/mesoporous structures. The existence of a mesoporous structure was beneficial for mass transfer and accelerated the reaction rate [44]. The morphological properties of [HCP-CH2-Im][Cl]-X are summarized in Table S6 in accordance with the N2 isotherm; the [HCP-CH2-Im][Cl]-X is primarily composed of microporous structures and a small number of mesoporous structures. Figure S2a shows that the CO2 isotherms of [HCP-CH2-Im][Cl]-1 have relative pressure ranging from 0 bar to 1.0 bar at 273 K and 298 K. The CO2 uptakes of [HCP-CH2-Im][Cl]-1 are 1.79 mmol g−1 (273 K) and 1.20 mmol g−1 (298 K). Moreover, Figure S2b illustrates that CO2 uptakes exhibited almost no decrease after adsorption-desorption was run five times, demonstrating excellent reusability.

2.4. Optimization of Cycloaddition Reaction Conditions and the Universality of Catalysts

The catalytic activity of [HCP-CH2-Im][Cl]-1 was further studied to obtain the optimal reaction conditions for the cycloaddition of CO2 with epoxides. As shown in Figure 7, the effects of temperature, mass ratio of the catalyst to SO, and reaction time on the catalytic activity of [HCP-CH2-Im][Cl]-1 were investigated by using SO as substrate. The catalyst [HCP-CH2-Im][Cl]-1 exhibited high selectivity (99%) under all reaction conditions. In Figure 7a, the reaction temperature exerted a remarkable effect on the yields for the cycloaddition reaction. When the reaction temperature was increased within the range of 50 to 140 °C, the yield of cyclic carbonate increased from 5% to 99%, and then it became stable when the temperature was further increased to 150 °C. When the mass ratio of the catalyst to SO increased from 0 wt% to 25 wt% (Figure 7b), carbonate yield constantly increased from 1% to 99%. Figure 7c shows the effect of reaction time at 140 °C, 25 wt% mass ratio of the catalyst to SO, and 0.1 MPa CO2 on catalytic activity. Cyclic carbonate yield exhibited a gradual increase from 1 h to 5 h (yield from 62% to 99%), and remained unchanged when the reaction time was increased to 6 h. [HCP-CH2-Im][Cl]-1 exhibited 99% yield and 99% selectivity for the cycloaddition of CO2 with SO under optimal reaction conditions (0.1 MPa CO2, 140 °C, 25 wt% catalysts, 5 h).
A range of epoxides with important industrial applications was tested to determine the universality of [HCP-CH2-Im][Cl]-1 for CO2 cycloaddition reaction. All the examined substrates exhibited excellent yield and selectivity under optimal conditions as summarized in Table 2. The yields of the products from propylene oxide, epichlorohydrin, and epibromohydrin reached 99% within 4 h (entries 1–3). The catalytic activity of the catalyst [HCP-CH2-Im][Cl]-1 for SO was further studied at lower temperatures; it can be found that excellent catalytic yields (entry 4) were achieved by slightly extending the reaction time at lower temperatures. Moreover, although less reactive substrates with long chains and oxymethylene moiety required a longer reaction time, excellent yields were still obtained at 140 °C, 0.1 MPa CO2 (6 h, 91–99%, entries 5–8). The experimental results demonstrated that the catalyst [HCP-CH2-Im][Cl]-1 exhibited excellent catalytic activity for similar substrates. The reaction time of the [HCP-CH2-Im][Cl]-1 catalyst was compared with those of recently reported heterogeneous porous catalysts for the cycloaddition of CO2 with SO (Table S7). By contrast, [HCP-CH2-Im][Cl]-1 required a shorter reaction time than the same type of catalysts under the same reaction conditions.

