Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX
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Triptycene-Based Microporous Cyanate Resins for Adsorption/ Separations of Benzene/Cyclohexane and Carbon Dioxide Gas Gaoyang Deng and Zhonggang Wang* State Key Laboratory of Fine Chemicals, Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China S Supporting Information *
ABSTRACT: Triptycene-based cyanate monomers 2,6,14tricyanatotriptycene (TPC) and 2,6,14-tris(4-cyanatophenyl)triptycene (TPPC) that contain different numbers of benzene rings per molecule were synthesized, from which two microporous cyanate resins PCN−TPC and PCN−TPPC were prepared. Of interest is the observation that the two polymers have very similar porosity parameters, but PCNTPPC uptakes considerably higher benzene (77.8 wt %) than PCN-TPC (17.6 wt %) at room temperature since the higher concentration of phenyl groups in PCN-TPPC enhances the π−π interaction with benzene molecules. Besides, the adsorption capacity of benzene in PCN-TPPC is dramatically 7 times as high as that of cyclohexane. Contrary to the adsorption of organic vapors, at 273 K and 1.0 bar, PCN-TPC with more heteroatoms in the network skeleton displays larger uptake of CO2 and higher CO2/N2 selectivity (16.4 wt %, 60) than those of PCN-TPPC (14.0 wt %, 39). The excellent and unique adsorption properties exhibit potential applications in the purification of small molecular organic hydrocarbons, e.g., separation of benzene from benzene/cyclohexane mixture as well as CO2 capture from flue gas. Moreover, the results are helpful for deeply understanding the effect of porous and chemical structures on the adsorption properties of organic hydrocarbons and CO2 gas. KEYWORDS: cyanate resin, microporous organic polymer, porous organic polymer, gas adsorption, CO2 capture, vapor adsorption, benzene/cyclohexane separation
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INTRODUCTION The study of microporous organic polymers (MOPs) has become a subject of intense research with multidisciplinary participation in nanomaterials, polymer science, physical chemistry, organic chemistry, separation science and technology, etc.,1−3 due to their importance as the construction of micropores with pore size less than 2 nm can bring unprecedented properties such as large specific surface area and extremely low density to traditional polymers, leading to a variety of new applications in gas adsorption/separation,4−10 chemical sensing,11−13 heterogeneous catalysis,14,15 and ultralow dielectricity.16 Over the past decade, numerous MOPs have been synthesized with research focus on the CO2 capture with respect to global-warming issue.17,18 There are, however, only few studies related to the adsorption of organic compounds.19−21 As a matter of fact, MOPs entirely composed of organic groups have intrinsic affinity to organic molecules. Moreover, the adjustable pore sizes and facilely tailorable chemical structure endow MOPs with the possibility to selectively adsorb/separate a small molecular organic compound from the mixture. For example, benzene and cyclohexane are important petrochemical products obtained from crude petroleum and are widely applied in the synthetic industry for organic and polymeric materials, but the © XXXX American Chemical Society
purification of benzene from the benzene/cyclohexane mixture is extremely difficult by conventional distillation process since their boiling points are very close with a difference of only 0.6 °C at the ambient pressure. Seeking a highly efficient adsorbent that enables the separation of benzene or cyclohexane from their mixture is a challenging task. Microporous cyanate resins represent a new class of MOPs developed in our group22−26 since 2009 that potentially extend the applications of thermosetting cyanate resins27−31 to other high-tech fields, e.g., adsorption of gases and organic vapors. Using multifunctional cyanate monomers, upon heating, every three OCN groups are thermally cyclotrimerized to form a triazine ring, which acts as net node in the hyper-cross-linked microporous cyanate resin networks. The abundant nitrogen and oxygen elements can enhance the affinity of the pore surface toward CO2 molecule owing to the dipole−quadrupole interaction. Besides, in comparison with other triazine-based microporous polymers like CTFs,32 which are usually prepared at a very high temperature of over 400 °C in a corrosive ZnCl2 medium, the microporous cyanate resins are obtained under a Received: October 3, 2017 Accepted: November 8, 2017
