Promoting and Tuning Porosity of Flexible Ether ... - ACS Publications

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Promoting and Tuning Porosity of Flexible Ether-Linked Phthalazinone-Based Covalent Triazine Frameworks Utilizing Substitution Effect for Effective CO2 Capture Kuanyu Yuan,†,‡,§ Cheng Liu,*,†,‡,§ Lishuai Zong,†,‡,§ Guipeng Yu,∥ Shengli Cheng,†,‡,§ Jinyan Wang,†,‡,§ Zhihuan Weng,†,‡,§ and Xigao Jian*,†,‡,§ †

State Key Laboratory of Fine Chemicals and ‡Department of Polymer Science and Materials, Dalian University of Technology, Dalian 116024, China § Liaoning Province Engineering Research Centre of High Performance Resins, Dalian 116024, China ∥ College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China S Supporting Information *

ABSTRACT: Five porous ether-linked phthalazinone-based covalent triazine frameworks (PHCTFs) were successfully constructed via ionothermal polymerizations from flexible dicyano monomers containing asymmetric, twisted, and Nheterocyclic phthalazinone structure. All the building blocks could be easily prepared by simple and low-cost aromatic nucleophilic substitution reactions, showing the large-scale application potential of thermal stable phthalazinone structure in constructing porous materials. Generally, the flexible building blocks are avoided to prevent the networks from collapsing in constructing high surface area porous materials. Our experimental results revealed that the introduction of the substituents can effectively decrease the probability of the network interpenetration from the longer struts and the intermolecular/intramolecular intercalation from the increased degree of conformation freedom in the flexible ether-linkage, the BET surface areas of PHCTFs increasing from 676 to 1270 m2 g−1. Meanwhile, the effects of introducing different sizes (methyl or phenyl group) and amounts (one or two) of substituents on the porosities of the target polymer networks were also investigated in detail. The high CO2 adsorption capacity of 10.3 wt % (273 K, 1 bar) can be ascribed to the strong affinity of the electron-rich N,O-containing networks with CO2. Excitingly, PHCTF-5 demonstrates the high CO2/N2 selectivity up to 138 (273 K, 1 bar), according to the ideal adsorbed solution theory (IAST) for the higher proportion of Vmicro accompanied the electron-rich heteroatoms characteristic. Such high CO2 adsorption capacity and good separation properties are superior to those of many other microporous organic polymers. These properties along with easily up-scalable synthesis make porous PHCTFs promising candidates applied in gas sorption and separation field. KEYWORDS: phthalazinone, covalent triazine framework, flexible and facile, substituents, gas adsorption and separation

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INTRODUCTION

poor physicochemical stability, MOPs are constructed from linking well ever-changing organic building blocks by pure covalent bonds with good mechanical, thermal, and physicochemical stability, through homo- or heteropolymerization. These merits such as high specific surface area, low skeletal density, and permanent porosity and flexibility for rational design endow MOPs with various potential applications in gas storage and separations,7 sensors,8 catalysis,9 drug delivery,10 photoelectricity,11 and energy fields.12 So far, a various kinds of MOP materials have been successfully developed, such as polymers of intrinsic microporosity (PIMs),13 conjugated

The imperatives of global warming and ocean acidification, due to the escalating concentration of greenhouse gas CO2 for the extensive consumption of fossil fuels, have attracted great attention and widespread public concerns over the last decades.1 Therefore, it seems that appropriate CO2 capture and storage (CCS) technologies with low-cost, efficient, and durable performance are of great importance in both environmental and economic aspects. Recently, porous solid-state adsorbents acting through the van der Waals interaction are becoming much more promising candidates owing to their low energy requirements without causing environment hazards,2 such as activated carbons,3 zeolites,4 metal−organic frameworks (MOFs),5 and microporous organic polymers (MOPs).6 Different from MOFs with © XXXX American Chemical Society

Received: February 7, 2017 Accepted: April 4, 2017 Published: April 4, 2017 A

DOI: 10.1021/acsami.7b01783 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Design of the phthalazinone-based building blocks.

Scheme 1. Preparation Routes of the Phthalazinone-Based Building Blocks and PHCTFs

building blocks. Therefore, the monomer structures finely controlled by the rigid node−strut topology strategy at the molecular level will be more effective in constructing MOPs.31,32 With respect to amorphous MOPs, it is reported that the increase of the length of linkages may result in an apparent drop for the surface areas of MOPs owing to the network interpenetration.33,34 However, Zhang et al. successfully introduced an efficient strategy to reduce the probability of network interpenetration by using the bulky dendritic building blocks.35 Meanwhile, Jiang et al. found that more inter- and intramolecular intercalation appeared due to the incremental extent of conformation freedom in the longer struts, resulting in more efficient space filling in the frameworks and the reduced micropore volume for networks with longer struts.36 In our previous work, we have successfully constructed a series of phthalazinone-based covalent triazine frameworks (PHCTFs) with relatively high specific surface areas from full rigid, twisted, asymmetric, and N,O-heterocycle-containing phthalazinone-based building blocks.37 Meanwhile, the micropore size distribution of the afforded PHCTF-2’s seemed to be shifted systematically to smaller pore diameters by changing the conformation of phthalazinone-based building blocks. Although the PHCTFs in the above-mentioned works possessed satisfactory CO2 adsorption properties, such full rigid phthalazinone containing building blocks with different conformations still have two shortages including multistep synthesis and toxic cyanide reagents, limiting their applications in large-scale construction of MOPs. Generally, the interconnected network of channels should be formed to manufacture a permanent stable pore structure; thus, it is essential to use rigid building blocks in order to keep the

