Authors: Chunqing Ji | Kongzhao Su | Wenjing Wang | Jianhong Chang | El-Sayed M. El-Sayed | Lei Zhang | Daqiang Yuan
Source: CCS Chemistry (article, 2021-10-04)
Field: Materials Science — Materials Chemistry

Tunable Cage-Based Three-Dimensional Covalent Organic Frameworks Open AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022Tunable Cage-Based Three-Dimensional Covalent Organic Frameworks Chunqing Ji, Kongzhao Su, Wenjing Wang, Jianhong Chang, El-Sayed M. El-Sayed, Lei Zhang and Daqiang Yuan Chunqing Ji State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 , Kongzhao Su State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Wenjing Wang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 , Jianhong Chang State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012 , El-Sayed M. El-Sayed State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 Chemical Refining Laboratory, Refining Department, Egyptian Petroleum Research Institute, Nasr City 11727 , Lei Zhang College of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118 and Daqiang Yuan *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002 University of the Chinese Academy of Sciences, Beijing 100049 Fujian Science and Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou 350002 https://doi.org/10.31635/ccschem.021.202101453 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail It is extremely challenging to construct three-dimensional (3D) crystalline covalent organic frameworks (COFs) with flexible building blocks and to further explore their tunable or adaptive characteristics due to crystallization and structure determination difficulties. Herein, we constructed three crystalline isostructural 3D-OC-COFs based on a newly synthesized flexible organic cage ( 6NH2-OC.4HCl) through a novel in situ acid–base neutralization strategy. Interestingly, network conformations can be fine-tuned by rationally introducing different substituents into the starting dialdehydes during the assembly process, resulting in one expanded structure ( 3D-OC-COF-H) and two different contracted structures ( 3D-OC-COF-OH and 3D-OC-COF-Cl), which can be ascribed to the hinge-like motions of organic cage building blocks. By virtue of the unique pore environments and different functional groups, we further employed these COFs for CO2 capture, of which 3D-OC-COF-OH exhibited superior CO2/CH4 separation performance. This study not only demonstrates a new strategy for constructing 3D cage-based COFs, but also identifies a novel factor that affects flexible building blocks for realizing structural diversity. Download figure Download PowerPoint Introduction Covalent organic frameworks (COFs) are a growing class of crystalline porous materials that have garnered extensive interest for their auspicious employment in numerous applications, including gas adsorption and separation,1–5 heterogeneous catalysis,6–10 energy storage,11–13 chemical sensing,14–16 and so on.17–20 One of the unique characteristics of COFs is the atomic precision with which molecular building blocks are integrated into two-dimensional (2D) or three-dimensional (3D) extended architectures via diverse covalent bond linkages.21–23 In past decades, reports of well-defined and predictable COFs have concentrated on various rigid building blocks to investigate structural and functional diversity due to their limited rotation, which is advantageous for undergoing a reversible dynamic equilibrium process.24–29 On the contrary, constructing and determining COFs with flexible building blocks to further explore their distinctive tunable or dynamic behavior has obtained relatively less attention owing to their crystallographic defects and various conformations.30–33 Meanwhile, compared with 2D COFs, covalently connected 3D networks are rarely reported due to the lack of extra weak interlayer interactions beneficial for synthesis.34–37 Therefore, incorporating flexible units into a 3D COF system is a significant and challenging task, particularly in the investigation of high-quality construction