Porosity Enhancement of Carbazolic Porous Organic Frameworks

Article pubs.acs.org/cm

Porosity Enhancement of Carbazolic Porous Organic Frameworks Using Dendritic Building Blocks for Gas Storage and Separation Xiang Zhang, Jingzhi Lu, and Jian Zhang* Department of Chemistry, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States S Supporting Information *

ABSTRACT: We report a facile synthesis of carbazolic porous organic frameworks (Cz-POFs) via FeCl3 promoted oxidative polymerization. Using bulky, dendritic building blocks with high connectivity, the porosity of Cz-POFs was significantly enhanced. Specifically, Cz-POF-1 and CzPOF-3 show high surface areas of 2065 and 1927 m2 g−1, respectively. These surface areas are 3.1 and 2.1 times larger than those of Cz-POF-2 and Cz-POF-4 constructed from less branched building blocks, respectively. At 1 bar and 273 K, Cz-POF-3 exhibits the highest CO2 uptake (21.0 wt %) and CH4 uptake (2.54 wt %), while Cz-POF-1 has the highest H2 uptake (2.24 wt %) at 77 K. These values are among the highest reported for porous organic polymers. In addition, Cz-POFs exhibit good ideal CO2/N2 selectivities (19−37) and CO2/CH4 selectivities (4.4−7.1) at 273 K, showing great promise for gas storage and separation applications.

1. INTRODUCTION The rapid consumption of fossil fuels has caused huge concerns in climate change and energy security.1 Greenhouse gas mitigation and the emerging clean energy economy require urgent development of novel porous materials that can be used as physical adsorbents in gas storage and separation. Indeed, cheap and easily accessible porous materials with excellent gas uptake performance play pivotal roles in carbon capture and sequestration (CCS) technology2 and clean energy (H23 and CH44) storage. The past decade has witnessed a fast growth of several classes of highly porous materials. For example, metal− organic frameworks (MOFs), prepared from the assembly of organic linkers with metal ions or inorganic clusters based on reticular chemistry,5 represent the most studied crystalline porous materials. Because of the tunability of both organic linkers and inorganic clusters, a large number of highly porous MOFs have been prepared, and several MOFs in fact hold the current record of highest surface area and gas uptake capacity among all the synthetic porous materials.6 However, the higher density compared to pure organic materials due to the presence of metals and the limited physicochemical stability against moisture and other harsh conditions (high temperature, acidic gases, etc.) are disadvantageous for MOFs in practical applications of gas uptake.7 Therefore, recently developed porous organic polymers, synthesized via polymerization of monomers with multiple linkages through strong covalent bonding, have received a great deal of attention.8 In addition to their intrinsic high physicochemical stability, porous organic polymers also possess permanent porosity, low density, and wide structural tunability, and have shown great potential in not only gas storage and separation9 but also other fields such as catalysis10 and energy applications.11 In the past few years, © XXXX American Chemical Society

several classes of porous organic polymers have been developed, and notable examples include PIMs,12 HCPs,13 CTFs,14 PPNs/PAFs,15 CMPs,16 etc. For gas storage and separation, the porosity parameters of porous materials such as surface area, pore size, and pore volume are important factors that dictate the adsorption capacity and/or gas separation selectivities. Surface area, in particular, is one of the most important porosity parameters and largely determines the adsorption capacity. Although some porous organic polymers such as PAF-1/PPN-3 exhibit fairly high surface areas that are comparable to the most porous MOFs,15a,c it is generally difficult to synthetically control the surface area of porous organic polymers. Increasing the length of linkages often leads to a significant decrease in surface area of porous organic polymers, presumably due to the network interpenetration.15b,17 Since incorporating bulky organic linkers has proven to be an effective strategy to prevent network interpenetration in MOFs,18 we hypothesize that bulky, dendritic building blocks with high connectivity can efficiently decrease the probability of interpenetration in porous organic polymers and, consequently, lead to an increase of surface area and pore volume. To test our hypothesis, we chose carbazole as the terminal moiety of the dendritic building block for the following reasons. First, carbazole is a relatively large molecule, and it serves as an efficient moiety in constructing bulky, dendritic building blocks with high connectivity. Second, the 3and 6-positions of carbazole are highly reactive,19 and upon oxidative polymerization, carbazolic porous organic polymers Received: May 13, 2014 Revised: June 16, 2014

