Covalent Organic Frameworks Constructed from Flexible Building


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Covalent Organic Frameworks Constructed from Flexible Building Blocks with High Adsorption Capacity for Pollutants Yang Li, Weiben Chen, Wenjing Hao, Yusen Li, and Long Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00983 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Covalent Organic Frameworks Constructed from Flexible Building Blocks with High Adsorption Capacity for Pollutants Yang Li, † Weiben Chen,† Wenjing Hao,† Yusen Li,† and Long Chen*,†,§ †

Department of Chemistry, Tianjin Key Laboratory of Molecular Optoelectronic Science, School

of Science, Tianjin University, Tianjin, 300072, P. R. China. §

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin,

300072, P. R. China. *

E-mail: [email protected]

KEYWORDS: two-dimensional, covalent organic frameworks, pollutant sorption, Rhodamine B, iodine adsorption

ABSTRACT: Two imine-based two-dimensional covalent organic frameworks (2D COFs: TPTAzine-COF and TPT-TAPB-COF) which exhibit large surface areas and good crystallinity were synthesized from flexible building blocks. Both of them exhibit prominent adsorption capacity for Rhodamine B (970 mg g-1) and volatile iodine (225 wt%) with good recyclability.

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INTRODUCTION As a burgeoning kind of porous materials, covalent organic frameworks (COFs) feature the characteristics of designable topologies and geometry, permanent porosity and tailor-made functionalities. Various applications have been explored including heterogeneous catalysis,1,2 sensing,3 gas separation and storage,4 optoelectronics,5,6,7 drug delivery,8 etc. The reticular chemistry9 and dynamic covalent chemistry10 were adopted in the design and synthesis for COFs, and most of reported COFs are constructed by borate or imine linkages,11,12 and exhibit controllable geometries and tunable pore sizes.13-18 Similar to the conjugated microporous polymers (CMPs), COFs were usually synthesized from rigid building blocks to guarantee the porous structure could be maintained upon activation and post-synthesis. Furthermore, exploration of the diversity of COFs using flexible building blocks has been rarely reported.19,20 For instance, Zheng and co-workers synthesized a series of flexible building blocks for the construction of 2D COFs based on 2,4,6-triaryloxy-1,3,5-triazine.21,22 Zhou and co-workers developed another semi-flexible monomer 1,4-cyclohexanediamine,23 which has two possible chair (dominant) or boat conformations.24 As demonstrated in these examples, the flexible geometry of these novel building blocks greatly promoted the development of diversified COFs with various functions. Wastewater which contained organic compounds produced from food industries, dyeing, and textile gave rise to a series of pollution problems. Organic dyes with non-biodegradability and chemically stability are common pollutants present in wastewater.25 Their toxicity to aquatic life generally cause mutagenic and carcinogenic problems to human beings. Adsorption is considered to be an effective and low-cost technological method for wastewater treatment. However, developing qualified adsorbents with high adsorption capacities remains challenging.26 COFs

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feature large porosity with specific open porous channels are recently demonstrated to be an ideal platform for removing and degrading organic pollutant.27 Furthermore, COFs constructed via flexible linkers usually could broaden the lattice sizes and thus might be facilitate for pollutants removal. The variety of flexible linkers could also extend the functionality and applications for COFs.28 Herein, we selected a heteroatom-rich flexible linker 2,4,6-tris-(4-formylphenoxy)-1,3,5-triazine (TPT-CHO)29 as the main building block to synthesize two COFs (TPT-Azine-COF and TPTTAPB-COF, Scheme 1). Hydrazine or rigid 1,3,5-tris(4’-aminophenyl)benzene (TAPB) were used as comonomers in the synthesis of COFs under solvothermal conditions. These two COFs not only exhibit superior adsorption performance for Rhodamine B (RhB, 970 mg g-1), but also showcase high absorption capacity (225 wt%) for iodine vapor, which indicating good application potential for pollutant treatment.

