Imidazolium-Based Porous Organic Polymers - ACS Publications

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Imidazolium-based Porous Organic Polymers: Anion ExchangeDrived Rapid Capture and Luminescent Probe of CrO 2

72-

Yanqing Su, Yangxin Wang, Xiaoju Li, Xinxiong Li, and Ruihu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05918 • Publication Date (Web): 01 Jul 2016 Downloaded from http://pubs.acs.org on July 3, 2016

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ACS Applied Materials & Interfaces

Imidazolium-based Porous Organic Polymers: Anion Exchange-Drived Rapid Capture and Luminescent Probe of Cr2O72Yanqing Su, †,‡ Yangxin Wang, ‡ Xiaoju Li,* †,‡ Xinxiong Li, ‡ and Ruihu Wang*‡ †

Fujian Key Laboratory of Polymer Materials, College of Materials Science and

Engineering, Fujian Normal University, Fuzhou, Fujian, 350007, China. ‡

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China.

KEYWORDS. porous organic polymers; imidazolium; anion exchange; pollutant removal; luminescent probe

ABSTRACT. A series of imidazolium-based porous organic polymers (POP-Ims) were synthesized through Yamamoto reaction of 1,3-bis(4-bromophenyl)imidazolium bromide and tetrakis(4-bromophenyl)ethylene. Porosities and hydrophilicity of such polymers may be well tuned by varying the ratios of two monomers. POP-Im1 with the highest density of imidazolium moiety exhibits the best dispersity in water and the highest efficiency in removing Cr2O72-. The capture capacity of 171.99 mg g-1 and the removal efficiency of 87.9% were achieved using equivalent amount of POP-Im1 within 5 min. However, no Cr2O72- capture was observed using non-ionic analogue (POP-TBE) despite of its large surface area and abundant pores, suggesting that anion 1

ACS Paragon Plus Environment


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exchange is the driving force for the removal of Cr2O72-. POP-Im1 also displays excellent enrichment ability and remarkable selectivity in capturing Cr2O72-. Cr(VI) in acid electroplating wastewater can be removed completely using excess POP-Im1. In addition, POP-Im1 can serve as a luminescent probe for Cr2O72- due to the incorporation of luminescent tetraphenylethene moiety.

1. Introduction

The contamination of water resources is one of global major concerns as enormous quantities of wastewater containing heavy metal ions have been discharged. Cr(VI) has been widely used in electroplating, printing, pigment and other industries. As one of toxic pollutants in wastewater, Cr(VI) oxoanions can cause harm to humans even at low concentrations due to their bioaccumulation through food chain.1,2 Various operation technologies and materials, such as chemical precipitation, electrolysis method, liquid-liquid extraction, liquid membrane separation, adsorption using active carbon, bioadsorbents and resins, have been reported for the detection and/or removal of Cr(VI),3-6 but they usually encounter one or more problems, such as low adsorption capacities, slow kinetics, inferior selectivity and poor chemical stabilities. Consequently, it is highly desirable to explore new materials with high efficiency, fast speed and good stabilities under varied conditions to carry out rapid capture and removal of Cr(VI). As an important class of porous materials, metal-organic frameworks (MOFs), 2


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especially cationic MOFs, have recently been employed for the removal and detection of chromate (CrO4-) 7,8 or dichromate (Cr2O72-) 9-12 in water. However, most of them possess poor stabilities and are apt to decompose during water treatment owing to reversible nature of coordination bonds.13 In contrast, porous organic polymers (POPs), which feature relatively stable covalent bonds, possess higher chemical stability and better tailorability.14, 15 Similar to MOFs, the modular nature of POPs synthesis allow to elegantly modulate the structures and porous properties of POPs through judicious selection of building units and synthetic methods, which provides great opportunity to incorporate luminescent groups and other functional groups into polymers at molecular level.16, 17 In cationic POPs, the positive charge is totally in the host framework, and charge-balancing counter anions, such as halides, reside in the cavities or channels of POPs.18 The electrostatic interaction between the host frameworks and counter halide anions is weak, and counter halide ions contribute to improving dispersity of cationic POPs in water, resulting in fast exchange of Cr(VI) oxoanions in water by the counter halide anions.19 Meantime, the interaction between the host frameworks and different counter anions may generate different luminescent properties. The detectable luminescent responses transduced by guest-host interactions may be used to examine the existence of Cr(VI) oxoanions based on their fluorescent quenching effect.20, 21 Consequently, the cationic POPs can be expected to be a class of promising materials for the luminescent detection and removal of Cr(VI) oxoanions. Recently, a few cationic POPs containing imidazolium groups in their host 3