and structural diversity. Given the limited amount of research in this area, very few flexible building blocks have been identified to date for constructing dynamic 3D COFs, such as macrocycle γ-cyclodextrin (γ-CD),38 bicyclooxacalixarenes,39 and so on.40 Generally, these building blocks feature flexible C-O single bonds that provide more possibilities for structural diversity. Meanwhile, dynamic behaviors of these 3D COFs have been identified in response to external stimuli, such as solvent or vapor. For instance, Cooper and co-workers39 designed a dynamic 3D-CageCOF-1, demonstrating small-pore and large-pore phases in empty or dimethylformamide (DMF)-loaded conditions. Wang et al.40 recently presented a novel flexible 3D COF ( FCOF-5) which underwent a reversible vapor-triggered shape transformation under tetrahydrofuran (THF) vapor. However, achieving diversified structures during the in situ assembly process in 3D COFs with flexible building blocks remains unprecedented (Scheme 1). Scheme 1 | General schematic representation of the strategy for constructing 3D-OC-COFs with flexible building blocks. Download figure Download PowerPoint Herein, we rationally designed and synthesized an amino-functionalized covalent organic cage (OC) named 6NH2-OC.4HCl, exhibiting C–O single bonds in its backbone and D3h-symmetric conformation. Hinge-like motions of oxygen-bridged phenyls could be directly observed from single-crystal structures of a series of molecular units (Im-6NH2-OCs), which were synthesized by condensation with different benzaldehyde monomers. The corresponding crystalline 3D-OC-COFs were then synthesized using a new direct method called in situ acid–base neutralization. It is worth noting that network conformations of these isostructural frameworks were tuned by employing different functionalized dialdehydes and the adaptive ability of the flexible cage during the assembly process. 3D-OC-COF-H with BDA-H adopted an expanded large-pore structure while 3D-OC-COF-OH ( BDA-OH) and 3D-OC-COF-Cl ( BDA-Cl) exhibited diverse contracted small-pore structures. All three 3D-OC-COFs exhibited high chemical and thermal stability, and permanent porosity but with a disparity in aperture sizes caused by structural distortion and polarity groups, which inspired us to explore the gas adsorption behavior of these materials. In particular, 3D-OC-COF-OH exhibited the best CO2/CH4 separation performance through dynamic breakthrough experiments. Experimental Section Synthesis of 6NH2-OC.4HCl 6NO2-OC was synthesized according to a previously reported method.41 To a 100-mL round bottom flask, 6NO2-OC (134 mg, 0.18 mmol) and SnCl2 dihydrate (2.52 g, 13.3 mmol) were added, followed by 20 mL concentrated HCl. The flask was degassed for 30 min before room-temperature stirring of the solution under argon for 24 h. The obtained white precipitate was filtered, followed by washing using concentrated HCl (30 mL). The resultant product was recrystallized using ethanol (EtOH), followed by vacuum drying at 70 °C, affording white microcrystalline powder of 6NH2-OC.4HCl (108.1 mg, 84.6%). Mp > 400 °C; 1H NMR (400 MHz, dimethyl sulfoxide (DMSO)-d6, δ) (ppm): 6.48 (s, 6H), 6.12 (s, 3H), 5.12 (s, 3H), 4.49 (s, 12H). 13C NMR (125 MHz, D2O, δ) (ppm): 156.22, 151.53, 118.16, 117.21, 115.81, 102. Infrared (IR) (KBr, 4000–400 cm−1) ν: 3365 (m), 2864 (vs), 1963 (w), 1597 (s), 1517 (vs), 1446 (s), 1327 (s), 1220 (vs), 1122 (s), 1001 (m), 912 (w). Electrospray ionization mass spectrometry (ESI-MS): [M]+ calcd for C30H24O6N6, 564.1752, 565.1782, 566.1809; found, 564.1749, 565.1794, 566.1843. Synthesis of Im-6NH2-OC-H To 6NH2-OC.4HCl (0.483 g, 0.68 mmol) in EtOH (25 mL) and benzaldehyde (1 mL), 2–3 drops of pyridine (Py) were added, and the mixture obtained was subjected to heating under reflux for 12 h under N2 atmosphere. The precipitate was filtrated, washed with EtOH three times, and then recrystallized with ethyl acetate (EA) to afford yellow rod-like crystals of Im-6NH2-OC-H, yielding 0.53 g (71.4%). 