microporous polymers (CMPs),14 covalent organic frameworks (COFs),15 hyper-cross-linked polymers (HCPs),16 porous aromatic frameworks (PAFs),17 and covalent triazine-based frameworks (CTFs).18,19 Compared with noble-metal-catalyzed coupling polymerizations, CTFs first developed by Thomas et al.18,19 were constructed by the trimerization reaction of carbonitriles from cheap and readily available starting materials, using ZnCl2 as the catalyst and solvent under ionothermal conditions. For CTFs, the nitrogen-rich triazine moiety not only constructs their permanent porosity but also endows such MOPs with strong affinity between the frameworks and gas molecule. More and more researches revealed that the incorporation of electron-rich (such as N or O)20 scaffolds significantly increases the CO2 adsorption capacities of MOPs due to electrostatic interactions between the electron-rich atoms and the carbon atoms of the CO2 molecule, resulting from dipole−induced dipole and dipole−quadrupole interactions.21,22 CTFs synthesized by Cooper et al. showed CO2 capacity of 136.4 mg g−1 with a CO2/N2 selectivity of 16.6 at 273 K, 1 bar.23 POP-3 reported by Pan et al. displayed the CO2 capacity of 110 mg g−1 with a CO2/N2 selectivity of 27.1 at 273 K, 1 bar.24 Similarly, nitrogen- or oxygen-rich microporous polybenzimidazoles,25,26 microporous polyamine,27 microporous cyanate resins,28 microporous phloroglucinol polymers,29,30 and others showed satisfactory CO2 capture performance. Apart from the physicochemical nature of the pore wall, the porosity parameters (such as pore volume, pore size, and surface area) of MOPs are also of vital important factors for the gas adsorption capacity and separation of MOPs.28 Moreover, the porosity parameters crucially rely on the structures of B


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NMR (100 MHz, DMSO-d6, δ, ppm) 161.0, 158.5,156.0, 147.4, 145.7, 135.2, 134.7, 133.2, 132.9, 132.1, 131.4, 128.4, 127.6, 126.9, 120.5, 119.0, 118.9, 110.2, 106.1; FT-IR (KBr, cm−1) 2229 (−CN), 1663 (CO), 1596, 1499 (Ar), 1249 (Ar−O−Ar); EI-MS m/z = 441.1 ([M + H]+, 100%); Anal. Calcd for C28H16N4O2 (%): C 76.35; H 3.66; N 12.72. Found: C 75.87; H 3.57; N 12.71. Synthesis of 2-(4-Cyanophenyl)-4-[3-methyl-4-(4cyanophenoxyl)phenyl]-phthalazinone (MDHPZ-DN). MDHPZ-DN was synthesized by a method similar to that for DHPZ-DN except using MDHPZ instead of DHPZ. Yield: 87%; mp: 187−188 °C. 1H NMR (400 MHz, CDCl3, δ, ppm) 8.65−8.59 (m, 1H), 8.04−7.99 (d, 2H), 7.92−7.81 (m, 3H), 7.80−7.75 (d, 2H), 7.67−7.62 (d, 2H), 7.60−7.57 (s, 1H), 7.54−7.49 (d, 1H), 7.16−7.12 (d, 1H), 7.06−7.01 (d, 2H), 2.30 (s, 3H); 13C NMR (100 MHz, DMSO-d6, δ, ppm) 161.3, 158.5,153.6, 147.5, 145.8, 135.2, 133.4, 133.2, 130.8, 129.6, 127.6, 126.9, 121.4, 119.2, 119.0, 117.8, 110.3, 105.5, 16.2; FT-IR (KBr, cm−1) 2926 (−CH3), 2224 (−CN), 1669 (CO), 1600, 1497 (Ar), 1250 (Ar−O−Ar); EI-MS m/z = 455.2 ([M + H]+, 100%); Anal. Calcd for C29H18N4O2 (%): C 76.64; H 3.99; N 12.33. Found: C 76.27; H 3.43; N 