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frameworks have been reported, but their applications have mainly focused on heterogeneous catalysis, gas sorption and separation,22-25 the exploration for the capture and detection of contaminant anions has not been reported hitherto. In our continuous efforts to seek for new types of materials for the removal of Cr2O72-,

9,13

herein, we report a series of durable imidazolium-based POPs (POP-Ims). These POP-Ims not only show large capture capacity, rapid speed and high selectivity in the removal of Cr2O72- through anion exchange, but also can serve as a luminescent probe for Cr2O72-. 2. Experimental Section 2.1 General Information 1,3-Bis(4-bromophenyl)imidazolium ethylene

26,27

bromide25

and

tetrakis(4-bromophenyl)-

were synthesized according to the literature methods, other chemicals

were commercially available. Solid-state

13

C CP/MAS NMR was performed on a

Bruker SB Avance III 500 MHz spectrometer with a 4-mm double-resonance MAS probe, a sample spinning rate of 8.0 kHz, a contact time of 2 ms and pulse delay of 5 s. FTIR spectra were recorded on KBr pellets by using Perkin-Elmer Instrument. Liquid UV-Vis spectroscopic analyses were performed on a Lambda35 UV-Vis spectrophotometer. The photoluminescence excitation and emission spectra were performed on an FLS920 Edinburgh Analytical instrument at room temperature. Nitrogen adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP 2020 system. The samples were degassed at 120 oC for 10 h 4


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before the measurements. Specific surface areas were calculated from the adsorption data using Brunauer-Emmett-Teller (BET) methods. Field-emission scanning electron microscopy (SEM) was performed on JEOL JSM-7500F operated at an accelerating voltage of 3.0 kV. Transmission electron microscope (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were obtained with TECNAI G2F20 instrument. Inductively coupled plasma (ICP) analysis was measured on Jobin Yvon Ultima2. Thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449C by heating samples from 25 to 800 oC in a dynamic air atmosphere with a heating rate of 10 °C min-1. Thermogravimetric-mass spectrometric (TG-MS) analysis was carried out on NETZSCH STA 449C spectrometer by heating samples from 35 to 800 oC in a dynamic nitrogen atmosphere with a heating rate of 10 °C∙min-1. Elemental analyses were performed on Vario EL III Elemental Analyzer. 2.2 Synthesis of POP-Im1 To a solution of 2,2′-bipyridine (565 mg, 3.63 mmol), bis(1,5-cyclooctadiene)nickel(0) (1.0 g, 3.63 mmol) and 1,5-cyclooctadiene (0.45 mL, 3.63 mmol) in anhydrous DMF (40 mL), 1,3-bis(4-bromophenyl)imidazolium bromide (367 mg, 0.80 mmol) and tetrakis(4-bromophenyl)ethylene (259 mg, 0.40 mmol) were added under nitrogen atmosphere. The mixture was stirred at 80 oC overnight. After cooling to room temperature, Aqueous HBr solution (3 mol L-1, 20 mL) was added, the resulting mixture was stirred for 6 h. The precipitate was collected by filtration, 5



