Substituent Effects on the Gas Sorption and Selectivity Properties of

Article pubs.acs.org/Macromolecules

Substituent Effects on the Gas Sorption and Selectivity Properties of Hexaphenylbenzene and Hexabenzocoronene Based Porous Polymers Christina M. Thompson, Gregory T. McCandless, Sumudu N. Wijenayake, Obada Alfarawati, Mohammad Jahangiri, Atef Kokash, Zachary Tran, and Ronald A. Smaldone* Department of Chemistry, University of Texas, Dallas, 800 W. Campbell Rd., Richardson, Texas 75080, United States S Supporting Information *

ABSTRACT: Herein we report a series of porous organic polymers (POPs) synthesized through a Sonogashira crosscoupling reaction containing hexaphenylbenzene (HEX) and hexabenzocoronene (HBC) with varying peripheral substitution. We observed vastly different gas sorption properties depending on substituent size and the extent of conjugation in the monomer core structure with BET surface areas ranging from 320 m2/g for HBC-POP-4 to 1140 m2/g for HEX-POP-3. In order to more clearly understand the effects of functional group substitution on the properties of these materials, we have characterized these POPs using N2, CO2, and H2 sorption measurements, powder X-ray diffraction, FT-IR spectroscopy, TGA, and EDX.

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onene (HBC) scaffolds because of their extended conjugation as well as their reliable and scalable synthetic accessibility. The functional properties of HBC and its derivatives have been investigated in numerous reports in the literature.14 Our most recent work10 involved the use of HBC as a component in copolymer mixtures with tetrakis(4-ethynyl)tetraphenylmethane (TPM). It was found that these polymers were microporous but displayed strong π-stacking in the structure, which is likely detrimental to its sorption properties. The π-stacking interactions can be disrupted in a number of waysone of which is to add substituent groups that can provide increased solubility to overcome π-stacking or can inhibit self-association through steric hindrance. In order to explore the possibility of having greater control over the polymer’s structure, as well as exploring the effects of the conjugated structure of HBC in greater detail, we have synthesized a series of HBC monomers with functional groups that will improve the solubility of the starting monomers and potentially disrupt the π-stacking capability of the HBC core. In addition to this, we have also synthesized copolymers of the hexaphenylbenzene (HEX) precursors of HBC with TPM for comparison. HEX based porous polymers have been synthesized on several other occasions, utilizing either metal-catalyzed coupling reactions15 or SNAr polymerizations.16 These previous reports showed that these polymers display high surface areas and gas storage capability and could be promising components

he processing, separation, and storage of gaseous materials such as hydrogen, methane, and carbon monoxide have become a major area of research interest over the past decade as a result of the drive for cleaner energy sources. To meet these challenges, a variety of new porous scaffolds have recently been developed, including highly ordered materials such as metal−organic frameworks1 (MOFs) and covalent organic frameworks2 (COFs) as well as microporous polymers such as polymers of intrinsic microporosity3 (PIMs), porous polymer networks4 (PPNs), porous organic polymers3c,5 (POPs), and porous aromatic frameworks6 (PAFs). Porous polymers are a robust class of microporous materials derived from structurally rigid organic molecules linked together by covalent bonds. These polymers are capable of adsorbing a wide variety of molecules with potential applications in industrial gas separations,3a,b,7 toxic vapor sequestration,8 and clean fuel storage.9 POPs have garnered great interest over the past several years as a result of their straightforward and modular synthesis, durability in the presence of water vapor, resistance to heat and corrosive conditions, and their tunable porosity. POPs can be either homo- or copolymeric in nature and are typically synthesized from rigid, sterically encumbered monomer units. Recently our group has investigated POPs containing large polycyclic aromatic hydrocarbon (PAH) monomers.10 PAH compounds have been studied as synthetic models of graphene and carbon nanotubes and are notable for their self-assembly11 and electrochemical properties.12 In addition, recent theoretical studies have indicated that compounds such as these could have promising properties for gas storage applications.13 Of this family of molecules, we have been interested in hexabenzocor© 2014 American Chemical Society

Received: August 13, 2014 Revised: October 21, 2014 Published: December 3, 2014 8645

dx.doi.org/10.1021/ma501663j | Macromolecules 2014, 47, 8645−8652

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Figure 1. Monomers and methods used to make the HBC and HEX-POPs described in this study.

