- Nano Express
- Open Access
Porous polymers bearing functional quaternary ammonium salts as efficient solid catalysts for the fixation of CO2 into cyclic carbonates
© The Author(s). 2016
Received: 14 March 2016
Accepted: 24 June 2016
Published: 1 July 2016
A series of porous polymers bearing functional quaternary ammonium salts were solvothermally synthesized through the free radical copolymerization of divinylbenzene (DVB) and functionalized quaternary ammonium salts. The obtained polymers feature highly cross-linked matrices, large surface areas, and abundant halogen anions. These polymers were evaluated as heterogeneous catalysts for the synthesis of cyclic carbonates from epoxides and CO2 in the absence of co-catalysts and solvents. The results revealed that the synergistic effect between the functional hydroxyl groups and the halide anion Br− afforded excellent catalytic activity to cyclic carbonates. In addition, the catalyst can be easily recovered and reused for at least five cycles without significant loss in activity.
Global climate change and excessive CO2 emission have attracted widespread public concern in recent years. Since CO2 is expected to be a highly abundant, quite cheap, nontoxic, and nonflammable C1 resource in the organic synthesis, the capture and utilization of CO2 to produce higher-value relevant chemicals, such as polycarbonates and cyclic carbonates, are receiving rising attention [1, 2]. In this context, the conversion of CO2 to cyclic carbonates via epoxide substrates is demonstrated to be an atom-economical reaction, and the products can serve as excellent aprotic polar solvents as well as intermediates in the production of pharmaceuticals and fine chemicals [3–5]. In the past few decades, extensive efforts have been devoted to develop efficient catalysts for the synthesis of cyclic carbonates, including salen-metal complexes [6–8], quaternary ammonium/phosphonium salts [9, 10], ionic liquids (ILs) [11, 12], molecular sieves , metal-organic frameworks [14, 15], and so on. Among them, ILs have become a class of promising candidates owing to their unique features of high thermal stability, variety of structures available, and easy shaping [16–18]. To simplify the separation process and improve the reusability of ILs, more efforts have been devoted to the supported IL catalysts [19, 20], which usually suffers from tedious preparation process, large mass transfer resistance, and the leaching of IL active sites. Thus, new strategies for the preparation of efficiently heterogeneous IL catalysts for the synthesis of cyclic carbonates are highly desirable.
Nanoporous polymeric materials have attracted increasing attention due to their versatile and tunable structures, high surface area, and tunable surface chemistry, which allow potential applications in gas storage, explosive detection, drug release, and catalysis [21, 22]. Particularly, ionic porous polymers obtained by the polymerization of monomeric ILs or copolymerization of ILs with other monomers have been investigated as innovative solid catalysts or catalyst supports [23, 24]. A wide range of ionic porous polymers have been developed and have shown excellent catalytic performances in numerous organic synthetic reactions, among which, the cycloaddition of CO2 with epoxide is a hot topic . For example, Wang et al.  used an ionothermal method to prepare a novel meso-macroporous hierarchical poly(ionic liquid), which was applied as highly efficient heterogeneous catalysts for the conversion of CO2 into cyclic carbonates at ambient pressure. Zhang et al. prepared imidazolium salt-modified porous hypercross-linked polymers and used them as highly efficient solid catalysts for synergistic CO2 capture and conversion . However, the present systems still suffer from long reaction time, high CO2 pressure, or high reaction temperature. Moreover, most of the present ionic porous polymers are based on imidazolium ILs, while the quaternary ammonium salt IL-based ionic porous polymers are scarce, although quaternary ammonium salts have been proved to be one of the most efficient catalysts for CO2 fixation. In addition, it has been reported that the functional groups such as hydroxyl and carboxyl are favorable for the cycloaddition reaction due to the synergistic effect with Br ions [28, 29]. These considerations prompted us to design new hydroxyl-containing quaternary ammonium salt-based ionic porous polymers as “task-specific” catalysts for the cycloaddition of CO2 with epoxides.
All chemicals in this work were used as received without further purification. Triallylamine (TAA); divinylbenzene (DVB), 3-bromo-1-propanol, 1-bromobutane, epichlorohydrin (ECH), and propylene oxide (PO) were purchased from Sigma Aldrich Reagent Co., LLC. Styrene oxide (SO), allyl glycidyl ether (AGE), and cyclohexene oxide (CHO) were provided by Aladdin Chemical Reagent Co., Ltd. Other reagents were laboratory-grade reagents from local suppliers.