12.38. Synthesis of 2-(4-Cyanophenyl)-4-[3,5-dimethyl-4-(4cyanophenoxyl)phenyl]-phthalazinone (DMDHPZ-DN). DMDHPZDN was synthesized by a method similar to that for DHPZ-DN except using DMDHPZ instead of DHPZ. Yield: 82%; mp: 213-214 °C. 1H NMR (400 MHz, CDCl3, δ, ppm) 8.65−8.59 (m, 1H), 8.04−7.99 (d, 2H), 7.92−7.81 (m, 3H), 7.81−7.76 (d, 2H), 7.65−7.59 (d, 2H), 7.42−7.39 (s, 2H), 6.96−6.91 (d, 2H), 2.21 (s, 6H); 13C NMR (100 MHz, CDCl3, δ, ppm) 160.7, 158.8, 151.2, 148.1, 145.4, 134.4, 132.7, 132.3, 132.2, 131.8, 130.4, 128.9, 126.1, 118.9, 118.5, 115.6, 110.9, 105.4, 16.5; FT-IR (KBr, cm−1) 2922 (−CH3), 2226 (−CN), 1669 (CO), 1599, 1502 (Ar), 1238 (Ar−O−Ar); EI-MS m/z = 469.2 ([M + H]+, 100%); Anal. Calcd for C30H20N4O2 (%): C 76.91; H 4.30; N 11.96. Found: C 76.67; H 4.08; N 12.14. Synthesis of 2-(4-Cyanophenyl)-4-[3-phenyl-4-(4cyanophenoxyl)phenyl]-phthalazinone (PDHPZ-DN). PDHPZ-DN was synthesized by a method similar to that for DHPZ-DN except using PDHPZ instead of DHPZ. Yield: 85%; mp: 215−216 °C. 1H NMR (400 MHz, CDCl3, δ, ppm) 8.67−8.61 (m, 1H), 8.04−7.98 (d, 2H), 7.93−7.86 (m, 3H), 7.82−7.76 (d, 3H), 7.69−7.64 (m, 1H), 7.58−7.53 (d, 2H), 7.52−7.47 (d, 2H), 7.40−7.28 (m, 4H), 7.02−6.96 (d, 2H); 13C NMR (100 MHz, DMSO-d6, δ, ppm) 161.3, 158.5, 152.3, 147.3, 145.7, 136.5, 135.0, 134.4, 133.2, 132.9, 132.5, 129.4, 128.9, 128.5, 126.9, 122.2, 110.3, 105.6; FT-IR (KBr, cm−1) 2230 (−CN), 1674 (CO), 1599, 1495 (Ar), 1259 (Ar−O−Ar); EI-MS m/z = 517.3 ([M + H]+, 100%); Anal. Calcd for C34H20N4O2 (%): C 79.06; H 3.90; N 10.85. Found: C 79.23; H 3.76; N 10.88. Synthesis of 2-(4-Cyanophenyl)-4-[3,5-diphenyl-4-(4cyanophenoxyl)phenyl]-phthalazinone (DPDHPZ-DN). DPDHPZDN was synthesized by a method similar to that for DHPZ-DN except using DPDHPZ instead of DHPZ. Yield: 75%; mp: 253−254 °C. 1H NMR (400 MHz, DMSO-d6, δ, ppm) 8.52−8.47 (m, 1H), 8.10−7.94 (m, 7H), 7.88−7.80 (s, 2H), 7.59−7.48 (m, 6H), 7.38− 7.22 (m, 6H), 7.82−7.75 (d, 2H); 13C NMR (100 MHz, DMSO-d6, δ, ppm) 160.8, 148.3, 147.2, 145.8, 136.7, 136.3, 134.4, 133.3, 129.5, 128.8, 128.3, 127.0, 119.0, 118.9, 116.8, 110.4, 104.4; FT-IR (KBr, cm−1) 2226 (−CN), 1677 (CO), 1601, 1501 (Ar), 1238 (Ar−O− Ar); EI-MS m/z = 593.2 ([M + H]+, 100%); Anal. Calcd for C40H24N4O2 (%): C 81.07; H 4.08; N 9.45. Found: C 81.18; H 4.08; N 9.45. General Synthesis Procedure for PHCTFs. PHCTF-3. DHPZDN (528.6 mg, 1.2 mmol) and ZnCl2 (1.64 g, 12 mmol) were mixed well and charged into a quartz tube (3 × 8 cm) under inert atmosphere. After being evacuated and sealed carefully, the tube was heated at 250 °C for 10 h, 300 °C for 10 h, 350 °C for 10 h, and 400 °C for 20 h in a muffle furnace, followed by cooling to room temperature. The obtained black product was ground into powder and then washed with deionized water several times. Furthermore, the residual metal salt was eliminated through stirring the powder in 100 mL of HCl solution (2 M) for 24 h. Subsequently, the material was washed with deionized water and alcohol, extracted by Soxhlet using