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washed with DMF, methanol, dichloromethane and cyclohexane. The resultant yellow product was further Soxhlet extracted with dichloromethane and then dried in vacuo at 80

o

C. Yield: 81 % (300 mg). Elemental analysis calcd (%) for C28H19BrN2

(462.08): C 72.58, H 4.13, N 6.05. Found: C 66.83, H 4.98, N 5.32. Solid-state

13

C

NMR spectrum (500 Hz): δ 140, 127. IR (KBr pellet, cm− 1): 3634 (w), 3022 (vw), 1601 (s) , 1493 (vs), 1394 (w), 1335 (vw), 1311 (vw), 1285 (vw), 1249 (m), 1185 (w), 1105 (w), 1066 (vw), 1002 (m), 951 (w), 829 (s), 764 (m), 697 (m), 610 (vw), 512 (vw). 2.3 Synthesis of POP-Im2 The procedure was similar to that of POF-Im1 except that 1:1 molar ratio of 1,3-bis(4-bromophenyl) imidazolium

bromide (243 mg, 0.53 mmol) and

tetrakis(4-bromophenyl)ethylene (343 mg, 0.53 mmol) was used. Yield: 87 % (290 mg). Elemental analysis calcd (%) for C41H27BrN2 (626.14): C 78.47, H 4.34, N 4.46. Found: C 73.83, H 5.05, N 3.49. Solid-state 13C NMR spectrum: δ 140, 127. IR (KBr pellet, cm-1): 3634 (w), 3022 (vw), 1601 (s) , 1493 (vs), 1394 (w), 1335 (vw), 1311 (vw), 1285 (vw), 1249 (m), 1185 (w), 1105 (w), 1066 (vw), 1002 (m), 951 (w), 829 (s), 764 (m), 697 (m), 610 (vw), 512 (vw). 2.4 Synthesis of POP-Im3 The procedure was similar to POF-Im1 except that 1:2 molar ratio of 1,3-bis(4-bromophenyl)imidazolium

bromide

(147

mg,

0.32

mmol)

and

tetrakis(4-bromophenyl)ethylene (414 mg, 0.64 mmol) was used. Yield: 85% (260 6


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mg). Elemental analysis calcd (%) for C67H43BrN2(954.26): C 84.18, H 4.53, N 2.93. Found: C 78.67, H 5.15, N 2.60. Solid-state 13C NMR spectrum: δ 140, 127. IR (KBr pellet, cm-1): 3641 (vw), 3028 (m), 1903 (vw), 1601 (s), 1494 (vs), 1394 (w), 1249 (w), 1176 (w), 1111 (w), 1-77 (vw), 1003 (m), 951 (vw), 816 (s), 749 (m), 693 (m), 621 (vw), 578 (vw), 498 (vw). 2.5 Synthesis of POP-TBE The

procedure

was

similar

to

that

POF-Im1

except

that

only

tetrakis(4-bromophenyl) ethylene (515 mg, 0.8 mmol) was used. Yield: 90% (230 mg). Elemental analysis calcd (%) for C26H16(328.13): C 95.09, H 4.91. Found: C 94.02, H 5.28, N 0.64. Solid-state 13C NMR spectrum: δ 140, 129, 124. IR (KBr pellet, cm-1): 3662 (vw), 3025 (s), 2938 (w), 2859 (vw), 1903 (vw), 1683 (m), 1609 (m), 1492 (vs), 1449 (vw), 1396 (m), 1272 (w), 1186 (vw), 1116 (w), 1080 (vw), 1004 (s), 971 (vw), 805 (vs), 749(s), 701 (m), 621 (vw), 581 (w), 501 (w). 2.6 Procedures for anion exchange 2.6.1 2:1 molar ratio of Br- to K2Cr2O7 As-synthesized POP-Im1 (21 mg, 0.04 mmol) was immersed in aqueous solution of K2Cr2O7 (0.002 mol L-1, 10 mL), the mixture was shaken at room temperature. The anion exchange process was monitored by liquid UV-vis spectroscopy based on typical absorption of Cr2O72- at 257 nm. 1.0 mL aqueous K2Cr2O7 solution was pipetted after appropriate time and was diluted using 4.0 mL deionized water to

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measure UV-vis adsorption intensity. The anion exchange capacity of POP-Im1 was calculated by the following formula:

