Microporous organic network@PET hybrid membranes: removal of minute organic pollutants dissolved in water

Abstract

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.

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Porous polymer membranes have been engineered for various purposes including the removal of environmental pollutants in water.¹ For example, polyethylene terephthalate (PET) fibers were prepared by electrospinning and further engineered to form macroporous membranes.² Due to chemical stability, conventional PET membranes with 0.1–10 μm pore widths have been applied for the removal of heterogeneous pollutants in water³ (Fig. 1). However, those PET membranes do not work for the removal of organic pollutants dissolved in water. Although nanofiltration technologies have attracted more attention in recent years for the removal of the dissolved species in water,⁴ decreasing pore width to sub 100 nm in PET membranes is technically difficult. Thus, the incorporation of secondary materials into PET membranes is an alternative approach to induce new functionality.^4,5

Fig. 1 Preparation of MON@PET hybrid membranes.

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 In addition, the powdery MON materials were investigated for the adsorptive removal of oil floating on the water.⁸ Recently, MON films have been fabricated on the solid supports including electrodes.⁹ However, the fabrication of MON related membranes is still rare.¹⁰ The introduction of MON materials to PET membrane can be a new approach for functional hybrid membranes with enhanced properties. However, as far as we are aware, this approach has not yet been reported.

As environmental regulation becomes more and more strict, the upper concentration limit of organic residues dissolved in water gradually decreases. For example, recently, the permissible exposure limit (PEL) of nitrobenzene, and phenol decreased to 1 and 5 ppm, respectively by the U. S. Occupational Safety & Health Administration (OSHA).¹¹ Thus, the removal methods for minute organic residues dissolved in water should be developed. Our research group has studied the MON based composite materials for environmental applications.¹² In this work, we report the engineering of MON@PET hybrid membranes and their adsorptive removal performance of aromatic pollutants dissolved in water.

Fig. 1 shows an engineering scheme for MON@PET hybrid membranes.

In the presence of PET membrane (disc shape with a 2.5 cm diameter and a 180–200 μm thickness), 1 eq. tetrakis(4-ethynylphenyl)methane and 2 eq. 1,4-diiodobenzene were reacted at 90 °C to form MON materials via the Sonogashira coupling. As the reaction progressed, MON materials gradually incorporated into the PET membrane, resulting in the color change of the PET membrane from white to brownish yellow. After 3 days, the resultant MON@PET hybrid membrane was retrieved and washed with solvents. The membrane was cut to a disc with a 1.3 cm diameter. We screened reaction time from 1 day to 2, 3, and 5 days. The obtained MON@PET membranes were denoted as MON@PET-1, MON@PET-2, MON@PET-3, and MON@PET-5, respectively. Fig. 2a shows the photographs of PET and MON@PET membranes. The brownish yellow color became gradually denser from MON@PET-1 to MON@PET-5. Infrared absorption (IR) spectroscopy of the control MON materials prepared by the Sonogashira coupling of tetrakis(4-ethynylphenyl)methane and 1,4-diiodobenzene showed the main peaks at 1506 and 832 cm⁻¹ (the peaks indicated by asterisks in Fig. 2b), corresponding to the vibrational stretching of C–C and C–H bonds in aromatic rings, respectively.¹³ The intensity of these peaks gradually increased from MON@PET-1 to MON@PET-5, indicating the loading of MON materials on the PET membrane (Fig. 2b).

Fig. 2 (a) Photographs, (b) IR absorption spectra, (c) UV-vis absorption spectra (obtained through the conversion of reflectance spectra) of PET, MON@PET-1, MON@PET-2, MON@PET-3, MON@PET-5, and control 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).

The absorption spectra obtained from reflectance spectroscopy showed that the absorption intensity in the visible light region gradually increased from MON@PET-1 to MON@PET-5 (Fig. 2c). The scanning electron microscopy (SEM) showed that the PET membrane consists of ∼20 μm diameter fibers and 1–10 μm gap widths (Fig. 2d). The MON materials in MON@PET-3 and the control MON materials showed spherical morphology with a 0.80 ± 0.18 μm diameter (Fig. 2e and f). The MON particles were mostly entrapped in the pores of PET membranes (Fig. 2e and S1 in the ESI†). According to the water contact angle measurement, water drop gradually adsorbed in the PET membrane (Fig. 2g). In comparison, the control MON particles showed superhydrophobicity (water contact angle > 150°) with a 152° water contact angle (Fig. 2i). The water contact angles increased gradually from 126° to 130, 138, and 143° for MON@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 of MON@PET-3 were further investigated. The analysis of N₂ sorption isotherm curves of the MON materials based on Brunauer–Emmett–Teller theory showed a high surface area of 876 m² g⁻¹, pore volume of 0.40 cm³ g⁻¹, and microporosity. Kr sorption isotherm measurements showed that microporosity (S_BET: 25 m² g⁻¹) of MON@PET-3 was induced through incorporation of MON materials¹⁴ (Fig. 3a and b and inset).