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dx.doi.org/10.1021/cm501717c | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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can be easily synthesized.20 Third, carbazole contains a nitrogen atom giving rise to an electron-rich polymer, which can significantly enhance interactions with gas molecules, increase gas uptake capacity, and/or enhance the separation selectivities.21 Herein, we describe the design and synthesis of two new carbazolic porous organic frameworks (Cz-POFs) constructed from two dendritic building blocks: Cz-1 and Cz-3 (Scheme 1).

number of scan was 8000. Mass spectra were recorded with the Applied Biosystem’s Voyager DE-Pro MALDI-TOF mass spectrometer. Thermogravimetric Analysis (TGA) was performed on a PerkinElmer STA 6000 thermogravimetric analyzer. Samples were heated from 50 to 800 °C at a rate of 10 °C/min under air. Fouriertransform infrared spectroscopy (FT-IR) was performed on a Nicolet 380 spectrometer. X-ray diffraction patterns were acquired from 3 to 30° using a Bruker-AXS D8 Discover diffractometer. Elemental analysis was performed by Atlantic Microlab, Inc. Gas adsorption isotherms were collected using a Micromeritics ASAP 2020-accelerated surface area and porosimetry analyzer after the samples had been degassed at 110 °C for 10 h under vacuum. The obtained adsorption− desorption isotherms were evaluated to give the pore parameters, including Brunauer−Emmett−Teller (BET) and Langmuir specific surface area (SABET and SALang), pore size, and pore volume. The pore size distribution was calculated from the adsorption branch with the nonlocal density function theory (NLDFT) approach. 2.2. Syntheses. Synthesis of Cz-1. The N-arylation reaction was proceeded with 3,3′,5,5′-tetrabromo-1,1′-biphenyl (0.50 g, 1.1 mmol), carbazole (0.95 g, 5.7 mmol), K2CO3 (0.95 g, 6.9 mmol), CuI (0.30 g, 1.6 mmol), and 1,10-phenanthroline (30 mg, 0.17 mmol) under argon. Anhydrous dimethylformamide (DMF) (13 mL) was injected into the reaction flask, and the resulting solution was degassed for 15 min under stirring. The mixture was heated at 160 °C for 18 h. After cooling down to room temperature, the mixture was poured onto water. The precipitate was washed with ethanol (100 mL) and further purified by recrystallization in toluene to give a white powder as the product. Yield: 0.32 g (36%). 1H NMR (CDCl3, 400 MHz, δ): 8.18 (d, J = 7.8 Hz, 8H), 8.06 (d, J = 1.8 Hz, 4H), 7.92 (s, 2H), 7.63 (d, J = 8.3 Hz, 8H), 7.46 (t, J = 7.5 Hz, 8H), 7.37 (t, J = 7.7 Hz, 8H). 13C NMR (CDCl3, 100 MHz, δ): 142.9, 140.5, 140.4, 126.4, 125.0, 124.4, 123.7, 120.6, 120.6, 109.6. Synthesis of Cz-2. A synthetic procedure similar to that of Cz-1 was adopted. The N-arylation reaction was proceeded with 4,4′-diiodo1,1′-biphenyl (4.5 g, 11 mmol), carbazole (4.0 g, 24 mmol), K2CO3 (4.0 g, 29 mmol), CuI (1.0 g, 5.2 mmol), and 1,10-phenanthroline (104 mg, 0.60 mmol) in anhydrous DMF (20 mL) and reacted at 160 °C for 18 h under argon. Yield: 2.8 g (52%). 1H NMR (CD2Cl2, 400 MHz, δ): 8.20 (d, J = 7.5 Hz, 4H), 7.95 (d, J = 8.6 Hz, 4H), 7.74 (d, J = 8.5 Hz, 4H), 7.54 (d, J = 8.1 Hz, 4H), 7.47 (t, J = 7.8 Hz, 4H), 7.34 (t, J = 7.4 Hz, 6H). Synthesis of Cz-3. A synthetic procedure similar to that of Cz-1 was adopted. The N-arylation reaction was proceeded with 3,3″,5,5″tetrabromo-5′-(3,5-dibromophenyl)-1,1′:3′,1″-terphenyl22 (1.00 g, 1.28 mmol), carbazole (1.90 g, 11.4 mmol), K2CO3 (1.90 g, 13.8 mmol), CuI (0.9 g, 4.72 mmol), and 1,10-phenanthroline (90 mg, 0.500 mmol) in anhydrous DMF (30 mL) and reacted at 160 °C for 3 days under argon. Yield: 0.93 g (66%). 1H NMR (CDCl3, 500 MHz, δ): 8.15 (d, J = 8.0 Hz, 12H), 8.01 (t, J = 1.7 Hz, 9H), 7.86 (s, 3H), 7.56 (d, J = 8.3 Hz, 12H), 7.40 (t, J = 7.7 Hz, 12H), 7.30 (t, J = 7.5 Hz, 12H). 13C NMR (CDCl3, 100 MHz, δ): 144.0, 141.6, 140.6, 140.1, 126.5 126.2, 124.9, 123.7, 120.5, 120.4, 109.6. Synthesis of Cz-4. A synthetic procedure similar to that of Cz-1 was adopted. The N-arylation reaction was proceeded with 1,3,5-tris(4iodophenyl)benzene (0.50 g, 0.73 mmol), carbazole (0.56 g, 3.4 mmol), K2CO3 (0.56 g, 4.0 mmol), CuI (0.16 g, 0.84 mmol), and 1,10phenanthroline (16 mg, 0.089 mmol) in anhydrous DMF (20 mL) and reacted at 160 °C for 18 h under argon. Yield: 0.39 g (67%). 1H NMR (CD2Cl2, 400 MHz, δ): 8.22 (d, J = 7.4 Hz, 6H), 8.14 (s, 3H), 8.11 (d, J = 8.6 Hz, 6H), 7.81 (d, J = 8.6 Hz, 6H), 7.59 (d, J = 8.2 Hz, 6H), 7.50 (t, J = 7.7 Hz, 6H), 7.36 (t, J = 7.4 Hz, 6H). Synthesis of Cz-POF-1. The synthesis of Cz-POFs was performed using a modified procedure reported by Han et al.20 Under argon protection, Cz-1 (216 mg, 0.524 mmol) was dissolved in anhydrous dichloromethane (30 mL). The monomer solution was added dropwise to a mixture of anhydrous FeCl3 (500 mg, 3.08 mmol) and anhydrous dichloromethane (20 mL). The resulting reaction mixture was stirred for 3 days at room temperature. Methanol (100 mL) was added and stirred for 1 h to quench the reaction. The obtained polymer was then washed with HCl (12 M) for 2 h, filtered,