2.5. Reusability of Catalysts

The reusability of catalysts is an important factor in economic and industrial applications. To investigate the reusability of the HIP catalyst [HCP-CH2-Im][Cl]-1, the SO was used as the substrate for cycloaddition reaction under optimal reaction conditions. After the reaction, ethyl acetate was added to separate the cyclic carbonates from the catalyst [HCP-CH2-Im][Cl]-1 via filtration. Then the catalyst [HCP-CH2-Im][Cl]-1 was dried under vacuum and applied in the next run. As shown in Figure 8a, the catalyst was run six times and its catalytic activity did not drop significantly (90–99% yield and 99% selectivity). The slight decrease in catalytic activity due to catalyst loss during the separation process demonstrates the excellent reusability of the [HCP-CH2-Im][Cl]-1. The stability of the catalyst [HCP-CH2-Im][Cl]-1 after six run times was studied via SEM, FTIR, and TGA. The recycled catalyst [HCP-CH2-Im][Cl]-1 showed a dense pore structure in the SEM image shown in Figure 8b. Furthermore, the characteristic peak of recycled catalyst presented no evident change in contrast with the FTIR spectrum of the fresh catalyst, and the newly emerged characteristic peaks 1790 cm−1 and 1672 cm−1 belong to cyclic carbonate (Figure 8c). As shown in Figure 8d, the stability of the catalyst was further examined through TGA. The thermal stability of the reused [HCP-CH2-Im][Cl]-1 catalyst after six run times was as good as that of the fresh catalyst. These results proved that the recycled catalyst [HCP-CH2-Im][Cl]-1 can meet the reaction temperature requirement.

2.6. Proposed Mechanism

The proposed catalytic mechanism of [HCP-CH2-Im][Cl]-1 in catalyzing CO2 conversion into cyclic carbonates is illustrated in Scheme 2. In all the aforementioned steps, the step to determine the CO2 cycloaddition rate is typically known as the epoxy ring opening step [45,46]. As shown in Scheme 2, when the epoxide entered the pores of the catalyst, the C-O bond of epoxide was activated by the interaction between the oxygen atom and imidazolium cation, accelerating the ring opening of the epoxide (Step 1) [25,47,48]. Meanwhile, the chloride anion of [HCP-CH2-Im][Cl]-1 attacked the α-carbon atom of the epoxy ring to produce the ring opening intermediate (Step 2). Subsequently, CO2 was adsorbed by the [HCP-CH2-Im][Cl]-1 and inserted into the strongly nucleophilic intermediate to generate cyclic esters (Step 3). Finally, cyclic carbonates were formed through the elimination of chloride ions and the intramolecular cyclization of acyclic ester (Step 4), followed by the regeneration of the catalyst for the next run.

3. Experimental Section

3.1. Materials

The commercial chemicals and reagents were used as received without further purification unless otherwise stated. Allyl chloride (98%), diphenyl (99%), dichloroethane (DCE, 99%), dimethoxy methane (FDA, 98%), 1-methylimidazole (99%), epichlorohydrin (AR), allyl glycidyl ether (99%), butyl glycidyl ether (98%), propylene oxide (99%), and o-Tolyl glycidyl ether (90%) were provided by Aladdin Chemical Reagent Co. LTD (Beijing, China). Sulfuric acid (98%), ferric chloride (FeCl3, 98%), and ethyl acetate (EtOAc, AR) were purchased from Sinopharm Chemical Reagent Co. LTD. Styrene oxide (98%), glycidyl phenyl ether (99%), and 1,2-ethylene dibromide (98%) were purchased from Energy Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Characterization

The IR spectra were obtained using an FT-IR (4000~400 cm−1) spectrometer (Nicolet Nexus FT-IR spectrometer (Madison, WI USA) at 4 cm−1 resolution and 32 scans. NMR spectra were acquired in CDCl3 on a Bruker AVANCE III 500 MHz spectrometer (Zurich, Switzerland) for 1H NMR; the particular NMR spectra can be found in the Supplementary Material (Figures S3–S10). 13C CP/MAS NMR spectra were measured on an Agilent-NMR-VnmrS 600 (Palo Alto, CA, USA). Thermogravimetric analysis (TGA-50H, Shimadzu, Kyoto, Japan) of the samples was carried out: they were heated from 50 to 800 °C at ramp 10 °C/min under Ar gas flow. Brunauer–Emmett–Teller (BET) pore volumes and surface areas were recorded with N2 adsorption at 77 K by using JWGB (JW-DEL 200 (Beijing, China). The crystal structure of the samples was examined by X-ray diffraction (XRD) on SmartLa. CHNS elemental analysis was performed on the Vario EL Cube. Field emission scanning electron microscope (FESEM; Hitachi S-4800, accelerated voltage: 5 kV (Tokyo, Japan) was used to observe the morphology. Transmission electron microscope (TEM) images of the samples were examined by Hitachi H-7650 (Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) of the [HCP-CH2-Im][Cl]-1 was determined by Thermo Fisher Scientific K-alpha+ (Waltham, MA, USA) equipped with Al K radiation (1486.68 eV).