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

The transmission electron microscopy (TEM) showed the quite homogeneous size distribution of MON particles with a 0.80 μm average diameter (Fig. 3c). Solid phase ¹³C nuclear magnetic resonance (NMR) spectroscopy showed the ¹³C peaks at 64, 85–95, and 116–151 ppm, corresponding to the benzyl carbon, internal alkyne, and aromatic rings, respectively. The powder X-ray diffraction (PXRD) studies revealed the amorphous characteristics of MON particles, matching well with the properties of MON materials prepared by the Sonogashira coupling in the literature¹⁵ (Fig. S2 in the ESI†).

Simple aromatic compounds are one of the most important intermediates in synthetic industry.¹⁶ Especially, nitrobenzene has been used for the production of explosives, pesticides, plastic, and dyes. At the same time, it is a priority pollutant listed by the US Environmental Protection Agency because of its carcinogenic toxicity.¹⁷ The removal of aromatic pollutants dissolved in water is a critical subject of environmental concerns.^16,17 Considering the organophilicity and high surface area, and microporosity of MON materials, we tested the removal performance of MON@PET hybrid membranes toward aromatic pollutants dissolved in water. We found that the MON@PET hybrid membranes work well for the adsorptive removal of nitrobenzene (the PEL of US-OSHA: 1 ppm) in water. Fig. 4 summarizes the results.

Fig. 4 (a) UV-vis absorption spectra of nitrobenzene in the water before (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 of MON@PET-3. (c) Rejection rate depending on the concentration of nitrobenzene (flow rate 5 mL min⁻¹, 1.32 cm² area) in water and (d) removal performance depending on the flow rate of nitrobenzene solution (7.8 ppm/5 mL) in the filtration by MON@PET-3.

We screened the MON@PET-1–5 membranes (1.32 cm² area) for the removal of nitrobenzene (3.9 ppm/5 mL) in water. The filtered solution was analyzed by UV-vis absorption spectroscopy. As shown in Fig. 4a, the rejection rate for nitrobenzene dissolved in water gradually increased from MON@PET-1 (60%) to MON@PET-5 (79%) at a 5 mL min⁻¹ flow rate. MON@PET-5 showed nearly same filtration performance with MON@PET-3. In comparison, the original PET membrane did not work for the removal of nitrobenzene in water (red curve in Fig. 4a). After filtration by MON@PET-3 and MON@PET-5, the concentration of nitrobenzene in water dropped below 1.0 ppm (0.83 ppm).¹⁸ The MON@PET-3 can be reused through simple washing. Even in the fifth filtration, the MON@PET-3 maintained 90% of its original performance (Fig. 4b). The IR and reflectance spectra confirmed no significant change in the chemical components of MON@PET-3 after five successive filtrations (Fig. S3 in the ESI†).

As shown in Fig. 4c, the rejection rate was dependent on the initial concentration of nitrobenzene in water. The rejection rate gradually decreased from 79% to 58, 36, and 33% at 3.9, 7.8, 15.6, and 31.2 ppm initial concentration of the nitrobenzene solution, respectively. As the flow rate decreased from 40 mL min⁻¹ to 20, 10, and 5 mL min⁻¹, the rejection rate increased from 32%, to 46, 52, and 58% for 7.8 ppm nitrobenzene in water, respectively (Fig. 4d). Although nanofiltration using the modified polymer membranes has been applied for the removal of dissolved species in water, it operates with relatively slow flux (12–158 L m⁻² h⁻¹)^4a under pressure. It is noteworthy that the main flux condition of MON@PET membrane is 2.3 × 10³ L m⁻² h⁻¹, which is faster by one order of magnitude, possibly due to the particulate packing of MON materials.

Next, we screened various aromatic pollutants in filtration by MON@PET-3 (7.8 ppm, 5 mL min⁻¹ flow rate through 1.32 cm² area) (Fig. 5a).

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

For reliable experiments, volatile organic compounds such as benzene and toluene and water insoluble organic compounds such as mesitylene, chlorobenzene, bromobenzene, and 1,3,5-tribromobenzene had to be excluded. To detect by UV-vis absorption spectroscopy, we screened aromatic compounds (Fig. 5b and c). In all the cases, the PET membranes showed no filtration performance towards aromatic compounds (7.8 ppm) dissolved in water. In comparison, MON@PET-3 showed 58 and 51% rejection rate for nitrobenzene and benzaldehyde, respectively. For 4-methylanisole, acetophenone, 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 understood based on the steric and hydrophobicity effects of adsorbates. The MON materials in the MON@PET-3 are hydrophobic and rich of benzene rings. Nitrobenzene and benzaldehyde are relatively planar molecules and less hydrophilic than phenol, inducing efficient interaction with MON materials in the membrane. Thus, these compounds showed the best filtration performance. 4-Methylanisole and acetophenone have sp³ carbon and are less hydrophilic than benzyl alcohol having sp³ carbon. Thus, 4-methylanisole and acetophenone showed better filtration performance than benzyl alcohol. Because of the 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 be applied for the fabrication of hybrid membranes. Notably, the chemical surrounding and physical properties of MON materials can be easily tuned through screening of various building blocks. Thus, we believe that more efficient and tailored membranes can be developed through the incorporation of various MON materials in the PET membranes.

Acknowledgements

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

Notes and references

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

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedure, PXRD patterns, and characterization data of recovered MON@PET-3 after five successive filtration. See DOI: 10.1039/c6ra13220k

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