Scheme 1. (a) Space-Filling Models of Carbazolic Building Blocks Placed in Cuboids, of Which Three Edges Illustrate the Monomers’ Dimension, and (b) Synthetic Method for Cz-POFs Using FeCl3 Promoted Oxidative Polymerizationa

a

Reaction conditions: FeCl3, CH2Cl2 (room temperature, 24 h).

Cz-1 and Cz-3 have high connectivities of 8 and 12, respectively, and an overall spherical morphology (Scheme 1a). Upon oxidative polymerization, the resulting Cz-POF-1 and Cz-POF-3 show a significant increase of surface area and remarkable gas adsorption capacity compared with those of CzPOF-2 and Cz-POF-4 that were constructed from less branched monomers (Cz-2 and Cz-4, respectively) (Scheme 1). Under low-pressure conditions, Cz-POF-1 and Cz-POF-3 show much increased CO2, CH4, and H2 storage capacity as well as good CO2/N2 and CO2/CH4 adsorption selectivity.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All chemicals were purchased from commercial suppliers and used without further purification, unless otherwise noted. Dichloromethane was dried and distilled with calcium hydride. Diethyl ether was dried and distilled with sodium. Solution 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on Bruker FT-NMR spectrometers (400 or 500 MHz). Solid-state cross-polarization magic angle spinning (CP/MAS) 13C NMR spectra were recorded on a Bruker Avance III three-channel spectrometer and acquired using CP-TOSS pulse sequences, which were cross-polarized and suppressed the spinning side bands. Spectrometer frequency was 100.6 MHz, cross-polarization time was 4 ms, rotor frequency was 8.0 kHz, relaxation delay was 1 s, and B



