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Microporous org

aDepartment of Chemistry, Sungkyunkwan

sson@skku.edubKorea Basic Science Institute, Daejeon 350-

† Electronic supplementary information (PXRD patterns, and characterization datsuccessive ltration. See DOI: 10.1039/c6r

Cite this: RSC Adv., 2016, 6, 83942

Received 21st May 2016Accepted 31st August 2016

DOI: 10.1039/c6ra13220k

www.rsc.org/advances

83942 | RSC Adv., 2016, 6, 83942–839

anic network@PET hybridmembranes: removal of minute organic pollutantsdissolved in water†

Eui Soon Kim,a Ju Hong Ko,a Sang Moon Lee,b Hae Jin Kimb and Seung Uk Son*a

This work shows that microporous organic network (MON) chemistry

can be applied for the engineering of hybrid membranes. While

a polyethylene terephthalate (PET) membrane did not work for the

removal of minute organic pollutants dissolved in water, the MON@-

PET hybrid membranes showed promising filtration towards aromatic

pollutants in water.

Porous polymer membranes have been engineered for variouspurposes including the removal of environmental pollutants inwater.1 For example, polyethylene terephthalate (PET) berswere prepared by electrospinning and further engineered toform macroporous membranes.2 Due to chemical stability,conventional PET membranes with 0.1–10 mm pore widths havebeen applied for the removal of heterogeneous pollutants inwater3 (Fig. 1). However, those PET membranes do not work forthe removal of organic pollutants dissolved in water. Althoughnanoltration technologies have attracted more attention inrecent years for the removal of the dissolved species in water,4

decreasing pore width to sub 100 nm in PET membranes istechnically difficult. Thus, the incorporation of secondarymaterials into PET membranes is an alternative approach toinduce new functionality.4,5

Recently, various microporous organic networks (MONs)have been prepared by the coupling of organic building blocks.6

The MON materials showed microporosity (pore size < 2 nm)and high surface area6 and were applied as gas adsorbents.6,7 Inaddition, the powdery MON materials were investigated for theadsorptive removal of oil oating on the water.8 Recently, MONlms have been fabricated on the solid supports includingelectrodes.9 However, the fabrication of MON relatedmembranes is still rare.10 The introduction of MONmaterials to

University, Suwon 16419, Korea. E-mail:

333, Korea

ESI) available: Experimental procedure,a of recovered MON@PET-3 aer vea13220k

46

PET membrane can be a new approach for functional hybridmembranes with enhanced properties. However, as far as we areaware, this approach has not yet been reported.

As environmental regulation becomesmore andmore strict, theupper concentration limit of organic residues dissolved in watergradually decreases. For example, recently, the permissible expo-sure limit (PEL) of nitrobenzene, and phenol decreased to 1 and 5ppm, respectively by the U. S. Occupational Safety & HealthAdministration (OSHA).11 Thus, the removal methods for minuteorganic residues dissolved in water should be developed. Ourresearch group has studied the MON based composite materialsfor environmental applications.12 In this work, we report theengineering ofMON@PEThybridmembranes and their adsorptiveremoval performance of aromatic pollutants dissolved in water.

Fig. 1 shows an engineering scheme for MON@PET hybridmembranes.

In the presence of PET membrane (disc shape with a 2.5 cmdiameter and a 180–200 mm thickness), 1 eq. tetrakis(4-ethynylphenyl)methane and 2 eq. 1,4-diiodobenzene werereacted at 90 �C to form MON materials via the Sonogashiracoupling. As the reaction progressed, MON materials graduallyincorporated into the PET membrane, resulting in the color

Fig. 1 Preparation of MON@PET hybrid membranes.

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change of the PET membrane from white to brownish yellow.Aer 3 days, the resultant MON@PET hybrid membrane wasretrieved and washed with solvents. The membrane was cut toa disc with a 1.3 cm diameter. We screened reaction time from 1day to 2, 3, and 5 days. The obtained MON@PET membraneswere denoted as MON@PET-1, MON@PET-2, MON@PET-3, andMON@PET-5, respectively. Fig. 2a shows the photographs ofPET and MON@PET membranes. The brownish yellow colorbecame gradually denser from MON@PET-1 to MON@PET-5.Infrared absorption (IR) spectroscopy of the control MONmaterials prepared by the Sonogashira coupling of tetrakis(4-ethynylphenyl)methane and 1,4-diiodobenzene showed themain peaks at 1506 and 832 cm�1 (the peaks indicated byasterisks in Fig. 2b), corresponding to the vibrational stretchingof C–C and C–H bonds in aromatic rings, respectively.13 Theintensity of these peaks gradually increased from MON@PET-1to MON@PET-5, indicating the loading of MON materials onthe PET membrane (Fig. 2b).