A
DOI: 10.1021/acsami.7b15050 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
tribromotriptycene (1.0 mmol) were added; then the mixture was deoxygenated under N2 purge for 1 h. After 0.24 g of Pd(PPh3)4 (0.22 mmol) was added, the mixture was refluxed for 48 h and then filtrated, washed with water, and purified through silica gel column chromatography (CH2Cl2:hexane = 1:5). A white needle crystal was obtained in a yield of 62%. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.740−7.732 (m, 3H), 7.531−7.495 (m, 9H), 7.250−7.221 (m, 3H), 6.996−6.957 (m, 6H), 5.811−5.721 (m, 2H), 3.817−3.811 (m, 9H). FTIR (KBr, cm−1): 3030, 2952, 2833, 1608, 1582, 1517, 1468, 1178, 1153. Synthesis of 2,6,14-Tris(4-hydroxyphenyl)triptycene (TPPH). To a solution of 1.15 g of TPPM (2 mmol) in 35 mL of preprepared anhydrous dichloromethane, 10 mL of BBr3 (1 M, 10 mmol) of dichloromethane solution was added dropwise under a nitrogen atmosphere over 30 min. Then the resulting mixture was allowed to react for 12 h at room temperature. After being poured into 100 mL of ice water and stirred vigorously for 30 min, the precipitate was filtrated and washed with water; the solid material was dried under vacuum for overnight. A white solid product was obtained in a yield of 95%; mp >300 °C. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.622 (d, 6H), 7.531−7.415 (s, 3H), 7.370−7.256 (d, 3H), 7.251−7.210 (d, 3H), 6.972−6.841 (m, 4H), 5.218−5.184 (m, 2H), 5.357 (s, 3H). FTIR (KBr, cm−1): 3603, 3057, 2952, 1599, 1506, 1486, 1469, 1190, 1166, 698. Synthesis of 2,6,14-Tris(4-cyanatophenyl)triptycene (TPPC). TPPC was prepared with a similar procedure to TPC. A white needle crystal was obtained in a yield of 94%; mp 117−118 °C. 1H NMR (400 MHz, d-DMSO): δ (ppm) 7.641 (s, 3H), 7.597−7.549 (d, 6H), 7.545−7.516 (d, 3H), 7.354−7.306 (d, 6H), 7.247−7.201 (d, 3H), 5.646−5.596 (broad, 2H). 13C NMR (100 MHz, d-acetone): δ (ppm) 153.3, 147.2, 146.0, 140.9, 137.4, 130.0, 125.1, 123.6, 116.6, 109.5, 54.8, 53.9. FTIR (KBr, cm−1): 3058, 2957, 2269, 2241, 1593, 1511, 1493, 1180, 816. MS calculated for C41H23N3O3, 605.1739; found, 605.1744. Preparation of Cyanate Resin Networks (PCNs). Cyanate resin networks of PCN-TPC and PCN-TPPC were synthesized by a same method from the triptycene-based cyanate precursors TPC and TPPC, respectively. Here take PCN-TPC as an example to describe the procedures in details. To a mixture of 1.3 g of TPC and 15 g of diphenyl sulfone was added 0.013 g of nonylphenol. The resulting mixture was heated under a nitrogen atmosphere at 180 °C for 2 h, 190 °C for 6 h, 230 °C for 8 h, 250 °C for 12 h, 280 °C for 12 h, and 315 °C for 2 h. The obtained yellow solids were washed with CH2Cl2, THF, and methanol for three times and then were extracted with THF in a Soxhlet apparatus for 48 h. After drying at 120 °C for 24 h, the pale yellow solid was obtained with a yield of 85%. Elemental Anal. Calcd for PCN-TPC C23H11N3O12: C 73.21, H 2.94, N 11.13, O 12.72%; found: C 66.34, H 3.14, N 10.61, O 19.91%. Calcd for PCNTPPC C41H23N3O16: C 81.31, H 3.84, N 6.94, O 7.92%; found: C 73.82, H 3.56, N 7.50, O 15.12%. Instrumentation. Fourier transform infrared spectra (FTIR) were recorded using a Nicolet 20XB FT-IR spectrophotometer in the 400− 4000 cm−1 region. Samples were prepared by dispersing the complexes in KBr and compressing the mixtures to form disks. 1H NMR spectra were measured on a 400 MHz Varian INOVA NMR spectrometer, using TMS as an internal reference. Solid-state 13C CP/MAS (crosspolarization with magic angle spinning) spectra were recorded on a Varian Infinity-Plus 400 spectrometer at 100.61 MHz at an MAS rate of 10.0 kHz using zirconia rotors 4 mm in diameter using a contact time of 4.0 ms and a relaxation delay of 2.0 s. The 1H π/2 pulse was 3.75 μs, and two-pulse phase modulation (TPPM) decoupling was used during the data acquisition. Elemental analysis was determined with an Elementar Vario EL III elemental analyzer. Thermogravimetric measurements were performed in the nitrogen atmosphere on a NETZSCH TG 209 thermal analyzer at a heating rate of 10 °C min−1. The temperature range was from 40 to 800 °C. In order to eliminate the effect of possible adsorption of moisture from the atmosphere, all the samples were dried at 120 °C under vacuum for a day before the thermal measurements. Sorption measurements for all the gases and vapors were conducted on an Autosorb iQ (Quantachrome) analyzer.