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and washed with water and THF. The polymer was further purified using Soxhelet extraction with THF for 24 h, and then dried at 110 °C overnight. Yield: 207 mg (96%). Elemental analysis calcd (%) for C60H30N4: C, 89.31; H, 3.75; N, 6.94. Found: C, 85.38; H, 4.01; N, 6.50. Synthesis of Cz-POF-2. The polymer was synthesized following the same method described above for Cz-COP-1 using Cz-2 (253 mg, 0.524 mmol). Yield: 239 mg (96%). Elemental analysis calcd (%) for C36H20N2: C, 89.97; H, 4.20; N, 5.83. Found: C, 87.71; H, 4.38; N, 5.86. Synthesis of Cz-POF-3. The polymer was synthesized following the same method described above for Cz-COP-1 using Cz-3 (227 mg, 0.175 mmol). Yield: 213 mg, (96%). Elemental analysis calcd (%) for C96H48N6: C, 89.69; H, 3.77; N, 6.54. Found: C, 84.60; H, 3.84; N, 5.97. Synthesis of Cz-POF-4. The polymer was synthesized following the same method described above for Cz-COP-1 using Cz-4 (279 mg, 0.349 mmol). Yield: 223 mg (97%). Elemental analysis calcd (%) for C60H33N3: C, 90.10; H, 4.14; N, 5.76. Found: C, 87.03; H, 4.44; N, 5.42.

all surface area measurements performed after such acid treatment show excellent porosity (vide infra). Thermogravimetric analyses of Cz-POFs indicate they also have excellent thermal stability (Figure S1, Supporting Information). The thermal decomposition temperatures are up to 500 °C for CzPOF-1, 500 °C for Cz-POF-2, 505 °C for Cz-POF-3, and 480 °C for Cz-POF-4 at 5% weight loss (Figure S1, Supporting Information). The broad and featureless diffraction patterns from powder X-ray diffraction analysis reveal the amorphous nature of Cz-POFs (Figure S2, Supporting Information). Solidstate CP/MAS 13C NMR was used to characterize the structures of Cz-POFs (Figure S3−6, Supporting Information). In general, four broad resonance peaks at 140, 125, 120, and 110 ppm are consistent with previously reported CPOPs series.20 Specifically, the signal at ∼140 ppm corresponds to the phenyl carbons bonded with the nitrogen atom in the carbazolic moiety. The signal at ∼125 ppm can be attributed to other substituted phenyl carbon atoms. Finally, the signals at ∼120 and ∼110 ppm are due to the unsubstituted phenyl carbons. Elemental analyses of Cz-POFs give satisfactory results compared to the theoretical values (Experimental Section). These results are indicative of an efficient oxidative polymerization. It should be noted that incomplete polymerization in the preparation of porous organic polymers often results in a deviation between the observed and theoretical elemental analysis values.28 FT-IR spectra also indicate the successful polymerization. In general, the absorption peak at ∼725 cm−1 (assigned to the bisubstituted phenyl ring in carbazole monomer)29 deceases and the absorption peak at ∼800 cm−1 (assigned to the trisubstituted phenyl ring in carbazole polymer)29 increases (Figure S7, Supporting Information). It is notable that the regioselectivity of the oxidative polymerization is not well-defined since the carbazole molecule can be oxidized at the 3- or 6-position, or at both 3- and 6-positions. In particular, for monomers Cz-1 and Cz-3 with high connectivity, full oxidization at each carbazole moiety is unlikely due to the steric hindrance. 3.2. Porosity Measurements. Porosity parameters of CzPOFs were studied using N2 adsorption−desorption measurements at 77 K. The N2 isotherms of Cz-POFs (Figure 1) show rapid N2 uptake at low relative pressures (P/P0 < 0.05), which is typical for microporous materials. The gradual increase in N2 uptake (P/P0 = 0.05−0.9) is due to the presence of mesopores in the polymers. The apparent hysteresis between adsorption