Where Dt and Qt are exchange capacity, C0, A0 and Ct, At are the concentration and absorbance of aqueous K2Cr2O7 solution at the peak of 257 nm before and after anion exchange, respectively. V is the volume of the solution, m is the mass of sorbent. The same experiments were performed for Cr2O72- exchange using POP-Im2 and POP-Im3 except both adsorbent and K2Cr2O7 solution were halved. 2.6.2 4:1 molar ratio of Br- to K2Cr2O7 As-synthesized POP-Im1 (21 mg, 0.04 mmol) was immersed in aqueous solution of K2Cr2O7 (0.002 mol L-1, 5 mL), the mixture was shaken at room temperature. The process was monitored by liquid UV-vis spectroscopy at different time intervals. 1.0 mL aqueous K2Cr2O7 solution was pipetted after appropriate time and was diluted using 4 mL deionized water to measure the UV-vis adsorption intensity. 2.6.3 Anion exchange for extremely low concentration of Cr2O72As-synthesized POP-Im1 (21 mg, 0.04 mmol) was immersed in aqueous solution of K2Cr2O7 (0.002 mol L-1, 5 mL), the mixture was shaken at room temperature. The process was monitored by liquid UV-vis spectroscopy at different time intervals. 1.0 mL aqueous K2Cr2O7 solution was pipetted after appropriate time and was diluted using 4 mL deionized water to measure the UV-vis adsorption intensity. 2.6.4 Anion exchange for wastewater from electroplating industry 8


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As-synthesized POP-Im1 (200 mg) was immersed in wastewater from electroplating industry (3 mL), the mixture was shaken at room temperature for appropriate time. The resultant mixture was filtered, rinsed with deionized water and dried in air. The solid was used for IR measurement, and the filtrate was pipetted for UV-vis measurement and ICP analysis. 2.6.5 General procedure for selective exchange of Cr2O72As-synthesized POP-Im1 (21 mg, 0.04 mmol) was immersed in aqueous solution of K2Cr2O7 (0.002 mol L-1, 10 mL) together with n-fold (n = 2, 4, 8, 12, 20, 40) molar amount of disturbing anions of Cl-, NO3- or SO42-. After 10 min, 1.0 mL aqueous solution was pipetted and was diluted using 4 mL deionized water to measure the UV-vis adsorption intensity. 2.6.6 The adsorption isotherm experiments As-synthesized POP-Im1 (11 mg, 0.02 mmol) were immersed in 5 mL aqueous solution of Cr2O72- with the appropriate concentration (108-432 mg L-1). After shaken for 30 min, the resultant mixture was filtered, and the filtrate was pipetted for UV-vis measurement. The adsorption isotherm for Cr2O72- was analyzed by Tempkin adsorption isotherm,28 which was expressed as

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Where Ce is the concentration of Cr2O72- at exchange equilibrium (mg L-1), qe is the capture capacity of Cr2O72- by POP-Im1 at the equilibrium (mg g-1). A and b are constants.

3. Results and Discussion Palladium-catalyzed cross-coupling reactions are one type of common methods for the synthesis of imidazolium-based POPs, but it is incapable of removing palladium residues completely after the catalytic reactions,25 and the residual palladium species can strongly disturb the intrinsic performances of POPs. In contrast, nickle-catalyzed Yamamoto reaction29 of aryl-halogenides not only can avoid metal residues through simple treatment with inorganic acid, but also can generate POPs with large surface areas.30 Although several POPs were prepared through either homocoupling of single aromatic halides or copolymerization of different aromatic halides, cationic POPs based on Yamamoto reaction have not been reported yet. As shown in Scheme 1, POP-Im1, POP-Im2 and POP-Im3 were prepared through Yamamoto reaction of different molar ratio of 1,3-bis(4-bromophenyl)imidazolium bromide (BPIB) and tetrakis(4-bromophenyl) ethylene (TBPE). After the reaction was quenched by aqueous HBr solution, the precipitate was filtered and successively washed with deionized water, DMF, methanol and dichloromethane to remove Ni(II) salt and any other possible residues. The resultant yellow powder was further treated by Soxhlet extraction in dichloromethane to give the target products. For comparison, non-ionic POP-TBE was prepared with TBPE as a monomer under the identical 10


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reaction conditions. All samples are insoluble in water and common organic solvents. Their dispersibility in water is in close correlation with the density of imidazolium group (Figure S1). POP-Im1 with the highest density of imidazolium group manifests the best dispersity in water, while POP-TBE is hydrophobic owing to the absence of cationic group in its host framework.

