Dual-Functional Conjugated Nanoporous Polymers for Efficient

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Dual-Functional Conjugated Nanoporous Polymers for Efficient Organic Pollutants Treatment in Water: A Synergistic Strategy of Adsorption and Photocatalysis Bo Wang, Zhen Xie, Yusen Li, Zongfan Yang, and Long Chen* Department of Chemistry, Tianjin Key Laboratory of Molecular Optoelectronic Science, School of Science, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: A series of 9,9′-bifluorenylidene-based conjugated microporous/mesoporous polymers (BF-CMPs) were constructed via Suzuki polycondensation. The porosities and electronic properties of BF-CMPs can be readily tuned by selection of proper monomers. Among three BF-CMPs, PyBF-CMP exhibited an extraordinarily high adsorption capacity (1905 mg g−1) and excellent photocatalytic degradation performance of organic dyes in water. Moreover, Py-BFCMP featured good recyclability with both high adsorption degree (99%) and photodegradation efficiency (92%) of RhB even after 10 recycles.

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INTRODUCTION Water purification and remediation technologies have been recognized as one of the most effective ways to address the pervasive problems of the scarcity of clean water for human society.1 Wastewater produced in industrial process often contains organic compounds that are toxic and usually not amenable for biodegradation, which often causes severe environmental issues and is mutagenic and carcinogenic.2 Organic dyes, as one of the most common pollutants in wastewater, are chemically stable and usually nonbiodegradable in water.3 Furthermore, most organic dyes exhibit intensive absorption and reflection of sunlight entering in water, which severely interfere with the growth of beneficial bacterial in the aquatic ecosystem.4 Up to date, various methods like sorption, ion exchange, electrolysis, and photocatalysis have been employed to remove or degrade organic dyes in aqueous solution.5 Among them sorption is one of the most costeffective, eco-friendly methods. Unfortunately, conventional porous materials-based sorbents, including zeolites, activated carbon, and natural fibers, usually suffer from low adsorption capacities, poor removal efficiency, and moderate regeneration ability.6−9 Compared to the traditional sorbents, porous organic polymers (POPs) as a new type of porous materials are constructed solely by strong covalent bond linkages10−12 and exhibit large surface area, easy functionality, low mass density, and high stability.13 POPs have a myriad of potential applications in catalysis,14 gas storage,15−18 separations,19−22 drug delivery,23 sensing,24,25 etc. As an important category of POPs, conjugated micro-/mesoporous polymers (CMPs) featuring π-conjugated scaffold have drawn increasing research interest since the pioneering work by Cooper and co-workers.26 © XXXX American Chemical Society

Different from other reported POPs, CMPs feature large electron delocalized network structure,27−31 which facilitates efficient light absorption and charge carrier transportation. Thus, it has been demonstrated that CMPs could server as photocatalysts for organic reactions and water splitting.32−35 Therefore, to maximize the performance of CMPs toward removal of organic pollutants, both the porosity and photocatalytic activity of CMPs need to be finely optimized, which remains a great challenge. We herein use the “kill two birds with one stone” strategy to integrate the virtue of high porosity and photocatalytic activity of CMPs with “donor−acceptor” alternatively arranged in the porous skeleton for adsorption and degradation of organic dyes. 9,9′-Bifluorenylidene is an well-known electron acceptor unit which has been used as fullerene counterpart in bulky heterojunction solar cells36 and also building blocks for constructing hole-transporting material for perovskite solar cell.37 However, the chemical functionalization of 9,9′-BF has rarely been explored.36,37 Furthermore, 9,9′-BF-based polymer has not been reported yet. In this work, 3,3′,6,6′-tetraboronic acid−pinacol ester−9,9′-bifluorenylidene (3,6-BF-Bpin4), a key intermediate of functionalized 9,9′-BF, was newly synthesized. On the basis of this electron acceptor monomer, we designed and prepared a series of BF-CMPs using Suzuki polycondensation (Scheme 1), which can serve as not only highly efficient adsorbents but also active visible light photocatalysts for degradation of organic dyes. The electronic bandgap and pore structure can be tuned by selection of proper donor (e.g., Received: March 29, 2018 Revised: April 20, 2018