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Cz-POFs. Carbazolic building blocks Cz-n (n = 1, 2, 3, and 4) were first synthesized using a copper-promoted N-arylation reaction and used in subsequent oxidative polymerization. Compared to many other transition-metal coupling reactions such as palladium-catalyzed Suzuki,23 Sonogashira−Hagihara,24 and homocoupling of alkynes,25 oxidative polymerization provides a very cost-effective method for preparing porous organic polymers.26 Using a single monomer and cheap oxidants such as FeCl3, the homocoupling reaction can proceed at room temperature in high yield. Therefore, this method shows great promise for scale-up preparation of porous materials for practical applications. Indeed, oxidative polymerization as a facile method to prepare porous organic polymers has drawn extensive attention in the past several years.20,27 We first optimized the polymerization conditions such as solvent, temperature, injection rate, and reaction time using a commercially available monomer, 1,3,5-tri(9-carbazolyl)-benzene (Table S1, Supporting Information). The porosity of the resulting porous organic polymer, specifically, SABET, was used as the criterion to determine the best reaction condition. It was found that the injection of monomer solution over a period of 40 min to a FeCl3/dichloromethane mixture at room temperature followed by polymerization for 24 h gave rise to the polymer with the highest SABET, and therefore, such reaction conditions were used for the polymerization of Cz-n (n = 1, 2, 3, and 4). It should be noted that Cz-4 dissolves poorly in dichloromethane, but has higher solubility in chlorobenzene. Since it is preferable to consistently use the same solvent for all polymerization reactions to minimize the impact of the solvent, we performed the polymerization of Cz-4 in both dichloromethane and chlorobenzene. Since the SABET of Cz-POF-4 synthesized in both solvents were essentially identical, dichloromethane was used as the solvent for all reactions. Oxidative polymerization of Cz-n (n = 1, 2, 3, and 4) proceeded smoothly by treating with anhydrous FeCl3 in dry dichloromethane under argon at room temperature and yielded Cz-POF-n (n = 1, 2, 3, and 4). Cz-POFs are insoluble in common organic solvents such as dichloromethane, THF, acetone, methanol, and DMF, suggesting that they are composed of highly cross-linked networks. All Cz-POFs are chemically stable. In particular, they can resist a harsh acidic condition (12 M HCl) with no sign of any decomposition since

Figure 1. (a) Nitrogen adsorption (solid symbols) and desorption (open symbols) isotherms and (b) NLDFT pore size distributions of Cz-POFs. C



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heats of adsorption (Qst) using the virial method, as shown in Figures 2a,d and S12, Supporting Information, and Table 2. Cz-

and desorption is observed for all Cz-POFs, which is consistent with the fact that all polymer networks contain both meso- and microporosity, and it can be attributed to the pore network effects.12a The SABET for Cz-POFs was calculated using the N2 adsorption branch in the pressure range of P/P0 = 0.01−0.10 (Table 1). Cz-POF-1 and Cz-POF-3 show remarkably high Table 1. Porosity Parameters for Cz-POFs polymer

SABETa

SALangb

Vmicroc

Vtotald

Vmicro/Vtotal

Cz-POF-1 Cz-POF-2 Cz-POF-3 Cz-POF-4

2065 671 1927 914

2398 763 2256 1032

0.44 0.19 0.46 0.25

1.57 0.42 1.35 0.60

0.28 0.45 0.34 0.42

a Surface area (m2 g−1) calculated from the nitrogen adsorption branch based on the BET model. bSurface area (m2 g−1) calculated from the nitrogen adsorption branch based on the Langmuir model. cPore volume (diameters 2 nm), which is ∼10% more than those in Cz-POF-2 and Cz-POF-4. This result is significant. It suggests that the use of dendritic building blocks can not only lead to an increase of SABET but also modulate the pore metrics by increasing the ratio of mesopores/micropores. 3.3. Gas Uptake of CO2, CH4, and H2. Because of the considerable porosity and electron-rich feature of Cz-POFs, we were interested in assessing their performance in gas uptake. We first collected CO2 isotherms and calculated their isosteric

Figure 2. CO2 (a), CH4 (b), and H2 (c) uptake isotherms of Cz-POFs and isosteric heats of adsorption for CO2 (d), CH4 (e), and H2 (f).

POF-1 and Cz-POF-3 show excellent CO2 uptake of 202 and 210 mg g−1 at 273 K, respectively, significantly higher than those of Cz-POF-2 (77 mg g−1) and Cz-POF-4 (121 mg g−1). Notably, the CO2 uptake of Cz-POF-3 at 273 K and 1.0 bar represents one of the highest values for porous organic polymers reported to date,9 which can be attributed to its large SABET compared with Cz-POF-2 and Cz-POF-4. In principle, nitrogen-rich porous organic polymers exhibit high CO2 adsorption capacities due to the strong dipole−quadrupole interactions between CO2 and nitrogen sites.30 The tertiary amine group in carbazole acts as a Lewis base site, which is expected to induce a strong interaction between carbazole and CO2, which contains Lewis acidic, electron deficient carbon atoms. The Qst of Cz-POF-3 and Cz-POF-4 for CO2 at zero coverage was found to be 27.8 kJ mol−1, which is slightly higher than those of Cz-POF-1 (25.8 kJ mol−1) and Cz-POF-2 (24.8 kJ mol−1). Overall, the Qst values of Cz-POFs are within the desirable range for CO2 sorbents without a large energy penalty for regeneration,31 comparable to those of BILPs (∼26 kJ mol−1)32 and some functionalized CMPs (∼25−29 kJ mol−1).9a,33 CH4 and H2 are important potential alternative clean fuels for the next generation of automotive technology; therefore, we also studied the uptake of these two gases. The CH4 uptakes at 273 and 290 K up to 1 bar were measured (Figures 2b,e and S13, Supporting Information, and Table 2). All isotherms exhibit a gradual rise and reached a maximum of 10−25 mg g−1 at 273 K. Cz-POF-1 and Cz-POF-3 exhibit as much as twice the CH4 uptake as Cz-POF-2 and Cz-POF-4, demonstrating again D