much lower temperature at ambient pressure, and the tedious evacuating and tube-sealing operations are unnecessary. Triptycene with a three-bladed and rigid geometry has been recognized as an ideal building block for the synthesis of microporous polymers as the high-energy barrier of the [2,2,2] bridge-head structure prevents the 120° angle from twisting and thus effectively traps the internal free volume between three benzene arms and generate intrinsic micropores. In the recent years, various functionalized triptycenes have been successfully synthesized and utilized to prepared MOPs such as intrinsic microporous polymers (Trip-PIMs),33 hyper-crosslinked microporous polymers (TMPT and TMPS),34,35 star triptycene-based porous polymers (STPs),36 microporous polyimides (STPIs),37 and microporous benzimidazole-linked polymers (BILPs and TBIs).38,39 Nevertheless, the triptycenebased microporous cyanate resins have not been reported up to now. Herein, triptycene-based cyanate monomer 2,6,14-tricyanatotriptycene (TPC) and its derivative 2,6,14-tris(4cyanatophenyl)triptycene (TPPC) with an extended benzene ring per arm were designed and synthesized. Subsequently, two hyper-cross-lined cyanate resins (PCN-TPC and PCN-TPPC) were prepared through thermal cyclotrimerization reaction. Thus obtained network simultaneously contains triptycene and triazine cross-linking nodes. The high-density of rigid net nodes are expected to efficiently prop up the polymer segments to create micropores in the polymers. More importantly, the contents of heteroatoms and benzene rings in the two networks are much different, which provide us an opportunity to thoroughly understand the effect of chemical composition and porous structure on adsorption/separation of gases as well as aromatic and aliphatic hydrocarbons.
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EXPERIMENTAL SECTION
Materials. Cyanogen bromide, boron tribromide (BBr3), 4methoxyphenylboromic acid, and tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) were purchased from J&K Chemical Co., Ltd. Potassium carbonate (K2CO3), nonylphenol, and triethylamine were purchased from Shanghai Chemical Reagent Co. Dichloromethane and acetone were dehydrated with 4 Å molecular sieves for 3 days and purified by refluxing over phosphorus pentoxide. Triethylamine was purified by refluxing over calcium hydride and distilled prior to use. 2,6,14-Trihydroxytriptycene and 2,6,14-tribromotriptycene were synthessized according to ref 40. Other reagents are used as received. Synthesis of 2,6,14-Tricyanatotriptycene (TPC). A solution of 3.9 g of cyanogen bromide (37.0 mmol) in 50 mL of anhydrous acetone was stirred at −30 °C for 30 min; then a solution of 3.2 g of 2,6,14-trihydroxytriptycene (10.5 mmol) in 30 mL of acetone and a solution of 4.4 mL of triethylamine (31.5 mmol) in 30 mL of acetone were simultaneously added dropwise into the system over 1 h. After all additions were completed, the mixture was stirred rapidly at −10 °C for 1 h and at 10 °C for another hour. To obtain the crystal products, the resulting mixture was evaporated and washed with water three times to remove residue organic salt and then purified through silica gel column chromatography (CH2Cl2). A white needle crystal was obtained in a yield of 90%; mp 107−108 °C. 1H NMR (400 MHz, dacetone): δ (ppm) 7.730−7.672 (t, 2H, J = 7.70 Hz), 7.641−7.585 (m, 4H, J = 7.62 Hz), 7.130−7.091 (m, 3H, J = 7.12 Hz), 6.136−6.057 (d, 2H, J = 6.09 Hz). 13C NMR (100 MHz, d-acetone): δ (ppm) 151.6, 148.9, 148.5, 144.3, 143.8, 135.7, 113.1, 112.6, 109.4, 53.0, 52.6. FTIR (KBr, cm−1): 3067, 2973, 2259, 1599, 1467, 1267, 1196, 1141, 937. MS calculated for C23H11N3O3, 377.0800; found, 377.0809. Synthesis of 2,6,14-Tris(4-methoxyphenyl)triptycene (TPPM). To a two-phase mixture of 20 mL of dioxane and 1.87 g of K2CO3 aqueous solution (2 M, 13.5 mmol), 0.68 g of 4methoxyphenylboromic acid (4.5 mmol) and 0.48 g of 2,6,14B