3.3. Synthesis of HCP

A mixture of biphenyl (0.02 mol, 3.084 g) and FDA (5.3 mL) in DCE (50 mL) was stirred for 10 min at room temperature. FeCl3 (9.732 g, 0.06 mol) was then added and the mixture was heated to 80 °C for 24 h. The resultant polymers were filtered and washed with methanol, and further purified for 24 h via Soxhlet extractions with methanol as solvent. The obtained HCP was vacuum-dried at 80 °C for 24 h.

3.4. Synthesis of HCP-CH2-Cl-X

A mixture of HCP (1 g) and ethyl acetate (40 mL) was swelled at 60 °C and in 2 h. Then the mixture was cooled to room temperature, and allyl chloride (5.3 mL) was added to the mixture. The ethyl acetate (10 mL) and sulfuric acid (1.5 g) were added to the mixture by using a constant pressure separatory funnel, and the reaction was carried out at 50 °C for 24 h. After the reaction, the products were washed with ethyl acetate and deionized water and dried in a vacuum at 80 °C for 24 h to obtain HCP-CH2-Cl-1. The other HCP-CH2-Cl-X was synthesized similarly using the aforementioned procedures under different reaction conditions.

3.5. Synthesis of [HCP-CH2-Im][Cl]-X

A mixture of the HCP-CH2-Cl-1 (1 g) and ethyl acetate was heated to 50 °C for 2 h for the swelling of the polymers and then 1-methylimidazole (0.02 mol, 1.64 g) was added to the mixture; the reaction was heated to 80 °C for 24 h. The resultant polymers were washed with ethyl acetate and further dried under vacuum at 80 °C for 24 h. The other [HCP-CH2-Im][Cl]-X was synthesized following the same procedure.

3.6. General Catalytic Procedure for CO2 Cycloaddition to Epoxides

In a typical run, styrene oxide (SO, 5 mmol) and [HCP-CH2-Im][Cl]-1 (25 wt%) were charged into a high-pressure stainless steel autoclave (Figure S11), which was pressured with CO2 (0.1 MPa), and the reactant was heated to 140 °C for 5 h. After the reaction and cooling to room temperature, the remaining CO2 was released slowly. The product was diluted with ethyl acetate and the liquid phase was separated via filtration. The ethylene dibromide (0.5 mmol, 36 µL) was added as the internal standard after the rotary evaporation of ethyl acetate. The yield and selectivity were analyzed via 1H NMR.

4. Conclusions

A new strategy for synthesizing hypercrosslinked imidazolium-based ionic polymers with high ionic content and higher specific surface area from porous HCPs was developed through addition reaction and quaternization. The specific surface area and ionic content of HIPs could be adjusted by optimizing addition reaction conditions (the mass ratio of allyl chloride to HCP, reaction temperature, the ratio of H2SO4/HCP, and reaction time). FT-IR, solid-state 13C NMR, XPS, and SEM mapping demonstrated that the HIPs were successfully prepared. The obtained HIP [HCP-CH2-Im][Cl]-1 with high ionic content and higher specific surface area not only possessed good CO2 uptake but also exhibited outstanding catalytic yields for most epoxy substrates (99% selectivity and 91–99% yield) at atmospheric pressure in a shorter reaction time (4–6 h) without cocatalyst, solvent, and additive. In addition, the catalyst [HCP-CH2-Im][Cl]-1 demonstrated excellent reusability and stability after six run times. The high ionic content of HIPs provides more reaction sites and high microporosity that could enrich CO2, which contributes to the efficient conversion of CO2 into cyclic carbonates. This work provides a new strategy to synthesize imidazolium-based HIPs with high catalytic activity from porous HCPs for the cycloaddition of CO2 with epoxides at atmospheric pressure. Furthermore, we determined that the coordination of ionic content and specific surface area is a crucial factor for high catalytic activity for the cycloaddition of CO2 with epoxides.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal12010062/s1, Figure S1: XRD patterns of HCP, HCP-CH2-Cl-1, [HCP-CH2-IM][Cl]-1; Figure S2: (a) CO2 adsorption isotherms of [HCP-CH2-Im][Cl] (1) at 273 K and 298 K, (b) recycling performance of [HCP-CH2-Im][Cl] (1) at 273 K. Figures S3–S10: 1H NMR spectra of cyclic carbonates. Table S1: The effect of mass ratio of allyl chloride/HCP on specific surface area and chloromethyl content of HCP-CH2-Cl-X; Table S2: The effect of mass ratio of H2SO4/HCP on specific surface area and chloromethyl content of HCP-CH2-Cl-X; Table S3: The effect of reaction time on specific surface area and chloromethyl content of HCP-CH2-Cl-X; Table S4: The effect of reaction temperature on specific surface area and chloromethyl content of HCP-CH2-Cl-X; Table S5: The synthesis conditions of HCP-CH2-Cl-X; Table S6: Textual properties of [HCP-CH2-Im][Cl]-X; Table S7: Activity of porous ionic polymers in the cycloaddition of CO2 with styrene oxide.

