Article pubs.acs.org/JPCC
Triazine-Based Microporous Polymers for Selective Adsorption of CO2 Muhammad Saleh,†,‡ Seung Bin Baek,‡ Han Myoung Lee,‡ and Kwang S. Kim*,‡ †
Department of Chemistry, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, Korea Centre for Superfunctional Materials, Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 689-798, Korea
‡
S Supporting Information *
ABSTRACT: Propeller-shaped triazine was used to synthesize microporous polycarbazole materials through an inexpensive FeCl3-catalyzed reaction using direct oxidative coupling (PCBZ) and extensive cross-linking (PCBZL) polymerization routes. PCBZL has a Brunauer−Emmett−Teller specific surface area of 424 m2 g−1 and shows larger CO2 uptake (64.1 mg g−1 at 273 K, 1 atm). Selective adsorption of CO2 over N2 calculated using the ideal adsorbed solution theory shows that both PCBZ (125) and PCBZL (148) exhibit selectivity at 298 K, which is significantly higher than PCBZ (110) and PCBZL (82) at 273 K. These values of selectivity are among the highest reported for any triazine-based microporous material. By introducing the electron-rich carbazole structure into the nitrogen fertile triazine-based system, the adsorption enthalpy is increased drastically, which in turn contributes to high selective adsorption values. The larger existing binding energy between CO2 and propeller specifies more stable and favorable interactions between adsorbent and adsorbate, which transforms into reasonable adsorption capacity at low pressure and eventually high selectivity. These polymeric networks also show moderate working capacity with high regenerability factors. The combination of a simple inexpensive synthesis approach, high thermal/chemical stability, and reasonable selective adsorption make these materials potential candidates for CO2 storage and separation applications.
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INTRODUCTION Carbon dioxide (CO2) is considered to be one of the major constituents of the greenhouse gases that contributes to global warming.1 The anthropogenic CO2 emissions are mainly due to the burning of fossil fuels and chemical processes.2 Recently, new and intriguing tools have been developed and introduced for CO2 capture, storage, and utilization.1 Liquid alkaline scrubbing solutions like aqueous solution of monoethanolamine have good adsorption properties and are currently being used in commercial processes, but the cost of their regeneration process is sufficiently large.3 Solid state adsorbents are a good alternative to these technologies owing to their operation over a broad adsorption temperature range. Additionally, the production of smaller amounts of waste during cycling and the ease of disposal of spent solid adsorbents without causing environmental hazards make them promising candidates. Recently, microporous organic polymers (MOPs) have garnered interest due to their applications in the fields of gas adsorption and chemical separations.4−11 They have advantage over other materials due to their low cost, high specific surface area (SSA), low skeletal density, and high thermochemical stability.11 This MOP class contains a variety of materials that include polymers of intrinsic microporosity,12 hyper-crosslinked polymers,13 porous aromatic frameworks,14 conjugated microporous polymers,15 covalent organic frameworks,16 and covalent triazine frameworks (CTFs).17 Among them, CTFs are important due to their high stability and existence of both © XXXX American Chemical Society
crystalline and amorphous structures. Although both structural types of CTFs have been studied for gas sorption, very few methods are available for the synthesis of triazine based porous materials. At present, various synthesis methods are available where the trimerization/condensation approaches have been used for synthesis of porous structures with reasonable gassorption properties.18−23 Thus, traizine as a building block has been used to construct microporous polymers and for gas sorption studies under various pressure conditions. However, the adsorption properties of CO2, CH4, and N2 at different temperature and the systematical study of temperature influence on separation of CO2/CH4 and CO2/N2 are still unexplored. Porous organic materials containing nitrogen in the backbone are considered to be pioneer materials because they provide excellent selectivity of CO2 over nitrogen. Because the texture properties of these materials are mainly governed by the design or selection of a specific monomer, we used the wellknown triazine-based material 2,4,6-tricarbazolo-1,3,5-triazine (TCT), which has a noncoplanar propeller-like structure due to the steric effect of the peripheral carbazole (CBZ) rings. This type of materials has been employed as an organic lightemitting diode.24 The monomer utilized in the polymerization Received: September 11, 2014 Revised: February 17, 2015
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DOI: 10.1021/jp509188h J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Scheme for the synthesis of noncoplanar propeller structure tricarbazole cyanurate.