1H NMR spectra were collected on a Varian Mercury plus 400-MHz spectrophotometer at ambient temperature using D2O as solvent. CHN elemental analysis was performed on a vario EL cube elemental analyzer. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a FT-IR instrument (Nicolet 360, KBr discs) in the 4000–400 cm−1 region. Thermogravimetric (TG) analysis was carried out with a STA409 instrument in nitrogen at a heating rate of 15 °C min−1. The nitrogen adsorption-desorption isotherms were measured on a BELSORP-MINI instrument at liquid nitrogen (77 K) temperature. The samples were evacuated at 423 K for 1.5 h before measuring. The specific surface areas were evaluated using the Brunauer-Emmett-Teller (BET) method, and the pore distribution was calculated by the BJH method from adsorption branches of isotherms. Transmission electron microscopy (TEM) analysis was performed on a JEM-2100 (JEOL) electron microscope operating at 200 kV. Scanning electron microscopy (SEM) images were recorded on a SUPERSCAN SSX-550 electron microscope (Shimadzu, Japan) operating at 20 kV. The morphology and the bromine (Br) element distribution were characterized by a Hitachi S-4800 field emission scanning electron microscope accompanied by energy-dispersive X-ray spectrometry.
Synthesis of catalysts
Synthesis of HTA and BTA
Triallylamine (20 mmol, 2.74 g) and 3-bromo-propanol (20 mmol, 2.78 g) were dissolved in ethanol (15 mL). The mixture was then stirred at 80 °C for 24 h under nitrogen atmosphere. On completion, the solvent was removed by distillation and the solid product was washed with ethyl acetate three times to remove the unreacted substrates. After drying under vacuum, hydroxypropyl functionalized triallylamine (HTA) was obtained. 1H NMR (400 MHz, D2O, TMS) δ(ppm) = 1.93 (t, 2H, CH2), 3.21 (m, 2H, CH2), 3.51 (m, 2H, CH2), 3.62 (d, 6H, CH2), 5.81 (m, 6H, CH2), 5.94 (d, 3H, CH). The butyl functionalized triallylamine (BTA) was prepared accordingly in the same way and then was characterized by 1H NMR (400 MHz, D2O, TMS) δ(ppm) = 1H NMR (400 MHz, D2O, TMS) δ(ppm) = 0.86 (t, 3H, CH3), 1.26 (m, 2H, CH2), 1.69 (m, 2H, CH2), 3.13 (t, 2H, CH2), 3.72 (d, 6H, CH2), 5.59 (d, 6H, CH2), 5.90 (m, 3H, CH).
Synthesis of ionic porous polymers DVB-HTA and DVB-BTA
DVB (1.3 g, 10 mmol) and HTA (0.58 g, 2 mmol) were dissolved in 20 mL ethyl acetate and 4 mL methanol, respectively. These two solutions were mixed, and azodisisobutyronitrile AIBN (0.05 g) was added into it. After stirring at room temperature for 3 h, the mixture was solvothermally treated at 70 °C for 24 h. The yellow solid product DVB-HTA was filtered, washed with methanol three times, and dried under vacuum at 50 °C for 24 h. CHN elemental analysis for DVB-HTA found the following (wt.%): C 86.87, H 8.64, N 1.07. The ionic porous polymer DVB-BTA was prepared in the same way by reacting DVB with BTA. CHN elemental analysis for DVB-BTA found the following (wt.%): C 87.73, H 8.91, N 1.13.
Typical procedure for cycloadditions
As a typical example, ECH (20 mmol) and catalyst DVB-HTA-Br (0.05 g) were added into a 50-mL stainless steel autoclave equipped with a magnetic stirrer. After the reaction mixture was heated to 120 °C, CO2 was then charged into the reactor until the desired pressure of 1.2 MPa was reached. The reactor was cooled to ambient temperature after reacting 6 h, and the resulting mixture was filtered and the filtrate was analyzed by gas chromatography (GC) that was equipped with a FID and a DB-wax capillary column (SE-54 30 m × 0.32 mm × 0.25 μm). Biphenyl was used as an internal standard to calculate the catalytic conversion. GC-MS (SCIONSQ-456-GC) was used to analyze the purity and structure of the products. For the catalyst recycling, the filtered solid catalyst was directly used in the next run after washing with diethyl ether and drying.
Results and discussion
Preparation and characterization of catalysts
Cycloaddition of CO2 and ECH catalyzed by various catalysts
In summary, we have developed a new type of quaternary ammonium salt-based ionic porous polymer, DVB-HTA, by the copolymerization of DVB and hydroxyl functionalized quaternary ammonium salts. The DVB-HTA was a highly cross-linked porous material with a large surface area of 708 m2/g and abundant Br− anions and acted as efficient heterogeneous catalysts for the transformation of CO2 and epoxides into cyclic carbonates under metal-solvent-free conditions. The excellent catalytic performance of DVB-HTA results from the synergistic effect between the Br− active centers and functional –OH groups. Moreover, the DVB-HTA had good recyclability, attributed to the durable highly cross-linked framework structure. This catalyst is potentially useful in industrial applications due to its low cost, excellent catalytic efficiency, and good stability.
Financial support was provided by the Doctoral Fund of Ministry of Education of China, the Guangdong Government (S20120011226), and the MOST of China (2014AA020512).
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