Facile Synthesis of Fluorinated Microporous Polyaminals for

Mar 31, 2016 - Solvothermal synthesis of hierarchically nanoporous organic polymers with tunable nitrogen functionality for highly selective capture o...
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Facile Synthesis of Fluorinated Microporous Polyaminals for Adsorption of Carbon Dioxide and Selectivities over Nitrogen and Methane Guiyang Li,†,‡ Biao Zhang,† and Zhonggang Wang*,† †

Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡ Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China S Supporting Information *

ABSTRACT: Monoaldehyde compounds, benzaldehyde, 4methylbenzaldehyde, 4-fluorobenzaldehyde, and 4-trifluoromethylbenzaldehyde, were utilized to react with melamine respectively to yield four hyper-cross-linked microporous polyaminal networks, PAN-P, PAN-MP, PAN-FP, and PANFMP, via a facile “one-step” polycondensation without adding any catalyst. It is found that relative to non-fluorinated polymers the fluorinated ones show the increased BET specific surface areas from 615 to 907 m2 g−1. Moreover, the incorporations of methyl and trifluoromethyl on the phenyl rings can effectively tailor the pore sizes from 0.9 to 0.6 nm. The polar C−F bond and nitrogen-rich polyaminal skeleton result in high CO2 adsorption enthalpies (38.7 kJ mol−1) and thereby raise the CO2 uptake up to 14.6 wt % (273 K, 1 bar) as well as large CO2/N2 and CO2/CH4 selectivities of 78.1 and 13.4 by the ideal adsorbed solution theory, respectively. The facile and scalable preparation method, low cost, and large CO2 adsorption and selectivities over N2 and CH4 endow the resultant microporous polyaminals with promising applications in CO2-capture from flue gas and natural gas.

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INTRODUCTION Microporous organic polymers (MOPs) are a new family of functional polymer materials having attracted great attention due to their potential applications in capture of carbon dioxide (CO2), hydrogen storage, adsorption of volatile organic compounds, heterogeneous catalysis, optics, and chemical sensing.1−4 Compared to the inorganic microporous materials such as zeolites5,6 and metal−organic frameworks (MOFs),7,8 besides the comparable specific surface area, most of the MOPs are free of any metal elements and therefore possess low skeleton density and excellent physicochemical stability. Moreover, the abundant synthetic strategies of organic and polymer chemistry provide the flexibility in molecular design and functionalization of MOP materials. Over the past decade, numerous microporous polymers with amorphous and ordered structures have been reported. However, they are usually constructed from multifunctional aromatic compounds with special geometrical configurations like those based on tetraphenyladamantane,9,10 tetraphenylmethane,11 tetraphenylsilane,12 triphenylamine,13 triphenylbenzene,14 porphyrin,15 and polyhedral oligomeric silsesquioxanes (POSS). 16 The multistep reactions and tedious purification procedures led to the preparation of MOPs being rather time-consuming and costly, which limits their scale-up production. In this regard, the development of an facile preparation method for MOPs using low-cost and commer© XXXX American Chemical Society

cially available raw materials is essentially important for the actual industrial application. The condensation of aromatic multialdehyde with triamine melamine has been successfully employed to prepare hypercross-linked microporous polyaminal networks.17−21 In this case, the intermediate imine bond (−CN−) generated further reacts with the active amino group to eventually form stable aminal linkage (−NH−CH−NH−). It is rational to deduce from the above studies that the polymerization between melamine and monoaldehyde compounds would also give rise to hyper-cross-linked polyaminal. The rigid multibranched net nodes are responsible for propping the linking struts to generate microporous structure. This approach is of significant importance since it opens up a facile way for abundant and cheap monoaldehyde compounds to prepare microporous materials. Moreover, melamine is a cheap chemical material widely applied in coating and plastic industries. The review of literature reveals that porous adsorbents bearing amine groups including porous silica,5,6,22−27 metal− organic frameworks (MOFs),7,8,28 and porous organic polymers29−32 have greatly improved adsorption capacity of CO2 and its adsorption selectivity over other gases because of the Received: January 21, 2016 Revised: March 26, 2016

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DOI: 10.1021/acs.macromol.6b00147 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis Routes to the Microporous Polyaminal Networks

