Bottom-up Approach for the Synthesis of a Three-Dimensional

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Bottom-up Approach for the Synthesis of a Three-Dimensional Nanoporous Graphene Nanoribbon Framework and Its Gas Sorption Properties Yearin Byun, and Ali Coskun Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00246 • Publication Date (Web): 16 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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Chemistry of Materials

Bottom-up Approach for the Synthesis of a ThreeDimensional Nanoporous Graphene Nanoribbon Framework and Its Gas Sorption Properties Yearin Byun† and Ali Coskun*,†,‡ † Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daehak-ro 291, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daehak-ro 291, Yuseong-gu, Daejeon 305-701, Republic of Korea ABSTRACT: We present a new polymerization strategy, that is catalyst-free Diels-Alder cycloaddition polymerization and subsequent FeCl3-catalyzed intramolecular cyclodehydrogenation reaction, to introduce graphene nanoribbons up to 2 nm in length and 1.1 nm in width into a graphene nanoribbon framework (GNF). The first graphene nanoribbon framework showed high thermal stability up to 400oC in air with relatively narrow pore size distribution and exhibited BET surface area of 679 m2 g-1. GNF possesses high affinity for H2 (Qst 7.7 kJ mol-1, 1.03 wt% at 77 K, 1 bar), CO2 (Qst = 28.7 kJ mol-1, 94.6 mg g-1 at 273 K, 1 bar), and CH4 (Qst = 24. 1 kJ mol-1, 11.5 mg g-1 at 273 K, 1 bar). The enhancement in gas affinities was attributed to the unique combination of large π-surface area arising from graphene nanoribbons and small pores (~5.8 Å) in GNF. The application of GNF can also be extended to natural gas purification process with exceptional CO2/CH4 (5:95) selectivity of 62.7, which is being the highest value reported to date at 298 K. Unlike previous studies which focus mostly on increasing the affinity of CO2 towards the sorbent in order to tune CO2/CH4 selectivity, our approach takes advantage of the kinetic diameter difference between CO2 (3.30 Å) and CH4 (3.80 Å), thus offering low-cost efficient alternative for natural gas purification process.

1. INTRODUCTION Nanoporous polymers1-5 attracted significant deal of attention in recent years due to their permanent porosity, chemical tunability, physicochemical stability and exceptional gas sorption properties. In particular, C-C bond formation reactions – namely, Pd-catalyzed Sonogashira-Hagira6-11 and SuzukiMiyura cross-coupling reactions,12, 13 Ni-catalyzed Yamamototype Ullmann reaction,14-21 cyclotrimerization22-24 and FriedelCrafts reactions25 – played a significant role in the synthesis of several nanoporous polymers, including, conjugated microporous polymers (CMPs),6-9, 12, 16 porous aromatic frameworks (PAFs),14, 15, 17, 22, 26, 27 porous polymer networks (PPNs),18, 20 porous organic polymers (POPs)28-31 and hypercrosslinked polymers (HCPs).25, 32-34 Especially, PAFs and PPNs showed exceptional surface areas up to 6461 m2 g-1 along with good physicochemical stability.18 Versatility of CC polymerization reactions accompanied by the chemical tunability and modularity of monomeric units offered significant degree of flexibility in designing these frameworks for the desired applications, including gas storage,15, 28, 34 optoelectronics,12 drug delivery27, sensing,35 and catalysis.36, 37 Incorporation of polycyclic aromatic hydrocarbons, e.g., graphene nanoribbons (GNRs) into nanoporous polymers, however, has been a significant challenge using these C-C polymerization reactions mainly due to their low solubility and high affinity to restack to form graphitic layers due to interlayer π-π stacking and van der Waals interactions. Graphene nanoribbons, defined as two-dimensional nanometer-wide strips of graphene, have attracted much attention in recent years due to their high electron delocalization along the backbone and high charge carrier mobility. GNRs with different sizes and shapes have

Figure 1. General synthetic strategy for the formation of graphene nanoribbon frameworks (GNFs).