Scheme 1. Synthesis of Modified HBC and HEX Monomers 1b,c and 3b,c

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(ASAP) 2020 surface area analyzer. Ultrahigh-purity-grade N2, H2, CO2, and He gases (obtained from Airgas Corp.) were used in all adsorption measurements. N2 and H2 isotherms were measured using a liquid nitrogen bath (77 K). CO2 isotherms were measured using either an ice water bath (273 K) or in a room temperature water bath (296 K) whose temperature was measured prior to use for accuracy. The pore volume of each material was estimated from the Dubinin− Raduskevich (DR) model with the assumption that the adsorbate is in the liquid state and that the adsorption involves a pore-filling process. Pore size distributions were determined using an nonlocalized density functional theory (NLDFT) carbon slit-pore model in the Micromeritics Software Package. Heats of adsorption values were computed by the Micromeritics ASAP software package using a variant of the Clausius−Clapeyron equation. Experiments. 1b. To a round-bottom flask containing 3b (225 mg, 0.301 mmol) in degassed CH2Cl2 (15 mL) was added FeCl3 (1.56 g, 9.65 mmol) as a solution in MeNO2 (8 mL). This was stirred at room temperature under a continuous flow of N2 for 1 h before being quenched with MeOH (50 mL). The resulting slurry was filtered and washed with CH2Cl2 (100 mL), MeOH (100 mL), and acetone (100 mL) to yield 1b as a light-brown powder. This compound was not sufficiently soluble to obtain a 1H NMR (180 mg, 79% yield). MALDIToF MS (m/z): Calculated for [C58H24Br2]+: 732.5. Found: 732.8. 1c. To a round-bottom flask containing 3a (500 mg, 0.724 mmol) in dry, degassed CH2Cl2 (30 mL) was added tert-butyl chloride (603 mg, 6.52 mmol). To this was added 0.1 mL of a solution of FeCl3 (2.84 g, 14.48 mmol) in MeNO2 (15 mL). The reaction was then heated to 40 °C under a continuous stream of nitrogen for 2 h before being cooled to room temperature. Once cooled, the remaining FeCl3 solution was added, and the reaction was stirred for 5 h. The reaction was then poured into MeOH (200 mL), resulting in a light yellow precipitate. This solid was recrystallized from THF/hexanes to yield 1c as a light yellow solid (247 mg, 28% yield). 1H NMR (500 MHz, CDCl3, ppm): δ 9.0 (s, 4H), 8.5 (d, 8H, J = 13.8 Hz), 2.0 (s, 36H). MALDI-ToF MS (m/z): Calculated for [C58H60Br2]+: 901.2. Found: 901.4. 3b. To a microwave vial containing 4 (390 mg, 0.686 mmol) in diphenyl ether (2 mL) was added 1,2-bis(4-methylphenyl)acetylene (141 mg, 0.686 mmol). This was microwave-heated (250 °C and 300 W) for 1 h. After that time the vial was cooled with a positive pressure of air, and the resulting purple slurry was filtered and washed with diphenyl ether to yield hexaphenylbenzene (3b) as white needle crystals (410 mg, 80% yield). 1H NMR (500 MHz, CDCl3, ppm): δ 7.0 (d, 4H, J = 8.4 Hz), 6.7−6.6 (m, 20H), 2.15 (s, 12H). 13C NMR (500 MHz, CDCl3, ppm): δ 140.3, 140.0, 139.5, 137.2, 134.6, 133.0, 131.1, 129.7, 127.5, 119.2, 21.1. MALDI-ToF-MS (m/z) Calculated for [C46H36Br2]+: 744.9. Found: 745.2. 3c. To a glass pressure tube containing 3a (1 g, 1.44 mmol) in CH2Cl2 (6 mL) was added tert-butyl chloride (6 mL, 72 mmol) followed by a solution of FeCl3 (20 mg, 0.12 mmol) in MeNO2 (1 mL). The tube was sealed and then heated at 40 °C in an oil bath for 72 h. After that time the reaction was filtered, and the resulting white solid was purified by column chromatography (EtOAc:hexanes, 0:100 → 30:70 v:v, SiO2) to give the final product as a white solid (604 mg, 45%). 1H NMR (500 MHz, CDCl3, ppm): δ 6.986−7.003 (d, 4H, J = 8.5 Hz), 6.859−6.876 (d, 8H, J = 8.5 Hz), 6.717−6.734 (d, 4H, J = 8.5 Hz), 6.649−6.666 (d, 8H, J = 8.5 Hz), 1.153 (s, 36H). 13C NMR (500 MHz, CDCl3, ppm): δ 148.0, 140.5, 139.9, 139.1, 137.3, 133.2, 130.9, 129.6, 123.4, 119.2, 34.1, 31.2. MALDI-ToF MS (m/z): Calculated for [C58H60Br2]+: 917.9. Found: 917.8. 4. To a round-bottom flask containing 1,3-bis(4-bromophenyl)-2propanone (1 g, 2.73 mmol) and 4,4′-dimethylbenzil (652 mg, 2.73 mmol) in CH2Cl2 (5 mL) was added KOH (76 mg, 1.36 mmol) as a solution in MeOH (10 mL). The reaction was then heated to reflux for 1 h before being cooled in an ice bath. The resulting purple solid was filtered and washed with ice cold MeOH (50 mL) to provide 4 as a purple solid (1.3 g, 84% yield). 1H NMR (500 MHz, CDCl3, ppm): δ 7.4 (d, 4H, J = 8.7 Hz), 7.1 (d, 4H, J = 8.7 Hz), 7.0 (d, 4H, J = 8.4 Hz), 6.8 (d, 4H, J = 8.4 Hz), 2.35 (s, 6H, 3H). 13C NMR (500 MHz, CDCl3, ppm): δ 199.6, 155.1, 139.0, 131.6, 131.3, 129.8, 129.6, 129.2,