Figure 1. FTIR spectra (left) and solid-state 13C MAS NMR spectra (right) for the four polyaminal networks. with 4 Å molecular sieves. Other reagents were of reagent grade and used as received. Instrumentation. Fourier transform infrared spectra (FTIR) of synthesized products were recorded using a Nicolet 20XB FT-IR spectrophotometer in 400−4000 cm−1. Samples were prepared by dispersing the complexes in KBr to form disks. Solid-state 13C MAS (with magic angle spinning) spectra were measured on a Varian Infinity-Plus 400 spectrometer at 100.61 MHz at an MAS rate of 10.0 kHz. Wide-angle X-ray diffractions (WAXD) from 5° to 60° were performed on a Rigku D/max-2400 X-ray diffractometer (40 kV, 200 mA) with a copper target at a scanning rate of 2°/min. Field-emission scanning electron microscopy (FE-SEM) experiments were carried on a Nova NanoSEM 450. Thermogravimetric analysis curves were recorded on a NETZSCH TG 209 thermal analyzer by heating the samples (ca. 8 mg) up to 800 °C with ramping rate of 10 °C min−1 under nitrogen flow. Adsorption and desorption measurements for all the gases and vapors were conducted on an Autosorb iQ (Quantachrome) analyzer. Prior to measurements, the samples were degassed at 120 °C under vacuum overnight. Adsorption and desorption isotherms of nitrogen were measured at 77 K. CO2 adsorption−desorption isotherms were measured at 273 and 298 K up to 1.0 bar. N2 and CH4 adsorption isotherms at 273 K were measured in order to evaluate the adsorption selectivities of CO2/N2 and CO2/CH4. Preparation of Microporous Polyaminal Networks. The polymerizations of the four polyaminal networks were carried out in a similar procedure, so only the polymerization of PAN-P is described here as an example. A dry Schlenk flask equipped with a stirrer and a condenser was degassed using two evacuation−argon-backfill cycles. Under argon flow, melamine (0.50 g, 3.96 mmol), benzaldehyde (0.76 g, 7.16 mmol), and 25 mL of DMSO were added and heated at 180 °C for 72 h. Finally, the system was cooled down, and the solid was isolated and washed successively with DMF, methanol, and THF. The

strong interaction between pore wall and CO2 molecule. The recent studies on microporous polyaminal networks show that the large amount of secondary amines and N-hetero moieties in the networks also play a significant role in enhancing the CO2 gas adsorption capability.17−21 Based on the above-mentioned reasons, in the present work, monofunctional benzaldehyde and 4-methylbenzaldehyde were chosen as raw materials to polymerize with melamine in order to create novel microporous polyaminals (PAN-P and PANMP) via a simple one-step polymerization without adding any catalyst. For comparison, commercially available 4-fluorobenzaldehyde and 4-trifluoromethylbenzaldehyde were also polymerized with melamine to produce fluorine-containing polyaminal networks (PAN-FP and PAN-FMP). The introduction of strong polar C−F bonds in PAN-FP and PAN-FMP will further enhance the affinity of pore wall toward CO2 molecule.33−35 In addition, the presence of different substituents on the phenyl rings is expected to tune the microporous structure and pore size. The influence of chemical structure of monomers on the porosity parameters of polymers and their relationships with the adsorptions of CO2 gas and selectivities over N2 and CH4 are studied in detail.

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EXPERIMENTAL SECTION

Materials. Melamine, benzaldehyde, 4-methylbenzaldehyde, 4fluorobenzaldehyde, and 4-trifluoromethylbenzaldehyde were purchased from J&K Chemical Co., Ltd. Dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), N,N-dimethylformamide (DMF), and other reagents were purchased from Shanghai Chemical Reagent Co. DMSO was purified by distillation under reduced pressure and dehydrated B

DOI: 10.1021/acs.macromol.6b00147 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules resultant product was extracted with THF in a Soxhlet apparatus for 24 h and dried at 120 °C under vacuum to constant weight.

