Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Naphthyl Substitution-Induced Fine Tuning of Porosity and Gas Uptake Capacity in Microporous Hyper-Cross-Linked Amine Polymers Shuangshuang Hou and Bien Tan* Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China S Supporting Information *
ABSTRACT: Exploring the effect of functional group substitution on the porous structure and gas adsorption performance of polymer materials is becoming much fascinating. Here, three novel polymers, based on various amine building blocks, were efficiently prepared by the solvent knitting method. It is intriguingly found that with the gradual functional group substitution of phenyl by naphthyl in building blocks both porosity and gas uptake capacity of these hyper-cross-linked polymers can be finely tuned, with SBET increased from 874 to 1717 m2 g−1, and CO2 adsorption capacity tuned from 12.76 to 18.85 wt % (273.15 K/1.00 bar). Besides, these polymers could reveal CO2/N2 selectivity up to 24.9 and CO2/CH4 selectivity of 6.3 at 298.15 K. This work has proved that increasing the number of naphthyl in precursors will be in favor of knitting microporous with improved porosity and enhanced gas uptake ability, and naphthyl outweighs phenyl in their contributions to bettering the porosity parameters and gas uptake capacity of synthesized polymers, which is crucially important for the further research.
1. INTRODUCTION Largely responsible for global warming, the ever-increasing anthropogenic emission of CO2 has drawn a great deal of public concern.1 For the sake of attaining efficient CO2 capture and storage (CCS), developing viable adsorbents seems to be highly desirable in the 21st century.2,3 Typically, those reported CO2 adsorbents can be classified into materials based on graphite/ graphene, zeolite, MOFs, silica, polymer, clay, etc., which capture CO2 primarily by physical interaction, and materials based on solid amine, alkali metal carbonate, immobilized ion liquid, alkali metasilicates, etc., which adsorb CO2 primarily by chemical binding.4−7 Meanwhile, the preparation of low-cost materials from waste resources also enhances the competitiveness of CO2 adsorptive separation in practical applications.7−10 While in a variety of proposed microporous solid adsorbents microporous organic polymers (MOPs), which emerged as a big family of state-of-the-art materials, have been at the forefront of scientific research.11,12 Because of the distinctive features such as low skeleton density, large surface area, tunable pore properties, good thermal stability, and synthetic diversity, they have potentially been strong candidates for the postcombustion CCS.13,14 As a subject of intense research, functional modification on MOPs, with regard to modulating framework−CO2 interactions, is perceived as an effective approach to bring about dramatic improvements in gas adsorption performance. Years have witnessed the efforts that concentrated on incorporating © XXXX American Chemical Society
polar CO2-philic groups into the pore walls to develop novel functionalized MOPs. Practices of introducing polar groups including amino,15−17 nitro,18 carboxyl,19,20 hydroxyl,21,22 sulfony and sulfonate groups23−25 or heteroatoms such as nitrogen,26,28 oxygen,27,28 and sulfur,28 by means of either postmodification methods or the direct polymerization of functional monomers, into the polymer skeleton, have significantly increased the CO2-sorption capacity. It is worthwhile to note that owing to the inducement of basicity and charge delocalization into the frameworks,29 nitrogen atom can be greatly helpful in improving the binding affinity toward CO2 molecules via the promoted dipole−quadrupole interaction and Lewis acid−Lewis base electrostatic interactions,14,30−34 thus providing plenty of opportunities to enhance the isosteric heat, CO2 uptake capacity, and gas sorption selectivity of polymer networks.33,35,36 For example, Cooper and co-workers reported an amine-based microporous CMP-1NH2 with a higher isosteric heat of adsorption for CO2 than its parent CMP-1.19 El-Kaderi et al. by homocoupling aniline-like building units prepared a series of nanoporous ALPs with CO2 uptake up to 5.37 mmol g−1 at 273.15 K/1.00 bar.37 Our group demonstrated the synthesis of pyrrole-based microporous organic polymer with CO2/N2 selectivity of 117 at 273 K, Received: February 5, 2018 Revised: March 21, 2018
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DOI: 10.1021/acs.macromol.8b00274 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