in membrane based gas separation applications. The HEX monomers are similar to HBC in their size and symmetry but lack the extended π-conjugation and planarity. Shown in Figure 1 are the HBC and HEX monomers used to make the POPs in this study.

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EXPERIMENTAL SECTION

Synthesis and Characterization. The synthesis of the HBC and HEX-POPs was carried out using a Sonogashira copolymerization with TPM. This polymerization method was chosen as it is an efficient, reliable reaction that has been demonstrated to be effective for making microporous polymers.17 Since it is a cross-coupling reaction, we can control the distribution of the TPM units within the copolymer to ensure A−B monomer arrangement. Infrared spectroscopy of the POPs (Figure S4) indicates that the polymerization has occurred as we observe the expected combination of acetylene C−H signals (∼3280 cm−1) from the unreacted TPM end groups as well as large peaks for aliphatic C−H stretches (2850−2950 cm−1) that can be attributed to the tert-butyl groups in HBC-POP-4 and HEX-POP-3. The acetylene C−H signals observed in the POPs are significantly decreased from those observed in TPM alone (Figure S4). The synthesis of HBC monomers 1a, 2, 3a, and TPM was performed using previously reported protocols.14a,18 Compounds 1b,c, 3b,c, and 419 were synthesized using modifications of known procedures and are shown in Scheme 1. The use of a convergent synthetic strategy allowed us to generate multiple monomers using only a few reactions. Hexaphenylbenzene (3a), which can be synthesized using a previously published procedure,18 can be alkylated through a Friedel−Crafts reaction using tert-butyl chloride and iron(III) chloride. Through the use of increased (i.e., noncatalytic) amounts of iron(III) chloride, a tandem Scholl oxidation will proceed to produce compound 1c from 3a in one pot.19 General Methods. All reagents were purchased from commercial suppliers (Sigma-Aldrich and Fisher Scientific) and used as received. TPM,5a 2,18 1a,14a and HBC-POP-1,210 were synthesized as previously reported. Microwave reactions were carried out in a CEM Discover microwave reactor. Powder X-ray diffraction (PXRD) analyses of polymers HBC-POP-1−4 and HEX-POP-1−3 were carried out on a Bruker D8 Advance diffractometer with a sealed tube radiation source (Cu Kα, λ = 1.541 84 Å), a no background sample holder, and a Lynxeye XE detector. Data were collected over a 2θ range of 5°−50° in Bragg−Brentano geometry with a generator setting of 40 kV and 40 mA, step size of 0.02°, and exposure time per step of 2 s. The thermogravimetric analyses were performed using a Mettler Toledo DSC/TGA 1 instrument under a nitrogen atmosphere with a heating rate of 10 °C min−1 from 30 to 800 °C. Elemental analyses20 were performed by MicroAnalysis, Inc. FT-IR spectra were obtained using a Nicolet 380 FT-IR with a SmartOrbit diamond attenuated total reflectance (ATR) cell. MALDI-ToF measurements were carried out on a Shimadzu Biotech Axima Confidence instrument. All spectra were obtained using tetracyanoquinodimethane (TCNQ) as a matrix. The general procedure for sample preparation was as follows: To a mortar was added TCNQ (10 mg) and the compound of interest (1 mg). This mixture