Fig. 2 (a) Photographs, (b) IR absorption spectra, (c) UV-vis absorptionspectra (obtained through the conversion of reflectance spectra) ofPET, MON@PET-1, MON@PET-2, MON@PET-3, MON@PET-5, andcontrol MON materials. SEM images and water contact angles of PET(d and g), MON@PET-3 (e and h), and control MON materials (f and i).

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The absorption spectra obtained from reectance spectros-copy showed that the absorption intensity in the visible lightregion gradually increased from MON@PET-1 to MON@PET-5(Fig. 2c). The scanning electron microscopy (SEM) showedthat the PET membrane consists of�20 mm diameter bers and1–10 mm gap widths (Fig. 2d). The MON materials inMON@PET-3 and the control MON materials showed sphericalmorphology with a 0.80 � 0.18 mm diameter (Fig. 2e and f). TheMON particles were mostly entrapped in the pores of PETmembranes (Fig. 2e and S1 in the ESI†). According to the watercontact angle measurement, water drop gradually adsorbed inthe PET membrane (Fig. 2g). In comparison, the control MONparticles showed superhydrophobicity (water contact angle >150�) with a 152� water contact angle (Fig. 2i). The water contactangles increased gradually from 126� to 130, 138, and 143� forMON@PET-1, MON@PET-2, MON@PET-3, and MON@PET-5,respectively (Fig. 2h and S1 in the ESI†).

The MON materials formed during the preparation ofMON@PET-3 were further investigated. The analysis of N2

sorption isotherm curves of the MON materials based on Bru-nauer–Emmett–Teller theory showed a high surface area of 876m2 g�1, pore volume of 0.40 cm3 g�1, and microporosity. Krsorption isotherm measurements showed that microporosity(SBET: 25 m2 g�1) of MON@PET-3 was induced through incor-poration of MON materials14 (Fig. 3a and b and inset).

The transmission electron microscopy (TEM) showed thequite homogeneous size distribution of MON particles witha 0.80 mm average diameter (Fig. 3c). Solid phase 13C nuclearmagnetic resonance (NMR) spectroscopy showed the 13C peaksat 64, 85–95, and 116–151 ppm, corresponding to the benzylcarbon, internal alkyne, and aromatic rings, respectively. Thepowder X-ray diffraction (PXRD) studies revealed the amorphous

Fig. 3 (a) N2 adsorption–desorption isotherm curves at 77 K, (b) poresize distribution diagram based on DFT method, (c) TEM image, and (d)solid phase 13C NMR spectrum of control MON materials. Kr adsorp-tion isotherm curve and pore size distribution diagram of PET andMON@PET-3 at 77 K (inset of a and b).

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characteristics of MON particles, matching well with the prop-erties of MON materials prepared by the Sonogashira couplingin the literature15 (Fig. S2 in the ESI†).

Simple aromatic compounds are one of the most importantintermediates in synthetic industry.16 Especially, nitrobenzenehas been used for the production of explosives, pesticides,plastic, and dyes. At the same time, it is a priority pollutantlisted by the US Environmental Protection Agency because of itscarcinogenic toxicity.17 The removal of aromatic pollutantsdissolved in water is a critical subject of environmentalconcerns.16,17 Considering the organophilicity and high surfacearea, and microporosity of MON materials, we tested theremoval performance of MON@PET hybrid membranes towardaromatic pollutants dissolved in water. We found that theMON@PET hybrid membranes work well for the adsorptiveremoval of nitrobenzene (the PEL of US-OSHA: 1 ppm) in water.Fig. 4 summarizes the results.

We screened the MON@PET-1–5 membranes (1.32 cm2 area)for the removal of nitrobenzene (3.9 ppm/5 mL) in water. Theltered solution was analyzed by UV-vis absorption spectroscopy.As shown in Fig. 4a, the rejection rate for nitrobenzene dissolvedin water gradually increased from MON@PET-1 (60%) toMON@PET-5 (79%) at a 5 mL min�1

ow rate. MON@PET-5showed nearly same ltration performance with MON@PET-3.In comparison, the original PET membrane did not work forthe removal of nitrobenzene in water (red curve in Fig. 4a). Aerltration by MON@PET-3 and MON@PET-5, the concentration

Fig. 4 (a) UV-vis absorption spectra of nitrobenzene in the waterbefore (3.9 ppm/5 mL) and after filtration by PET, MON@PET-1,MON@PET-2, MON@PET-3, and MON@PET-5. (b) Reuse tests ofMON@PET-3. (c) Rejection rate depending on the concentration ofnitrobenzene (flow rate 5 mL min�1, 1.32 cm2 area) in water and (d)removal performance depending on the flow rate of nitrobenzenesolution (7.8 ppm/5 mL) in the filtration by MON@PET-3.

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of nitrobenzene in water dropped below 1.0 ppm (0.83 ppm).18

The MON@PET-3 can be reused through simple washing. Evenin the h ltration, the MON@PET-3 maintained 90% of itsoriginal performance (Fig. 4b). The IR and reectance spectraconrmed no signicant change in the chemical components ofMON@PET-3 aer ve successive ltrations (Fig. S3 in the ESI†).

