Three-Dimensional Ionic Covalent Organic Frameworks for Rapid

Nov 28, 2017 - The obtained 3D COFs are built of 3-fold interpenetrated diamond net and show impressive surface area and CO2 uptakes. The ion-exchange...
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Three-Dimensional Ionic Covalent Organic Frameworks for Rapid, Reversible and Selective Ion Exchange Zonglong Li, Hui Li, Xinyu Guan, Junjie Tang, Yusran Yusran, Zhan Li, Ming Xue, Qianrong Fang, Yushan Yan, Valentin Valtchev, and Shilun Qiu J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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Journal of the American Chemical Society

Three-Dimensional Ionic Covalent Organic Frameworks for Rapid, Reversible and Selective Ion Exchange Zonglong Li,† Hui Li,† Xinyu Guan,† Junjie Tang,† Yusran Yusran,† Zhan Li,† Ming Xue,† Qianrong Fang,∗,† Yushan Yan,‡ Valentin Valtchev∗,†,∥ and Shilun Qiu∗,† †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, P. R. China Department of Chemical and Biomolecular Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, USA

‡

∥

Normandie Univ, ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et Spectrochimie, 6 Marechal Juin, 14050 Caen, France

Supporting Information Placeholder ABSTRACT: Covalent organic frameworks (COFs) have emerged as functional materials for various potential applications. However, the availability of three-dimensional (3D) COFs is still limited, and nearly all of them exhibit neutral porous skeletons. Here we report a general strategy to design porous positively charged 3D ionic COFs by incorporation of cationic monomers in the framework. The obtained 3D COFs are built of 3-fold interpenetrated diamond net, and show impressive surface area and CO2 uptakes. The ion exchange ability of 3D ionic COFs has been highlighted by reversible removal of nuclear waste model ions and excellent size-selective capture for anionic pollutants. This research thereby provides a new perspective to explore 3D COFs as a versatile type of ion exchange materials.

Covalent organic frameworks (COFs) are a class of crystalline polymers with periodic molecular orderings and inherent porosity.1-3 They can be precisely designed by molecular building blocks based on reticular chemistry through covalent bonds and exhibit periodic architectures, low densities, and well defined pores, making them excellent candidates for various applications.4-22 Since the pioneering work of Yaghi in 2005,1 the design and synthesis of two-dimensional (2D) COFs is already well established through several condensation reactions. At present the availability of three-dimensional (3D) COFs is limited and the exploration of 3D COFs are still considered to be a great challenge.23-28 We have recently prepared a series of 3D COFs, including 3D base-functionalized COFs for size-selective catalysis,29 3D crystalline polyimide COFs for drug delivery,30 and 3D COFs with dual linkages for bifunctional cascade catalysis.31 It must be noted, however, that almost all 3D COFs exhibit neutral skeletons, which limits to great extent their potential uses as functional materials and ionic exchangers in particular. In this study, we report a general strategy for producing positively charged 3D COFs with high crystallinity and porosity by the combination of linear ionic linkers and tetrahedral neutral building units. The new COFs, denoted 3D-ionic-COF-1 and 3Dionic-COF-2, feature 3-fold interpenetrated structures with the diamond (dia) topology. These 3D-ionic-COFs show high specific surface area and remarkable CO2 uptake capacities. Besides, we

Scheme 1. Strategy for preparing 3D-ionic-COFs. Condensation of tetrahedral tetrakis(4-formylphenyl)methane (TFPM, a) and linear ionic linkers, dimidium bromide (DB, b) or ethidium bromide (EB, c), to give 3D porous ionic COFs, 3D-ionic-COF-1 (d) or 3D-ionic-COF-2 (e) with the diamond (dia) topology (f).

demonstrate that the newly synthesized 3D-ionic-COFs can act as excellent ion exchange materials, which is exemplified by the quick removal of nuclear waste model ions and size-selective capture for anionic dye ions. To the best of our knowledge, this study is the first report of 3D porous ionic COFs with ability to perform rapid, reversible and selective ion exchange.