DOI: 10.1021/acsami.7b15050 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces Scheme 1. Synthesis Routes to Cyanate Monomers 2,6,14-Tricyanatotriptycene (TPC) and 2,6,14-Tris(4cyanatophenyl)triptycene (TPPC)
Prior to measurements, the samples were degassed at 120 °C under vacuum for 24 h. Adsorption−desorption isotherms of nitrogen were measured at 77 K. The surface areas were calculated according to the Brunauer−Emmett−Teller (BET) model in the relative pressure (P/ P0) range from 0.1 to 0.2 for PCN-TPC and from 0.05 to 0.15 for PCN-TPPC, respectively. Pore size and distributions were derived from the N2 adsorption isotherms using the nonlocal density functional theory (NLDFT). Microporous volume was calculated using the t-plot method based on the Halsey thickness equation, while the total porous volume was obtained from the N2 isotherm at P/P0 = 0.9. CO2 sorption measurements were conducted at the temperature of 273 and 298 K and the pressure up to 1.0 bar. N2 sorption isotherms at 273 K are measured, and the selectivity of CO2/N2 was calculated from the ratios of initial slopes and ideal adsorbed solution theory (IAST).
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RESULTS AND DISCUSSION Synthesis of Triptycene-Based Cyanate Monomers and Hyper-Cross-Linked Cyanate Resin Networks. The synthesis routes to the triptycene-based cyanate monomers 2,6,14-tricyanatotriptycene (TPC) and 2,6,14-tris(4-cyanatophenyl)triptycene (TPPC) are depicted in Scheme 1. Using 2,6,14-trihydroxytriptycene (TPH) as staring material, TPC was readily obtained by the reaction between TPH and cyanogen bromide with a high yield of 90%. The synthesis of 2,6,14-tris(4-cyanatophenyl)triptycene (TPPC) was first carried out by reacting 2,6,14-tribromotriptycene with 4-methoxyphenylboromic to give 2,6,14-tris(4-methoxyphenyl)triptycene (TPPM) through Suzuki coupling in the presence of tetrakis(triphenylphosphine) palladium and K2CO3. After demethylation with BBr3, the resultant 2,6,14-tris(4-hydroxyphenyl)triptycene (TPPH) was reacted with cyanogen bromide to produce 2,6,14-tris(4-cyanatophenyl)triptycene (TPPC). The chemical structures of the triptycene-based cyanate monomers were characterized by FTIR, 1H NMR, 13C NMR, and mass spectroscopy. As shown in Figure 1, the band at 2253
Figure 1. FTIR spectra for cyanate monomers and cyanate resin networks.
cm−1 for TPC and the bands at 2234 and 2269 cm−1 for TPPC are ascribed to the characteristic OCN groups,31 confirming the successful conversion of hydroxyls to cyanate ester groups. The absorptions at 2950 cm−1 is due to the presence of C−H in triptycene, whereas the bands at 3070 and 1598 cm−1 belong to C−H stretching and C−C stretching of the benzene rings, respectively. Figures S1−S4 illustrate the 1H NMR and 13C NMR spectra of TPC and TPPC. All the signals are well assigned to the protons and carbons in triptycene and benzene rings. In addition, their masses obtained from the molecular ion peaks in mass spectra are consistent with the calculated values according to the molecular formulas. C
DOI: 10.1021/acsami.7b15050 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
endotherm peaks at 107 and 118 °C and curing exotherm peaks at 250 and 301 °C are observed for TPC and TPPC monomers, respectively. As expected, the temperature corresponding to the onset of curing exotherms (Ti) for TPPC (Ti = 213 °C) is higher than that of TPC (Ti = 177 °C) owing to the larger molecular volume and steric hindrance of TPPC monomer. According to the DSC data of the curing exothermic peaks in Table S1, using nonylphenol as a catalyst, the thermal polymerization of TPC and TPPC monomers was carried out in the nitrogen atmosphere to produce two cyanate resins (PCN-TPC and PCN-TPPC), respectively, as illustrated in Scheme 2. In order to avoid rapid cross-linking and obtain the homogeneous networks, the polymerization temperatures of TPC and TPPC were initially set at around 180 and 215 °C for 2 h, respectively. Then the temperatures were raised stepwise to the final curing exothermic temperatures of 315 °C for TPC and 355 °C for TPPC and allowed to