Preparation of Microporous Polymeric Networks. Synthesis of PCBZL by Extensive Cross-Linking Approach. TCT (200 mg, 0.347 mmol) was dispersed in 20 mL of anhydrous DCE. To this dispersion, DMM (0.184 mL, 2.083 mmol) was added under a flow of nitrogen. Iron(III) chloride (338 mg, 2.083 mmol) was added and the reaction mixture was heated to 80 °C for 24 h. After cooling to room temperature, the solid product was collected by filtration and repeatedly washed with methanol, dilute HCl, distilled water, and methanol. The product was dried under vacuum at 90 °C for 24 h. Yield = 92% Synthesis of PCBZ by Oxidative Coupling Polymerization. TCT (200 mg, 0.347 mmol) was dispersed in 20 mL of DCE under the nitrogen environment. To this solution, we added iron(III) chloride (450 mg, 2.78 mmol), and the mixture was stirred at room temperature for 24 h. The resulting mixture was transferred to 100 mL of methanol. The precipitates were collected by filtration and washed with methanol, dilute HCl, distilled water, and methanol, sequentially. After washing, the precipitates were collected and dried under vacuum at 90 °C for 24 h. Yield = 90%, Instrumental Characterization. A Micromass Platform II mass spectrometer was used to determine the molecular weight. Melting points was obtained using a differential scanning calorimetric (DSC) measurement with a Shimadzu DSC-60 instrument. Fourier transformed infrared (FTIR) spectra were collected in KBr pellets using a Bruker FTIR. The 400 MHz 1H NMR spectrum and 100 MHz 13C NMR spectrum were measured in a CDCl3 solution with the Varian VNMRS 400 spectrometer. Elemental analysis was performed using Thermo Scientific Element Analyzer. Solid-state 13C cross-polarization magic-angle spin nuclear magnetic resonance (CP-MAS NMR) measurements were performed on a Bruker Avance II+ NMR 400 MHz spectrometer at a frequency of 100.62 MHz with 13 kHz spinning rate, and 1024 scans were signal averaged. Scanning electron microscopy (SEM) images of the product were taken on a field-emission scanning electron microscope (FESEM, JEOL, FEG-XL 30S) operating at an accelerating voltage of 5.0 kV. An energy-dispersive X-ray (EDX) detector was used to analyze the chemical elements of the samples operating at an accelerating voltage of 20 kV. X-ray diffraction (XRD) patterns were recorded from 10 to 80° on a Riguka, Japan, RINT 2500 V X-ray diffractionmeter using Cu Kα irradiation (λ = 1.5406 Å). A Seiko thermogravimetric/ differential thermal analyzer-6300 was used to collect thermogravimetric analysis (TGA) data by heating the samples at 5 °C min−1 to 800 °C in a nitrogen atmosphere. Gas adsorption measurements were performed using a Belsorp mini II (Micromeritics, Japan) device. Before each measurement, a weighed sample was heat-treated at 150 °C under vacuum for 16 h. Brunauer−Emmett−Teller (BET)
was synthesized in a single step using cyanuric trichloride, CBZ, and n-butyl lithium. The synthesized monomer was polymerized in the presence of the FeCl3 catalyst by direct oxidative coupling25 and as well as extensive cross-linking approach.26 The focus of the present work is to (i) synthesize and well characterize the chemical and porous structures of the two microporous polymers and (ii) study the influence of pore morphology and building blocks on the adsorption capacities, enthalpies of adsorption, and the selective adsorption of CO2 over N2 and CH4 in the microporous polymers.