networks from collapsing.38 However, recently, Ding et al.39 and Yang et al.40 reported that the flexible building blocks could also be used to constructed MOPs with good CO2 adsorption and separation properties. In this paper, we designed and synthesized five phthalazinone moiety building blocks (Figure 1) with different substituent groups (methyl or phenyl) and flexible ether linkage by simple and low-cost aromatic nucleophilic substitution reaction. Then, a series of high surface area PHCTFs containing different sizes and amounts of appended groups were successfully constructed by an ionothermal polymerization of aromatic nitriles using ZnCl2 as Lewis acid and porogenic agent41 (Scheme 1). Our researches also indicate that the rigid structure building blocks may not be required as reported. At the same time, this work exhibits another two considerations. On the one hand, the simple and low-cost synthesis method shows the potential applications of phthalazinone structure in large-scale constructing MOPs, and the different substituent groups further expand the designability of the phthalazinone moiety. On the other hand, we considered that the introduction of the bulky appendant groups could effectively decrease the probability of the network interpenetration owing to the longer struts and the inter-/intramolecular intercalation from the enhanced degree of conformation freedom due to the presence of the flexible linkages and the longer struts.

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EXPERIMENTAL SECTION

Materials. 4-(4-Hydroxyphenyl)phthalazin-1(2H)-one (DHPZ), 4(3-phenyl-4-hydroxyphenyl)-phthalazin-1(2H)-one (PDHPZ), 4-(4hydroxy-3-methylphenyl)phthalazin-1(2H)-one (MDHPZ), 4-(3,5diphenyl-4-hydroxyphenyl)-phthalazin-1(2H)-one (DPDHPZ), and 4-(4-hydroxy-3,5-dimethylphenyl)phthalazin-1(2H)-one (DMDHPZ) were kindly offered by Dalian Polymer New Materials Co., Ltd. Anhydrous zinc chloride (ZnCl2) was obtained from J&K Scientific Ltd. (PR China), purified and distilled over thionyl chloride to remove water, and then dried at 180 °C for 24 h in a vacuum oven. All other chemicals and regents were obtained from commercial suppliers and used without any further treatment. General Methods. Fourier transform infrared (FT-IR) spectra were collected on a Nicolet 20 DXB FT-IR spectrophotometer in the 400−4000 cm−1 region by mixing the samples with KBr and compressing them into disks. 1H NMR and 13C NMR spectra were recorded on a Bruker spectrometer using tetramethylsilane (TMS) as an internal standard at 400 and 100 MHz, respectively. Elemental analyses were performed with a Vario EL III CHNOS Elementalysator from Elementaranalysesyteme GmbH. Field-emission scanning electron microscopy (FE-SEM) images were recorded on a Nova NanoSEM 450 after the samples coated with gold. The powder Xray diffraction (XRD) patterns of the samples were obtained on a SmartLab (9) diffractometer using Cu Kα radiation (45 kV, 200 mA) with the 2θ ranged from 5 to 80° and a scan speed of 8° min−1. Transmission electron microscopy (TEM) images were taken on a Tecnai12 transmission electron microscopy. Thermal gravimetric analyses (TGA) were carried out on a Mettler TGA/SDTA851 thermogravimetric analysis instrument (nitrogen atmosphere) at a heating rate of 20 °C min−1 from 30 to 800 °C. The Autosorb iQ (Quantachrome) analyzer was used to evaluate the adsorption properties of N2, CH4, and CO2 with the samples degassed at 150 °C for 12 h before testing under high vacuum. The pore size distributions and specific surface areas were calculated according to the nonlocal density functional theory (NLDFT) and Brunauer−Emmet− Teller (BET) model (P/P0 ranging from 0.01 to 0.1). Synthesis of 2-(4-Cyanophenyl)-4-[4-(4-cyanophenoxyl)phenyl]phthalazinone (DHPZ-DN). DHPZ-DN was synthesized in a similar method as described in the reported procedure.42 Yield: 93%; mp: 194−195 °C. 1H NMR (400 MHz, DMSO-d6, δ, ppm) 8.50−8.44 (m, 1H), 8.06−7.95 (m, 6H), 7.93−7.87 (d, 2H), 7.85−7.75 (m, 3H); 13C C


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Figure 2. (a) FT-IR spectra of PHCTFs; (b) PXRD patterns of PHCTFs; (c) FE-SEM image of PHCTF-3; (d) TEM image of PHCTF-3. methanol and acetone, respectively, and finally dried at 150 °C in vacuum oven. Yield: 88%. PHCTF-4. The procedure for PHCTF-3 was followed using the precursor compound MDHPZ-DN (545.4 mg, 1.2 mmol) and ZnCl2 (1.64 g, 12 mmol). Yield: 85%. PHCTF-5. The procedure for PHCTF-3 was followed using the precursor compound DMDHPZ-DN (562.2 mg, 1.2 mmol) and ZnCl2 (1.64 g, 12 mmol). Yield: 83%. PHCTF-6. The procedure for PHCTF-3 was followed using the precursor compound PDHPZ-DN (516.6 mg, 1 mmol) and ZnCl2 (1.36 g, 10 mmol). Yield: 86%. PHCTF-7. The procedure for PHCTF-3 was followed using the precursor compound DPDHPZ-DN (592.7 mg, 1 mmol) and ZnCl2 (1.36 g, 10 mmol). Yield: 90%.

clearly according to the integral values. These above-mentioned characteristic results agree with the proposed structures of the designed target building blocks containing phthalazinone moiety. Simultaneously, the TGA measured (Figure S12b) prior to the polymerization showed the relative good thermal stability of the building blocks (only 5% weight loss when heated to 400 °C), ensuring continuous heating at high polymerization temperature, generally. Synthesis and Characterization of the PHCTFs. The novel PHCTFs with different substituent groups (methyl or phenyl) and flexible ether linkage were all fabricated by the procedure of ionothermal polymerization, using molten ZnCl2 as the Lewis acid catalyst and porogenic agent as reported by Thomas et al.18 Nevertheless, the method of gradient increased temperature45 was adopted to minimize the carbonization and decomposition of the monomers, compared with the method that the reactants were heated to the high temperature by one step. As for the molar ratio of the ZnCl2 to the monomer, Wang et al.’s46 recent research exhibited that more ZnCl2 would lead to the decrease in BET surface areas for the dilutive effect of excess amount of solvents, as reported by Bettina et al.47 However, we considered that the lower monomer concentration (higher salt content) might be favorable to the high polymerization degree for such large and complex building blocks in this paper, so we still preferred the ZnCl2 to monomer molar ratio of 10:1 for better porosities of the PHCTF-1a37 at the same ratio in our previous work, consistent with the observations made by Kuhn et al.48 After polymerization, all black polymers were ground into fine powders in agate mortar. Then, these black powders were all immersed in diluted HCl and washed thoroughly by deionized water to remove most of the residual ZnCl2. The inductive coupled plasma emission spectrometer (ICP) measurements of the PHCTFs indicated a negligible residual quantity of ZnCl2 (Table S1). All of the resultant PHCTFs were insoluble in most organic solvents, for instance chlorobenzene, chloroform, tetrahydrofuran (THF), N-methlypyrrolidone (NMP), dimethyl sulfoxide

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RESULTS AND DISCUSSION Synthesis and Characterization of Phthalazinone Moiety Building Blocks. Previous studies43,44 have demonstrated that the bisphenol-like DHPZ and its substituted derivatives all have active lactam N−H and O−H groups, which can react with activated halogenated monomers in the existence of a base via nucleophilic displacement reaction. Thus, the five phthalazinone-based aromatic dicyano building blocks with different substituent groups (methyl or phenyl) were synthesized by the simple one-step aromatic nucleophilic substitution reaction of 4-chlorobenzonitrile and substituted phthalazinone derivatives with good yield, as exhibited in Scheme 1. The chemical structures of phthalazinone-based dicyano monomers were approved by 1H NMR, 13C NMR (Figures S1−S10), FT-IR (Figure S11), and EI-MS. In the FTIR spectra, the adsorption peaks ranged from 3200 to 3500 cm−1 have disappeared, indicative of the complete consumption of N−H and O−H groups. The strong adsorption at near 1669 cm−1 ascribes to the lactam CO of phthalazinone, and the emergence of a strong adsorption peak at about 1250 cm−1 confirms the formation of aromatic ether linkage. Meanwhile, the characteristic absorption peaks of −CN groups can also be observed clearly at near 2232 cm−1. Also, in the 1H NMR spectra shown in Figures S1−S10, all protons could be assigned D


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Figure 3. (a) N2 adsorption (closed)/desorption (open) isotherms at 77 K and (b) pore size distributions for PHCTF-3−PHCTF-7, compared with PHCTF-1a.