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DOI: 10.1021/acs.macromol.8b00669 Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. (a) Synthetic Routes of BF-CMPs and the Pictures of BF-CMPs Powder (from Left to Right: Py-BF-CMP, TPE-BFCMP, and TPA-BF-CMP); (b) a Simple Illustration of the Degradation of RhB Organic Dye

carried out on X-ray diffractometer model Rigaku SmartLab 9 kW diffractometer (Japan) at 40 kV and 200 mA with a Cu-target tube and a Kβ filter. Field emission scanning electron microscopies (FE-SEM) were performed on a Hitachi Limited Model S-4800 microscope operating at an accelerating voltage of 3.0 kV. High-resolution transmission electron microscopies (HRTEM) were performed on a JEOL model JEM-2100F microscope. The nitrogen and carbon dioxide adsorption and desorption isotherms were measured at 77 and 273 K using a Bel Japan Inc. model BELSOPR-Max analyzer, respectively. The samples were thermally activated at 120 °C for 8 h under vacuum (10−5 bar) before analysis. Surface areas were calculated from the nitrogen adsorption data. The pore size distributions were calculated from the nitrogen adsorption branch using the nonlocal density functional theory (NLDFT). Cyclic voltammetric experiments were carried out using a CHI 660E electrochemical workstation. The cyclic voltammograms were acquired at room temperature with the scan rate of 100 mV s−1. The potentials are reported versus the Fc/Fc+ redox couple as a standard. Elemental analysis (C, H, and N) was performed on a PerkinElmer 240C elemental analyzer. The residual Pd contents were determined by inductively coupled plasma mass spectrometry (ICP-MS) techniques using an Agilent model 7700X ICP-MS. The samples were prepared by nitrohydrochloric acid digestion before measurement.

pyrene (Py), tetraphenylethene (TPE), and triphenylamine (TPA)) as comonomer. The alternatively “donor−acceptor” arrangement (i.e., Py-BF, TPE-BF, and TPA-BF) in the BFCMP skeletons further varied the adsorption range and photodegradation performance toward organic dyes. Among all BF-CMPs, Py-BF-CMP exhibited the best performance on adsorption and photodegradation of RhB. Remarkably, the maximum adsorption capacity reach up to 1905 mg g−1, which outperforms most of the reported porous polymer based sorbents. For example, Py-BF-CMP can completely purify wastewater containing RhB (initial concentration is 75 mg L−1) by the synergistic effect of adsorption and photodegradation.

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EXPERIMENTAL SECTION

Materials. 4,7-Dibromo-2,1,3-benzothiadiazole (BT-Br2), Pd(PPh3)4, 1,3,6,8-tetrabromopyrene (Py-Br4), phenanthrene-9,10dione, and tris(4-bromophenyl)amine (TPA-Br3) were purchased from commercial sources. Rhodamine B (RhB) was purchased from Tianjin Jiangtian Chemical Company. All chemicals were of analytical grade and used without further purification. Tetrakis(4-bromophenyl)ethylene (TPE-Br4) was prepared according to the literature via a modified procedure (see the Supporting Information). Physical Measurements. FT-IR spectra were collected in transmission mode on a Bruker Alpha spectrometer using KBr pellets with a scan range of 400−4000 cm−1. The UV−vis absorption spectra were obtained from PerkinElmer Lambda 750 spectrophotometer equipped with integration sphere with standard procedure. The 1H NMR spectra of all organic compounds were recorded on a Bruker AVANCE III-400 NMR spectrometer. Solid-state NMR experiments were performed on a Bruker WB AVANCE II 400 MHz NMR spectrometer. The 13C cross-polarization magic angle spinning (CP/ MAS) spectra were recorded with a 4 mm double-resonance MAS probe and at a MAS rate of 10.0 kHz with a contact time of 2 ms (ramp 100) and a pulse delay of 3 s. The thermal properties of all the CMPs were evaluated to using a thermogravimetric analysis (TGA) with a differential thermal analysis instrument (TA Instruments TGA Q50 analyzer) over the temperature range from 20 to 800 °C under a N2 atmosphere with a heating rate of 10 °C/min using an empty Al2O3 crucible as the reference. Powder X-ray diffraction measurements were