been prepared with various fabrication strategies such as exfoliation, direct chemical vapor deposition, chemical synthesis, and unzipping of carbon nanotubes.38 Among these methods, recently, bottom-up approach has attracted significant interest because different topologies and widths of GNRs can be manufactured in precision.39 While bottom-up synthetic methods can precisely control the edge shapes and widths of GNRs, previous studies have been limited to produce twodimensional nanoarchitectures and mainly studied in the area of electrochemistry such as photovoltaic devices and lightemitting diodes.38 Before extending the application of GNRs beyond electrochemistry and transferring their unique properties into nanoporous polymers, we have to first consider their inherent property, which is their high affinity to restack to form graphitic layers. In order to prevent restacking, we propose to introduce a permanent ‘spacer’ such as porosity between GNRs within nanoporous polymers. Herein, we used a

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bottom-up approach39 (Figure 1) for the preparation of a graphene nanoribbon framework (GNF) and introduced an efficient, regioselective, catalyst-free, low-cost C-C polymerization reaction based on well-known Diels-Alder cycloaddition reaction between dicyclopentanedienone and arylacetylene derivative, followed by FeCl3-catalyzed intramolecular cyclodehydrogenation reaction for the preparation of these newclass of nanoporous polymers, incorporating GNRs up to ~2 nm in length and ~1.1 nm in width, that is being the largest aromatic subunit incorporated into nanoporous polymers to date. GNF-1 was found to be extremely stable up to 400oC in air and exhibited surface area of 679 m2 g-1. GNF-1 showed much higher affinity towards H2 (Qst= 7.7 kJ mol-1, 1.03 wt% at 77 K, 1 bar) compared to the nanoporous polymers with smaller π-surface area such as PAFs and PPNs. GNF-1 also exhibited high affinity towards CO2 (Qst = 28.7 kJ mol-1, 94.6 mg g-1 at 273 K, 1 bar) and CH4 (Qst = 24.1 kJ mol-1, 11.5 mg g-1 at 273 K, 1 bar), which was attributed to the increased microporosity of the framework following aromatization of preGNF and to the formation of GNRs within the framework. Moreover, GNF-1 showed the highest IAST selectivity of 62.7 for CO2/CH4 (5:95) mixture at 298 K, indicating its potential use in natural gas purification process. This high CO2/CH4 selectivity could be explained by the molecular sieving effect considering relatively larger kinetic diameter of CH4 (3.80 Å) compared to that of CO2 (3.30 Å).

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Scheme 1. (Top) Synthesis of the model compounds 5 (cis:trans ratio of 17:83) and 6, as a mixture of isomers. (Bottom) Synthesis of pre-GNF via Diels-Alder cycloaddition polymerization of 3 and 4 and subsequent FeCl3catalyzed oxidative cyclodehydrogenation and de-tertbutylation (retro-Friedel-Crafts) reactions to form GNF-1.

trans-5 (83)

FeCl 3

o-Xylene

CH3NO 2, Toluene 25oC, 24 h

cis-5 (17)

6

O

O

O

O

145 oC, 24 h 48%

O

DBU EtOH, 80oC, 1 h

O

O

47% 1

2

3

2. EXPERIMENTAL SECTION 2.1. General. All chemicals and solvents were purchased from Sigma-Aldrich and used without further purification. Air-sensitive liquids were transferred via syringe and were injected into the reaction flask through rubber septa. Moisture sensitive solids were transferred in a dry glovebox under argon. 1H NMR spectra were obtained using either Bruker DMX 300 MHz or Varian VNMR 600 MHz. 13C NMR spectra were obtained using Bruker Avance 400 MHz NMR instrument. High-resolution mass spectra (HR-MS) were obtained on Bruker autoflex III Matrix Assisted Laser Desorption Ionization Mass Spectrometer. Solid-state UV-Vis spectra of polymers were measured on Jasco V-570. FT-IR and Raman spectra were recorded on Shimatzu IRTracer-100 and ARAMS Dispersive-Raman Spectrometers, respectively. Xray diffraction patterns of the polymers with 2ϴ ranging from 5 to 80o were obtained using a Rigaku D/MAX-2500 Multipurpose High Power X-ray diffractometer. All Argon, H2, CO2, and CH4 adsorption and desorption isotherms were performed with a Micromeritics Triflex system and the data were analyzed using the Triflex data analysis software. 2.2. Synthesis. 2.2.1 pre-GNFs. Tetrakis(4-ethynylphenyl)tetraphenylmethane, 4 (0.028 g, 0.069 mmol), 2,8-di-tert-butyl-4,6,10,12tetraphenyldicyclopentanepyrene-5,11-dione, 3 (0.1 g, 0.138 mmol), and o-xylene were added (to obtain monomer concentrations of 20, 40 and 70 mM) into a flame-dried Schlenk flask equipped with a magnetic stir bar and a water-cooled reflux condenser. The reaction mixture was refluxed for 24 h. The formation of dark red solid was observed during the course the polymerization reaction. The desired product was filtered and washed with H2O (2 x 100 mL), Methanol (2 x 100 mL), Chloroform (2 x 100 mL), and Acetone (2 x 100 mL). Then, the solid was subjected to Soxhlet extraction with Methanol in order to completely remove trapped o-xylene. After drying