1H NMR (400 MHz, DMSO-d6, δ): 8.78 (s), 7.92 (dd), 7.50 (d), 7.49 (d), 7.30 (s), 7.12 (s), 5.55 (s). High-resolution mass spectrometry (HRMS) (ESI-MS) (m/z): [M + Na]+ calcd for C72H48O6N6, 1115.3528, 1116.3560, 1117.3591; found, 1115.3536, 1116.3494, 1117.3567. Anal. Calcd for C72H48O6N6: C, 61.43; H, 4.32; N, 13.98. Found: C, 60.15; H, 4.28; N, 13.98. Synthesis of Im-6NH2-OC-OH To 6NH2-OC.4HCl (0.483 g, 0.68 mmol) in EtOH (25 mL) and salicylaldehyde (1 mL), 2–3 drops of Py were added, and the mixture obtained was subjected to heating under reflux for 12 h under N2 atmosphere. The precipitate was filtrated, washed with EtOH three times, and then recrystallized with dichloromethane (DCM) and hexane to afford yellow rod-like crystals of Im-6NH2-OC-OH, yielding 0.64 g (79.3%). 1H NMR (400 MHz, DMSO, δ): 13.52 (s, 6H), 9.20 (s, 6H), 7.97 (s, 3H), 7.65 (d, J = 6.7 Hz, 6H), 7.43 (t, J = 7.8 Hz, 6H), 7.29 (s, 6H), 7.07–6.91 (m, 12H), 5.57 (d, J = 8.7 Hz, 3H). Anal. Calcd for C72H48O12N6.C2H6O.15 H2O: C, 48.78; H, 4.61; N, 4.61. Found: C, 48.68; H, 3.31; N, 5.12. Synthesis of Im-6NH2-OC-Cl To 6NH2-OC.4HCl (0.483 g, 0.68 mmol) in EtOH (25 mL) and 2-chlorobenzaldehyde (1 mL), 2–3 drops of Py were added, and the mixture obtained was subjected to heating under reflux for 12 h under N2 atmosphere. The precipitate was filtrated, washed with EtOH three times, and then recrystallized with DCM and hexane to afford yellow tabular-like crystals of Im-6NH2-OC-Cl, yielding 0.55 g (62.4%). 1H NMR (400 MHz, DMSO, δ): 9.07 (s, 6H), 8.21 (d, J = 7.0 Hz, 6H), 7.57 (dt, J = 8.0, 4.9 Hz, 12H), 7.49 (t, J = 6.6 Hz, 6H), 7.41 (d, J = 5.6 Hz, 6H), 7.20 (s, 6H), 5.58 (d, J = 8.5 Hz, 3H). Anal. Calcd for C72H42O6N6Cl6: C, 66.5; H, 3.23; N, 6.47. Found: C, 66.16; H, 3.29; N, 6.89. Synthesis of 3D-OC-COF-H To a mixture of 6NH2-OC.4HCl (37.4 mg, 0.05 mmol) and BDA (20.1 mg, 0.15 mmol) in a Pyrex tube measuring o.d. × i.d. = 10 × 8 mm, o-dichlorobenzene (o-DCB) (1 mL), methanol (MeOH) (1 mL), Py (40 μL) and H2O (50 μL) were added, followed by thorough sonication of the resultant mixture for 20 min. Then, the tube was flash-frozen in a liquid nitrogen (77 K) bath, followed by degassing through three freeze–pump–thaw cycles and flame sealed. Upon heating at 120 °C for 3 days, a resultant yellow precipitate was collected by filtration, followed by immersion in anhydrous THF (20 mL) and then acetone for 12 h. The solvent used in activation was decanted and freshly replenished three times a day. After solvent exchange, the COF solid was transferred to a sand core funnel and rinsed completely with EtOH (50 mL). Care was taken in this step to keep the sample from drying out, and the moist solid was transferred to a filter paper for further supercritical carbon dioxide activation (scCO2). A fluffy yellow solid was afforded in 86.9% yield (42 mg). Anal. Calcd for C54H30O6N6·5H2O: C, 68.35; H, 4.22; N, 8.86. Found: C, 68.44; H, 4.22; N, 9.72. Synthesis of 3D-OC-COF-OH To a mixture of 6NH2-OC.4HCl (37.4 mg, 0.05 mmol) and BDA-OH (24.9 mg, 0.15 mmol) in a Pyrex tube measuring o.d. × i.d. = 10 × 8 mm, mesitylene (1 mL), n-butanol (n-BuOH) (1 mL), Py (40 μL) and H2O (50 μL) were added, followed by thorough sonication of the resultant mixture for 20 min. Then, the tube was flash-frozen in a liquid nitrogen (77 K) bath, followed by degassing through three freeze–pump–thaw cycles and flame sealed. Upon heating at 120 °C for 3 days, a resultant yellow precipitate was collected by filtration, followed by immersion in anhydrous THF (20 mL) and then acetone for 12 h. The solvent used in activation was decanted and freshly replenished three times a day. After solvent exchange, the COF solid was transferred to a sand core funnel and rinsed completely with EtOH (50 mL). Care was taken in this step to keep the sample from drying out, and the moist solid was transferred to a filter paper for further scCO2. A fluffy yellow solid was afforded in 84.7% yield (45 mg). Anal. Calcd for C54H24O6N6Cl6·2H2O: C, 58.85; H, 2.54; N, 7.63. Found: C, 58.74; H, 2.97; N, 8.41. Synthesis of 3D-OC-COF-Cl To a