Table 1. Specific Surface Areas and Pore Volume of the PHCTF-3−PHCTF-7 and PHCTF-1a samples

SBET (m2 g−1)a

SLan (m2 g−1)b

Vtotal (cm3 g−1)c

Vmicro (cm3 g−1)d

Vmicro/Vtotal

PHCTF-1a PHCTF-3 PHCTF-4 PHCTF-5 PHCTF-6 PHCTF-7

1062 676 1270 1015 1109 835

1499 1040 1917 1504 1707 1209

0.56 0.49 0.79 0.61 0.77 0.70

0.44 0.17 0.46 0.36 0.30 0.20

79% 35% 58% 59% 39% 29%

a Calculated in the pressure range of 0.01−0.1 bar. bCalculated in the pressure range of 0.05−0.3 bar. cTotal pore volume at P/P0 = 0.9. dMicropore volume was calculated by the t-plot method.

diameter under 1 nm for all samples (Figures 2d and S13− S16). Besides, the alternating areas of light and dark contrast also indicate the disordered porous structure characteristics consistent with the above-mentioned PXRD results. Porous Properties of PHCTFs. The porosity properties of obtained phthalazinone-based polymer networks were investigated by N2 adsorption/desorption measured at 77 K (Figure 3a). A steep increase at lower relative pressures, observed from the isotherms (P/P0 < 0.001), indicates the abundant micropore structure16 of PHCTFs. The loosing packing of the smaller particles shown in FE-SEM (Figures 2c and S13− 16) may lead to adsorption on the outer surface of PHCTFs, which results in a continuous adsorption increase (P/P0 = 0.1− 0.9). Meanwhile, the adsorption increasing at the higher relative pressure (P/P0 > 0.9) indicates the existence of the meso- and macrostructures and interparticulate voids53 in these porous materials. According to International Union of Pure and Applied Chemistry (IUPAC) classification,54 PHCTF-3− PHCTF-6 prepared from the flexible building blocks display a type I isotherms with type IV character, and PHCTF-7 is absolutely type IV isotherm, compared with the fully reversible isotherm of PHCTF-1a. This appearance of the hysteresis loop can be attributed to the presence of mesopore structures and the swelling induced by adsorbate molecule in the soft polymer skeleton due to the introduction of the ether linkage, as well as the contribution of the pores blocked by narrow openings restricting the access of adsorbate.55,56 Table 1 lists the specific surface areas (SBET) of PHCTFs calculated in the relative pressure range of 0.01−0.1 bar. SBET of PHCTF-3 from the building block with longer strut and flexible ether linkage displays the smaller value of 676 m2 g−1 compared with PHCTF-1a (1062 m2 g−1) because of the network interpenetration and intermolecular/intramolecular intercalation as reported.33,34 Nevertheless, the enhancement of the BET surface areas of PHCTF-4−PHCTF-7 implies the effectiveness

(DMSO), dimethylformamide (DMF), and dimethylacetamide (DMAc), indicating a good physical-chemical stable nature. The chemical structures of PHCTFs were examined by FT-IR. The strong characteristic −CN adsorption peaks for phthalazinone-based building blocks at about 2232 cm−1 (Figures 2a and S11) are almost totally disappeared after polymerization, indicating the relatively high degree of trimerization. Furthermore, the characteristic peaks of the lactam CO of phthalazinone located around 1620 cm−1. Meanwhile, the characterized adsorption peaks of triazine rings appeared around 1560 and 1417 cm−1, which maybe not very obvious for all of the porous polymer networks due to the partial carbonization and decomposition (or fragmentation).22,49 In addition, the increase of the C/N ratio from the loss of the nitrogen content by the element analysis (Table S1) also reflects the partial graphitization of polymers. Thermal gravimetric analysis (TGA) (Figure S12a) measurements were conducted to evaluate the thermal stability of the resultant PHCTFs. These polymers exhibited high stability above 410 °C (the onset decomposition temperature) and high char yield at 800 °C under nitrogen, due to their rigid skeletons and the nature of their cross-linking structures. Additionally, the slight mass drops before the onset of decomposition are attributed to the trapped solvent and gas within the micropore structure, just like most of the reported MOPs.50 The crystalline nature of the polymeric networks was confirmed by the powder X-ray diffraction (PXRD) (Figure 2b). As expected, there was only one broad diffraction peak at about 25°, indicating the amorphous nature of the polymers, which was in good accordance with the reported results.51 FE-SEM was used to observe the surface morphologies of samples, as shown in Figures 2c and S13−S16. All of the polymers are composed of loose tiny particles with irregular shapes and rough surfaces, similar to other reported researches.28,52 Meanwhile, the porous channels exhibited in the TEM images are uniform with a pore E


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Figure 4. Molecule simulations are carried out using Gaussian 09W software: (a) DHPZ-DN; (b) DMDHPZ-DN; and (c) DPDHPZ-DN.