was ground together to form a uniform solid. An aliquot (1 mg) of this solid was then transferred to a separate vial and chloroform (1 mL) was added. The resulting suspension was then spotted onto the MALDI plate and analyzed. NMR spectra were carried out on a Bruker 500 MHz Advance spectrometer. Scanning electron microscope (SEM) images were acquired on a Zeiss SUPRA40 SEM instrument, and energy dispersive X-ray spectroscopy (EDX) was carried out using an Oxford Instruments EDX detector. The samples were prepared on 15 mm aluminum stubs using doublesided adhesive carbon spectro tab. The uncoated samples were imaged at a working distance of 10 mm and a voltage of 20 kV using a secondary electron detector. Porosity Analysis and Gas Sorption Measurements. Lowpressure gas adsorption experiments (up to 760 Torr) were carried out on a Micromeritics Accelerated Surface Area and Porosimetry System 8647


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Figure 2. N2 adsorption and desorption isotherms for HBC and HEX-POPs. (A) Filled circles represent the adsorption isotherm and open circles represent desorption. (B) Bar graph illustrating the BET surface areas of each POP. (C) Micropore size distribution obtained from low pressure N2 adsorption (P/P0 < 0.1). sonicated in CH2Cl2 for 30 min, filtered, sonicated in DMSO for 30 min, filtered, and then sonicated in acetone for 30 min and refiltered. The resulting solid was then dried at 150 °C under high vacuum for 12 h. A light brown powder was obtained (34 mg, 35%). Elemental analysis (%) Calcd: C 95.09, H 4.91; Found: C 77.54, H 4.42. EDX: Br 0.3%. HEX-POP-2. To a glass pressure tube containing TPM (17 mg, 0.04 mmol) in toluene (3 mL) was added 3b (60 mg, 0.08 mmol) followed by Et3N (1 mL). This was degassed with nitrogen for 15 min before Pd(PPh3)4 (10 mg, 0.008 mmol) and CuI (2 mg, 0.008 mmol) were added, and the tube was sealed. The reaction was then heated at 95 °C for 20 h. After that time the reaction was filtered, and the resulting light brown solid was washed with CH2Cl2 (100 mL), followed by MeOH (100 mL) and water (100 mL) before being sonicated in CH2Cl2 for 30 min, filtered, sonicated in DMSO for 30 min, filtered, and then sonicated in acetone for 30 min and refiltered. The resulting solid was then dried at 150 °C under high vacuum for 12 h. A light brown powder was obtained (23 mg, 40%). Elemental analysis (%) Calcd: C 94.42, H 5.58; Found: C 71.49, H 3.58. EDX: Br 0.7%. HEX-POP-3. To a glass pressure tube containing TPM (14 mg, 0.033 mmol) in toluene (3 mL) was added 3c (60 mg, 0.065 mmol) followed by Et3N (1 mL). This was degassed with nitrogen for 15 min before Pd(PPh3)4 (8 mg, 0.0065 mmol) and CuI (2 mg, 0.0065 mmol) were added, and the tube was sealed. The reaction was then heated at 95 °C for 20 h. After that time the reaction was filtered, and the resulting light brown solid was washed with CH2Cl2 (100 mL), followed by MeOH (100 mL) and water (100 mL) before being sonicated in CH2Cl2 for 30 min, DMSO for 30 min, and acetone for 30 min before being refiltered. The resulting solid was then dried at 150 °C under high vacuum for 12 h. A light brown powder was obtained (30 mg, 47%). Elemental analysis (%) Calcd: C 92.88, H 7.12; Found: C 74.68, H 5.61. EDX: Br 1.3%.