Scheme 1. Schematic illustration for the synthesis of POP-Ims and POP-TBE.

The chemical structures and compositions of POP-Im1, POP-Im2, POP-Im3 and POP-TBE were determined by solid-state 13C NMR, FTIR and elemental analyses. In the solid-state

13

C NMR spectrum of POP-TBE (Figure 1a), the broad peak at 140

ppm corresponds to the carbon atoms of vinyl group in tetraphenylethene unit, and the broad peaks at 129 and 127 ppm may be ascribed to the carbon atoms of aromatic rings.22, 31 The solid-state 13C NMR spectra of POP-Im1, POP-Im2 and POP-Im3 are similar to POP-TBE except for the gradual overlap of aromatic peaks with the increment of imdazolium group. The FTIR spectra of POP-Im1, POP-Im2, POP-Im3 and POP-TBE are almost identical with each other (Figure 1b). Thermogravimetric analyses (TGA) show that they are stable up to 300 °C in air (Figure S2). The initial weight losses of 5.52%, 2.66% and 2.26% before 150 oC are observed in POP-Im1, 11



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POP-Im2 and POP-Im3, respectively, which are attributed to the release of the captured water molecules, this is further confirmed by ion current at m/z=18 in the thermogravimetric-mass spectrometric (TG-MS) curves (Figure S3). No other guest molecules, such as DMF, are detected in POP-Im1, POP-Im2, POP-Im3. In comparison with these ionic polymers, there is no significant weight loss before 250 o

C in POP-TBE. Elemental analyses display that the contents of imidazolium group in

POP-Im1, POP-Im2 and POP-Im3 are 1.90, 1.25 and 0.93 mmol g-1, respectively.

Figure 1. (a) Solid-state

13

C NMR spectra of POP-Ims and POP-TBE; (b) FTIR

spectra of monomers, POP-Ims, POP-TBE and POP-Im1-Cr; (c) N2 adsorption isotherms of POP-Ims and POP-TBE measured at 77 K.; (d) Pore size distributions of POP-Im2, POP-Im3 and POP-TBE.

The porosities of POP-Ims and POP-TBE were investigated by nitrogen adsorption experiments at 77 K (Figure 1c). N2 adsorption of POP-Im1 is negligible, which is 12


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probably attributed that the existence of abundant Br- blocks the possible foraminule. As for POP-Im2, POP-Im3 and POP-TBE, N2 physisorption measurements express a combination of type I and IV isotherms.32,

33

Their specific surface areas are

193.24±2.51, 618.29±8.58 and 1045.28±10.38 m2 g-1, respectively, suggesting that the incorporation of more imidazolium group in the host framework results in decrement of the surface area. The severe hysteresis between the adsorption and desorption branch in the isotherm of POP-TBE indicates the swelling of framework structure and/or the existence of mesopores.23,

34

The pore size distribution for

POP-Im2, POP-Im3 and POP-TBE were calculated using nonlocal density functional theory (Figure 1d). POP-Im2 and POP-Im3 mainly consist of micropores, while both micropores and mesopores are found in POP-TBE. Scanning electron microscopy (SEM) images show that POP-Ims and POP-TPE are composed of tiny particles (Figure 2a, b and Figure S4). High-annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) images clearly demonstrate homogeneous distribution of nitrogen atoms and Br- throughout POP-Im1, POP-Im2 and POP-Im3 (Figure 2c, d and Figure S5). It should be mentioned that no nickel residues were detected in EDX spectra of POP-Im1, POP-Im2 and POP-Im3 (Figure S6), which is much different from the imidazolium-based

ionic

polymers

synthesized

through

Suzuki-Miyaura cross-coupling reaction.25

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palladium-catalyzed


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Figure 2. SEM images of (a) POP-Im1 and (b) POP-TBE; HAADF-STEM and EDX elemental mapping images for POP-Im1 (c and d).