size substituent groups increases the pore size through restricting the interpenetration of networks28 to some extent for PHCTF-4−PHCTF-7 with two methyl or large phenyl group. As for PHCTF-7, both restriction of the two big phenyl groups and the longer strut from the above-mentioned molecule simulations result in the biggest mesopore size as demonstrated in PSDs. Simultaneously, the decrease of the pore size in the microporous region with the introduction of the phenyl group ascribes to the effective pore division. The pore volumes were also calculated and listed in Table 1. It is reasonable that the total pore volume of PHCTF-3 decreases to 0.49 compared with 0.56 of PHCTF-1a since the form of the stacked structure for its long strut and the flexible ether linkage would reduce the free volumes. Due to the incorporation of the different substituent groups into the building blocks, the total pore volumes are also increased by restricting the interpenetration of networks. As same as the order of the BET surface areas, PHCTF-4 with relative smaller size and less amount of substituent group has the highest total pore volume, although the total pore volumes of the networks containing large phenyl groups are relatively smaller than those of PHCTF-4 for the pore volume occupation effect. It is still noteworthy that the total volumes of PHCTF-6 and PHCTF-7 with relative large phenyl appended group show an increase trend compared with PHCTF-5, indicating the support effect of the phenyl group to the polymer skeleton with larger freedom and flexibility from the longer arms and the ether linkage. These results are opposite to the report that bigger side groups on the pore surface will make the pore volumes of porous polymer networks become smaller.57 As for Vmicro/Vtotal, the results of PHCTF-3−PHCTF-7 show an obvious decrease with the appearance of the more mesopore showed in nitrogen adsorptions compared with PHCTF-1a. For PHCTF-6 and PHCTF-7, though they exhibited the effective pore division of the phenyl group, the mesoporous structure turned to increase from the results of the nitrogen adsorptions, so the results of Vmicro/Vtotal exhibit a great decrease, especially for PHCTF-7. CO2 Capture and Ideal Selectivities of CO2/N2 and CO2/CH4 Gas Pairs. Considering the microporous natures and

of the introduction of the bulky substituent groups in weakening the above-mentioned negative effects, and the changes of the size and amount of the substituent groups demonstrate the different influences the BET surface areas of the PHCTFs. The highest surface area of PHCTF-4, 1270 m2 g−1, seems to indicate that the relatively smaller size and fewer amounts are much more effective. This is due to the fact that substituent groups might still occupy some certain free volume, though they could decrease the network interpenetration. The results of the molecule simulations carried out by Gaussian 09W, as shown in Figure 4, clearly reveal that the torsion angle (θ7, Figure 4c) of ether linkage changes to nearly 180° resulted from the existence of the two larger phenyl groups relative to methyl group (θ5, Figure 4b) or hydrogen (θ3, Figure 4a) (θ3 ≈ θ5), leading to a little increase of the molecular size of DPHPZDN. Thus, the relatively more mesoporous distribution of PHCTF-7 may be responsible of the greater degree of hysteresis upon desorption presenting in the isotherm, and the BET surface area of PHCTF-7 displays obvious decrease due to the pore occupation effect from the two phenyl groups. On the basis of their N2 adsorption isotherms, the pore size distributions (PSDs) of PHCTFs were calculated by the nonlocal density functional theory (NLDFT). Figure 3b exhibits that the dominant pores still locate in the microporous region with a diameter of less than 2 nm and that all the polymer networks have similar pore distributions in this region. However, apart from microporous structure, PHCTF-3− PHCTF-7 also show clear mesporous distribution compared with the detection of only a small amount of mesopores in PHCTF-1a, because of the larger molecule sizes of the building blocks (Figure 4). Such results are coherent with those revealed in the N2 adsorption/desorption isotherms with the obvious hysteresis loops in Figure 3a. At the same time, it is interesting to observe the differences of the mesoporous distribution of the polymer networks for the substituent groups with different sizes and amounts. Just as it has the highest surface area, PHCTF-4 with only one methyl shows the minimum mesporous size. Compared with PHCTF-3, the methyl can divide the relative large mesopores into small size, but a greater amount or larger F


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Figure 5. CO2 adsorption isotherms for (a) PHCTF-3, (b) PHCTF-4, (c) PHCTF-5, (d) PHCTF-6, and (e) PHCTF-7 measured at 273 and 298 K.

Table 2. CO2 Uptakes and CO2 Selectivities over N2 and CH4 at 273 K of PHCTF-3−PHCTF-7 CO2 uptake (wt %)a

CO2 selectivity(initial slope)b

CO2 selectivity (IAST)c

sample

273 K

298 K

CO2/N2

CO2/CH4

CO2/N2

PHCTF-1ad PHCTF-3 PHCTF-4 PHCTF-5 PHCTF-6 PHCTF-7

10.2 8.4 10.3 9.6 10.2 9.3

6.2 5.4 6.9 5.9 6.6 6.2

22 39 40 67 22 42

4 9 9 8 8 7

29 35 138 15 47

CO2/CH4 9 9 8 8 7

(8) (9) (7) (8) (7)

a

CO2 uptake at 1 bar. bSelectivities of CO2/N2 and CO2/ CH4 calculated from initial slope. cCO2/N2 and CO2/ CH4 selectivities calculated based on the IAST method from a gas mixture ratio of 0.15/0.85 and 0.05/0.95 (0.5/0.5) at 1 bar, respectively. dRef 37.

Figure 6. Isosteric heat of adsorption plots (Qst) of CO2 adsorption (a) and the virial plots of CO2 for PHCTF-3−PHCTF-7 (b).