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Scheme 1. (a) Synthetic route of the TPT-COFs; Top views of (b) TPT-TAPB-COF and (d) TPT-Azine-COF; Side views of (c) TPT-TAPB-COF and (e) TPT-Azine-COF. Inset: photograph of the bulk TPT-TAPB-COF and TPT-Azine-COF. EXPERIMENTAL Synthetic conditions for TPT-COFs were carried out by screening various solvent combinations to obtain highly crystalline and porous materials (Table S1-S2, Figure S1). Synthesis of TPT-Azine-COF: Hydrazine hydrate (0.108 mmol, 6.6 µL), TPT-CHO (0.072 mmol, 31.8 mg) were added to glass tube, then 1 mL of mixed solvent of n-butanol/odichlorobenzene (2:1 by volume) and 3 M acetic acid (0.2 mL) were added. The tube was degassed under vacuum at 77 K and sealed. The tube was heated at 393 K and kept for three days. Upon cooling to room temperature, the product was isolated by filtration and washed with tetrahydrofuran and acetone. The resulted powder was dried under vacuum for 10 h in 92% yield. Synthesis of TPT-TAPB-COF: The procedure is similar to the above instead of using TAPB (0.036 mmol, 12.6 mg), TPT-CHO (0.036 mmol, 15.9 mg), and 1.2 mL of mixed solvent of nbutanol/o-dichlorobenzene/3M acetic acid (5:5:2 by volume). Pale yellow powder was obtained in 89% yield. RESULTS AND DISCUSSION Characterization. The crystallinity of TPT-Azine-COF and TPT-TAPB-COF were verified with powder X-ray diffraction (PXRD) analysis. The extended structures for COFs were modeled using Materials Studio package to obtain optimized structural parameters (Table S7-S8). The obtained PXRD pattern of TPT-Azine-COF shown an intensive diffraction peak at 3.35o, with

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other diffraction peaks appeared at 5.90o and 9.05o, which were assignable to the (100), (110) and (210) diffractions, respectively. Meanwhile, the PXRD profile of TPT-TAPB-COF exhibited prominent peak at 3.95o and relatively weak peaks at 6.85o, 7.90 o and 10.50o, which were indexed to (100), (110), (200) and (210) diffractions, respectively (Figures 1a and 1d). The simulated diffraction patterns in the hexagonal P6/m and P6 space group with eclipsed AA stacking provide good description of TPT-Azine-COF and TPT-TAPB-COF, respectively. The final unit cell parameters were obtained by performing Pawley refinement (Rp = 2.72% and Rwp = 3.49% for TPT-Azine-COF; Rp = 5.19% and Rwp = 6.80% for TPT-TAPB-COF, Supporting Information).

Figure 1. (a, d) Experimental PXRD profiles (red curve: TPT-Azine-COF; blue curve: TPTTAPB-COF), Pawley refined (black) and their difference (orange), the simulated eclipsed AA (magenta) pattern and staggered AB (cyan) pattern; Unit cells of AA stacking modes for (b)

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TPT-Azine-COF and (e) TPT-TAPB-COF; Unit cells of AB stacking modes for (c) TPT-AzineCOF and (f) TPT-TAPB-COF. Fourier transform infrared (FT-IR) spectra of the TPT-Azine-COF displayed the characteristic peaks for C=N bonds (1628 cm-1), C=O bonds (1701 cm-1) and the N-H vibration bands (3439 cm-1, 3329 cm-1) of the corresponding precursors were disappeared.30 Similar findings were observed for TPT-TAPB-COF with the C=N strctching at 1626 cm-1, implying the occurrence of the aldimine condensation (Figure S2a-b). The more detailed structure analysis of COFs was validated by the solid-state NMR spectroscopy. The characteristic signals at 162 ppm for TPTAzine-COF and 158 ppm for TPT-TAPB-COF further confirm the constitution of the imine bonds (Figure S3).31 In contrast, the signal of C=N bonds of the triazine units is located at 173 ppm for both COFs.22,32 Elemental analysis of the activated COF samples indicated that the contents of C, H, N were in good accordance with the theoretical values calculated from the chemical formulas of C8H5N2O for TPT-Azine-COF and C16H10N2O for TPT-TAPB-COF. Both TPT-COFs displayed spherical particles morphologies as visualized by scanning electron microscopy (SEM, Figure S4a-b). Interestingly, transmission electron microscopy (TEM) images further unveiled that the TPT-TAPB-COF actually adopted hollow capsule morphology while TPT-Azine-COF was solid particles (Figure S4c-d). Porosity measurement and thermal properties. The permanent porosity of the 2D TPT-COFs was assessed by the nitrogen adsorption/desorption measurements at 77 K. The sorption curves of two TPT-COFs were conformed to type IV isotherm (Figure 2a), implying that TPT-COFs were mesoporous. The Brunauer–Emmett–Teller (BET) surface areas of TPT-Azine-COF and TPT-TAPB-COF were determined to be 1020 m2 g-1 and 957 m2 g-1, respectively. In addition, the pore volumes were 0.65 cm3 g-1 for TPT-Azine-COF and 0.57 cm3 g-1 for TPT-TAPB-COF,

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which were evaluated from N2 adsorption amount at P/P0 = 0.99. Uniform pore size distribution (2.50 nm for TPT-Azine-COF; 2.33 nm for TPT-TAPB-COF) was simulated based on the nonlocal density functional theory (NLDFT) (Figure 2b-c). These results were correspond to the values of d-spacing observed from the PXRD pattern at 2θ = 3.35o (2.63 nm for TPT-AzineCOF) and 2θ = 3.95o (2.24 nm for TPT-TAPB-COF). Both TPT-COFs were insoluble in common solvents like THF, MeOH, DMF, acetone and water, etc. All these as-synthesized COF samples exhibited great robustness with retained PXRD patterns after soaked in these solvents for 3 days (Figure S5). Thermogravimetric analysis indicated that TPT-Azine-COF and TPTTAPB-COF were thermally stable up to ca. 310 and 340 °C under a nitrogen atmosphere, respectively (Figure S6a).