mixture of 6NH2-OC.4HCl (37.4 mg, 0.05 mmol) and BDA-Cl (30.5 mg, 0.15 mmol) in a Pyrex tube measuring o.d. × i.d. = 10 × 8 mm, o-DCB (1 mL), n-BuOH (1 mL), Py (80 μL) and H2O (100 μL) were added, followed by thorough sonication of the resultant mixture for 20 min. Then, the tube was flash-frozen in liquid nitrogen (77 K) bath, followed by degassing through three freeze–pump–thaw cycles and flame sealed. Upon heating at 120 °C for 3 days, a resultant yellow precipitate was collected by filtration, followed by immersion in anhydrous THF (20 mL) and then acetone for 12 h. The solvent used in activation was decanted and freshly replenished three times a day. After solvent exchange, the COF solid was transferred to a sand core funnel and rinsed completely with EtOH (50 mL). Care was taken in this step to keep the sample from drying out, and the moist solid was transferred to a filter paper for further scCO2. A fluffy yellow solid was afforded in 84.7% yield (45 mg). Anal. Calcd for C54H30O12N6·17H2O: C, 51.42; H, 5.12; N, 6.66. Found: C, 51.10; H, 5.15; N, 6.40. Column breakthrough experiments The mixed-gas breakthrough separation experiment was performed on a self-built real-time apparatus coupled with a mass spectrometer (Pfeiffer GSD320) using gas mixtures of CO2/CH4 (50:50, v/v) and CO2/N2 (85:15, v/v). Supercritical activated 3D-OC-COF powders were loaded into a Φ 3 × 120 mm2 stainless steel column with silica wool filling the void spaces. Before carrying out the breakthrough experiment, the column was heated to 120 °C for 12 h under a high-purity He atmosphere (10 mL/min) for further activation to ensure that the adsorption column was filled with He gas. After cooling to room temperature, the flow of He was then turned off, and a mixed gas (CO2/CH4 or CO2/N2) was allowed to flow through the column. The flow rate of mixed gas was controlled at a rate of 2 cm3 min−1 by a flowmeter at the inlet, and the composition of outlet gas was real-time monitored by mass spectrometry. After that, the samples for the breakthrough experiment were in situ regenerated under He gas flow at 120 °C overnight. Results and Discussion A prism-like OC ( 6NH2-OC.4HCl) was synthesized in the form of hydrochloride for the first time by reducing the 6NO2-OC precursor using SnCl2 and HCl, affording 84.6% yield (see Supporting Information Scheme S1). The synthesized cage was verified by 1H NMR and ESI time-of-flight mass spectrometry (ESI-TOF-MS) (see Supporting Information Figures S1–S3). It is worth noting that the system still retained four HCl molecules, as confirmed by single-crystal X-ray diffraction (see Supporting Information Figure S4) and acid–base titration (see Supporting Information Table S1). Thermogravimetric analysis (TGA) indicated that 6NH2-OC.4HCl was stable up to 410 °C (see Supporting Information Figure S5). Moreover, this hydrochloride cage could be easily gram-scale synthesized and maintained without oxidation at ambient atmosphere after months. Notably, the as-synthesized compound had good water solubility, and single crystals of 6NH2-OC.4HCl were obtained from its water solution by slow diffusion of EtOH. Single-crystal X-ray crystallography revealed that the 6NH2-OC.4HCl adopted a D3h-symmetric prism-shaped skeleton with two phloroglucinol units in parallel and six amino groups at its vertices (see Supporting Information Figure S4). All characteristics mentioned above demonstrate that 6NH2-OC.4HCl can serve as a suitable building block for constructing 3D cage-based COFs. To ascertain the viability of this idea, we initially synthesized an imine-linked molecular analog to probe the reactivity of the 6NH2-OC.4HCl monomer (see Supporting Information Schemes S2–S4). In this regard, the 6NH2-OC.4HCl monomer was reacted with 6 equiv benzaldehydes, and imine-functionalized cage models, Im-6NH2-OCs, were readily obtained in high yields and subsequently characterized by means of 1H NMR and ESI-TOF-MS, which corroborated that 6NH2-OC.4HCl was an appropriate monomer for 3D COF construction (see Supporting Information