purified by Soxhlet extractor with ethanol for 24 h, and finally dried in a vacuum oven at 70 °C for 48 h. The obtained polymer material was obtained as a black solid. Yield: 140%. Synthesis of Polymer 2. This polymer was synthesized by employing the same method as described above for polymer 1 and prepared by treating NPB (0.2 mmol, 0.1178 g) with anhydrous AlCl3 (12.8 mmol, 1.7088 g) in CH2Cl2 (8 mL). The obtained polymer material was obtained as a black solid. Yield: 130%. Synthesis of Polymer 3. This polymer was synthesized by employing the same method as described above for polymer 1 and prepared by treating α,β-TNB (0.2 mmol, 0.1378 g) with anhydrous AlCl3 (16 mmol, 2.1360 g) in CH2Cl2 (8 mL). The obtained polymer material was obtained as a brown solid. Yield: 134%. 2.3. Characterization. The FT-IR spectra were recorded in the wavenumber range of 4000−500 cm−1 by using a Bruker VERTEX 70 FT-IR spectrometer under ambient conditions. Solid-state 13C crosspolarization/magic-angle spinning nuclear magnetic resonance (13C CP/MAS NMR) spectra were performed on a WB 400 MHz Bruker Avance II spectrometer and collected with a spinning frequency of 20 kHz by using a 2.5 mm double-resonance MAS probe. The fieldemission scanning electron microscopy (FE-SEM) images were recorded by employing an FEI Sirion 200 field-emission scanning electron microscope operating at 10 kV. The transmission electron microscopy (TEM) images were recorded on a Tecnai G2 F30 microscope (FEI Corp. Holland) operating at 200 kV, while all the samples were dried in a vacuum oven at 70 °C for about 24 h and then sputter-coated with platinum before this measurement. The elemental analysis was obtained on a Vario Micro cube elemental analyzer (Elementar, Germany). Thermogravimetric analysis (TGA) was performed from room temperature to 850 °C, with a PerkinElmer Instrument Pyris1 TGA heated at a rate of 10 °C min−1 under a nitrogen atmosphere. Before analysis, all the samples were degassed at 120 °C for at least 8 h under vacuum of 10−5 bar. Ultrahigh purity gases were used for all measurements, and the free volume was measured using helium. Gas (H2, N2, CH4, and CO2) sorption properties and specific surface area of samples were measured by employing a Micromeritics ASAP2020 surface area and porosity analyzer. Pore size distribution was calculated by N2 adsorption isotherms employing a Tarazona nonlocal density functional theory (NLDFT) model assuming slit pore geometry. Total pore volumes (Vtotal) were derived from nitrogen sorption isotherms at relative pressure P/P0 = 0.995.
which was much better than that of polymer hyper-cross-linked from benzene under the same conditions.28 Simulations employing density functional theory (DFT) calculations have previously indicated that more negative charge distribution over moieties of a monomer could help in improving the CO2 uptake capacity.38 Polycyclic aromatic hydrocarbons (PAHs) are a vital class of organic compounds with two or more laterally fused benzene rings.39−41 Attributed to the composed π-conjugated systems in structure, they are advantageously rich in electrons41,42 and therefore probably better choices for producing MOPs with outstanding CO2 uptake performance. Moreover, naphthalene, with a wide range of sources, is the most significant PAH if applied over preparing special chemicals in industry. However, of all the functional groups, naphthyl originated from naphthalene, with more active sites and electron-rich merit, has not received much attention with respect to the preparation of porous materials for CO2 adsorbents, to the best of our knowledge. Hence, it is not hard to speculate that building blocks bearing varied number of naphthyls, if considered as starting precursors, perhaps will be greatly in favor of creating novel MOPs, and the resultant polymers that stemmed from naphthyl-containing building blocks in turn are also more likely to exhibit marvelous properties. Inspired by the existing progress and ideas, this work focuses on the construction of microporous hyper-cross-linked amine polymers with various number of naphthyl groups. However, such microporous organic hyper-cross-linked amine polymers with high SBET and comparable CO2 adsorption performance have rarely been reported because of lacking efficient methods. Actually there are a series of synthetic methods including the Scholl coupling reaction,43 the knitting method with FDA as external cross-linker,44 and the solvent knitting method45 that have been successively reported for the sake of creating novel polymers with fascinating structures and multifunctional applications. Notably, the solvent knitting method, among the reported methods, based on the Friedel−Crafts alkylation reaction mechanism, has won particular acceptance for its distinguishing characteristics such as being simple, one-step, and cost-effective as well as offering more available chances for polymer networks with high surface area, narrow pore size distribution, good gas uptake performance, and so forth. Herein, the solvent knitting method was employed as a straightforward and reliable polymerization technique here.