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stretching bands of the imine group at 1622 cm-1 for 3D-ionicCOF-1 and 1624 cm-1 for 3D-ionic-COF-2. The 13C crosspolarization magic-angle-spinning (CP/MAS) NMR spectra (Figures S7 and S8 in SI) confirm the presence of carbon from the C=N bonds at 164 ppm for 3D-ionic-COF-1 and 162 ppm for 3Dionic-COF-2. These results prove the covalent bonds establishment. According to the thermogravimetric analysis (TGA), both 3D-ionic-COFs are stable up to about 450 °C under nitrogen (Figures S9 and S10 in SI).

Figure 1. PXRD patterns of 3D-ionic-COF-1 (a) and 3D-ionicCOF-2 (b) with the observed profile in black, refined in red, the difference (observed minus refined) in dark yellow, and calculated in blue. We addressed this issue by involving the combination of tetrahedral neutral knots and linear cationic building units for the construction of ionic interfaces based on Schiff base reactions. As shown in Scheme 1, tetrakis(4-formylphenyl)methane (TFPM, Scheme 1a) was synthesized for an ideal tetrahedral linker, and dimidium bromide (DB, Scheme 1b) and ethidium bromide (EB, Scheme 1c) were chosen for linear ionic building units. The condensation of TFPM and DB or EB resulted in two novel 3D porous ionic COFs, 3D-ionic-COF-1 (Scheme 1d) and 3D-ionicCOF-2 (Scheme 1e), respectively. On the basis of this design, TFPM can be defined as a 4-connected node, and MB or EM can act as a 2-connected node. The association between the 2- and 4connected building units results in 3D networks with the dia topology (Scheme 1f).32 In addition, since the tetrahedral centers are separated by long linear linkers, the resulting structures tend to be interpenetrated networks.33 The synthesis of 3D-ionic-COFs was carried out by solvothermal reaction of TFPM (21.6 mg, 0.05 mmol) and DB (37.9 mg, 0.1 mmol) or EB (39.3 mg, 0.1 mmol) in a mixture of dioxane and acetic acid, followed by heating at 85 °C for 3 days. The yield of dark red crystalline solid was 83% for 3D-ionic-COF-1 and 80% for 3D-ionic-COF-2. Complementary methods were subsequently employed for detail structural characterization of new COFs. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) study reveals that 3D-ionic-COFs exhibit rodlike morphology with the particle size of about 1 µm (Figures S1S4 in Supporting Information (SI)). Fourier transform infrared (FT-IR) spectra (Figures S5 and S6 in SI) show the characteristic