Figures S6–S11). In addition, a single X-ray diffraction experiment was further performed to better understand the structural conformation of molecular units after reversible condensation reaction ( Supporting Information Table S4). As depicted in Figures 1a–1f, Im-6NH2-OCs also possessed a similar prism-shaped skeleton with a shape-persistent cage core compared with 6NH2-OC.4HCl. However, diverse torsional structures were observed mainly due to the characteristic rotation and torsion of the chemical bond, especially the flexible single bond in OC arms. The extent of hinge-like motions was further quantified by the degree of torsion angle (DTA). The maximum DTA in one direction was 19.9° ( Im-6NH2-OC-H), higher than 17.2° ( Im-6NH2-OC-OH) and 17.3° ( Im-6NH2-OC-Cl), indicating that conformation of OCs was affected by introducing different substituents during the self-assembly process (Table 1). Figure 1 | Crystal structures of Im-6NH2-OC-H (a and d), Im-6NH2-OC-OH (b and e), and Im-6NH2-OC-Cl (c and f) from side and top views and corresponding planes (the plane goes through benzene ring of 1,5-difluoro-2,4-diaminobenzene in OC) and lines (the straight line that goes through C–O single bond in phloroglucinol, the plane and the line are both illustrated to pass through the same oxygen atom) are shown in three direction. Solvent molecules are omitted for clarity. Schematic representation of the synthesis of 3D-OC-COFs (g). Download figure Download PowerPoint Table 1 | The Calculation of DPL and DTA Im-6NH2-OC Im-6NH2-OC-H Im-6NH2-OC-OH Im-6NH2-OC-Cl PH-1LH-1 PH-2LH-2 PH-3LH-3 POH-1LOH-1 POH-2LOH-2 POH-3LOH-3 PCl-1LCl-1 PCl-2LCl-2 PCl-3LCl-3 Degree between planea and lineb (DPL) 70.1° 78.0° 89.8° 72.8° 80.2° 82.3° 88.4° 72.7° 89.3° Degree of torsion angle (DTAc) 19.9° 12.0° 0.2° 17.2° 9.8° 7.7° 1.6° 17.3° 0.7° a"Plane" means the plane goes through benzene ring of 1,5-difluoro-2,4-diaminobenzene in OC. b"Line" means the straight line that goes through C–O single bond in phloroglucinol. Both plane and line pass through the same oxygen atom. cDTA = 90° – DPL. With this knowledge in mind, we attempted to synthesize 3D COFs by assembling the 6NH2-OC.4HCl motif as a D3h-symmetric building block with dialdehydes as a C2 linker. Remarkably, the catalyst was found to be the most critical factor in producing highly crystalline COFs. In particular, two different catalytic methods were utilized for COF preparation (see Supporting Information Scheme S5). One uses acetic acid catalysis after neutralizing 6NH2-OC.4HCl with triethylamine (TEA), while the other uses in situ neutralization of 6NH2-OC.4HCl with base catalysts (TEA or Py).42 However, we failed to obtain crystalline COF materials by acetic acid catalysis, which is widely employed to synthesize imine-based COFs (see Supporting Information Figure S12).43,44 This may be ascribed to the laborious control of base addition to neutralize HCl molecules in the hydrochloride cage. Meanwhile, the poor solubility and easy oxidation of 6NH2-OC also demonstrated that this strategy is undesirable (see Supporting Information Figure S13). Moreover, in the form of hydrochloride, 6NH2-OC.4HCl shows excellent water solubility and can be stored without oxidative deterioration under ambient atmosphere for a long time. Therefore, controlling alkali volume through the in situ method is the most effective and convenient way to reduce the variables correlated with constructing crystalline frameworks. Several attempts have been made to screen the optimal synthetic conditions, including solvents, reaction temperatures, reaction times, and catalysts (see Supporting Information Figures S14–S17 and Tables S2–S3). Finally, crystalline 3D-OC-COFs were successfully prepared using solvothermal reactions at 120 °C for 3 days (Figure 1g; see Supporting Information Schemes S6–S8). Typically, 3D-OC-COF-H was synthesized as a bright yellow powder by heating suspensions of 6NH2-OC.4HCl and BDA-H in a mixed solvent system of o-DCB/MeOH (1:1, by volume) in the presence of 10 M Py (0.09 mL) as