polymerize at this high temperature for 2 h to ensure a sufficient cross-linking. The chemical structures of the two cyanate resin networks were confirmed by FTIR, solid-state 13C CP/MAS NMR spectroscopy, and elemental analysis. As shown in Figure 1, the disappearance of the cyanate absorption at 2234−2269 cm−1 suggests that the conversions of cyanate groups to the triazine rings are complete. The characteristic absorption of triazine can be found at 1563 and 1365 cm−1.22−26 The solid-state 13C CP/ MAS NMR spectra (Figure 3) further prove the formation of cyanate resin networks since the peak at 173 ppm corresponds to the carbons of triazine rings.22−25 The resonance at around 153 ppm is ascribed to the O-substituted phenyl carbons in the networks. The peak at 53 ppm belongs to the bridgehead aliphatic carbons derived from the triptycene-based monomers, while the other signals at 110−149 ppm are attributed to the phenyl carbons. The calculated contents of nitrogen and hydrogen are roughly consistent with values measured by elemental analysis, but the three are a big deviation for carbon and oxygen elements (Table S2), probably caused by the adsorbed moisture and CO2 upon exposing to air in the course of measurement operation, which have been often observed in the previous reports.41,42 The resultant solid products are insoluble in common organic solvents, such as tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF), suggesting the hyper-cross-linked network structures. To investigate the thermal stability of PCNs, thermal gravimetric analyses (TGA) were conducted in the nitrogen atmosphere in the range from 40 to 800 °C (Figure S5). PCN-TPPC shows the initial decomposition temperature of over 410 °C. Compared to PCN-TPPC, the higher content of aliphatic triptycene in the network leads to the decreased decomposition temperature to some extent. The weight losses for both samples at the temperature below 130 °C are probably caused by the remained solvent or absorbed moisture from air. Porous Structures of Cyanate Resin Networks. The sorption isotherms of PCNs of nitrogen were measured at 77 K (Figure 4a), from which the porosity parameters were calculated and are listed in Table 1. For each sample, the nitrogen uptake exhibits a steep rise at the very low relative pressure (P/P0 < 0.01), suggesting the existence of substantial micropores in the network.43 Besides, in the desorption curve an obvious step at the relative pressure of around 0.45 was observed, indicating that both PCN-TPC and PCN-TPPC possess some mesopores. The deductions above are further confirmed by the analyses of pore size distributions through
Figure 2. DSC curves for cyanate monomers of (a) TPC and (b) TPPC.
Scheme 2. Synthesis Routes to Microporous Cyanate Resin Networks
Figure 3. Solid-state 13C CP/MAS NMR spectra for (a) PCN-TPC and (b) PCN-TPPC. Asterisks (∗) indicate peaks arising from spinning side bands.
Figure 2 shows dynamic scanning thermograms (DSC) recorded at a heating rate of 10 °C/min for the two cyanate monomers in the presence of nonylphenol. The melting D
DOI: 10.1021/acsami.7b15050 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. (a) Adsorption (filled) and desorption (empty) isotherms of nitrogen at 77 K for PCN-TPC (+200) and PCN-TPPC. (b) Pore size distribution curves obtained by the NLDFT method for PCN-TPC (+0.5) and PCN-TPPC. (c) Computer simulation of porous structure by Material Studio software for PCN-TPC (top) and PCN-TPPC (bottom).
Table 1. Porosity Parameters for Cyanate Resins Obtained by N2 Adsorption sample
SBETa (m2 g−1)
VTotalb (cm3 g−1)
VMicroc (cm3 g−1)
PCN-TPC
686
0.55
0.12
PCN-TPPC
662
0.48
0.17
Table 2. Uptakes of Benzene and Cyclohexane at 298 K for Porous Cyanate Resin Networks
pore sized (nm)
sample
benzenea (wt %)
cyclohexanea (wt %)
Abenzene/Acyclohexaneb
0.55, 0.74, 1.60, 3.26 0.50, 0.65, 1.54, 3.02
PCN-TPC PCN-TPPC
17.6 77.8
7.4 10.7
2.4 7.2
a
a
Measured at 298 K and P/P0 = 0.9. bThe ratio of adsorption capacity for benzene to cyclohexane at 298 K.