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RESULTS AND DISCUSSION Synthesis and Characterization of Polyaminal Networks. Microporous polyaminal networks (PAN-P and PANMP) and their fluorinated counterparts (PAN-FP and PANFMP) were prepared via one-step polycondensations from benzaldehyde, 4-methylbenzaldehyde, 4-fluorobenzaldehyde, and 4-trifluoromethylbenzaldehyde with melamine (Scheme 1). The chemical structures of monoaldehyde monomers are similar except that the para-hydrogen atom (PAN-P) and paramethyl (PAN-MP) are replaced by fluorine atom (PAN-FP) and trifluoromethyl (PAN-FMP), respectively. The polymerizations were carried out in dimethyl sulfoxide at 180 °C. In the course of polymerizations no any catalyst was added. The formation of aminal linkages in the polymer networks were confirmed by FTIR and solid-state 13C MAS NMR spectroscopy. FTIR spectra (Figure 1, left) show that after polymerizations the absorption of aldehyde groups at 1690 cm−1 disappears, and instead, the stretching vibrations of secondary amine (N−H) at 3410 and 1195 cm−1 and methylene (CH) at 2917 cm−1 belonging to the aminal linkages appear,20 confirming the formation of aminal linkage. In addition, the characteristic absorptions of triazine rings are observed at 1556, 1481, and 1367 cm−1. The bands at 1103 and 1042 cm−1 are due to the stretching vibrations of aromatic C−F bond in PAN-FP, whereas the peak at 1127 cm−1 is assigned to the CF3 group in PAN-FMP.36,37 For PAN-MP and PAN-FMP, the characteristic band of imine linkage (CN) at 1620 cm−1 cannot be observed.38,39 However, the careful observation show that PAN-P and PAN-FP appear a very slight shoulder peak appear at around 1620 cm−1, indicating that trace amount of imine linkages may remain. In solid-state 13C MAS NMR spectra (Figure 1, right), the carbon signals of triazine rings and aromatic rings appear at 165 and 100−135 ppm, respectively.20 The resonances at 38−65 ppm are assigned to the tertiary carbons of aminal linkages. The signals of fluorinated bonds are probably overlapped with those of aromatic carbons, but it is seen that relative to PAN-MP, the signal of aminal linkages in PAN-FMP apparently shifts from 52 ppm to low field (57 ppm) due to the strong electron-withdrawing effect of CF3 groups. Wide-angle X-ray diffractions indicate that the four polyaminal networks are amorphous in nature (Figure S1, Supporting Information) owing to the kinetically controlled polymerization mechanism. The field-emission scanning electron microscopy (FE-SEM) is used here to microscopically observe the morphology of samples (Figure 2). PAN-P, PANFP, and PAN-MP exhibit loose cotton-like morphology, whereas PAN-FMP is composed of tiny particles with irregular shape and sharp edges. In addition, the resultant products are not soluble in common organic solvents such as dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, Nmethylpyrrolidone, chloroform, and tetrahydrofuran because of the hyper-cross-linked structures. Porous Structure of Polyaminal Networks. The porosity parameters of polyaminal networks were investigated by physical sorption of nitrogen at 77 K (Figure 3). The isotherms of all the polymers exhibit a steep rise of uptake at the very low relative pressure (P/P0 < 0.01), indicative of the typical characteristic of microporous materials.1 In addition, it is seen that the nitrogen uptakes continuously increase in the P/P0

Figure 2. FE-SEM images of PAN-P, PAN-FP, PAN-MP, and PANFMP.

Figure 3. Adsorption (filled) and desorption (empty) isotherms of nitrogen at 77 K.

range from 0.1 to 0.8 which, according to the classification of IUPAC, is attributed to the combination of type I and IV adsorptions.40 As a result, the four polymers should consist of both microporous and mesoporous structures. Besides, the loose packing of small particles give rise to interparticulate voids as reflected in the significant increase of nitrogen uptake when the P/P0 values exceeds 0.8. On the other hand, the previously reported microporous organic polymers usually display significant adsorption−desorption hysteresis which is attributed to the “softness” of polymer segments and deformation of the pore structure in the course of measurements in liquid nitrogen.1−4 Nevertheless, it is found that the four polymers prepared exhibit nearly reversible adsorption− desorption isotherms, suggesting that that the network architectures constructed with triazine and benzene rings through aminal linkages are sufficiently rigid. The specific surface areas and porosity parameters calculated from the sorption isotherms of PAN-P, PAN-FP, PAN-MP, and PAN-FMP are listed in Table 1. The BET surface area (SBET) and microporous surface area (SMicro) of PAN-P are 682 and C