a catalyst. By employing hydroxy- or on and 3D-OC-COF-OH and 3D-OC-COF-Cl were also prepared under similar synthetic of and (see Supporting Information Figure 3D cage-based COFs were for their chemical immersion in various chemical environments of organic and X-ray diffraction experiments indicated that COF samples maintained their after under these (see Supporting Information Figures methods were used to and 3D-OC-COFs at the atomic on 13C NMR 3D-OC-COFs demonstrated characteristic at ( ( and ( 3D-OC-COF-Cl), indicating the presence of (see Supporting Information Figure Meanwhile, in COFs demonstrated new to the of imine-linked materials (see Supporting Information Figure to these revealed high thermal at a °C under nitrogen atmosphere (see Supporting Information Figure In addition, based on and Supporting Information Figures 3D-OC-COFs possessed similar the high was further indicated by see Supporting Information Figures Figure 2 | of 3D-OC-COF-H 3D-OC-COF-OH and 3D-OC-COF-Cl and structural with different torsion angle of 3D-OC-COF-H and 3D-OC-COF-OH and and 3D-OC-COF-Cl and The observed are shown in the are presented in the of and 3D-OC-COF-Cl based on the are in and and the between observed and is illustrated in Download figure Download PowerPoint To provide into the structural of the prepared 3D COFs, analysis was The COF samples were activated with supercritical CO2 to and The presence of and diffraction in the indicated a crystalline which was different from of the starting (see Supporting Information Figures on the Structure and that 3D frameworks with D3h-symmetric units as a and as were of and were for building initially extended structures with the energy using the by Materials of (see Supporting Information Figures and Tables illustrated that 3D-OC-COFs a with 3 or 3D-OC-COF-H demonstrated two at and to and 3D-OC-COF-OH two at and corresponding to and 3D-OC-COF-Cl two at and to and of the was out for the models, and were obtained (a = = = = = = = and = for a = = = = = = = and = for a = = = = = = = and = for Notably, 3D-OC-COF-H without substituents an extended structure similar to previously reported By introducing such as or a of contracted structure was due to interactions between the two Meanwhile, 3D-OC-COFs with diverse structures were mainly from rotation and torsion of the chemical bond, especially the flexible single bond in OC and this behavior can also be verified in After further by the functional of Materials the torsion angle was for and the torsion degree of bond in In the degree of torsion angle was while was to and by introducing different sizes of × × and × × groups, see Supporting Information Figures and Tables 3D-OC-COFs tunable behaviors in the self-assembly process, these resultant materials structures after activation without The permanent porosity exhibited by 3D cage-based COFs was using nitrogen gas analysis at Remarkably, both solvent and COF activation methods were found to nitrogen gas (see Supporting Information Figures and Solvent with and followed by supercritical activation of carbon afforded the As illustrated in Figure 3D cage-based COFs exhibited a with in a relatively The of 3D-OC-COF-H and 3D-OC-COF-Cl may be with ability under conditions, in this of Moreover, a of indicated the of for COF were to be ( ( and ( 3D-OC-COF-Cl), which were higher than 6NH2-OC.4HCl, were due to in solid (see Supporting Information Figures and This revealed that the cage into the extended cage-based could not only new porosity cage but also made the porosity (the of 6NH2-OC.4HCl) a pore was 3D-OC-COF-H exhibited two at and and while 3D-OC-COF-OH and 3D-OC-COF-Cl at and (Figure The of pore sizes was from and structure Figure 3 | gas adsorption and and pore of Download Cage ; Covalent bond ; John Cage ; Nanotechnology ; Computer science ; Chemistry ; Materials science ; Engineering ; Organic chemistry ; Structural engineering ; Art ; Art history Covalent Organic Framework Applications ; Metal-Organic Frameworks: Synthesis and Applications ; Advanced Photocatalysis Techniques