As shown in Fig. 4c, the rejection rate was dependent on theinitial concentration of nitrobenzene in water. The rejectionrate gradually decreased from 79% to 58, 36, and 33% at 3.9, 7.8,15.6, and 31.2 ppm initial concentration of the nitrobenzenesolution, respectively. As the ow rate decreased from 40 mLmin�1 to 20, 10, and 5 mL min�1, the rejection rate increasedfrom 32%, to 46, 52, and 58% for 7.8 ppm nitrobenzene inwater, respectively (Fig. 4d). Although nanoltration using themodied polymer membranes has been applied for the removalof dissolved species in water, it operates with relatively slow ux(12–158 L m�2 h�1)4a under pressure. It is noteworthy that themain ux condition of MON@PET membrane is 2.3 � 103 Lm�2 h�1, which is faster by one order of magnitude, possiblydue to the particulate packing of MON materials.

Next, we screened various aromatic pollutants in ltration byMON@PET-3 (7.8 ppm, 5 mL min�1

ow rate through 1.32 cm2

area) (Fig. 5a).For reliable experiments, volatile organic compounds such

as benzene and toluene and water insoluble organiccompounds such as mesitylene, chlorobenzene, bromo-benzene, and 1,3,5-tribromobenzene had to be excluded. Todetect by UV-vis absorption spectroscopy, we screened aromaticcompounds (Fig. 5b and c). In all the cases, the PET membranesshowed no ltration performance towards aromaticcompounds (7.8 ppm) dissolved in water. In comparison,MON@PET-3 showed 58 and 51% rejection rate for nitroben-zene and benzaldehyde, respectively. For 4-methylanisole, ace-tophenone, and phenol, MON@PET-3 showed 42, 27, and 24%rejection rate, respectively. While MON@PET-3 showed 11%rejection rate toward benzyl alcohol, it did not work for 1,4-hydroquinone and benzoic acid. These trends can be under-stood based on the steric and hydrophobicity effects of adsor-bates. The MON materials in the MON@PET-3 are hydrophobic

Fig. 5 (a) Illustration of filtration by PET and MON@PET-3 membranesfor organic pollutants dissolved in water. (b) Removal performance oforganic pollutants (7.8 ppm) dissolved in water by PET andMON@PET-3 membranes. (c) Organic compounds tested in this study.

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and rich of benzene rings. Nitrobenzene and benzaldehyde arerelatively planar molecules and less hydrophilic than phenol,inducing efficient interaction with MON materials in themembrane. Thus, these compounds showed the best ltrationperformance. 4-Methylanisole and acetophenone have sp3

carbon and are less hydrophilic than benzyl alcohol having sp3

carbon. Thus, 4-methylanisole and acetophenone showedbetter ltration performance than benzyl alcohol. Because ofthe hydrophilicity of 1,4-hydroquinone and benzoic acid,MON@PET-3 did not work for the removal of these compounds.

In conclusion, this study shows that MON chemistry can beapplied for the fabrication of hybrid membranes. Notably, thechemical surrounding and physical properties of MON mate-rials can be easily tuned through screening of various buildingblocks. Thus, we believe that more efficient and tailoredmembranes can be developed through the incorporation ofvarious MON materials in the PET membranes.

Acknowledgements

This work was supported by grants NRF-2012-R1A2A2A01045064(Midcareer Researcher Program) through the National ResearchFoundation of Korea.

Notes and references

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11 Please refer to information on the web site: https://www.osha.gov.

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13 D. L. Pavia, G. M. Lampman and G. S. Kriz, Introduction toSpectroscopy, Thompson Learning, Inc., 3rd edn, 2001.Although PET materials have aromatic C–C and C–Hbonds, the corresponding IR peaks of PET are very weak.

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The IR absorption spectroscopy of MON powder showeda vibration peak of terminal alkyne at 3300 cm�1. Theenergy dispersive X-ray spectroscopy of MON powder andMON@PET-3 veried the existence of iodide. Theseobservations imply the existence of defects in networking(Fig. S4 in the ESI†).

14 According to elemental analysis, the MON contents inMON@PET membranes gradually increased from 2.0 w%(MON@PET-1) to 2.9 (MON@PET-2), 3.7 (MON@PET-3),and 5.7 w% (MON@PET-5). Refer to Table S1 in the ESI.†

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18 According to the ICP-AES analysis, the possible Pd leachingthrough ltration was not observed. The activated carbon(Cat # 29, 259-1, Aldrich. Co., 14 mg, the same weight ofthe MON@PET-3 with a 1.3 cm diameter) which waspacked on a cotton showed 55% rejection rate for 3.9 ppmnitrobenzene (Fig. S5 in the ESI†).

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