As listed in Table 2, the CO2 adsorption data were calculated. Among all of the polymers studied at 273 K, 1 bar, PHCTF-4 displays the highest CO2 capacity of 10.3 wt %. The value of PHCTF-6 with a pendant phenyl unit is slightly lower at 10.2 wt %. PHCTF-3, PHCTF-5, and PHCTF-7 demonstrated 8.4, 9.6, and 9.3 wt % CO2 storage capacities, respectively. The order of the CO2 storage capacities of the polymers reveals that the PHCTFs possessing higher specific surface area and more micropore volume tended to exhibit the higher CO2 adsorption capacity for their similar structure of the building blocks. It is

the relatively high surface areas of the PHCTFs, we were interested in assessing their gas sorption and separation properties. Figure 5 displays the CO2 adsorption−desorption isotherms measured at 273 and 298 K (0−1 bar), respectively. For all the porous polymer networks, the rapidly improved amounts of CO2 adsorbed in the initial stage can be ascribed to the favorable interaction between the polymer skeleton and CO2 molecule,58 and the sustainable increasing of CO2 uptake with the exerting pressure indicates that it is far from the equilibrium or saturated state in the range of exerting pressure. G


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Table 3. KH, A0, and Q0 Values of CO2 Adsorption in PHCTF-3−PHCTF-7

interesting that these values of CO2 adsorption capacity are comparable to those reported for porous aromatic frameworks TPI-1 (10.7 wt %, 809 m2 g−1),59 PON-1 (10.8 wt %, 1400 m2 g−1),60 and PAF-1 (9.1 wt %. 5600 m2 g−1).61 Excitingly, these polymers are even superior to those of BPL, a commercially available carbon, with a CO2 adsorption capacity of 9.15 wt %.20 It is remarkable that PHCTF-4 and PHCTF-6 exhibited little increase of CO2 adsorption capacities compared with that of PHCTF-1a (10.2 wt %), although both of them have higher BET surface areas, especially for PHCTF-4, and more electronrich oxygen sites (added ether linkage). This result can be ascribed to the decrease of the Vmicro/Vtotal from 79% of PHCTF-1a to 58 and 39%, respectively, of PHCTF-4 and PHCTF-6 for the appearance of the more mesoporous. Because of the 0.36 nm molecular size of CO2, the above results seem to reflect that the small size pores tend to be more effective for CO2 capture at low pressure. Some important physicochemical parameters, such as the first viral coefficient, Henry’s law constant, and enthalpy of adsorption of CO2 gas, were examined to further understand the effect of porous structure on CO2 adsorption. To measure the binding affinity of PHCTFs for CO2, we calculated the isosteric enthalpies of adsorption (Qst) from CO2 adsorption isotherms at 273 and 298 K using the Clausius−Clapeyron equation. Then, these were plotted as a function of the adsorbed amount of CO2 gas illustrated in Figure 6. The Qst values drop rapidly from the initial high enthalpies of adsorption with the increase CO2 adsorption amounts, indicating that the CO2 molecule has a stronger affinity toward the polymer skeleton than CO2 itself, and all these porous materials demonstrated a high Qst (near or higher than 30 kJ mol−1) at low coverage. The high Qst values of PHCTF-3− PHCTF-7 are comparable to those of many other MOPs, for instance HCP-1 (24 kJ mol−1),62 NOPs (28−37 kJ mol−1),63 and CMP (27−33 kJ mol−1).57 However, there is an abnormal result in that PHCTF-3 with the lowest CO2 capacity possesses the highest Qst value. We speculate that the existence of the exposed and electric-rich ether oxygen linkage of the building block, DHPZ-DN, may be responsible for such strange phenomenon. The adjacent substituent groups of the ether oxygen might in some certain extent hinder the interaction between the CO2 molecule and this electric field on the network surface. This may be another reason for the above results that the CO2 adsorption capacities of PHCTF-4 and PHCTF-6 with higher BET surface areas and the electron-rich oxygen ether linkage exhibit little increase compared with PHCTF-1a. Because of the weak BET surface area and porosity of PHCTF-3, it still demonstrates the lower CO2 capacity compared with PHCTF-4−7. Overall, the Qst values of all the polymers not exceeding 50 kJ mol−1 demonstrate that such physical adsorbed interaction of CO2 in these polymers facilities the regeneration of the adsorbents. In addition, the virial plots of CO2 for PHCTF-3−PHCTF-7 exhibit fairly straight lines in Figure 6. The first virial coefficients (A0) which are the intercepts of the lines generally represent the interaction between CO2 and pore surface of the polymer networks, and the Henry’s law constants (KH) can be calculated by KH = exp(A0) equation. Then, the plot slope of ln KH versus 1/T can be obtained as the limiting enthalpy of adsorption, Q0. As shown in Table 3, it can be found that the sequence of their Q0 values is similar to the above-mentioned Qst results. Moreover, their Q0 values are also comparable with

sample PHCTF-3 PHCTF-4 PHCTF-5 PHCTF-6 PHCTF-7

T (K) 273 298 273 298 273 298 273 298 273 298

KH (mol g−1 pa−1) 7.86 2.75 7.79 3.18 6.64 2.84 6.38 2.89 6.44 2.89

× × × × × × × × × ×

10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−5

A0 ln(mol g−1 pa−1) −9.451 −10.500 −9.460 −10.356 −9.62 −10.47 −9.66 −10.45 −9.65 −10.45