128.9, 124.1, 121.8, 21.5. MALDI-ToF MS (m/z): Calcd for [C31H22Br2O]+: 570.0. Found: 571.3 [M + H]+. HBC-POP-3. To a glass pressure tube containing TPM (23 mg, 0.055 mmol) in toluene (3 mL) was added 1b (80 mg, 0.11 mmol) followed by Et3N (1 mL). This was degassed with nitrogen for 15 min before Pd(PPh3)4 (12 mg, 0.011 mmol) and CuI (2 mg, 0.011 mmol) were added, and the tube was sealed. The reaction was then heated at 95 °C for 20 h. After that time the reaction was filtered, and the resulting light brown solid was washed with CH2Cl2 (100 mL), followed by MeOH (100 mL) and water (100 mL) before being sonicated in CH2Cl2 for 30 min, DMSO for 30 min, and acetone for 30 min before being refiltered. The resulting solid was then dried at 150 °C under high vacuum for 12 h. A light brown powder was obtained (63 mg, 75%). Elemental analysis (%) Calcd: C 95.88, H 4.12; Found: C 67.69, H 3.40. EDX: Br 1.7%. HBC-POP-4. To a glass pressure tube containing TPM (23 mg, 0.055 mmol) in toluene (3 mL) was added 1c (100 mg, 0.11 mmol) followed by Et3N (1 mL). This was degassed with nitrogen for 15 min before Pd(PPh3)4 (13 mg, 0.011 mmol) and CuI (2 mg, 0.011 mmol) were added, and the tube was sealed. The reaction was then heated at 95 °C for 20 h. After that time the reaction was filtered, and the resulting light brown solid was washed with CH2Cl2 (100 mL), followed by MeOH (100 mL) and water (100 mL) before being sonicated in CH2Cl2 for 30 min, DMSO for 30 min, and acetone for 30 min before being refiltered. The resulting brown solid was then loaded into a Soxhlet apparatus and was continuously extracted with toluene for 120 h. The resulting solid was then dried at 150 °C under high vacuum for 12 h. A light brown powder was obtained (55 mg, 52%). Elemental analysis (%) Calcd: C 94.07, H 5.93; Found: C 74.68, H 4.99. EDX: Br 1.6%. HEX-POP-1. To a glass pressure tube containing TPM (30 mg, 0.0724 mmol) in toluene (3 mL) was added 3a (100 mg, 0.14 mmol) followed by Et3N (1 mL). This was degassed with nitrogen for 15 min before Pd(PPh3)4 (17 mg, 0.014 mmol) and CuI (3 mg, 0.014 mmol) were added, and the tube was sealed. The reaction was then heated at 95 °C for 20 h. After that time the reaction was filtered, and the resulting light brown solid was washed with CH2Cl2 (100 mL), followed by MeOH (100 mL) and water (100 mL) before being

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RESULTS AND DISCUSSION To determine the accessible surface areas and pore size distributions, nitrogen isotherms were obtained for each polymer at 77 K. From each isotherm (Figure 2) the surface 8648