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Macromolecules Table 1. Porosity Parameters of the Four Microporous Polyaminal Networks sample

SBET (m2/g)

SLangmuir (m2/g)

SMicro (m2/g)

VMicro (cm3/g)

VTotal (cm3/g)

pore size (nm)

PAN-P PAN-FP PAN-MP PAN-FMP

682 703 615 907

1003 1036 911 1351

318 331 248 340

0.15 0.16 0.12 0.16

0.61 0.61 0.53 0.89

0.84/2.12 0.93/2.16 0.66/2.25 0.63/2.20

318 m2 g−1, respectively. After incorporation of methyl substituents, the SBET and Smicro values of PAN-MP drop to 615 and 248 m2 g−1, and the total pore volume (VTotal) and micropore volume (VMicro) also decrease from 0.61 and 0.15 cm3 g−1 to 0.53 and 0.12 cm3 g−1, respectively. For PAN-MP, the steric hindrance of methyl groups is disadvantageous for the formation of homogeneous hyper-cross-linked structure, as reflected in its broader WAXD diffractive halo compared to that of PAN-P (Figure S1, Supporting Information), which results in a reduction of specific surface area and loss of pore volume to some extent. In contrast to PAN-P and PAN-MP, the fluorinated PAN-FP and PAN-FMP show the significantly larger BET surface areas of 703 and 907 m2 g−1, respectively, which are at a moderate level comparable to those earlier reported polyaminal frameworks (479−1377 m2 g−1).17−21 The high electronegativity of fluorine atoms effectively enhances the reactivity of aromatic aldehyde and facilitates the formation of hyper-cross-linked structure to generate large amount of pores. In addition, it is noteworthy that PAN-FMP possesses the largest surface area and porous volume among the four polymers, meaning that the strong electron-withdrawing effect of trifluoromethyl plays a more significant role in the formation of porous structure although it also has the steric hindrance similar to methyl in PAN-MP. Using nitrogen as probe, the comparisons of pore sizes and distributions for the four polyaminal networks were conducted through the analyses of the adsorption isotherms by the nonlocal density functional theory (NLDFT). Relative to PANP and PAN-FP, the microporous peaks of PAN-MP with methyls and PAN-FMP with trifluoromethyls apparently shift from around 0.9 to 0.6 nm (Figure 4). The smaller pore sizes of PAN-MP and PAN-FMP are due to that partial cavity spaces have been occupied by methyls and trifluoromethyls, respectively. Therefore, the experimental results reveal that pore size and distribution of microporous polymers can be readily tailored by the introduction of suitable substituent groups. Moreover, all the four polymers have the similar

mesoporous sizes centered at 2.2 nm. Compared to the other three samples, PAN-MP displays the larger mesoporous peak area, indicating that the steric effect of methyl groups in PANMP gives rise to more mesopores. CO2 Adsorption and Selectivities of CO2/N2 and CO2/ CH4. The CO2 adsorption−desorption isotherms measured at 273 and 298 K up to 1 bar are presented in Figure 5. For all the

Figure 5. Adsorption (filled) and desorption (empty) isotherms of CO2 at 273 and 298 K for PAN-P, PAN-FP, PAN-MP, and PAN-FMP.

four polyaminal networks, the CO2 uptakes appear a rapid rise at the low pressure region, suggesting the strong interaction between CO2 molecule and polymer skeleton. The CO2 uptakes continuously increase with pressure and have not reached saturation in the experimental pressure range, implying that the much large adsorption capacity could be achieved at the further increased CO2 feed pressure. The nearly complete reversibility of adsorption−desorption is quite beneficial to the practical operation in CO2 capture and regeneration of porous materials. The CO2 adsorption data are listed in Table 2. At 273 K and 1 bar, PAN-FP and PAN-FMP can uptake 13.1 and 14.6 wt % CO2, respectively, which are much higher than nonfluorinated PAN-P (11.1 wt %) and PAN-MP (11.6 wt %). Moreover, the CO2 adsorption capacities of PAN-FP and PANFMP are comparable or superior to many other porous polymers with even larger surface area, such as COF-103 (7.6 wt %, 3530 m2 g−1),11 CMP-0 (9.2 wt %, 1018 m2 g−1),13 PAF1 (9.1 wt %, 5640 m2 g−1),41 and microporous network-A (11.7 wt %, 4077 m2 g−1).42 In addition, it is noted that PAN-FMP can uptake 10.5 wt % CO2 under the ambient condition (298 K/1 bar), exceeding many other microporous polymers.1−4 To investigate the difference of affinity toward CO2 molecule between non-fluorinated and fluorinated polymer skeletons, the