3. RESULTS AND DISCUSSION The synthetic illustration of polymer networks based on the one-step Friedel−Crafts alkylation reaction, and corresponding amine monomers are sketched in Scheme 1. All the polymer materials exhibit difficult solubility in various common organic solvents such as 1-methyl-2-pyrrolidinone, 1, 2-dichloroethane, tetrahydrofuran, chloroform, benzene, and methanol after their purification. Characterizations like FT-IR and cross-polarization (CP) 13C MAS NMR were conducted to verify the structure of networks and their corresponding building blocks. The results indicate that the peaks in 1380−1250 cm−1 are the C−N stretching vibrations and the peaks in 3100−3000 cm−1 are the C−H stretching vibrations of benzene ring, which can be found in both polymeric networks (Figure 1) and their building blocks (Figures S2−S4, Supporting Information), while the three peaks at near 2920 cm−1 only found in these polymers (Figure 1) are the C−H stretching vibrations of methylene, which firmly confirm the existence of methylene group in chemical structure of polymers. Various types of carbon signals can be drawn from the 13C MAS NMR (Figure 2); for instance, the peaks about 130 ppm are attributed to the unsubstituted aromatic carbon and the peaks around 137 ppm belong to the substituted aromatic carbon, while in contrast with those of starting materials (Figure S5), the resonance peaks near 37
2. EXPERIMENTAL SECTION 2.1. Materials. Chemicals such as tetraphenylbenzidine (TPB), N,N′-bis(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) and N4,N4′-di-1-naphthalenyl-N4,N4′-di-2-naphthalenyl[1,1′-biphenyl]-4,4′-diamine (α,β-TNB) were purchased from Aladdin Chemical Reagent Corp. (Shanghai, China) and used as received. Dichloromethane (CH2Cl2), ethanol, hydrochloric acid (HCl), and anhydrous aluminum chloride (AlCl3) were obtained from Sinopharm Chemical Reagent Ltd. Co. (Shanghai, China) and also used as received. Unless noted otherwise, all commercially available solvents and other chemicals were obtained from local suppliers and used without any further purification. 2.2. Preparation of Polymers. Synthesis of Polymer 1. Under a nitrogen atmosphere, TPB (0.2 mmol, 0.0978 g) was dispersed in CH2Cl2 (8 mL) for 30 min, and then anhydrous AlCl3 (9.6 mmol, 1.2816 g) was added to the solution; the mixture was allowed to react at 20 °C for 4 h, 30 °C for 8 h, 40 °C for 12 h, 60 °C for 12 h, and 80 °C for 24 h by vigorously stirring. After cooling to room temperature, the solid product was quenched by using 20 mL of HCl−H2O (v/v = 2:1), washed several times with deionized water and ethanol, further B
DOI: 10.1021/acs.macromol.8b00274 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Proposed Synthetic Routes to the Network Structures and Building Blocks
In view of the FT-IR and 13C MAS NMR results, there good reasons to believe that these microporous hyper-cross-linked amine polymers by the simple solvent knitting method were successfully synthesized. The FE-SEM was adopted to probe into the surface morphologies of polymers, and it is found that these hypercross-linked polymers are intrinsically amorphous with irregular blocks in shape (Figure 3), while the morphologies of building
Figure 1. FT-IR spectrum of materials from polymer 1 to polymer 3.
Figure 3. FE-SEM image (a) and TEM image (b) of polymer 1, FESEM image (c) and TEM image (d) of polymer 2, and FE-SEM image (e) and TEM image (f) of polymer 3.
blocks are also presented for contrast (Figure S1). The TEM was used to assess the textural property of these networks, and the results show that these polymers are porous in nature, with abundant pores randomly distributed all over the structure (Figure 3). The TGA curves indicate that all the polymers display an initial decomposition temperature at 400 °C or so under a nitrogen atmosphere. All the polymer networks have similar decomposition behaviors, and the critical weight loss in a high temperature region is the result of the destruction of polymeric networks, which rationally reflects the good physicochemical and thermal stability of these hyper-crosslinked amine polymers (Figure S6). The porosity parameters of these nitrogen-containing microporous organic polymers were measured by the physical sorption experiments of nitrogen adsorption and desorption isotherms at 77.3 K. For each case (Figure 4a), the appearance of a steep rise of nitrogen uptake at a very low relative pressure (P/P0 < 0.001) implies that abundant micropores are dominant in the materials.46 The observed hysteresis loops tend to be more obvious from polymer 1 to polymer 3 in the desorption branch of these isotherms in the middle pressure area, which
Figure 2. Cross-polarization (CP) 13C MAS natural abundance NMR spectrum of materials from polymer 1 to polymer 3.