4 FeCl 3

o-Xylene

CH3NO 2, Toluene 25oC, 72 h

GNF-1

145 oC, 24 h

pre-GNF

under vacuum, the desired polymer was obtained as a dark red solid (109.8 mg of pre-GNF 70 mM). FT-IR (powder): 2951, 1597, 1495, 1441, 1395, 1362, 1252, 1177, 1153, 1070, 1018, 882, 826, 752, 617, 525, 406 cm-1. Same procedure was repeated for monomer concentrations of 20 and 40 mM.

2.2.2 Synthesis of GNF-1. pre-GNF (50 mg, 0.16 mmol) was added into a flame-dried Schlenk flask. A suspension of FeCl3 (0.133 g, 8.17 mmol) in nitromethane (3 mL) and toluene (1 mL) mixture was purged with Ar for 10 min and added to the flask via syringe. The reaction mixture was stirred for 72 h under constant bubbling with Ar. The resulting product was filtered and washed with CH3NO2, 10% HCl, H2O (2 x 100 mL), Methanol (2 x 100 mL), Chloroform (2 x 100 mL), and Acetone (2 x 100 mL) to obtain GNF-1 as a black solid (40 mg). FT-IR (powder): 2959, 2876, 1595, 1574, 1568, 1385, 1362, 1315, 1263, 1240, 1229, 1219, 1171, 1159, 759, 739, 698, 455 cm-1.

3. RESULTS AND DISCUSSION 3.1. Synthesis of GNF-1. Our bottom-up approach (Scheme 1) for the preparation of the first GNF is based on two steps, (1) Diels-Alder cycloaddition reaction between tetrakis(4-ethynylphenyl)methane and dicyclopentanedienone