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RESULTS AND DISCUSSION Synthesis and Characterization. As shown in Scheme 1a, three BF-CMPs were synthesized via palladium-catalyzed Suzuki polycondensation,38,39 and the obtained BF-CMPs are all insoluble in common solvents with deep colors. The chemical structures of BF-CMPs were verified by solid state 13C cross-polarization magic angle spinning (CP/MAS) NMR spectra. As presented in Figure S1, chemical shifts between 125 and 150 ppm appeared for all BF-CMPs, which could be attributed to the aromatic rings in the polymer backbone as well as the ethylene groups in the BF segments (No. 1) and the TPE units (No. 8 in TPE-BF-CMP). Thermogravimetric analysis (TGA) demonstrated that these BF-CMPs were stable up to 300 °C (Figure S3). Powder X-ray diffraction (PXRD) measurement indicated that BF-CMPs were amorphous in B

DOI: 10.1021/acs.macromol.8b00669 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Nitrogen adsorption/desorption isotherms at 77 K of Py-BF-CMP (purple), TPE-BF-CMP (pink), and TPA-BF-CMP (orange) (the adsorption branches are labeled with filled symbols, and the desorption branches are labeled with empty symbols). (b) Pore size distribution curves of Py-BF-CMP (purple), TPE-BF-CMP (pink), and TPA-BF-CMP (orange) via NLDFT calculation.

of BF-CMPs are listed in Table S2. The Brunauer−Emmett− Teller (BET) specific surface areas are 1306, 777, and 590 m2 g−1 for Py-BF-CMP, TPE-BF-CMP, and TPA-BF-CMP, respectively. In addition, the total pore volume estimated from the amount of N2 gas adsorbed at P/P0 = 0.99 were 1.28, 1.09, and 0.82 cm3 g−1, respectively. The pore size distribution (PSD) profiles based on the nonlocal density functional theory (NLDFT) confirmed the hierarchical porous structure of these BF-CMPs (Figure 1b). The large porosity of these BF-CMPs

nature (Figure S4). The morphologies of these BF-CMPs particles were visualized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 2)both SEM and TEM images of BF-CMPs diameter less than 50 nm. The microporous structures of BF-CMPs were also visualized by high-resolution transmission electron microscopy (HR-TEM) (Figure 2d−f). The diffuse reflectance UV−vis spectra of BF-CMPs are displayed in Figure S6a, which all exhibited a broad intense absorption covering the whole visible light region. The bandgaps of BF-CMPs were determined by Kubelka−Munk transformed reflectance spectra (Figure S6b). The bandgap derived from the reflectance edges are 1.55, 1.98, and 1.89 eV for Py-BF-CMP, TPE-BF-CMP, and TPA-BF-CMP, respectively. The lowest bandgap of Py-BF-CMP might be ascribed to the stronger donor strength and better conjugation of Py than that of TPE and also the largest polymerization degree as proved by the porosity analysis discussed below. The electrochemical properties of BF-CMPs were measured by cyclic voltammetry (CV) according to the reported method (Figure S7).37 All BF-CMPs exhibited quasi-reversible reduction peaks originated from the reductive BF unit, while no oxidation signals could be observed. The calculated energy levels derived from the CV measurement and optical bandgaps are summarized in Table S1. Porosity Analysis. The porosities of the BF-CMPs were assessed by nitrogen sorption measurements performed at 77 K (Figure 1a). All BF-CMPs exhibited a rapid uptake of N2 at a relatively low pressure (P/P0 < 0.05), which features permanent micropores in these BF-CMPs. Meanwhile, type IV sorption characteristics with adsorption/desorption hysteresis at higher pressure range were also observed in these BF-CMPs, which indicated the presence of mesopores. The hysteresis loop shape of Py-BF-CMP is much larger than that of TPE-BF-CMP and TPA-BF-CMP. The wide hysteresis loop indicate a delay in both condensation and evaporation of N2 during the measurement, which implied ink-bottle-type pore structure in the 3D porous Py-BF-CMP network.41 The parameters of the porosity

Figure 2. SEM images of (a) Py-BF-CMP, (b) TPE-BF-CMP, and (c) TPA-BF-CMP (the scale bars are 500 nm) and high-resolution TEM images of the BF-CMP samples redispersed in EtOH, revealing the inherent porous structures of (d) Py-BF-CMP, (e) TPE-BF-CMP, and (f) TPA-BF-CMP (the scale bars are 10 nm). C

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Figure 3. (a) Adsorption kinetics of RhB (initial concentration C0 = 25 mg L−1) on Py-BF-CMP. (b) Reusability of BF-Py-CMP adsorbent for RhB in 60 min. (c) The Langmuir isotherm model. (d) The adsorption isotherms for RhB on the three BF-CMPS.