Q0 (kJ mol−1) 28.4 24.3 23.0 21.3 21.7

those of benzimidazole-linked polymers (27−29 kJ mol−1)25,64 or borazine-linked polymers (20−28 J mol−1).65 After the CO2 uptake properties were studied, the further evaluation of the separation performance toward N2 and CH4 is also necessary for these porous sorbents in CO2 capture application field, so the single-component adsorption isotherms of CH4 and N2 at 273 K were measured and compared with that of CO2, as illustrated in Figure 7a−e. All the porous PHCTFs exhibited higher CO2 uptakes than that of CH4 and N2. Afterward, the ratio of the initial slopes of the adsorption isotherms as one of the most common methods wildly used for porous materials was applied to estimate the selectivity obtained by a linear fit as shown in Figure S17. From the calculated selectivity as shown in Table 2, we can see that the CO2/N2 selectivities of PHCTF-3−PHCTF-7 at 273 K, 1 bar are in the range from 22 to 67, comparable with those of triazine-based PCTFs (9−22),41,66 APOPs (23.8−43.4),67 and polyimides SMPI-0 (30) and SMPI-10 (32).68 Here, the CO2 capacities of PHCTF-3−PHCTF-7 are not superior to PHCTF-1a as described above, yet the performance of the CO2/N2 selectivities improved a lot up to 67 for PHCTF-5. This can be ascribed to the incorporation of the electron-rich ether linkage, which enhances the electrostatic interaction between the surface of the network walls and the guest molecules. Because of the better Vmicro/Vtotal providing strong interaction with gas molecules, PHCTF-5 exhibits the higher CO2/N2 selectivity value than others. Just like the previous reports, the affinity of a gas toward polymer backbone is related to its critical temperature (Tc).69 Thus, the polymer networks have higher CH4 adsorption capacity because of the higher Tc value of CH4 (191 K) than N2 (126 K), although the CH4 molecule has the larger kinetic diameter (3.80 Å) than N2 (3.64 Å). Certainly, this will lead to the smaller selectivity of CO2/ CH4 than that of CO2/N2. However, the selectivities of PHCTF-3−PHCTF-7 in the range from 7 to 9 are still comparable to those of BILPs (8−17),25,26 MPIs (8−12)58 and APOPs (5.3−6.7).67 The flue gas contains 15% CO2, 75% N2, and 10% other gases at a pressure of about 1 bar; hence, preferential CO2 adsorption over N2 and CH4 for the porous polymers studied by ideal adsorbed solution theory (IAST) seems to be necessary for practical applications. Here, we investigate the adsorption selectivity of natural gas (CO2/CH4 = 0.05/0.95), flue gas (CO2/N2 = 0.15/0.85), and landfill gas (CO2/CH4 = 0.50/0.50) by IAST method, respectively, and the resultant single-site Langmuir−Freundlich curves of CO2, N2, and CH4 fitted well with the experimental pure gas isotherms, as H


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Figure 7. Adsorption isotherms of CO2, CH4, and N2 gases at 273 K for PHCTF-3 (a), PHCTF-4 (b), PHCTF-5 (c), PHCTF-6 (d), and PHCTF-7 (e); IAST selectivities for the 0.15/0.85 CO2/N2 mixture (blue squares), the 0.50/0.50 CO2/CH4 mixture (red circles), and the 0.05/0.95 CO2/CH4 mixture (brown triangles) for PHCTF-3 (a′), PHCTF-4 (b′), PHCTF-5 (c′), PHCTF-6 (d′), and PHCTF-7 (e′).

penetration from the longer struts and the inter-/intramolecular intercalation from the increased degree of conformation freedom in the flexible ether linkage, which favors the formation of high surface areas. Furthermore, the size (methyl or phenyl group) and amount (one or two) of substituents affected the porosities and the gas adsorption and separation properties of the target polymer networks. Such a strategy endows the easily prepared flexible dicyano building blocks with asymmetric, twisted, and N,O-heterocycle phthalazinone structure by simple and low-cost aromatic nucleophilic substitution reaction with great large-scale application potential in constructing porous materials. Meanwhile, the N,O-rich natures of PHCTFs impart the networks’ strong electrostatic interaction with CO2 molecule, thus high CO2 adsorption capacities. The CO2/N2 selectivity calculated by the ideal adsorbed solution theory (IAST) is up to 138 at 273 K, resulting from the higher proportion of Vmicro accompanied the electron-rich heteroatoms characteristic. These properties along

illustrated in Figure S18. Thus, a plot of the obtained ideal selectivities of CO2/N2 and CO2/CH4 versus the pressure (0− 1 bar) of the mixed gases for PHCTFs are achieved, as shown in Figure 7a′−e′ and Table 2. It is noted that PHCTF-5, exhibiting the highest selectivity of CO2/N2 up to 138, is superior to many other microporous polymers for its higher proportion of Vmicro accompanied the electron-rich heteroatoms characteristic. The ideal selectivity of CO2/CH4 gas pair calculated from IAST method is similar to the results based on the initial slope of the method.

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CONCLUSIONS A simple, facile, and effective strategy was developed to obtain high surface areas and engineer the porosity through the introduction of the substituents in constructing phthalazinonebased covalent triazine frameworks (PHCTFs) by flexible building blocks. The introduction of the substituents can effectively decrease the probability of the network interI


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with easily up-scalable synthesis indicate that these porous phthalazinone-based covalent triazine frameworks materials are promising functional materials for application in the gas sorption and separation field.

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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/acsami.7b01783. FT-IR, 1H NMR and 13C NMR spectra of the monomers, elemental analysis and ICP results of PHCTFs, TGA curves of PHCTFs and monomers, the FE-SEM and TEM images of PHCTF-4−7 and adsorption selectivity of CO2 over CH4 and N2 at 273 K of PHCTFs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-411-84986191. *E-mail: [email protected]. Phone: 86-411-83639223. ORCID

Cheng Liu: 0000-0003-4883-1471 Guipeng Yu: 0000-0001-8712-0512 Author Contributions

The authors declare no competing financial interest. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the financial support from the Fundamental Research Funds for the Central Universities (DUT16LK14 and DUT16RC(3)056), National Natural Science Foundation of China (Nos. 51473025 and 51673033), and the National High Technology Research and Development Program of China (No. 2015AA033802).

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