Author Contributions

Conceptualization, X.L. (Xu Liao), B.P. and J.L.; methodology, X.L. (Xu Liao) and B.P.; investigation, X.L. (Xu Liao); resources, X.L. (Xu Liao), B.P. and J.L.; data curation, X.L. (Xu Liao), B.P., R.M., J.H., L.K. and X.G.; writing—original draft preparation, X.L. (Xu Liao); writing—review and editing, X.L. (Xu Liao), X.L. (Xiaoyan Luo) and J.L.; visualization, X.L. (Xu Liao) and J.L.; supervision, X.L. (Xiaoyan Luo) and J.L.; project administration, J.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support from the National Natural Science Foundation of China (21803021, 21246008) and the Natural Science Foundation of Fujian Province (2020J01065).

Data Availability Statement

The data presented in this study are available on request from the corresponding author via e-mail: [email protected] (J.L.).

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21803021, 21246008) and the Natural Science Foundation of Fujian Province (2020J01065). The authors give thanks for the support of the Instrumental Analysis Center, Huaqiao University.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Catalysts 12 00062 sch001 550
Scheme 1. Preparation of imidazolium-based hypercrosslinked ionic polymer [HCP-CH2-Im][Cl]-X (X: different reaction conditions).
Scheme 1. Preparation of imidazolium-based hypercrosslinked ionic polymer [HCP-CH2-Im][Cl]-X (X: different reaction conditions).
Catalysts 12 00062 sch001
Catalysts 12 00062 g001 550
Figure 1. Effects of reaction conditions on specific surface area and content of the chloromethyl group of HCP-CH2-Cl: (a) mass ratio of ally chloride to HCP, mass ratio of H2SO4/HCP = 1.5, 50 °C, 24 h; (b) mass ratio of H2SO4/HCP, mass ratio of ally chloride to HCP = 5, 50 °C, 24 h; (c) reaction time, mass ratio of ally chloride to HCP = 5, mass ratio of H2SO4/HCP = 1.5, 50 °C; (d) reaction temperature, mass ratio of ally chloride to HCP = 5, mass ratio of H2SO4/HCP = 1.5, 24 h.
Figure 1. Effects of reaction conditions on specific surface area and content of the chloromethyl group of HCP-CH2-Cl: (a) mass ratio of ally chloride to HCP, mass ratio of H2SO4/HCP = 1.5, 50 °C, 24 h; (b) mass ratio of H2SO4/HCP, mass ratio of ally chloride to HCP = 5, 50 °C, 24 h; (c) reaction time, mass ratio of ally chloride to HCP = 5, mass ratio of H2SO4/HCP = 1.5, 50 °C; (d) reaction temperature, mass ratio of ally chloride to HCP = 5, mass ratio of H2SO4/HCP = 1.5, 24 h.
Catalysts 12 00062 g001
Catalysts 12 00062 g002 550
Figure 2. FT-IR spectra of (a) HCP, (b) HCP-CH2-Cl-1, and (c) [HCP-CH2-Im][Cl]-1.
Figure 2. FT-IR spectra of (a) HCP, (b) HCP-CH2-Cl-1, and (c) [HCP-CH2-Im][Cl]-1.
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Catalysts 12 00062 g003 550