Anion exchange experiments were performed initially with POP-Im1 in aqueous K2Cr2O7 solution. The exchange process was monitored by liquid UV-vis spectroscopy based on typical absorption of Cr2O72- at 257 nm. After equivalent amount of POP-Im1 (21 mg, 0.04 mmol) was placed into aqueous K2Cr2O7 solution (0.02 mmol, 10 mL), Cr2O72- concentration in solution decreased quickly by 86.1% and 87.9% after 2 min and 5 min, respectively. The color of Cr2O72- solution becomes light. There are no further variations in Cr2O72- concentration and solution color with the extension of exchange time (Figure 3a and Figure S7), indicating that the anion exchange approaches equilibrium within 5 min. The overall capture capacity of POP-Im1 for Cr(VI) is 171.99 mg g-1 (Figure 3b), which is in accordance with the 14


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result (171.52 mg g-1) from inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. After anion exchange of POP-Im1, the resultant claybank powder (denoted as POP-Im1-Cr) was collected (Figure 3a insert). The characteristic band of Cr2O72- at 939 cm-1 can be observed in the FTIR spectrum of POP-Im1-Cr (Figure 1b). It’s worth mentioning that the removal efficiency of 87.9% within 5 min for equivalent amount of POP-Im1 is much superior to our previously reported Ag2(btr)2·2ClO4, which only provides a 73% removal efficiency in 24 h.9 The capture rate and capacity of POP-Im1 toward Cr2O72- are also superior to most of reported cationic MOFs materials and the commercial anion exchange resins. For examples, the double-layer structure MOF, FIR-53, exhibits an uptake capacity of 103 mg g-1 in 16 h. Three-dimensional cationic MOF, 1-SO4, was reported to completely remove Cr2O72- in 72 h.12

The macroporous anion-exchange resins, D301, D314 and D354,

have exhibited 20% and 99% removal efficiency in 100 ppm of acidic Cr(VI) solution in 5 min and 30 min, respectively, and their capture capacities at 30 min are 152.52, 120.48 and 156.25 mg g-1, respectively.35 Styrene-divinyl benzene copolymer, DEX-Cr, displays higher capture capacity of 248 mg g-1 in 60 min than POP-Im1, but only provide a capture capacity of 30 mg g-1 in 5 min.36 Cr2O72- capture experiment was further conducted using two equivalent amount of POP-Im1 (21 mg, 0.04 mmol) in aqueous K2Cr2O7 solution (0.01 mmol, 5 mL). It was found that 96.2% of Cr2O72- was removed within 2 min (Figure S8). The removal 15



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efficiency reaches to 98.1% after 5 min, which is consistent with ICP-AES analysis result (98.6%). Meanwhile, the aqueous solution of K2Cr2O7 was destained obviously, manifesting the substantial removal of Cr2O72- (Figure 3c). To investigate the effect of K2Cr2O7 concentration on the capture process, the adsorption isotherms of K2Cr2O7 describing the relationship of capture capacity (qe) and equilibrium concentration (Ce) were examined. As shown in Figure S9, qe and lnCe are in a linear relation, which displays that the adsorption behavior follows the Tempkin adsorption isotherm.28 The value of lnA and b are 0.35 and 49.41, respectively, and the R2 is 0.9948. When equivalent amount of POP-Im2 and POP-Im3 were employed for Cr2O72capture, the equilibrium was also reached within 5 min. The overall capture capacities of POP-Im2 and POP-Im3 are 107.21 and 74.45 mg g-1, respectively, and their removal efficiencies are 83.6% and 76.2%, respectively. It’s obvious that the capture capacity and removal efficiency of POP-Ims are highly dependent on the densities of the imdazolium moiety.

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Figure 3. (a) UV-vis spectra of aqueous K2Cr2O7 solution during exchange with equivalent amount of POP-Im1. Insert: pictures of POP-Im1 and POP-Im1-Cr; (b) Capture capacity of POP-Ims for Cr2O72- as a function of time; (c) Color change of aqueous K2Cr2O7 solution during anion exchange using two equivalent of POP-Im1; (d) UV-vis spectra of aqueous K2Cr2O7 solution during exchange with POP-TBE.