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Table 2. H2, CO2, CH4, and N2 Uptakes and Isosteric Heats of Adsorption for Cz-POFs CO2 uptakea

a

CH4 uptakea

H2 uptakea

N2 uptakea

polymer

273 K

290 K

Qst

273 K

290 K

Qst

77 K

87 K

Qst

273 K

290 K


202 77 210 121

129 58 134 80

25.3 24.8 27.8 27.8

22.9 11.4 25.4 10.4

15.6 6.0 17.1 6.1

19.0 20.9 20.2 19.3

22.4 9.7 20.7 10.3

14.9 7.3 15.4 7.4

8.1 7.8 8.0 8.5

11.5 4.5 11.0 3.5

8.0 3.1 7.9 3.2

One bar; unit of gas uptake, mg g−1; unit of Qst, kJ mol−1.

estimate the CO2/N2 and CO2/CH4 selectivities. On the basis of Henry’s law constants, the CO2/N2 selectivities were calculated from the single-component adsorption isotherms to be 19−37 at 273 K (Figure S15−18, Supporting Information and Table 3). Although these values are lower than those of

the impressive impact of dendritic building blocks with high connectivity. Qst values for CH4 were also calculated using the virial method to be fairly consistent within all four Cz-POFs (19.0 to 20.9 kJ mol−1 at zero coverage). These values rank among the highest Qst values for CH4 of porous organic polymers, comparable to those of HCP-3 (20.8 kJ mol−1)13a and PPNs (∼15−18 kJ mol−1).15a The H2 uptakes at 77 and 87 K up to 1 bar (Figures 2c,f and S14, Supporting Information, and Table 2) were measured. Cz-POF-1 shows an impressive H2 uptake of 2.24 wt % (273 K, 1 bar), more than twice that of Cz-POF-2 (0.97 wt %). A similar increase in H2 uptake was also observed for Cz-POF-3 compared to that of Cz-POF-4, indicating the importance of increasing surface area for the enhancement of H2 uptake. The Qst of Cz-POFs for H2 at zero coverage ranges from 7.8 to 8.5 kJ mol−1, similar to those of nitrogen containing PPFs (6.8−8.2 kJ mol−1),17 TzFs (tetrazine-based organic frameworks) (7.8−8.2 kJ mol−1),34 and BILPs (∼7.8 kJ mol−1).28b,32 3.4. Selective CO2 Capture at Low Pressure. After the porosity and gas uptake properties of Cz-POFs were studied, we investigated their performance in selective CO2 capture over N2 and CH4 to assess their potentials in practical gas separation applications. Single component adsorption isotherms for CO2, N2, and CH4 were collected at 273 K up to 1.0 bar (Figures 3

Table 3. CO2/N2 and CO2/CH4 Selectivity of Cz-POFs selectivity [initial slope]a

selectivity [IAST]b

polymer

CO2/N2

CO2/CH4

CO2/N2

CO2/CH4


19 26 23 37

4.4 4.7 4.4 7.1

17 21 20 26

5.4 7.1 5.6 7.2

Selectivity (mol mol−1) was calculated by the initial slope method at 273 K. bSelectivity (mol mol−1, 1 bar) was calculated by the IAST method at a mole ratio of 15:85 for CO2/N2 and a mole ratio of 5:95 for CO2/CH4 at 273 K. a