Figure 3. (a) TGA curves of HCP, HCP-CH2-Cl-1, and [HCP-CH2-Im][Cl]-1, (b) 13C CP/MAS NMR spectrum of [HCP-CH2-Im][Cl]-1, (c) survey scan, and (d) N 1 s XPS spectra of [HCP-CH2-Im][Cl]-1.
Figure 3. (a) TGA curves of HCP, HCP-CH2-Cl-1, and [HCP-CH2-Im][Cl]-1, (b) 13C CP/MAS NMR spectrum of [HCP-CH2-Im][Cl]-1, (c) survey scan, and (d) N 1 s XPS spectra of [HCP-CH2-Im][Cl]-1.
Catalysts 12 00062 g003
Catalysts 12 00062 g004 550
Figure 4. SEM images and elemental (N and Cl) mapping images of HCP (a–d), HCP-CH2-Cl-1 (e–h), and [HCP-CH2-Im][Cl]-1 (i–l).
Figure 4. SEM images and elemental (N and Cl) mapping images of HCP (a–d), HCP-CH2-Cl-1 (e–h), and [HCP-CH2-Im][Cl]-1 (i–l).
Catalysts 12 00062 g004
Catalysts 12 00062 g005 550
Figure 5. TEM images of [HCP-CH2-Im][Cl]-1 (a) 10 nm; (b) 20 nm; (c) 50 nm).
Figure 5. TEM images of [HCP-CH2-Im][Cl]-1 (a) 10 nm; (b) 20 nm; (c) 50 nm).
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Catalysts 12 00062 g006 550
Figure 6. (a) N2 adsorption-desorption isotherms, (b) pore size distribution of [HCP-CH2-Im][Cl]-X.
Figure 6. (a) N2 adsorption-desorption isotherms, (b) pore size distribution of [HCP-CH2-Im][Cl]-X.
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Catalysts 12 00062 g007 550
Figure 7. Effect of yield and selectivity as a function of (a) temperature, styrene oxide (5 mmol), 0.1 MPa CO2, 5 h, [HCP-CH2-Im][Cl]-1 (25 wt%); (b) mass ratio of catalyst to SO, styrene oxide (5 mmol), 0.1 MPa CO2, 140 °C, 5 h; (c) reaction time, styrene oxide (5 mmol), 0.1 MPa CO2, 140 °C, [HCP-CH2-Im][Cl]-1 (25 wt%).
Figure 7. Effect of yield and selectivity as a function of (a) temperature, styrene oxide (5 mmol), 0.1 MPa CO2, 5 h, [HCP-CH2-Im][Cl]-1 (25 wt%); (b) mass ratio of catalyst to SO, styrene oxide (5 mmol), 0.1 MPa CO2, 140 °C, 5 h; (c) reaction time, styrene oxide (5 mmol), 0.1 MPa CO2, 140 °C, [HCP-CH2-Im][Cl]-1 (25 wt%).
Catalysts 12 00062 g007
Catalysts 12 00062 g008 550
Figure 8. (a) Reusability of [HCP-CH2-Im][Cl]-1 for CO2 cycloaddition with epichlorohydrin. Reaction conditions: styrene oxide (5 mmol), CO2 (0.1 MPa), catalyst (25 wt%), 140 °C, 5 h, (b) SEM image, (c) FT-IR spectrum, (d) TG curves of [HCP-CH2-Im][Cl]-1 after having been run 6 times.
Figure 8. (a) Reusability of [HCP-CH2-Im][Cl]-1 for CO2 cycloaddition with epichlorohydrin. Reaction conditions: styrene oxide (5 mmol), CO2 (0.1 MPa), catalyst (25 wt%), 140 °C, 5 h, (b) SEM image, (c) FT-IR spectrum, (d) TG curves of [HCP-CH2-Im][Cl]-1 after having been run 6 times.
Catalysts 12 00062 g008
Catalysts 12 00062 sch002 550
Scheme 2. Plausible mechanism for [HCP-CH2-Im][Cl]-1 catalyzed CO2 cycloaddition with epoxides.
Scheme 2. Plausible mechanism for [HCP-CH2-Im][Cl]-1 catalyzed CO2 cycloaddition with epoxides.
Catalysts 12 00062 sch002