Figure 4. (a) UV−vis absorption spectra of RhB aqueous solution at different illumination time intervals with Py-BF-CMP (the initial concentration of RhB is 75 mg L−1). (b) Photocatalytic degradation kinetics of RhB using Py-BF-CMP. (c) Recycling experiments of photocatalytic degradation of RhB in water using Py-BF-CMP under visible light (λ > 450 nm) irradiation. (d) Effect of different scavengers, isopropanol (IP), ammonium oxalate (AO), sodium azide (NaN3), benzoquinone (BQ), and the absence of oxygen (under Ar) on the degradation of RhB over Py-BF-CMP under visible light irradiation for 30 min.

To afford a quantitative assessment of the adsorption performance of the BF-CMPs, the adsorption processes were monitored by the absorbance variation of RhB at 554 nm. As shown in Figure 3a and Figure S9, upon adding a catalytic amount of BF-CMPs (4 mg) to an aqueous RhB solution (25 mg L−1, 20 mL), the dye molecules could be fully adsorbed within 90 min. The UV−vis absorption spectra of the supernate taken at different time intervals are displayed in Figure S10, which clearly indicated the fast uptake of RhB into the porous BF-CMP networks. The removal percentage of RhB reached approximately 97%, 96%, and 89% within 45 min for Py-BFCMP, TPE-BF-CMP, and TPA-BF-CMP, respectively. To

also guarantees high performance for CO2 sorption, with capacities (273 K, 1.0 bar) of 141.4, 92.3, and 80.5 mg g−1 for Py-BF-CMP, TPE-BF-CMP, TPA-BF-CMP, respectively (Figure S8), which outperform most CMPs42 prepared by Suzuki polycondensation and comparable to the state-of-the-art COFs reported to date (Table S3).43 Dye Adsorption and Recycling Assessment. The high surface area, large pore volume, and hydrophobic nature of the conjugated polymer scaffold enable BF-CMPs to be potential adsorbent for organic pollutant in water. Rhodamine B (RhB), a common organic cationic dye, is widely used as standard guest to evaluate the adsorption properties of the adsorbents. D

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using BF-Py-CMP as the photocatalyst were performed for ten cycles (Figure 4c). The degradation degree of RhB was still higher than 90% after the tenth cycle, indicating the high stability and recyclability of Py-BF-CMP in water. Mechanism Investigation. It is well accepted that superoxide radical (•O2−), singlet oxygen (1O2), photogenerated hole (h+), and •OH are possible reactive species in photocatalytic degradation of organic pollutants.46−50 In an attempt to figure out which reactive species were involved in the dye degradation using Py-BF-CMP as the catalyst, a series of control experiments with corresponding scavengers using PyBF-CMP as the catalyst were conducted. Sodium azide (NaN3, the scavenger of 1O2), isopropanol (IP, the •OH radical scavenger), benzoquinone (BQ, the •O2−scavenger), and ammonium oxalate (AO, the h+ scavenger) were separately added to the photocatalysis system to verify the catalytic mechanism. As shown in Figure 4d and Figure S20, the presence of BQ and the absence of oxygen cause a prominent activity decrease. In contrast, the addition of IP, AO, and NaN3 only causes a slight decrease in the degradation efficiency of RhB. These results reflected that •O2− is the primary active species in the photodegradation of RhB, which was consistent with the reported examples.51,52 It is reported that the •O2− is formed by the reaction of electrons with dissolved O2.47 The photocurrent response measurement (Figure S21) reveals that Py-BF-CMP could generate photoinduced electrons upon visible light irradiation, which further confirms the generation of superoxide radical (•O2−).