porous polymers with highly polar functional groups such as azo-COPs (73−124),35 HMP Py-1 (117),13b and MPIs (41− 102),28c they are comparable to triazine-based PCTFs (9− 22),36 polyimides SMPI-0 (30),37 and SMPI-10 (32),37 etc. It is clear that the CO2/N2 selectivity decreases as the SABET and CO2 adsorption capacity increases, a common phenomenon observed in selective gas uptake.9b Here, the decrease of selectivity can be attributed to the larger percentage of mesopores in Cz-POF-1 and Cz-POF-3, which enhances the adsorption capacity but does not improve the selectivity due to the lack of micropores that can provide strong interaction with gas molecules. Indeed, as described above, Cz-POF-2 and CzPOF-4 contain a higher percentage of micropores (Table 1). This finding is consistent with the general understanding regarding the relationship between porosity and selectivity, that is, a larger porosity usually leads to an inferior selectivity.9b Similar trend was also observed in CO2/CH4 selectivities. CzPOF-2,4 with smaller SA BET exhibit higher CO 2 /CH 4 selectivities (4.7−7.1) than those of Cz-POF-1,3 (4.4). Note that CO2/CH4 selectivity is usually lower than CO2/N2 selectivity, due to the higher polarizability of CH4 than that of N2.38 An alternative method to estimate selectivities is based on the ideal adsorbed solution theory (IAST).39 This method allows for prediction of gas mixture behavior including selectivities as a function of pressure for different types of porous materials from single-component isotherms.40 Thus, we used IAST to predict binary gas mixture behavior at 273 K, with the mixture compositions similar to those of flue gas (CO2/N2 = 15:85) and natural gas (CO 2/CH4 = 5:95) (Figure S19−24, Supporting Information, and Table 3). Generally, the results are in good agreement with those obtained from the initial slope method. Good CO2/N2 adsorption selectivities at 273 K (17, 21, 20, and 26) were obtained for Cz-POF-1, Cz-POF-2,

Figure 3. CO2, CH4, and N2 adsorption isotherms of Cz-POFs at 273 K.

and S15−S18, Supporting Information). It should be noted that a detailed, systematic evaluation of carbazolic porous organic polymers for selective gas uptake has not been reported. Previous studies of CPOP series only provided empirical estimation of gas selectivities based on the adsorption capacity at 1 bar.20 We first used the initial slope ratio method to E



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Cz-POF-3, and Cz-POF-4, respectively (Table 3). Cz-POF-4 has the highest CO2/N2 selectivity of 26 at 1 bar. Most CzPOFs have constant CO2/N2 selectivities throughout the entire low-pressure range from 0 to 1 bar (Figure S23, Supporting Information). The IAST CO2/N2 selectivities are also well comparable with those of PPFs (11−20),17 CTFs (14−31),28a and PCTFs (14−41).36 The CO2/CH4 selectivity of Cz-POFs at 273 K ranges from 5.4 to 7.2 at 1 bar. Again, with higher percentage of micropores, Cz-POF-2,4 exhibit higher selectivities at 273 K (7.1 and 7.2, respectively) than those of Cz-POF1,3 (5.4 and 5.6, respectively), consistent with those estimated based on the initial slope calculations.

4. CONCLUSIONS In summary, we have introduced an efficient strategy to synthesize highly porous carbazolic porous organic frameworks (Cz-POFs) by using bulky, dendritic building blocks with high connectivity to decrease the probability of network interpenetration. Synthesized from dendritic Cz-1 and Cz-3, CzPOF-1 and Cz-POF-3 show high surface areas of 2065 and 1927 m2 g−1, which are 3.1 and 2.1 times larger than that of CzPOF-2 and Cz-POF-4 constructed from less branched building blocks, respectively. Cz-POFs exhibit excellent uptake of CO2, CH4, and H2. In particular, Cz-POF-3 has remarkable CO2 uptake (21 wt %) and CH4 uptake (2.54 wt %) at 273 K and 1 bar. Cz-POF-1 has an excellent H2 uptake (2.24 wt %) at 77 K and 1 bar. These values rank among the highest by porous organic polymers. Because of their easy synthesis, high gas uptake capacities, and excellent physicochemical stability, CzPOFs show great promise in small gas storage and separation applications.

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ASSOCIATED CONTENT

S Supporting Information *

Optimization of polymerization conditions, thermal gravimetric analysis, powder X-ray diffraction, solid-state CP/MAS 13C NMR, FT-IR, and gas adsorption studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Dewey Barich for the assistance in solid-state CP/MAS 13C NMR measurements. We acknowledge financial support from the University of Nebraska−Lincoln and Nebraska Center for Energy Sciences Research.

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REFERENCES

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Porosity Enhancement of Carbazolic Porous Organic Frameworks

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