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EXPERIMENTAL SECTION Materials and Methods. All chemicals were purchased from local suppliers and used without purification: iron(III) chloride (Alfa Aesar, 98%), n-butyl lithium in 1.6 M n-hexane (Aldrich, 99%), CBZ (Aldrich, 98%), dimethoxymethane (DMM, Alfa Aesar, 98%), cyanuric trichloride (Aldrich, 99%), anhydrous tetrahydrofuran (THF) (Aldrich, 99%), 1,2dichloroethane (DCE, Aldrich, 99%), methanol (Aldrich, 99%), hydrochloric acid (Aldrich, 35%), and ether (Aldrich, 99%). High-purity gases were used for the adsorption measurements (N2: 99.999%), (H2: 99.999%), (CH4: 99.5%), and (CO2: 99.5%). Synthesis of 2,4,6-Tricarbazolo-1,3,5-triazine. CBZ (3.0 g, 17.93 mmol) was dissolved in dry THF (25 mL) under a nitrogen atmosphere in a three-necked side-arm roundbottomed flask equipped with a stirring bar, a septum inlet, and a three-way stopcock. This mixture was cooled in dry ice bath and stirred for 10 min. To this solution, 1.6 M n-butyllithium/ hexane solution (12.2 mL, 17.93 mmol) was added dropwise, and a little exothermicity was noticed during addition. The cold bath was removed 15 min after the addition of n-butyllithium. In another three-necked round-bottomed flask equipped with a condenser, a slurry of cyanuric trichloride (1.0 g, 5.43 mmol) in dry THF (25 mL) was prepared. The CBZ−lithium solution was added dropwise to the cyanuric trichloride solution, and a large amount of solid product was precipitated. After stirring for 1 h at room temperature, this reaction mixture was refluxed for 6 h. After cooling to room temperature, 60 mL of water was added. The product was filtered off, washed with water and diethyl ether, and further purified using hot chlorobenzene. The faint yellow crystalline solid was finally oven-dried under vacuum at 100 °C to remove traces of chlorobenzene. Yield = 60%. EI-MS: m/z 576.206 found (cf. 576.2065 calculated for C39H24N6). Elemental analysis: Expected, C: 81.23%, H: 4.20%, N: 14.57%; Found, C: 80.90%, H: 4.09%, N:14.24%. 1H NMR (400 MHz, CDCl3) δ 7.45 (m, 12 H), 8.13 (m, 6 H), 9.02 (m, 6 H). The peak at 1.589 ppm corresponds to the residual water traces in CDCl3. 13C NMR (100 MHz, CDCl3) δ 138.78, 127.11, 126.51, 123.49, 119.76, 117.52. mp 452 °C. B
DOI: 10.1021/jp509188h J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. (a) Synthesis of porous polymers by oxidative and extensive linker polymerization and (b) FESEM images of porous polymers.
Figure 3. (a) FTIR spectra of tricarbazolecyanurate and polymers and (b) 13CP-MAS NMR spectra of polymers. Asterisks show spinning sidebands.
monomer was prepared by the nucleophilic substitution of cyanuric trichloride. (See Figure 1.) Previous reports have shown that nucleophilic substitution of cyanuric trichloride results in incomplete substitution due to both relatively low nucleophilicity and high steric requirements of diarylamines.27 Therefore, nucleophilic substitution of cyanuric trichloride with three equivalents of CBZ was used to prepare the noncoplanar propeller-shaped monomer (scheme shown in Figure 1). The system temperature was raised slowly in several steps so that the process of CBZ linking to the cyanuric moiety, which
surface area was investigated using N2 adsorption−desorption isotherms measured at 77 K. Pore-size distributions were calculated using nonlocal density functional theory (NLDFT). CO2, N2, and CH4 storage adsorption isotherms were measured at 273 and 298 K and up to 1 atm.
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RESULTS AND DISCUSSION To build a porous architecture, we employed polymerization of a building block with noncoplanar geometry to form 3D porous organic frameworks. Keeping this in mind, a propeller shape C
DOI: 10.1021/jp509188h J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 4. (a) N2 adsorption isotherms measured at 77 K for polymeric networks and (b) their pore size distributions.
samples were prepared on a SiO2 substrate. Solid-state 13C CPMAS NMR was used to investigate the structure of the polymers (Figure 3b). The characteristic triazine carbon resonance peak is observed at ∼164.3 ppm.29 The resonance peaks around 137.0 and 125.4 ppm are attributed to the substituted aromatic carbons, while signal peaks at 118.3 ppm are ascribed to the unsubstituted aromatic carbons. The additional methylene carbon introduced in the PCBZL polymer displays a characteristic peak at 39.4 ppm. XRD was used to determine the crystalline nature of the polymeric networks. XRD patterns show only two broad diffraction signals at 2 theta angles of 12 and 23°; this clearly shows that the polymers are mainly amorphous in nature and have a small crystallization character (Figure S6 in the SI). The peak at ∼12° is ascribed to the chain-to-chain distance of densely packed polymer chains, while the peak at ∼23° is assigned to more loosely packed polymer chains.30 To investigate the thermal stabilities of the porous networks, we conducted thermogravimetric measurements by TGA under a nitrogen atmosphere over the temperature 30−800 °C. Both samples exhibit