Table
Table 1. Cycloaddition of CO2 with styrene oxide.
Table 1. Cycloaddition of CO2 with styrene oxide.
EntrySample aIL Content b (mmol g−1)SBET c (m2 g−1)Yield d (%)Sel. d (%)
1HCP-1153599
2HCP-CH2-Cl-1-530799
3[HCP-CH2-Im][Cl]-12.1038595>99
4[HCP-CH2-Im][Cl]-20.9951089>99
5[HCP-CH2-Im][Cl]-30.483867499
6[HCP-CH2-Im][Cl]-40.8550388>99
7[HCP-CH2-Im][Cl]-51.651837299
8[HCP-CH2-Im][Cl]-61.8140684>99
9[HCP-CH2-Im][Cl]-71.262957299
a Reaction condition: 5 mmol styrene oxide, catalyst (25 wt%), CO2 (0.1 MPa), 140 °C, 4 h. b IL content calculated by elemental analysis. c BET specific surface area. d Yield and selectivity were determined by 1H NMR.
Table
Table 2. Cycloaddition of CO2 to various epoxides catalyzed by [HCP-CH2-Im][Cl]-1.
Table 2. Cycloaddition of CO2 to various epoxides catalyzed by [HCP-CH2-Im][Cl]-1.
Entry aSubstrateProductTime (h)Yield b (%)Sel. b (%)
1 Catalysts 12 00062 i001 Catalysts 12 00062 i00249999
2 Catalysts 12 00062 i003 Catalysts 12 00062 i00449999
3 Catalysts 12 00062 i005 Catalysts 12 00062 i006499>99
4 Catalysts 12 00062 i007 Catalysts 12 00062 i0085
8
20		99
94 (120 °C)
93 (100 °C)		99
>99
99
5 Catalysts 12 00062 i009 Catalysts 12 00062 i010692>99
6 Catalysts 12 00062 i011 Catalysts 12 00062 i012691>99
7 c Catalysts 12 00062 i013 Catalysts 12 00062 i014695 c-
8 Catalysts 12 00062 i015 Catalysts 12 00062 i01669999
a Reaction condition: 5 mmol epoxides, [HCP-CH2-Im][Cl]-1 (25 wt%), CO2 (0.1 MPa), 140 °C. b Yield and selectivity were determined by 1H NMR. c Isolate yield by recrystallization.
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Liao, X.; Pei, B.; Ma, R.; Kong, L.; Gao, X.; He, J.; Luo, X.; Lin, J. Hypercrosslinked Ionic Polymers with High Ionic Content for Efficient Conversion of Carbon Dioxide into Cyclic Carbonates. Catalysts 2022, 12, 62. https://doi.org/10.3390/catal12010062

AMA Style

Liao X, Pei B, Ma R, Kong L, Gao X, He J, Luo X, Lin J. Hypercrosslinked Ionic Polymers with High Ionic Content for Efficient Conversion of Carbon Dioxide into Cyclic Carbonates. Catalysts. 2022; 12(1):62. https://doi.org/10.3390/catal12010062

Chicago/Turabian Style

Liao, Xu, Baoyou Pei, Ruixun Ma, Lingzheng Kong, Xilin Gao, Jiao He, Xiaoyan Luo, and Jinqing Lin. 2022. "Hypercrosslinked Ionic Polymers with High Ionic Content for Efficient Conversion of Carbon Dioxide into Cyclic Carbonates" Catalysts 12, no. 1: 62. https://doi.org/10.3390/catal12010062

APA Style

Liao, X., Pei, B., Ma, R., Kong, L., Gao, X., He, J., Luo, X., & Lin, J. (2022). Hypercrosslinked Ionic Polymers with High Ionic Content for Efficient Conversion of Carbon Dioxide into Cyclic Carbonates. Catalysts, 12(1), 62. https://doi.org/10.3390/catal12010062

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