ppm only found in prepared polymers are ascribed to the methylene carbon, which convincingly proves the formation of methylene linker that results from dichloromethane with functions of both economical solvent and external cross-linker. C
DOI: 10.1021/acs.macromol.8b00274 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. Nitrogen adsorption and desorption isotherms at 77.3 K (a) and pore distribution of pore size distribution calculated using DFT methods (slit pore models, differential pore volumes). Pore width (b) and inset (c) for the pore size distribution at lower pore width no more than 2 nm of samples.
the experimental pressure rang up to 1.13 bar, making it clear that there is still a larger CO2 capacity can be expected in the event of further elevating the CO2 pressure. Of the three polymers, polymer 1 with SBET 874 m2 g−1 and SLangmuir 1119 m2 g−1 and polymer 2 with SBET 1268 m2 g−1 and SLangmuir 1543 m2 g−1 can respectively adsorb 12.76 and 16.47 wt % CO2 at 273.15 K/1.00 bar, while in contrast, polymer 3 with SBET 1717 m2 g−1 and SLangmuir 2135 m2 g−1 exhibits a CO2 uptake capacity of 18.85 wt % at the same conditions most probably because of its accessibility of the electron-rich naphthyl groups. As the substitution of phenyl by naphthyl is increasing, the electrondonating naphthyl groups as moieties of the precursor, affected by the conjugated effect, will largely improve the electron-rich character of the whole building block, thus further strengthening the affinity of polymer toward CO2. Apart from this, it is also affected by other influencing factors such as the incorporation of nitrogen atoms, the pore size that less than 1 nm prefers to adsorbing CO2 molecules,47 and the ultramicropore whose diameter is comparable to the kinetic diameter of CO2 that is prone to increasing the interactions between CO2 molecules and the pore walls.48 Notably, the CO2 uptake ability of polymer 3 is comparable to many reported materials such as binaphthol-based HCPs (17.42 wt %),49 Ndoped nanocomposite Zn/Ni-ZIF-8-1000 (18.70 wt %),50 SMPs-7 (20.4 wt %),43 FCDTPA-K-500 (19.93 wt %),51 C1M1-Al (18.1 wt %),52 and PPN-6-CH2DETA (15.8 wt %, at 298 K/1.00 bar).53 Besides, the CO2 adsorption in polymer 3 is higher than that of the best top performing N-containing scaffolds, such as the N-TC-EMC (17.6 wt %),54 BILP-4 (15.8 wt %),55 FCDTPA-K-900 (15.00 wt %),51 MPI-6FA (13.5 wt %),56 FCDTPA (12.45 wt %),51 and MFB-600 (9.90 wt %).57 Moreover, the CO2 sorption value of polymer 3 is still better than those porous materials with either large total pore value or ultrahigh surface area including the CPOP-10 (SBET = 3337 m2 g−1, about 9.1 wt %, at 298 K/1.00 bar),58 COF-102 (SBET = 3620 m2 g−1, 6.8 wt %),59 PAF-1 (SBET = 4077 m2 g−1, 9.1 wt %),47 and CMPN-3 (2.36 cm3 g−1, 3.82 wt %).60 To have a better understanding of the relationship between polymer networks and CO2 molecules, the isosteric heat (Qst) of adsorption for the three polymer materials was calculated by fitting the CO2 adsorption isotherms collected at 273.15 and 298.15 K in terms of the Clausius−Clapeyron equation, which were found in the ranges of 28.19−26.91, 27.29−25.96, and 26.77−25.49 kJ mol−1 respectively for polymers 1, 2, and 3
clearly indicate the existence of growing mesopores. For the three polymers, there is almost no steep rise in the nitrogen adsorption isotherms at high relative pressures (P/P0 > 0.9), suggesting the near absence of macropores. The pore size distribution calculated by employing the nonlocal density functional theory (NLDFT) model (Figure 4b) shows that the three polymer networks have relatively narrow pore size distribution, with apparent peaks located primarily in the micropore region (