As a control experiment, non-ionic POP-TBE (20 mg) was also tested in aqueous K2Cr2O7 solution (0.02 mmol, 10 mL). In spite of the abundant micro- and meso-pores as well as large specific areas of POP-TBE, Cr2O72- concentration and solution color almost have no change after 12 h (Figure 3d and Figure S10), suggesting the incompetence of POP-TBE in removing Cr2O72-. Such observations conclude that the driving force for the removal of Cr2O72- using POP-Ims mainly results from the anion exchange rather than physisorption.

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Figure 4. (a) UV-vis spectra of 28.0 mg L-1 aqueous K2Cr2O7 solution during anion exchange with excess POP-Im1; (b) Color change of 28.0 mg L-1 aqueous K2Cr2O7 solution during anion exchange with excess POP-Im1; (c) Effect of single disturbing ion (Cl -, NO3- and SO42-) on capture capacity of POP-Im1; (d) Effect of multiple disturbing ions (Cl -, NO3- and SO42-) on capture capacity of POP-Im1.

Given the best performance of POP-Im1, the enrichment ability of Cr2O72- was further investigated in more dilute aqueous solution. As shown in Figure 4, when excess POP-Im1 (10 mg) was placed in 28.0 mg L-1 aqueous K2Cr2O7 solution. The color of solution changed from pale yellow to colorless within 2 min, and the characteristic absorption peak of Cr2O72- almost disappeared completely in UV-vis spectroscopy. ICP-AES analysis shows that the concentration of residual Cr(VI) is only 0.197 mg L-1 at 30 min, suggesting good enrichment ability of POP-Im1 for Cr2O72-.

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As well known, wastewater usually contains more than one type of anions, so it is indispensable to investigate the capturing selectivity for Cr2O72-. The selectivity of POP-Im1 for Cr2O72- was tested initially in the presence of single disturbing ion (Figure 4c). When POP-Im1 (21 mg, 0.04 mmol) was immersed in 10 mL aqueous solution containing 0.02 mmol Cr2O72- and 8-fold moles of Cl-, the removal efficiency for Cr2O72- almost has no change and is still as high as 87.9%. Notably, when 40-fold moles of Cl- was added, the removal efficiency only decreased slightly to 82.4%. Similarly, the excellent selectivity was also observed for 40-fold moles of SO42- with a removal efficiency of 81.5% for Cr2O72-. Comparing with Cl- and SO42-, NO3exhibited slightly stronger disturbance on the capture of Cr2O72-, and 60.0% Cr2O72was exchanged when 40-fold moles of NO3- was used as a disturbing anion. The influence of the existence of multiple disturbing anions was also studied. As shown in Figure 4d, the removal efficiency maintains as high as 80.8% in the presence of 2-fold moles of Cl-, SO42- and NO3-. When the dosage of multiple disturbing anions increased to 12 fold, the removal efficiency for Cr2O72- only decreased to 62.9%. These results show that POF-Im1 possesses superior selectivity for Cr2O72- over Cl-, NO3- and SO42-. The practical application of POF-Im1 was further investigated using wastewater from electroplating industry. When excess of POF-Im1 was immersed in the acidic electroplating wastewater contains 937 mg L-1 of Cr(VI), the yellow mixture was rapidly destained within 5 min, and the characteristic absorption peak of Cr2O72 19



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disappeared completely in UV-vis spectroscopy (Figure 5a), further elongation of the exchange time has no detectable effect on them. ICP-AES analysis shows that the concentration of residual Cr(VI) is 0.235 mg L-1, corresponding to removal efficiency of 99.97% It should be mentioned that the Cr(VI) residue is lower than the permissible limits of discharge for industrial wastewater (0.25 mg L-1).37 The FTIR spectrum in the exchanged POPs-Im1 (Figure 5b) is identical with that of as-synthesized POPs-Im1 except that the characteristic band of Cr2O72- appears at 939 cm-1, suggesting that the structural framework of POP-Im1 is well maintained after capturing Cr(VI) in the electroplating wastewater. The excellent stability of POP-Im1 and promising capture performances of Cr(VI) provide great promises for practical application for the removal of Cr(VI) oxoanions in the industrial wastewater.