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derivative followed by extrusion of –CO leads to formation of an aromatic framework (pre-GNF) with a close spatial arrangement of phenyl rings in one-pot reaction under catalystfree conditions, and (2) subsequent FeCl3-catalyzed intramolecular cyclodehydrogenation and de-tert-butylation reactions results in the formation of GNF-1 incorporating GNRs up to ~2 nm in length and ~1.1 nm in width. We have identified ditert-butyl-pyrene-4,5,9,10-tetraone, 1, as an ideal precursor for the preparation of corresponding dicyclopentanedienone derivative, 3, via Knoevagel condensation. tert-Butyl groups are introduced (1) to improve the solubility of 3 and also (2) to serve as porogens, which could be removed during the FeCl3catalyzed intramolecular cyclodehydrogenation reaction via retro-Friedel-Crafts reaction using toluene as a trapping agent. The compound 3 was synthesized (See Supporting Information, SI) by reacting di-tert-butyl-pyrene-4,5,9,10-tetraone with 1,3-diphenylpropanone in the presence of 1,8Diazabicyclo[5.4.0]undec-7-ene as a base in 47% yield. Tetrahedral building blocks, i.e., tetraphenylmethane, offer easy access to 3D nanoporous polymers with maximal void space formation, thus we choose tetrakis(4-ethynylphenyl)methane, 4, as the tetrahedral building block for the polymerization reaction. In order to examine the efficiency of the Diels-Alder reaction between compound 3 and arylalkynes, we have synthesized model compound 5 by reacting 3 with phenylacetylene in o-xylene at 145oC for 24 h. The resulting compound 5 was isolated as a mixture of isomers in 48% yield. In order to quantify the ratio of regioisomers, we have recorded 1H NMR spectrum (Figure S1) of compound 5. Based on the integration of tert-butyl peaks, we calculated the ratio of cis-5 to trans-5 as 17:83 indicating that the Diels-Alder reaction is highly regioselective favoring the sterically less demanding trans-5 isomer. The compound 5 was further characterized using high resolution mass spectroscopy. Following the Diels-Alder cycloaddition reaction, we aromatized the compound 5 using iron(III) chloride as a catalyst in nitromethane/toluene mixture at 25oC for 24 h to synthesize the compound 6. 1H NMR analysis of compound 6 (Figure S2) revealed complete aromatization and de-tert-butylation of both cis- and trans-5 regioisomers. Successful synthesis of model compounds prompted us to utilize this reaction for the catalyst-free C-C polymerization reaction between compound 3 and 4 to form pre-GNF. The Diels-Alder cycloaddition polymerization reaction was carried out at 145oC in o-xylene for 72 h under Ar atmosphere in a flame-dried Schlenk flask. Due to the instability of the compound 3, pre-heated oil bath was used prior to the polymerization. Immediate formation of gel-like solid was observed within 1 h. During the polymerization, we have also observed a significant color change from dark green to dark orange, which suggests the success of the polymerization reaction. We propose that the trans:cis ratio of 83:17 in compound 5 could be further increased in the pre-GNF due to the steric requirements of the cis-isomer. Therefore, we believe that trans isomer will dominate the formation of pores in the pre-GNF, thus minimizing the role of cis isomer in the pore formation. Previous studies on the bottom-up synthesis of GNRs have demonstrated that higher monomer concentrations produce higher molecular weight polymers in the Diels-Alder cycloaddition polymerization.40 Thus, in our studies, we have also varied concentrations of the monomers and correlate it with the Branauer, Emmett, Teller (BET) surface areas of pre-GNFs in order to optimize the reaction conditions for the Diels-Alder cycloaddition polymerization. The effect of monomer concen-

trations (20, 40, 70 mM) on surface area of resulting preGNFs was evaluated by using BET surface area analysis. Interestingly, increasing monomer concentration resulted in an increase in the BET surface area presumably due to the efficiency of the Diels-Alder reaction at high monomer concentrations. Increasing the monomer concentration beyond 70 mM was limited by the solubility of monomers in o-xylene. Based on the results of the BET analysis, we chose pre-GNF synthesized at 70 mM monomer concentration in o-xylene for the preparation of GNF-1 and also for the gas sorption measurements. Graphitization of pre-GNF to form GNF-1 was achieved via the Scholl reaction using FeCl3 both as an oxidant and a Lewis acid in nitromethane/toluene mixture at 25oC for 72 h. We have found that the addition of toluene into the reaction mixture is critical in order to promote the retro-DielsAlder reaction during the cyclodehydrogenation reaction of both compound 5 and pre-GNF. We have also observed a significant color change from dark orange to black during the course of the reaction, which clearly indicates the formation of GNRs within GNF-1. We propose that the cyclodehydrogenation of pre-GNF, as in the case of compound 5, produced GNRs with 18 membered aromatic rings upon elimination of 12 hydrogen atoms, thus in-situ generating GNRs up to 2 nm in length and 1.1 nm in width. 3.2. Spectroscopic Characterization of preGNF and GNF-1. The successful graphitization of precursor pre-GNF into GNF-1 was confirmed by various analytical techniques, including UV-Vis, Raman, solid-state cross polarization magic-angle spinning (CP-MAS) 13C NMR spectroscopies, and powder X-ray diffraction (PXRD). We have also carried out Fourier transform infrared spectroscopy (FT-IR), thermo gravimetric analysis (TGA), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) analysis for both pre-GNF and GNF-1 (see SI). We have examined (Figure 2a) the effect of monomer concentration on the formation of pre-GNF by using UV-Vis spectroscopy. Increasing monomer concentration from 20 to 70 mM, resulted in a red-shift in the UV-Vis spectra, indicating clearly that higher monomer concentrations are required for efficient formation of pre-GNF. UV-Vis spectra of preGNF and GNF-1 exhibited (Figure 2b) significant structural changes associated with the graphitization process. Increasing the π-conjugation of GNF-1 produced a significantly broader absorption spectrum in the entire UV-Vis region, which is in good agreement with the previously reported41 UV-Vis data for graphene and GNRs, thus showing the efficiency of the cyclodehydrogenation reaction. Moreover, we have also observed significant color change during the transformation of pre-GNF (orange) into GNF-1 (black). We have used (Figure 2c) Raman spectroscopy, which is an important technique for the characterization of carbonbased materials such as graphene, GNRs and carbon nanotubes, in order to probe graphitization of the framework.42 In particular, the intensity ratio of D (defect) and G bands is critical in determining amount of defect in 2D carbon materials. While D peak originates from in-plane breathing mode of A1g symmetry caused by six-fold aromatic rings, the G peak is attributed to the E2g mode due to the sp2-hybridized carbon atoms within GNRs. Raman spectrum of pre-GNF exhibited very strong D band at 1608 cm-1 and a relatively weak G band at 1328 cm-1, which corresponds to the ID/IG ratio of 2.3. Following graphitization process, however, GNF-1 showed



