- Nano Express
- Open Access
Triphenylphosphine-based functional porous polymer as an efficient heterogeneous catalyst for the synthesis of cyclic carbonates from CO2
© The Author(s). 2017
- Received: 1 August 2017
- Accepted: 16 November 2017
- Published: 28 November 2017
A novel triphenylphosphine-based porous polymer (TPDB) with a high Brunauer–Emmett–Teller (BET) surface area was synthesized through Friedel–Crafts alkylation of triphenylphosphine and α-dibromo-p-xylene. Then, the functional hydroxyl groups were successfully grafted onto the polymer framework by post modification of TPDB with 3-bromo-1-propanol (BP) and triethanolamine (TEA). The resulting sample TPDB-BP-TEA was characterized by various techniques such as FT-IR, TG, SEM, EDS mapping, ICP-MS, and N2 adsorption–desorption. This new polymer was tested as the catalyst in the solvent-free cycloaddition reaction of CO2 with epoxides, which exhibited excellent performance, with high yield, selectivity, and stable recyclability for several catalytic cycles. The comparison experiment results demonstrate that the bromide ions and hydroxyl groups, as well as high surface area, are key factors in improving the catalytic activity of this new catalyst.
- Porous polymer
- Heterogeneous catalyst
- Fixation of CO2
- Cyclic carbonates
Ionic liquids (ILs) have been attracted significant attention as alternative reaction media/catalysts because of their specific properties, such as negligible volatility, excellent thermal stability, remarkable solubility, and the variety of structures [1–3]. Particularly, ILs could be designed and modified with various functional groups in their cations or anions to gain the functionalities required by target reactions [4, 5]. Many IL-catalyzed organic reactions have been reported, among which cycloaddition reactions are a hot topic [6, 7]. Since carbon dioxide (CO2) is a potentially abundant, cheap, non-toxic, nonflammable, and renewable carbon resource in organic synthesis, great effort has been made to develop effective processes for CO2 chemical fixation. Recently, the cycloaddition of CO2 with epoxides for the synthesis of valuable cyclic carbonates is expected to be one of the most promising strategies for effective fixation of CO2 [8–11]. The products cyclic carbonates have found extensive applications as aprotic solvents, precursors, fuel additives, and green reagents. Though ILs have demonstrated to be excellent catalysts for the cycloaddition of CO2 at metal-free/solvent-free conditions, these homogeneous catalysts inevitably suffered from some problems of catalyst recovery and product purification.
The porous materials with high surface area open up new possibilities for the design and synthesis of new heterogeneous catalysts [12–14]. During the last few decades, in addition to traditional porous zeolites and activated carbon, a number of useful porous materials such as metal organic frameworks (MOFs) [15, 16], covalent organic frameworks (COFs) [17, 18], and porous organic polymers [19, 20] were developed and applied as catalyst supports for heterogeneous catalysis. Among these porous materials, IL-containing porous organic polymers have attracted particular attention due to their low skeletal density, high chemical stability, and the capability of introducing a broad range of useful functional groups within the porous framework [21–23]. For example, He et al. have developed a series of novel heterogeneous catalyst by immobilizing imidazolium-based ILs on an FDU-type mesoporous polymer, which show a good catalytic activity in the CO2 cycloaddition reaction . However, multistep IL-modification method will inevitably suffer from the low IL loading amount and the inhomogeneous distribution of ILs. Besides the post modification strategy, direct synthesis of IL-containing polymer by radical polymerization is an alternative approach. For example, Wang and co-workers reported a template-free radical self-polymerization method to synthesize a mesoporous hierarchical poly(ionic liquid)s . The obtained poly (ionic liquid)s present high activity, easy recycling, and reuse in the cycloaddition of CO2. Although various ionic polymers with abundant functional species can be obtained, the high BET surface area and high IL loading amount still cannot be acquired simultaneously make this copolymerization technique embarrassing. Therefore, the incorporation of IL groups into porous organic polymer framework with a high stable content and large surface area is still a great challenge.
In this paper, we reported the synthesis of triphenylphosphine-based ionic porous polymer with high surface area, large pore volume, and abundant bromide ions and hydroxyl groups for the cycloaddition of CO2 with epoxides. First, triphenylphosphine (PPh3) and α-dibromo-p-xylene (DB) were reacted to form porous polymer (TPDB) through Friedel–Crafts alkylation with anhydrous FeCl3 as a promoter. Then, the TPDB can be easily functionalized by 3-bromo-1-propanol (BP) and triethanolamine (TEA), respectively, to afford functional porous polymer (TPDB-BP-TEA). TPDB-BP-TEA was characterized by employing FTIR, TG, SEM, EDS mapping, ICP-MS, and N2 adsorption–desorption. Systematic catalytic tests show that the porous polymer is excellent catalyst for cycloaddition of CO2 to epoxides, with the advantages of high activity and selectivity, easy recovery, and steady reuse.