Figure 2. Structural representations of 3D-ionic-COF-1: (a) single diamond network, (b) 3-fold interpenetrated diamond topology, and (c) 3-D porous structure with the ionic interface. The high crystallinity of 3D-ionic-COFs was confirmed by powder X-ray diffraction (PXRD) analysis (Figure 1). After a geometrical energy minimization by using the Materials Studio Software package34 based on the 3-fold interpenetrated dia topology, the unit cell parameters were determined. Both materials exhibit tetragonal unit cells with a = b = 37.6646 Å, c = 14.9093 Å and α = β = γ = 90° for 3D-ionic-COF-1; and a = b = 37.7384 Å, c = 14.8184 Å and α = β = γ = 90° for 3D-ionic-COF-2. Simulated PXRD patterns show good match with the experimental ones (Figure 1, black and blue curves). Furthermore, the full profile pattern matching (Pawley) refinements were performed from the experimental PXRD patterns. Strong PXRD peaks at 4.43, 6.65, 7.90, 9.41, 10.91, 13.23, 14.47, and 15.93° two theta for 3D-ionicCOF-1 can be assigned to the (200), (220), (121), (400), (321), (341), (521), and (422) Bragg peaks of P-4 (No. 81); and 4.49, 6.70, 7.95, 9.45, 10.67, 13.43, 14.55, and 15.97° two theta for 3Dionic-COF-2 can be assigned to the (200), (220), (121), (400), (321), (341), (521), and (422) Bragg peaks of P-4 (No. 81), respectively. The refinement results revealed unit cell parameters nearly equivalent to the predicted ones with excellent agreement factors (a = b = 37.6985 Å, c = 14.8969 Å, α = β = γ = 90°, wRp = 2.26%, and Rp = 1.08% for 3D-ionic-COF-1; a = b = 37.7592 Å, c = 14.8057 Å, α = β = γ = 90°, wRp = 3.20%, and Rp = 1.59% for 3D-ionic-COF-2). Besides the 3-fold interpenetrated dia network, we also considered other alternative structures based on the noninterpenetrated and 2-fold interpenetrated dia net, which were set up from the space groups of P43212 (No. 96) and Pnn2 (No. 34), respectively. However, the resepctive calculated patterns did not match to the experimental PXRD patterns (Figures S11-18 and Tables S1-6 in SI). On the basis of the above results, 3D-ionicCOFs were proposed to have the expected architectures with 3fold interpenetrated dia topology, which show microporous cavities with a diameter of about 8.3 Å and well aligned ionic interface (Figure 2 and Figures S19-21 in SI). Notably, compared with non-ionic COF-320 with 9-fold interwoven diamond net constructed from linkers with approximate length,33 the 3D-ionicCOFs exhibit dramatically decreased interpenetration due to the steric effect and charge interaction of DB or EB linker.

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Journal of the American Chemical Society To investigate the chemical stability of 3D-ionic-COFs, we treated these COFs in different solvents, including tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dimethylsulfoxide (DMSO), boiling water, aqueous HCl (1 M and 3 M) and NaOH (1 M and 3 M) solutions. After 24 hours, these samples were collected, washed with THF, and dried under vacuum at 100 °C for 8 hours. We found 3D-ionic-COFs exhibited negligible weight loss upon treatment in above solvents, and no change in the peak position and intensity of their PXRD patterns (Figures S22 and S23 in SI) was observed.

Figure 3. (a) N2 adsorption-desorption isotherms and (b) CO2 uptakes for 3D-ionic-COF-1 and 3D-ionic-COF-2. Note: full symbols adsorption, empty symbols desorption. Nitrogen (N2) adsorption-desorption isotherms were conducted at 77 K to evaluated the porosity of 3D-ionic-COFs. As can be seen in Figure 3a, a sharp increase in gas uptake is observed at low pressure (below 0.1 P/P0) for both 3D-ionic-COFs, which reveals their microporous nature. The inclination of the isotherm in the 0.8-1.0 P/P0 range shows the presence of textural mesopores, which are a consequence of the agglomeration of COF crystals.30 The Brunauer-Emmett-Teller (BET) surface area is 966 m2 g-1 for 3D-ionic-COF-1 and 880 m2 g-1 for 3D-ionic-COF-2 (Figures S24 and S25 in SI). The pore-size distributions of 3Dionic-COFs were calculated by nonlocal density functional theory (NLDFT). Both 3D-ionic-COFs showed a narrow pore width (8.6 Å for 3D-ionic-COF-1 and 8.2 Å for 3D-ionic-COF-2, Figures S26 and S27 in SI), which was in good agreement with the pore size predicted from their crystal structures (8.3 Å). Furthermore, carbon dioxide (CO2) adsorption was studied to evaluate the potential of 3D porous ionic COFs in CO2 capture. CO2 capacities of 161 mg g-1 at 273 K and 93 mg g-1 at 298 K for 3D-ionic-COF1 and 133 mg g-1 at 273 K and 76 mg g-1 at 298 K for 3D-ionicCOF-2 were recorded (Figure 3b). These values are among the highest for COF-type material, close to the results obtained with TpPa-1 (156 mg g-1),7 [HO2C]100%-H2P-COF (174 mg g-1),35 and PyTTA-BFBIm-iCOF (177 mg g-1).36 Given the high crystallinity, porosity, and stability of 3D-ionic-