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Figure 2. Spectroscopic characterization of pre-GNF and GNF-1. (a) UV-Vis absorption spectra of pre-GNF synthesized at 20, 40 and 70 mM monomer concentrations. (b) UV-Vis spectra of pre-GNF and GNF-1. Inset: Photographic image of pre-GNF and GNF-1 powders. (c) Raman spectrum of pre-GNF measured at 514 nm (2.14 eV) and of GNF-1 measured at 785 nm (1.58 eV) on a powder sample. (d) Solidstate CP-MAS 13C NMR spectra and (E) powder X-ray diffraction patterns of pre-GNF and GNF-1.

relatively weaker D band and a strong G band, which leads to the ID/IG ratio of 0.5, thus further proving that graphitization occurs during the FeCl3-catalyzed cyclodehydrogenation process. We have also recorded (Figure 2d) solid-state CP-MAS 13 C-NMR spectra in order to probe the structural changes following graphitization and also to prove the successful de-tertbutylation of the framework. The solid-state CP-MAS 13C NMR spectra of pre-GNF showed carbon peaks at 147.8, 143.2, 131.8, 67.8, 37.6, and 34.8 ppm. The chemical shifts located at 34.8, 37.6 and 67.8 ppm were attributed to the primary and quaternary carbon atoms of tert-butyl moiety and to the quaternary carbon core of tetraphenylmethane, respectively. The broad carbon peaks located at 147.8, 143.2, 131.8 pm were attributed to the aromatic sp2-hybridized carbon atoms of pre-GNF. Following graphitization, we have observed significant changes in the 13C NMR spectrum of GNF-1. The coalescence of carbon peaks associated with the sp2-hybridized carbon atoms clearly indicates successful graphitization process. The significant attenuation of the quaternary (37.6 ppm) and primary (34.8 ppm) carbon peaks of tert-butyl moiety also indicates near quantitative removal of tert-butyl groups. Broad features in the PXRD patterns of pre-GNF and GNF-1 revealed (Figure 2e) that while the resulting polymers are mostly

amorphous, there is a slight long-range order due to the reversibly of Diels-Alder cycloaddition reaction at high temperatures. We believe that continuous extrusion of C=O during cycloaddition reaction serves as an irreversible kinetic trap for the polymerization reaction and promotes amorphous nature of the framework. We have also carried out FT-IR analysis (Figure S3) to determine the extent of polymerization and terminal groups in pre-GNF. The complete disappearance of alkyne stretching band at 3285 cm-1 along with –C=O stretching band at 1686 cm-1 supports the fact that both monomers completely reacted to form pre-GNF. Furthermore, using FT-IR analysis (Figure S4), we have also verified the graphitization of preGNF into GNF-1. The strong bands at 752 and 696 cm-1 represent the C-H out-of-plane deformation of a mono-substituted benzene moiety. The intensity of these C-H bands is reduced significantly after the formation of GNF-1. The intensity of aromatic C-H stretching bands at 3024 and 3053 cm-1 are also decreased following graphitization of pre-GNF. The C-H stretching band of tert-butyl group was observed at 2951cm-1. Attenuation of this stretching band further verified the successful de-tert-butylation reaction. We have also investigated (Figure S5) thermal stability of pre-GNF and GNF-1 using TGA. Both pre-GNF and GNF-1 showed exceptional thermal stability up to 400oC in air, indicating their potential application for gas capture and storage applications. Bulk-scale mor-