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measurements, it appears that there is no significant difference in the Br content between the HEX-POPs, produced from uniformly very soluble monomers, and the HBC-POPs. This indicates the degree of polymerization is likely not the main determinant for the observed porosity in these polymers. Our second hypothesis for the relatively low porosity of HBC-POPs was that the strong π-stacking interactions in the polymers, as previously observed by PXRD,10 would serve to limit pore formation. To test the second hypothesis, we obtained PXRD data for each of the POPs synthesized as part of this study (Figure 3a). In general, the HEX-POPs show very little long-range order, which is to be expected since most POPs reported in the literature are amorphous in character.22 The HBC-POPs, however, do display some ordered character. HBC-POP-1−3 show a broad reflection at 2θ ∼26° that is attributed to face-toface π-stacking that can be expected to occur between the HBC components of the polymers. A significantly different diffraction pattern is observed with HBC-POP-4, indicating that there is some periodic order within the polymer. Our first consideration was that the peaks could be attributed to residual HBC (1c) trapped in the pores. However, even after significant washing with a variety of solvents, and an extended washing in a Soxhlet extractor (see Experiment section for details), these peaks could not be removed.23 It should be noted that of all the HBC monomers 1c is the most soluble in organic solvents. To clarify this issue, we obtained diffraction data on bulk 1c itself, shown in Figure 3b. A comparison of the diffraction patterns of 1c and HBC-POP-4 shows a poor correlation (especially the low angle diffraction region), indicating that the peaks observed in the polymer are not exactly the same as pure monomer 1c. While there are some possible similarities between the two diffraction patterns, the individual 2θ values at each peak’s highest intensity are significantly different between the monomer and the polymer; peak broadening does not cause major shifts in 2θ. It may be possible that small oligomers are trapped within the framework and cannot be removed using any practical extraction method. These impurities (if present), which are trapped in a different environment than the bulk materials, could possibly contribute to the order observed. We considered the possibility of trapped TPM in the network; however, our IR spectra do not show significant amounts of the acetylene C−H peaks that would be expected were large quantities of this compound present. A comparison of the diffraction pattern of

areas of HBC-POP-1−4 and HEX-POP-1−3 were calculated using both BET and Langmuir models (Table 1), and the BET Table 1. BET and Langmuir Surface Area Values and Pore Volume Calculations for Each of the POPs Tested in This Series pore volume (cm3/g) POP

BET surface area (m2/g)

Langmuir surface area (m2/g)

single point

Horvath− Kawazoe

HBC-POP-1 HBC-POP-2 HBC-POP-3 HBC-POP-4 HEX-POP-1 HEX-POP-2 HEX-POP-3

670 500 360 320 610 600 1140

730 500 390 350 670 620 1290

0.36 0.32 0.19 0.41 0.46 0.28 0.67

0.38 0.35 0.20 0.52 0.47 0.28 0.69

surface areas are plotted as a bar graph for comparison with one another (Figure 2b). At first glance, there are several apparent trends. Overall, the HEX-POPs have higher surface areas than any of the HBC-POPs other than HBC-POP-1. Interestingly, the surface areas of the HBC and HEX-POPs appear to be affected in opposite ways to increasing the size of the peripheral groups. Alkyl functional group effects on porous polymers have been previously studied on several occasions.21 In COFs, it has been found that longer alkyl chains can decrease surface area but improve the hydrogen storage capacity.21a Alkyl substituent studies on PIMs21b have shown a similar trend, with the exception of branched alkyl chains, which display higher surface areas compared with linear chains of the same carbon content. In our case, the effects of increasing pendant group size were dependent on the structure of the monomer core (i.e., HEX vs HBC). Figure 2b shows that the HBC-POPs decrease in surface area as larger functional groups are added, while in contrast, HEX-POP-3, functionalized with four tert-butyl groups, displays the highest overall surface area observed in this report at 1140 m2/g. In our previous report10 regarding HBC-POPs-1,2, we hypothesized two main reasons for the relatively low BET surface areas. The first was that the generally poor solubility of the HBC starting materials caused the formation of short oligomers due to poor cross-coupling conversion. If this were the case, then the addition of solubilizing side groups would generally improve the porosity regardless of core PAH structure. However, based on bromine analysis from EDX