COFs, we investigated their potential application for the removal of dangerous ions in nuclear waste, such as radioactive technetium (Tc-99). Taking into account their harmfulness, permanganate has been chosen as the model ion to study pertechnetate uptake since both are group 7 oxoanions.37 Typically, the ionic removal was performed with 10.0 mg of aqueous solution of KMnO4 (20.0 mL) with 3D-ionic-COF-1 or 3D-ionic-COF-2 (20.0 mg) at room temperature for 30 min, and the concentration of MnO4- was monitored by using UV-Vis spectrometer. As shown in Figure 4a and 4b, almost 100% of MnO4- can be removed by 3D-ionicCOFs in less than 20 min. We also compared the ion exchange performances of other ion exchange materials under the same condition including PVBTAH-ZIF-838 and LDHs39. In contrast, they show a low capability in removing the MnO4- ions, and after 30 min only 80% and 40% of the MnO4- ions can be removed for PVBTAH-ZIF-8 and LDHs, respectively (Figure 4b and Figure S28-30 in SI). After the reactions, the results of PXRD and N2 adsorption confirmed the structural integrity of 3D-ionic-COFs, thus revealing the high stability of new COF materials (Figures S31–S36 in SI). The COF crystals can be easily isolated from the reaction mixture by a simple filtration, recovered in aqueous solution of NaBr, and reused at least five times with almost no loss of activity (Figure 4c and Figure S37 in SI).

Figure 4. (a) UV-vis spectra of MnO4- aqueous solution in the presence of 3D-ionic-COF-1 as a function of time, (b) comparison of the ion exchange performances of different materials, (c) the recyclability study of 3D-ionic-COF-1, and (d) UV-vis spectra of MO/MB aqueous solution in the presence of 3D-ionic-COF-1 monitored with time. Beyond the fast and reversible ion exchange, 3D-ionic-COFs can be also employed to selectively capture anionic dye pollutants on the basis of a size exclusion effect. Organic dyes are among the major categories of water pollutants and their removal is crucial because of their toxicity to mankind and aquatic living organisms.40 To evaluate the potential in size-selective ion capture of 3D-ionic-COFs, two commercially widely applied organic dyes, methyl blue (MB) and methyl orange (MO) with different dimensions (13.9 × 14.4 × 24.5 Å for MB vs. 5.4 × 7.8 ×15.2 Å for MO, Figure S38 in SI) were employed. The uptake experiments were performed by mixing 3D-ionic-COFs and two dyes with the same molar ratio, and UV-vis spectra were recorded periodically for the filtered solution. As shown in Figure 4d and Figure S39 in SI, 3Dionic-COFs almost completely capture the MO molecules in about 20 minutes, whereas the MB molecules remain in the solution, which is a sound proof of size discrimination ability of 3D-ionicCOFs. Furthermore, 3D-ionic-COFs can be reused for dye ion capture and show similar activities (Figures S40 and S41 in SI). In

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addition, LDHs was also used to separate MO/MB in aqueous solution as a control; however, it fails to effectively separate dye molecules via a size exclusion effect (Figure S42 in SI). In conclusion, we have designed and synthesized two 3D porous ionic COFs with 3-fold interpenetrated dia topology by using two linear cationic monomers, DB or EB, and a tetrahedral building unit, TFPM. These 3D-ionic-COFs showed high specific surface area, impressive CO2 uptakes, quick removal of nuclear waste model ions, and excellent size-selective capture for dye ions. 3D ionic COFs successfully prepared in this work may not only expand the development of COFs with functional structures, but also promote the applications of COFs as efficient functional materials.

ASSOCIATED CONTENT Supporting Information Synthetic procedures, SEM, TEM, FTIR, solid state 13C NMR, TGA, BET plots, and pore size distribution. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (21571079, 21621001, 21261130584, 21390394, 21571076, and 21571078), and “111” project (B07016). V.V. and Q.F. acknowledge the Thousand Talents program (China).

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