Figure 4. Pore size distribution curves for the polyaminals calculated by NLDFT. D

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Macromolecules Table 2. CO2 Uptake, Q0 and Adsorption Selectivity of Polyaminal Networks CO2 uptakea (wt %) sample

273 K

298 K

PAN-P PAN-FP PAN-MP PAN-FMP

11.1 13.1 11.6 14.6

7.9 9.2 9.0 10.5

gas mixture of CO2/N2 was studied using the ideal adsorbed solution theory (IAST) method according to the mathematical integration.49 The gas mixture composition of flue gas is set as CO2/N2 = 0.15/0.85. Figure 6a−d shows that the corresponding single-site Langmuir−Freundlich curves (solid lines) for CO2 and N2 can well fit the experimental pure component isotherms of the corresponding gas. Thus, by IAST method, the selectivities of CO2/N2 are obtained and plotted as a function of pressure of the mixing gases up to 1 bar (Figure 6a′−d′). The comparison of data in Table 2 show that at 1 bar the CO2/ N2 selectivity of fluorinated PAN-FP (78.1) is the highest among the four polymers. Moreover, the fluorinated PAN-FMP also exhibits the higher selectivity (59.1) than the nonfluorinated PAN-MP (48.2). The enhanced selective adsorption of CO2/N2 for PAN-FP and PAN-FMP are consistent with their higher sorption enthalpies of CO2 than the nonfluorinated counterparts. Subsequently, the selective adsorption of binary gas mixtures of CO2/CH4 is also evaluated by IAST method in order to assess the potential applications in stripping CO2 from nature gas and landfill gas. The CO2/CH4 ratio in the gas mixtures of natural gas and landfill gas are set as 0.05/0.95 and 0.50/0.50, respectively. Similar to N2 and CO2, the single-site Langmuir− Freundlich curves of CH4 (solid black lines) for all the four polymers also fit well with the experimental pure component isotherms (Figure 6a−d). The data in Table 2 show that the CO2/CH4 selectivities for PAN-P, PAN-FP, and PAN-MP are 11.0−11.9 (natural gas) and 12.3−13.4 (landfill gas) at 1.0 bar, which are comparable or superior to many other porous adsorbents like porous polymer frameworks (PPFs) (8.6− 11.0)38 and porous coordination networks (PCNs) (7.2− 14.0).50,51 However, compared to the other three polymers, PAN-FMP displays a lower CO2/CH4 selectivity probably because the trifluoromethyls have some favorable interaction with CH4 gas although the real reason is not clear yet.

CO2 selectivityc Q0b

(kJ/mol) 36.8 38.7 37.4 38.2

CO2/N2 55.9 78.1 48.2 59.1

CO2/CH4 11.0 11.4 11.9 7.7

(12.6) (12.3) (13.4) (7.2)

a

CO2 uptake at 1 bar. bLimiting enthalpy of adsorption of CO2. Adsorption selectivity calculated by IAST method from 0.15/0.85 gas mixture for CO2/N2 and from 0.05/0.95 (0.5/0.5) gas mixtures for CO2/CH4 at 273 K and 1.0 bar. c