further investigate the adsorption behavior of RhB on BFCMPs, the obtained equilibrium adsorption data were fitting against both the Langmuir and Freundlich isotherm models.44,45 The fitting data are summarized in Tables S4 and S5. The correlation coefficients of Langmuir isotherm model (RL2: 0.999, 0.999, and 0.995) were higher than those of Freundlich isotherm model (RF2: 0.983, 0.919, and 0.722), which indicated that the adsorbed RhB molecules are likely on a monolayer and homogenized coverage on the pores (Figure 3c and Figure S11).44 Thus, the adsorption capacity of each BF-CMP was calculated according to the Langmuir equation (eq 1, section 11; Supporting Information),44 based on the initial and residual concentration of RhB solution determine from a calibration curve (Figure 3c). According to the Langmuir equation,44 the maximum adsorption capacities of Py-BF-CMP, TPA-BF-CMP, and TPE-BF-CMP for RhB were calculated to be 1905, 1024, and 926 mg g−1, respectively. These results confirmed that PyBF-CMP was the most promising candidate for adsorbing RhB among the BF-CMPs and other reported POPs (Table S6). To verify the universality of Py-BF-CMP for other organic dyes, the adsorption behaviors of Congo Red (CR) and Methyl Orange (MO) were also conducted for Py-BF-CMPs. Similarly, the maximum adsorption capacities on CR and MO based on the Langmuir isotherm model were 1267 and 637 mg g−1, respectively (Figures S12 and S13). The best adsorption capacity of Py-BF-CMP on RhB is resonable not only due to the largest porosity but also because the volume of RhB fits better with the micropore structure than that of CR and MO (Figure S14). In an attempt to investigate the recyclability of Py-BF-CMP, cyclic adsorption−regeneration tests on RhB dye solution were carried out. The adsorption efficiency of Py-BFCMP negligibly decreased even after ten recycles (Figure 3b), which suggests that Py-BF-CMP can serve as an excellent candidate for wastewater remediation. In a word, Py-BF-CMP exhibited superior adsorption performance of both cationic and anionic water-soluble organic dyes. Photocatalytic Degradation of RhB. Taking good use of the good dispersibility of BF-CMPs in water (Figure S15), narrow bandgap, and excellent porosity, the photocatalytic activity of BF-CMPs were further explored by degradation of water-soluble RhB. As for the control test in the absence of photocatalysts, little change was observed in the UV−vis absorption spectra (Figure S16), which indicated that selfphotobleaching of RhB is negligible. According to the different adsorption capacity of BF-CMPs, different initial concentrations of RhB solution (75, 55, and 40 mg L−1) were chosen for Py-BF-CMP, TPE-BF-CMP, and TPA-BF-CMP, respectively. The contribution of photodegradation can be distinguished from the simultaneous adsorption in the dark (Figure S16). The results are summarized in Figure 4a,b as well as Figures S17 and S18. Py-BF-CMP showed the highest photocatalytic activity toward the degradation of RhB among all the BF-CMPs. After visible light (>450 nm) irradiation for 30 min, more than 81% of RhB was degraded in the presence of Py-BF-CMP. The result might be attributed to the highest BET surface area (1306 m2 g−1) and narrowest bandgap (1.55 eV).46 The RhB degradation efficiency of Py-BF-CMP is comparable to or even higher than that of benzothiadiazole-based CMP nanoparticles,46 porous carbon nitride (C3N4),47 and POMbased CMPs (Bn-Anderson-CMP)48 under similar photocatalytic conditions even with less catalysts (0.2 mg L−1). Furthermore, Py-BF-CMP showed excellent stability during the photodegradation process (Figure S19). Recycling experiments

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CONCLUSIONS In summary, three BF-CMPs with dual functions of both adsorption and degradation of organic dyes have been facilely prepared through palladium-catalyzed Suzuki polycondensation. The porosity and electronic properties can be tuned by selection of proper building blocks. Among the three BFCMPs, Py-BF-CMP exhibited the best performance on both adsorption and degradation of RhB dye with good reusability, which might be attributed to the synergistic effect of its higher BET surface area and narrower bandgap. The high activity of Py-BF-CMP was exemplified in the reductive activation of molecular oxygen for the degradation of RhB under a visible light source (λ > 450 nm) at room temperature in water. Our current work represented an unprecedented example in the respect of removing organic water-soluble dyes by both adsorption and photodegradation, which provide a new “kill two birds with one stone” strategy to develop faster and more efficient porous materials for water purification and remediation.

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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/acs.macromol.8b00669. Experimental details and characterization of the monomer and BF-CMPs (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.C.). ORCID

Long Chen: 0000-0001-5908-266X E

DOI: 10.1021/acs.macromol.8b00669 Macromolecules XXXX, XXX, XXX−XXX

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Author Contributions

B.W. and Z.X. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51522303 and 21602154) and the National Key Research and Development Program of China (2017YFA0207500).

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DOI: 10.1021/acs.macromol.8b00669 Macromolecules XXXX, XXX, XXX−XXX