Figure 2. (a) Nitrogen sorption isotherms (red curve: TPT-Azine-COF; blue curve: TPT-TAPBCOF). Pore size distribution of (b) TPT-Azine-COF and (c) TPT-TAPB-COF. Adsorption of Rhodamine B (RhB). The great chemical and thermal stabilities and the large porosity are propitious to pollutant sorption. The adsorption experiments of TPT-COFs towards the water-soluble dyes (methylene blue (MB) or Rhodamine B (RhB)) were investigated. For a typical sorption experiment (e.g. for RhB), small amount of COF samples (2 mg) were stirred in 10 mL dye solution with varied initial concentrations. The absorbance variation of RhB was

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monitored at 554 nm by UV-vis spectroscopy at different time intervals during the adsorption processes. As shown in Figure 3c-e, the efficiency of removing RhB could reached 84%, 94% within 40 min for TPT-Azine-COF and TPT-TAPB-COF respectively. After 90 min, nearly complete adsorption of RhB had been realized (94% for TPT-Azine-COF, 97% for TPT-TAPBCOF), indicating that TPT-COFs were efficient adsorbents in water treatment. Based on Langmuir and Freundlich isotherm models, the equilibrium adsorption of dye can be simulated. All the fitting parameters were summarized in Tables S3 and S4. According to the correlation coefficients, Freundlich isotherm model (RF2, 0.745, 0.946) reveal a poor fit in the equilibrium adsorption, but the Langmuir isotherm model affording higher values of R2 (RL2, 0.951, 0.997) was more adequate for the evaluation of adsorption capacity of the TPT-COFs. The obtained adsorption data was fitting well with the monolayer Langmuir isotherm models (Figures 3a and S7), which suggested that the adsorbed RhB were more likely homogeneously dispersed on the porous scaffold. Based on the Langmuir equation,33 the calculated maximum adsorption capacity of RhB dye for TPT-Azine-COF and TPT-TAPB-COF were 725 mg g-1 and 970 mg g-1 (Figure 3a), respectively, which greatly outperform the benchmark adsorbent of activated carbon (98 mg g-1).34 For further comparison, a similar COF (N3-COF)35 built up with non-flexible building block (4,4',4''-(1,3,5-triazine-2,4,6-triyl)tribenzaldehyde) was prepared. The resynthesized control sample N3-COF exhibit comparable BET surface area (1010 m2 g-1) with the analog TPTAzine-COF (1020 m2 g-1), but the adsorption capacity of RhB dye are lower than that of TPTAzine-COF and TPT-TAPB-COF. These results confirmed that TPT-COFs were promising candidate for adsorbing RhB among the reported COFs (Table S5).

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Figure 3. (a) The Langmuir isotherm model for RhB using TPT-Azine-COF (red) and TPTTAPB-COF (blue) as adsorbents. (b) Iodine uptake curves using TPT-Azine-COF (red) and TPT-TAPB-COF (blue) as adsorbents at 348 K. (c) Adsorption rates of RhB using TPT-AzineCOF (red) and TPT-TAPB-COF (blue) as adsorbents. Photographs of RhB adsorption of (d) TPT-Azine-COF and (e) TPT-TAPB-COF at different time intervals (0, 20, 40, 60, and 90 min). The PXRD patterns for COFs before and after RhB adsorption were recorded (Figure S8). Good retention of the PXRD patterns was obeserved for TPT-TAPB-COF. In contrast, the PXRD signal of TPT-Azine-COF exhibit obvious decrease, which probably due to the partial amorphisation and the mass loss of the COFs samples during the cycling process. To evaluate the recyclability of two TPT-COFs, repeated adsorption-regeneration experiments of RhB dye were carried out. Both TPT-COFs maintain well adsorption capacity after at least ten recycles (Figure 4), which indicated that TPT-COFs could be good candidate as efficent adsorbents in water treatment. Interestingly, when two cationic dyes RhB and methylene blue were mixed for the

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adsorption experiment, TPT-Azine-COF and TPT-TAPB-COF selectively remove RhB with a higher efficiency (Figure S9).