Figure 3. (A) PXRD patterns of the HEX and HBC-POP derivatives used in this work. (B) Comparison of the PXRD patterns of HBC-POP-4 and 1c. 8649


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Figure 4. (A) CO2 adsorption isotherms at 273 K. (B) CO2 heat of adsorption measurements from isotherms measured at 273 and 298 K. (C) Hydrogen adsorption isotherms at 77 K. (D) Ambient pressure CO2/N2 selectivity at 298 K.

higher storage and surface area but poorer gas sieving properties. In general, it appears that the addition of tert-butyl groups affects the properties of the POPs the most significantly. In previously reported work, the use of bulky substituents was effective in templating the formation of larger pores in POPs by reducing the polymer chain interpenetration through steric hindrance.5f We believe this principle is what we observed in the improved porosity of HEX-POP-3. For HBC-POPs, it appears that the peripheral substitutions are not able to overcome the other intermolecular organization forces (such as π-stacking), leading the substituents to fill the void space and decrease the observed surface area as the size of the substituent increases.

TPM and HBC-POP-4 is also provided in the Supporting Information (Figure S6). Shown in Figures 4a and 4c are the adsorption isotherms for CO2 at 273 K and H2 at 77 K, respectively. Based on the isotherms alone, the overall storage capacities appear to mirror the BET surface areas as would be expected for a series of polymers whose adsorption processes rely on physisorption through van der Waals contact alone, rather than dipolar interactions (e.g., amine or alcohol functionalized polymers). The adsorption isotherms for CO2 were also obtained at 298 K in order to calculate adsorption enthalpies (Figure 4b). HBC and HEX-POPs display excellent isosteric enthalpies at low loadings (24−26 kJ/mol) regardless of overall surface area. These POPs have excellent storage capacity for CO2 with HEXPOP-3 the highest at nearly 18 wt % and compare favorably with other porous materials with much higher BET surface areas.2b,24 Hydrogen storage capacity is modest for both HEX and HBC-POPs, following the same correlation of capacity vs BET surface area as CO2 across the board with a maximum of 1.4 wt % for HEX-POP-3. While the highest bulk storage correlates with the largest surface areas, the highest CO2/N2 selectivities at 298 K and ambient pressure (eq 1) are observed with HBC-POP-1−3 (Figure 4d). S=

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CONCLUSIONS We have reported here a series of porous copolymers of the tetrahedral monomer TPM and functionalized hexabenzocoronene and hexaphenylbenzenes. The N2, CO2, and H2 sorption properties of each of these POPs were reported, and the effects of both extended π-conjugation and the steric bulk of the peripheral functional groups were discussed. We observed significant effects on the properties as a result of these structural modifications, in both their gas-sorption performance and the microscopic structure. Side group size and π-stacking ability were more important factors based on surface area trends and direct observation by PXRD analysis. The use of tert-butyl groups appears to have the most dramatic effects on both HBC and HEX polymers, with remarkable long-range order being observed in the former (HBC-POP-4) and improved overall

CO2(quantity adsorbed)/P N2(quantity adsorbed)/P

(1)

This observation is likely a result of the small pores (6−8 Å) that are formed with the HBC-POPs, whereas the HEX-POPs tend to form larger micro- to mesopores that are conducive to 8650


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porosity and storage capacity for the latter (HEX-POP-3). We plan to continue to explore the effects of expanded π-aromatic monomers as well as side group functionalization on the synthesis of porous polymers and other porous materials in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Thermogravimetric analyses, full CO2 (at 273 and 298 K) and H2 (77 K) isotherms, including desorption isotherms, infrared spectroscopy measurements and EDX Pd images, and additional PXRD comparison (TPM vs HBC-POP-4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (R.A.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Texas, Dallas, for startup funds, the American Chemical Society’s Petroleum Research Fund (52906-DNI10), and the NSF Major Research Instrumentation Program (CHE-1126177) for their support. We acknowledge Benjamin Batchelor and the Voit group for their assistance with obtaining TGA measurements.

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REFERENCES

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Macromolecules

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Substituent Effects on the Gas Sorption and Selectivity Properties of

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