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phology of pre-GNF and GNF-1 was investigated using SEM (Figure S6) and HR-TEM (Figure S7) analysis. Both pre-GNF and GNF-1 formed micron-sized irregular shaped particles. Interestingly, we observed an increase in particle size upon aromatization reaction presumably due to interparticle reactions. Moreover, we were able to visualize several layers of graphene nanoribbons within the polymer using HR-TEM, which further supports our proposed graphitic structure.

3.3 Gas Sorption Studies of pre-GNF and GNF-1. In order to evaluate pore structure of pre-GNF and GNF-1, we have carried out (Figure 3) low-pressure Ar physisorption measurements at 87 K. All the adsorption measurements were carried out after activating the polymers at 200oC for 6 h. We have demonstrated that pre-GNF and GNF-1 are predominantly microporous and exhibited typical type-I reversible sorption profile. The hysteresis upon desorption could be attributed to the pore network effects, interaction of gas molecules with micropore surfaces and to the swelling of the framework as commonly observed for CMPs and PIMs. In line with the previously reported39 studies on the synthesis of GNRs, wherein increasing monomer concentration in the Diels-Alder polymerization resulted in higher molecular weight polymers, we have also observed a similar trend, that is increasing surface area with increasing monomer concentration presumably due to the increased efficiency of the polymerization reaction. The BET surface areas of pre-GNFs were found to be 560, 610.4, 672 m2 g-1 for the monomer concentrations of 20, 40 and 70 mM, respectively. We selected pre-GNF synthesized at 70 mM concentration for graphitization and for further analysis due to its high surface area compared to other preGNFs. In order to probe the changes in the pore structure of pre-GNF following graphitization reaction, we have also carried out Ar adsorption-desorption experiments on GNF-1 at 87 K. Following graphitization, although we have observed slight increase in surface area from 672 to 679 m2 g-1, the micropore surface area increased substantially from 212 to 430 m2 g-1. We believe that the partial reorganization of GNRs within GNF-1 along with the removal of tert-butyl moieties rendered the pores more accessible to Ar and contributed positively to the increase in microporosity of the framework while decreasing the average pore diameter from 0.77 to 0.58 nm. Permanent porosity and high microporosity of pre-GNF and GNF-1 prompted (Table 1) us to investigate their affinity towards H2, CO2, and CH4, which are apolar molecules with kinetic diameters of 2.90, 3.30 and 3.80 Å, respectively. While high microporosity of the GNF-1 is expected to serve well for its high-affinity towards CO2 and H2, hydrophobicity of GNF-1 is expected to favor CH4 gas. The low-pressure H2 adsorption-desorption isotherms of pre-GNF and GNF-1 were measured (Figure 4a, d) at 77 and 87 K. The H2 isotherms at 77 K showed an uptake capacity of 0.91 wt% and 1.03 wt% for pre-GNF and GNF-1, respectively. At 87 K, however, the H2 uptake decreased to 0.65 wt% for pre-GNF and 0.77 wt% for GNF-1. Lack of hysteresis in the

Figure 3. (a) Argon adsorption-desorption isotherms (87 K) of pre-GNF synthesized at different monomer concentrations. (b) Argon uptake measurements of pre-GNF and GNF-1 at 87 K. Inset: Pore volume vs pore size. Filled (■) and empty (□) symbols represent gas adsorption and desorption, respectively.