NLDFT method (Figure 4b). For PCN-TPC, there are not only microporores centering at 0.74/1.60 nm and mesopore at 3.26 nm, but also an ultramicroporous peak22,44 at 0.55 nm. The presence of ultramicropores is especially favorable for the adsorption of small gas molecules like CO2. It is interesting to observe that each arm in triptycene monomer of PCN-TPPC has one more benzene ring than that
of PCN-TPC, but the two samples have the similar porosity parameters including BET surface areas, pore sizes, and distributions as reflected in the simulated porous structure and packing of polymer segments of the two porous cyanate resins using Materials Studio software (Figure 4c), assuming that the cross-linking reactions are complete and all the reactive groups are interlinked according to the geometrical structure of triptycene-based monomers. In fact, since the thermal polymerizations of cyanate monomers are kinetically controlled, the interpenetration of networks is inevitable so that the spaces propped up by the rigid net nodes are divided into micrpores
Brunauer−Emmett−Teller surface area. bTotal pore volume determined from the N2 adsorption isotherms at P/P0 = 0.9. c Micropore volume derived using the t-plot method based on the Halsey thickness equation. dPore size derived from N2 adsorption isotherms using the NLDFT method.
Figure 5. Adsorption isotherms of benzene (■) and cyclohexane (●) at 298 K for PCN-TPC and PCN-TPPC. E
DOI: 10.1021/acsami.7b15050 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 6. (a) Adsorption (filled) and desorption (empty) isotherms of CO2 and (b) variation of CO2 isosteric enthalpies with the adsorbed amount for PCN-TPC and PCN-TPPC.
opportunity to investigate in detail the effect of porous structure and chemical composition on adsorption of CO2 gas and organic vapors. Adsorption/Separation of Benzene and Cyclohexane in Microporous Cyanate Resins. The sorption isotherms of benzene and cyclohexane in the two polymers were measured at room temperature (Figure 5), and the results are presented in Table 2. At P/P0 = 0.9 and 298 K, PCN-TPPC can uptake 77.8 wt % benzene, which exceeds carbon material F42C (39.5 wt %),45 metal azolate framework MAF-2 (20.6 wt %),46 and many other microporous organic polymers such as cyanate resins (CEs, 34.4−58.5 wt %),23 polybenzimidazole networks (PBIs, 23.1−54.4 wt %),21 and porous aromatic framework (PAF-2, 13.8 wt %)19 and is comparable to PAF-11 (87.4 wt %),47 microporous polyimides (NPIs, 41.5−90.5 wt %),48 poly(Schiff-base) networks (PSNs, 46.8−86.1 wt %),49 and polyaminals (PANs, 73.6 wt %).50 In addition, for both samples, the uptakes of are significantly higher than cyclohexane. Particularly for PCN-TPPC, its adsorption capacity (77.8 wt %) is 7 times as high as that of cyclohexane (10.7 wt %). The outstanding adsorption selectivity of benzene over cyclohexane endows PCN-TPPC with promising potential in the benzene/cyclohexane separation application. The above results show that compared to PCN-TPC, PCNTPPC exhibits apparent advantages in both adsorption capacity and selectivity for benzene and cyclohexane vapors. The reason should be attributed only to their difference in chemical structure as their specific surface areas and porosity parameters are very similar. As seen in Scheme 2, each cyanate monomer for PCN-TPPC has six benzene rings, being twice that for PCN-TPC. The abundant phenyl groups in the PCN-TPPC network provide more affinity sites for aromatic benzene molecule by virtue of the strong π−π interaction. CO2 Adsorptions in Porous Cyanate Resin Networks. The sorption isotherms of CO2 gas in PCN networks were measured at 273 and 298 K in the pressure range from 0 to 1.0 bar (Figure 6a). The adsorption and desorption in PCNs are nearly reversible, indicative of the physicosorption in nature. At 273 K and 1 bar, the adsorption capacity of CO2 in PCN-TPC reaches 16.4 wt % (Table 3). Under the same measurement condition, this value is the highest among the microporous
Table 3. CO2 Adsorption and CO2 Selectivities of Porous Cyanate Resin Networks CO2 uptake (wt %)a
CO2/N2 selectivityc
sample
273 K
298 K
Qstb (kJ mol−1)
initial slope
IAST
PCN-TPC PCN-TPPC
16.4 14.0
8.8 6.8
34.6 31.0
60 39
45 32
a
CO2 uptake at 1.0 bar. bIsosteric enthalpy of CO2 adsorption. Calculated selectivities from the initial slope and IAST methods for CO2/N2 at 273 K. c
Figure 7. Five cycles of CO2 adsorption at 298 K for PCN-TPC.
and even ultramicropores (