Figure 4. UV-vis spectra of RhB adsorption reusability for (a) TPT-Azine-COF and (c) TPTTAPB-COF upon 10 cycles; (b) and (d) are the corresponding bar graph of adsorption reusability. Iodine adsorption experiments. The large porosity and high adsorption performance of TPTCOFs on RhB remind us to investigate the adsorption capacity of iodine vapor for COFs. Radionuclide 129I is a kind of harmful radioactive waste in nuclear waste disposal.36 It is reported that the presence of heteroatoms and large π-conjugated scaffold was beneficial for the adsorption of iodine.37 The adsorption tests was performed by exposing TPT-COFs to excess iodine at 348 K under ambient pressure.38 An apparent color change of the COFs from white to brown was observed upon standing for a short time (Figure S11). The color did not show further variation after one day, suggesting that the loading amounts of iodine was becoming saturated. The equilibrium iodine uptake was determined to be 219 wt%, 225 wt% for TPT-Azine-COF and TPT-TAPB-COF respectively (Figure 3b). A weight loss step appeared between 90 and 300 °C

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in the thermogravimetric analysis (TGA) curve of the iodine-loaded polymer [email protected], and the calculated iodine mass were close to the value of equilibrium iodine adsorption (Figure S6b). The theoretical capacities for iodine sorption were calculated to be 3.20 g g-1 for TPT-AzineCOF and 2.81 g g-1 for TPT-TAPB-COF respectively. Thus, it is about 68% and 80% of the pore volumes of these two COFs were filled upon one day sorption. The iodine uptake capacity of TPT-TAPB-COF is comparable to most of the recent reported outstanding COFs and even better than some conjugated microporous polymers (e.g. SCMP-1, PAF-1, etc.) and zeolite imidazole framework (e.g. ZIF-8) (Table S6). Compared with TPT-COFs, no obvious change appeared in the FT-IR graph of [email protected] (Figures S2c and S2d), which confirmed that this process is physical adsorption. The aromatic networks of TPT-Azine-COF and TPT-TAPB-COF with nitrogen rich units provide a number of binding sites to the guest iodine molecules,39,40 thereby leading to a higher equilibrium uptake capacity of iodine. CONCLUSIONS In summary, two 2D COFs built up with flexible building blocks were synthesized by aldimine condensation of flexible triazine-based aldehydes. These TPT-COFs show high crystallinity, large surface areas and excellent porous characters, and exhibit high adsorption capacities for organic dyes and volatile iodine. It is could be envisioned that introducing flexible building blocks into COFs scaffolds could greatly expand the diversity of new structures41 and functions for COF based materials. ASSOCIATED CONTENT

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Supporting Information Experimental details and characterization data of both the monomers and TPT-COFs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L. Chen) Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51522303, 21602154), the National Key Research and Development Program of China (2017YFA0207500) and the Natural Science Foundation of Tianjin (17JCJQJC44600). REFERENCES (1) Xu, H.; Gao, J.; Jiang, D. Stable, Crystalline, Porous, Covalent Organic Frameworks as a Platform for Chiral Organocatalysts. Nat. Chem. 2015, 7, 905-912. (2) Lin, S.; Diercks, C. S.; Zhang, Y.-B.; Kornienko, N.; Nichols, E. M.; Zhao, Y.; Paris, A. R.; Kim, D.; Yang, P.; Yaghi, O. M.; Chang, C. J. Covalent Organic Frameworks Comprising Cobalt Porphyrins for Catalytic CO2 Reduction in Water. Science 2015, 349, 1208-1213. (3) Ding, S.-Y.; Dong, M.; Wang, Y.-W.; Chen, Y.-T.; Wang, H.-Z.; Su, C.-Y.; Wang, W. Thioether-Based Fluorescent Covalent Organic Framework for Selective Detection and Facile Removal of Mercury(II). J. Am. Chem. Soc. 2016, 138, 3031-3037.

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(39) Wang, P.; Xu, Q.; Li, Z.; Jiang, W.; Jiang, Q.; Jiang, D. Exceptional Iodine Capture in 2D Covalent Organic Frameworks. Adv. Mater. 2018, 30, 1801991. (40) Zhao, Y. Emerging Applications of Metal-Organic Frameworks and Covalent Organic Frameworks. Chem. Mater. 2016, 28, 8079-8081. (41) Ma, Y-X.; Li, Z-J.; Wei, L.; Ding, S-Y.; Zhang, Y.-B.; Wang, W. A Dynamic ThreeDimensional Covalent Organic Framework, J. Am. Chem. Soc. 2017, 139, 4995-4998.

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