H2 adsorption-desorption isotherms proves physisorptive mechanism. We have also calculated the isosteric heats of adsorption (Qst) from the H2 adsorption data at 77 and 87 K by using the Clausius-Clapeyron equation. At zero coverage, the Qst values of H2 adsorption for pre-GNF and GNF-1 were found to be 6.6 and 7.7 kJ mol-1. For both polymers, Qst values slightly decreased as a function of increasing amount of adsorbed H2 (Figure S8). We attribute the increase in the Qst value to the increased π-surface area upon graphitization, high microporosity and also to the small average pore diameter (5.8 Å) of the GNF-1. Interestingly, a recent theoretical study suggested43 that decreasing interlayer separation in carbon materials results in an increase in H2 binding free energy. It is also proposed that an ideal carbon-based sorption material with favorable adsorption free energies for H2 should possess inter-

Table 1. BET surface areas, H2, CO2, and CH4 uptake capacities and isosteric heat of adsorption (Qst) of the polymers (at zero coverage), pre-GNF and GNF-1. Polymers

pre-GNF

GNF-1

BET, Ar (m2 g-1)

Micropore area (m2 g-1)

Micropore volume (cm3 g-1)

Average pore diameter (nm)

672 679

212 430

0.08 0.16

0.77 0.58

H2 adsorption (wt%) 77 K 0.91 1.03

87 K 0.65 0.77

Qst for H2 (kJ mol-1)

6.6 7.7

CO2 adsorption (mg g-1) 273 K 75.1 94.6


Qst for CO2

(kJ mol-1)

298 K

39.0 53.3

29.8 28.7

CH4 adsorption

(mg g-1)

273 K 9.5 11.5

Qst for CH4

(kJ mol-1)

298 K

4.5 5.4

22.7 24.1


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Figure 4. H2, CO2 and CH4 adsorption and desorption isotherms of pre-GNF and GNF-1. (a, d) H2 adsorption and desorption isotherms at 77 K and 87 K, (b, e) CO2 adsorption and desorption isotherms at 273 K and 298 K and (c, f) CH4 adsorption and desorption isotherms at 273 K and 298 K. Filled (■) and empty (□) symbols represent gas adsorption and desorption, respectively.

layer distance between 6.0-7.0 Å, these values are in good agreement with the average pore diameters of pre-GNF (7.7 Å) and GNF-1 (5.8 Å). Interestingly, we have also observed an increase in Qst value of H2 due to a decrease in average pore diameter upon graphitization of pre-GNF. Remarkably, Qst value of H2 for GNF-1 was found to be enhanced compared to other carbon-based physisorption materials such as activated carbon and PAF-1. The commercially available activated carbon AX-21 with a BET surface area of 2800 m2 g-1 showed adsorption free energy of 4.66 kJ mol-1.44 Similarly, PAF-1 (SABET = 5600 m2 g-1) showed Qst of 4.6 kJ mol-1 for H2.14 One explanation for this enhancement could be the unique combination of large π-surface area arising from GNR moieties and small pores (~5.8 Å) in GNF-1. Moreover, GNF-1 also showed higher Qst of H2 adsorption compared to common ZnMOFs (4.1-6.2 kJ mol-1).45-48 We have also investigated the affinity of pre-GNF and GNF-1 towards CO2 gas. The CO2 adsorption-desorption isotherms at 278 and 298 K are shown in Figure 4b and 4e, respectively. At 278 K, the CO2 uptake capacities of pre-GNF and GNF-1 were found to be 75.1 and 94.6 mg g-1 at 1 bar, respectively. CO2 uptake capacities were decreased to 39.0 and 53.3 mg g-1 at 1 bar, 298 K for pre-GNF and GNF-1, respectively. We have also calculated Qst of CO2 adsorption for pre-GNF and GNF-1. The Qst values were found (Figure S8) to be 29.8 and 28.7 kJ mol-1. These values are higher compared to activated carbon-based materials (10.5-16.2 kJ mol1 49 ) and comparable to nitrogen containing nanoporous polymers such as BILP50, azo-COP51, ALP52, and CTF53, thanks to the high microporosity of GNF-1. High physicochemical stability of GNF-1 along with its high affinity towards CO2 renders GNFs as strong candidates for post-combustion CO2 capture applications.