limiting adsorption enthalpies at zero surface CO2 coverage (Q0) were calculated from their virial plots at two temperatures (Figure S2, Supporting Information). The data in Table 2 show that the Q0 values of PAN-FP and PAN-FMP (38.2−38.7 kJ mol−1) are indeed higher than those of PAN-P and PAN-MP (36.8−37.4 kJ mol−1). Moreover, their Q0 values also surpass many other porous polymers like benzimidazole-linked polymers (BILPs) (26.5−28.8 kJ mol−1),14,43 hyper-crosslinked polymers (HCPs) (20−24 kJ mol−1),4,44 and borazinelinked polymers (BLPs) (20.2−28.3 kJ mol−1),45 but lower than amine-containing porous silicas (55−145 kJ mol−1)46−48 and MOFs (71 kJ mol−1).7 In addition to the relatively larger surface areas and nitrogen-rich polyaminal skeletons, the incorporation of abundant polar C−F bonds in PAN-FMP probably play a positive role in its largest CO2 uptakes at both 273 and 298 K among the four polymers owing to the enhanced dipole−quadrupole interaction.33−35 The single component adsorption isotherms of N2 and CH4 at 273 K and their single-site Langmuir−Freundlich fitting curves (solid) are presented in Figure 6a−d. It is seen that the CO2 uptakes for all the four polymers are considerably higher than N2 and CH4. At first, the adsorption selectivity of binary

Figure 6. Experimental adsorption isotherms for CO2, CH4, and N2 at 273 K, and their single-site Langmuir−Freundlich fitting curves (solid) (a−d), and IAST selectivities for 0.15/0.85 CO2/N2 mixture (■, blue), 0.50/0.50 CO2/CH4 mixture (●, red), and 0.05/0.95 CO2/CH4 mixture (▲, green) (a′−d′) for the four microporous polyaminals. E

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(7) McDonald, T. M.; Lee, W. R.; Mason, J. A.; Wiers, B. M.; Hong, C. S.; Long, J. R. Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal−Organic Framework mmenMg2(dobpdc). J. Am. Chem. Soc. 2012, 134, 7056−7065. (8) Li, P. Z.; Zhao, Y. L. Chem. - Asian J. 2013, 8, 1680−1691. (9) Lu, W.; Yuan, D.; Zhao, D.; Schilling, C. I.; Plietzsch, O.; Muller, T.; Bräse, S.; Guenther, J.; Blümel, J.; Krishna, R.; Li, Z.; Zhou, H. C. Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation. Chem. Mater. 2010, 22, 5964−5972. (10) Shen, C. J.; Yu, H.; Wang, Z. G. Synthesis of 1,3,5,7-Tetrakis(4Cyanatophenyl)-Adamantane and Its Microporous PolycyanurateNetwork for Adsorption of Organic Vapors, Hydrogen and Carbon Dioxide. Chem. Commun. 2014, 50, 11238−11241. (11) Furukawa, H.; Yaghi, O. M. Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc. 2009, 131, 8875−8883. (12) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortés, J. L.; Côté, A. P.; Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Designed Synthesis of 3D Covalent Organic Frameworks. Science 2007, 316, 268−272. (13) Ren, S. J.; Dawson, R.; Laybourn, A.; Jiang, J. X.; Khimyak, Y.; Adams, D. J.; Cooper, A. I. Functional Conjugated Microporous Polymers: from 1,3,5-Benzene to 1,3,5-Triazine. Polym. Chem. 2012, 3, 928−934. (14) Rabbani, M. G.; El-Kaderi, H. M. Synthesis and Characterization of Porous Benzimidazole-LinkedPolymers and Their Performance in Small Gas Storage and Selective Uptake. Chem. Mater. 2012, 24, 1511−1517. (15) Wang, X. S.; Liu, J.; Bonefont, J. M.; Yuan, D. Q.; Thallapally, P. K.; Ma, S. Q. A Porous Covalent Porphyrin Framework with exceptional uptake capacity of saturated hydrocarbons for Oil Spill Cleanup. Chem. Commun. 2013, 49, 1533−1535. (16) Wu, Y.; Wang, D. X.; Li, L. G.; Yang, W. Y.; Feng, S. Y.; Liu, H. Z. Hybrid Porous Polymers Constructed from Octavinylsilsesquioxane and Benzene Via Friedel−Crafts Reaction: Tunable Porosity, Gas Sorption, and Postfunctionalization. J. Mater. Chem. A 2014, 2, 2160− 2167. (17) Schwab, M. G.; Fassbender, B.; Spiess, H. W.; Thomas, A.; Feng, X. L.; Müllen, K. Catalyst-Free Preparation of Melamine-Based Microporous Polymer Networks through Schiff Base Chemistry. J. Am. Chem. Soc. 2009, 131, 7216−7217. (18) Shunmughanathan, M.; Puthiaraj, P.; Pitchumani, K. MelamineBased Microporous Network Polymer Supported Palladium Nanoparticles: A Stable and Efficient Catalyst for the Sonogashira Coupling Reaction in Water. ChemCatChem 2015, 7, 666−673. (19) Ansari, M. B.; Jeong, E. Y.; Park, S. E. Styrene Epoxidation in Aqueous over Triazine-Based Microporous Polymeric Network as a Metal-Free Catalyst. Green Sustainable Chem. 2012, 2, 1−7. (20) Li, G. Y.; Zhang, B.; Yan, J.; Wang, Z. G. Tetraphenyladamantane-Based Polyaminals for Highly Efficient aptures of CO2 and Organic Vapors. Macromolecules 2014, 47, 6664−6670. (21) Laybourn, A.; Dawson, R.; Clowes, R.; Iggo, J. A.; Cooper, A. I.; Khimyak, Y. Z.; Adams, D. J. Branching out with Aminals: Microporous Organic Polymers from Difunctional Monomers. Polym. Chem. 2012, 3, 533−537. (22) Chaikittisilp, W.; Khunsupat, R.; Chen, T. T.; Jones, C. W. Poly(allylamine)- Mesoporous Silica Composite Materials for CO2 Capture from Simulated Flue Gas or Ambient Air. Ind. Eng. Chem. Res. 2011, 50, 14203−14210. (23) Drese, J. H.; Choi, S.; Didas, S. A.; Bollini, P.; Gray, M. L.; Jones, C. W. Effect of Support Structure on CO2 Adsorption Properties of Pore-Expanded Hyperbranched Aminosilicas. Microporous Mesoporous Mater. 2012, 151, 231−240. (24) Kuwahara, Y.; Kang, D. Y.; Copeland, J. R.; Brunelli, N. A.; Didas, S. A.; Bollini, P.; Sievers, C.; Kamegawa, T.; Yamashita, H.; Jones, C. W. Dramatic Enhancement of CO2 Uptake by Poly(ethyleneimine) Using Zirconosilicate Supports. J. Am. Chem. Soc. 2012, 134, 10757−10760. (25) Chen, C.; Yang, S. T.; Ahn, W. S.; Ryoob, R. AmineImpregnated Silica Monolith with A Hierarchical Pore Structure:

CONCLUSIONS Four non-fluorinated and fluorinated microporous polyaminal networks were synthesized from melamine with aromatic monoaldehydes benzaldehyde, 4-methylbenzaldehyde, 4-fluorobenzaldehyde, and 4-trifluoromethylbenzaldehyde through a facile “one-step” polycondensation at a moderate temperature without using any catalyst. Their chemical structures were confirmed by FTIR and solid-state 13C MAS NMR spectroscopy. The resultant polyaminals cannot dissolve in common solvents, indicative of the hyper-cross-linking structure. The analyses of N2 adsorption isotherms at 77 K reveal that they are microporous materials with controllable pore sizes in the range from 0.6 to 0.9 nm through varying the substituents on the phenyl rings. The high reactivity of 4-fluorobenzaldehyde and 4-trifluoromethylbenzaldehyde results in the fluorinated polymer networks the significantly larger BET surface areas (907 m2 g−1) compared to the non-fluorinated ones (615 m2 g−1). The adsorption and desorption isotherms of CO2 gas in the four porous polymers are reversible, which characteristic is convenient for the regeneration of CO2 adsorbents. The polymers display CO2 adsorption capacities up to 14.6 wt % (273 K/1 bar) and selectivities for CO2/N2 and CO2/CH4 are 78.1 and 13.4 by the IAST method, respectively, being superior or comparable to many other microporous organic polymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00147. Two figures, including wide-angle X-ray diffractions patterns and virial plots of CO2 adsorptions (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation of China (Nos. 51273031, 51473026, and U1462125) for financial support of this research.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b00147 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b00147 Macromolecules XXXX, XXX, XXX−XXX