Materials and methods
All the chemicals were of chemical grade and used as purchased. Thermogravimetry (TG) analysis was conducted with a STA409 instrument at a heating rate of 10 K/min in nitrogen. Fourier-transform infrared (FT-IR) spectra were recorded on an Agilent Cary 660 FT-IR spectrometer in the 4000–400 cm−1 region with the tested samples pressed into KBr disks. Scanning electron microscopy (SEM) images were recorded on a SUPERSCAN SSX-550 electron microscope (Shimadz, Japan) operating at 20 kV. The phosphorus (P), oxygen (O), and nitrogen (N) element distribution were characterized by Hitachi S-4800 field emission scanning electron microscope accompanied by energy dispersive X-ray spectrometry. BELSORP-MINI instrument was used to measure the nitrogen sorption-isotherms at liquid nitrogen (77 K) temperature. 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. The P element content was determined by ICP-MS using Agilent 7700 spectrometer. CHN elemental analysis was performed on an elemental analyzer Vario EL cube.
Synthesis of TPDB
TPDB was prepared according to the previous literature . PPh3 (4 mmol, 1.05 g) and α-dibromo-p-xylene (DB, 4 mmol, 1.06 g) were dissolved in 20 mL 1,2-dichloroethane (DCE). Then, anhydrous FeCl3 (16 mmol, 2.59 g) was added in the above solution to catalyze the alkylation between PPh3 and DB. The reaction mixture was first stirred at 45 °C for 5 h and then reacted at 80 °C for another 48 h. On completion, the resulting brown gel was filtered out and Soxhlet extracted with DCE and methanol for 24 h, respectively. The cross-linked polymer TPDB was obtained after drying at 60 °C under vacuum condition.
Synthesis of TPDB-BP
The obtained polymer TPDB (1 g) was dispersed in 15 mL acetonitrile, and 3-bromo-1-propanol (BP, 0.8 g) was added into the solution. The reaction mixture was reacted at 80 °C for 24 h. The solid product TPDB-BP was filtered, washed with acetonitrile for three times, and dried at 60 °C under vacuum condition.
Synthesis of TPDB-BP-TEA
TPDB-BP (1 g) was dispersed in 15 mL acetone, and then, triethanolamine (TEA, 0.8 g) was added into it. The reaction mixture was reacted at 60 °C for 24 h. On completion, the solid product TPDB-BP-TEA was filtered and washed with acetone for three times, followed by drying in vacuum at 60 °C for 12 h. ICP-MS analysis result disclosed 3.7 wt% of P element within TPDB. CHN elemental analysis results found (wt%) C 64.91%, H 5.54%, and N 1.65%.
The cycloaddition reaction was performed in a stainless steel autoclave reactor (25 mL) with a magnetic stirrer. In a typical run, propylene oxide (PO, 20 mmol) and catalyst TPDB-BP-TEA (0.1 g) were placed in the autoclave reactor. CO2 was then charged to 1 MPa, and the reaction temperature was adjusted to 120 °C. The reaction mixture was reacted for 6 h, after which, the reactor was cooled to ambient temperature, and ethyl alcohol was added into it to dilute the reaction mixture. The solid catalyst was filtered out, and the filtrate was analyzed by gas chromatography (GC) using biphenyl as an internal standard to calculate the yield. GC was equipped with a FID and a DB-wax capillary column (SE-54 30 m × 0.32 mm × 0.25 μm). The GC spectra are shown in Additional file 1: Figures S1–S5.
Synthesis and characterization of catalysts
Catalytic performance of catalysts
Cycloaddition of CO2 and PO catalyzed by various catalysts
A porous organic polymer with large surface area, high density of ionic sites, and functional –OH groups is developed by Friedel–Crafts alkylation and post modification reaction. The resulting sample TPDB-BP-TEA could be used as the highly efficient heterogeneous catalyst for the synthesis of cyclic carbonates from cycloaddition of CO2 and epoxides under metal-free and solvent-free conditions. Relative high yields and selectivity are obtained over various substrates, and the catalyst can be facilely separated and reused with very steady activity. The abundant bromide ions and hydroxyl groups, the porous structure, and high surface area are revealed to be responsible for the catalyst’s excellent performances in cycloaddition of CO2. The approach in this work triggers an ideal pathway for an easy access to a series of porous, functionalizable polymers, which not only can be applied for chemical fixation of CO2 into fine chemicals, but is also promising for a myriad of potential catalytic applications.
The authors thank Guandong Provincal Department of Science and Technology and Shenzhen Science and Technology Innovation Commission for financial support.
This work was supported by the sponsorship from Shenzhen Science and Technology Research Grant (JCYJ20160527100441585, JCYJ20150331100628880, JCYJ20160510144254604, KQTD2014062714543296, JCYJ20150731091351923, JCYJ20170412150857, JCYJ20160330095448858, and JCYJ20150806112401354), Shenzhen Peacock Plan (KQTD2014062714543296), and Guangdong Key Research Project (nos. 2014B090914003 and 2015B090914002).
SW is the first author. SW, CT, and YW designed the experiment. SW and SC carried out the experiments and characterizations. BJ, HT, and HM helped to draft and correct the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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