Methane is highly popular as an alternative energy source due to its abundance and clean nature. Therefore, we have also investigated (Figure 4c, f) CH4 sorption behavior of pre-GNF and GNF-1. We have found CH4 uptake capacity of 9.5 and 11.5 mg g-1 at 1 bar, 278 K for pre-GNF and GNF-1, respectively. Upon raising the temperature to 298 K, the CH4 uptake capacities of pre-GNF and GNF-1 were decreased to 4.5 and 5.4 mg g-1, respectively. We have observed hysteresis in the CH4 adsorption desorption isotherms, which is attributed to the interaction of methane with micropore surfaces. This hysteresis was relatively more pronounced in the case of GNF-1 due to its higher microporosity. We have also calculated Qst of CH4 absorption, by using CH4 adsorption data at 273 and 298 K. The Qst values of CH4 adsorption were found (Figure S8) to be 22.7 and 24.1 kJ mol-1 for pre-GNF and GNF-1. Slight increase in the Qst value upon graphitization of pre-GNF was attributed to the increased microporosity of GNF-1. This Qst value is comparable, if not higher, to that of a Ni-based MOF (20 kJ mol-1), Ni2(dobdc), which showed highest volumetric methane uptake reported to date for any metal-organic framework at 25oC and 35 bar.54 We attributed this enhanced Qst to the small pore size as well as the hydrophobic nature of GNF1. Before natural gas can be used as fuel, it should be processed by a procedure called “natural gas sweeting” in order to remove the impurities such as CO2. In order to simulate natural gas conditions, we have calculated CO2/CH4 selectivity for pre-GNF and GNF-1 using Ideal Adsorbed Solution Theory (IAST) for CO2:CH4 mixture (5:95) at two different temperatures, 273 and 298 K (Figures S9, S10). We found exceptional CO2/CH4 selectivities of 52.5 and 62.7 at 1 bar, 298 K for preGNF and GNF-1. GNF-1 showed the highest CO2/CH4 selectivity compared to recently reported nitrogen containing nanoporous polymers such as, Li-POPs (17-38)29, PAF-30 (24.330.2)26, ALP (4-8)52 and NPOFs (3-12)55. The CO2/CH4 selec-


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tivity of GNF-1 is also very high compared to the activated carbon (3.7)56, thus showing clearly the promise of our bottom-up approach for the preparation of carbon-based frameworks. We have observed little temperature dependency in CO2/CH4 selectivity. Although GNF-1 showed high affinity towards both CO2 and CH4, we propose that high CO2/CH4 selectivity originates from molecular sieving effect associated with the larger kinetic diameter of CH4 compared to that of CO2, which limits the diffusion of CH4 into the small pores of GNF-1. GNF-1 showed higher selectivities compared to preGNF at both 273 and 298 K due to its smaller average pore diameter. These results indicate the potential of GNFs as lowcost, efficient solid-sorbents for natural gas purification process.

4. CONCLUSION We have introduced a new, efficient C-C polymerization reaction based on Diels-Alder cycloaddition polymerization, for the preparation of nanoporous polymers with a close spatial arrangement of phenyl rings in a one-pot reaction under catalyst-free conditions. Subsequent FeCl3-catalyzed cyclodehydrogenation reaction allowed us to form GNFs incorporating GNRs up to 2 nm in length and 1.1 nm in width. By using this strategy, we not only introduced the largest aromatic subunit into nanoporous polymers, but also addressed restacking problem of GNRs by introducing permanent pores. Exceptional physicochemical stability of GNFs indicates their potential use in gas storage and separation applications. Large π-surface area of GNRs combined with high microporosity of GNF-1 allowed us to obtain high Qst of 7.7 kJ mol-1 for H2 adsorption which is higher compared to nanoporous polymers with smaller aromatic subunits such as PAF-1 and activated carbons. Moreover, GNF-1 as low-cost, efficient solid-sorbent showed the highest CO2/CH4 selectivity reported to date, which is quite significant for natural gas purification process. Apart from gas storage and separation properties, applications including light-harvesting, energy storage, semiconductors, and sensing are also similarly promising with GNFs, which possess binary characteristics of graphene nanoribbons and nanoporous polymers. Considering the tunability and modularity of monomers, this new polymerization strategy could contribute to the development of new class of nanoporous polymers.

ASSOCIATED CONTENT Information regarding experimental details, FT-IR spectra, thermogravimetric analysis, SEM images, isosteric heats of adsorption, and IAST selectivity graphs. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013R1A1A1012282). This work was also partially supported by the National Research Foundation of Korea (NRF) Grant funded

by the Korea government (MEST) (NRF-2014R1A4A1003712 and BK21 PLUS program).

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Bottom-up Approach for the Synthesis of a Three-Dimensional

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