Figure 5. (a) UV-vis spectra of electroplating wastewater during anion exchange with excess POP-Im1. Insert: Color change during anion exchange; (b) FTIR spectra of POP-Im1 (black) and the exchanged POP-Im1 in the electroplating wastewater (red).

Tetraphenylethene is a typical chromophore, whose luminescence may be enhanced through restricting the rotation of peripheral phenyl groups using coordination or 20


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covalent bonds.26,

27, 38

The luminescent property of POP-Im1 was also explored.

Impressively, POP-Im1 exhibits a broad emission peak with maxima at 595 nm upon excitation at 365 nm (Figure 6), which are 195 and 105 nm red-shift comparing with that in TBPE (440 nm) (Figure S15a) and BPIB (495 nm) (Figure S15b), respectively. This red-shift is presumably attributed to the extended π-conjugation structure of POP-Im1.39 POP-Im1 emits a yellow light under UV irradiation, which is much different from that from TBPE and BPIB (Figure S15). It should be mentioned that the emission of POP-Im1-Cr is almost invisible to naked eyes (Figure 6 Insert). The solid-state luminescence spectra further show that the luminescence intensity of POP-Im1-Cr is largely impressed in comparison with POP-Im1. The dramatic luminescence quenching probably originates from the fact that the electron-transfer transitions of Cr2O72- decrease the energy of π-π* transition in the host framework of POP-Im1.9, 20 POP-Im1 may thus be considered a luminescent probe for Cr2O72-, an application that has not hitherto been reported for cationic POPs.

Figure 6. Room-temperature solid-state emission spectra upon excitation at 365 nm for a) POP-Im1 and b) POP-Im1-Cr. Insert: fluorescent images under UV irradiation. 21



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4. Conclusion The imidazolium-based POPs were prepared through Yamamoto reaction. The applications of such cationic POPs in capture and detection of toxic Cr2O72- have been explored for the first time. Owing to the presence of imidazolium in the host framework, POP-Im1 exhibits not only rapid speed, large capture capacity, high removal efficiency, good enrichment ability and excellent stability in removing Cr2O72-, but also remarkable selectivity for Cr2O72- even in the presence of large excess disturbing anions. Cr(VI) from the industrial electroplating wastewater can be removed completely using excess POP-Im1, the residual Cr(VI) is lower than the permissible limits of discharge for industrial wastewater. The control experiments have confirmed that such promising performances are derived from anion exchange between Br- and Cr2O72- rather than physisorption. POP-Im1 is also applied for detection of Cr2O72- based on luminescent quenching effect. In summary, this work not only provides a new approach for the synthesis of cationic POPs without metal residues, but also widens the application of POPs, which offers a new inspiration for the removal and detection of other anionic pollutant species using durable cationic POPs.

ASSOCIATED CONTENT Supporting Information. The dispersity in water and pore size distributions of POPs; The additional SEM, HAADF-TEM and EDX mapping images, color change and UV-vis spectra of aqueous K2Cr2O7 solution during anionic exchange; Fluorescence 22


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spectra for TBPE and BPIB. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author Ruihu Wang, E-mail: [email protected], Xiaoju Li, Email: [email protected]

Notes The authors declare no competing financial interest. Acknowledgment This work was supported by the National Natural Science Foundation of China (21471151, 21273239), Natural Science Foundation of Fujian Province (2015J01038, 2015J05041) and Program for Innovative Research Team in Science and Technology in Fujian Province University (IRTSTFJ). References (1) Aroua, M. K.; Zuki, F. M.; Sulaiman, N. M. Removal of Chromium Ions from Aqueous Solutions by Polymer-Enhanced Ultrafiltration. J. Hazard. Mater. 2007, 147, 752-758.

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