Resorcinarene Cavitand Polymers for the Remediation of

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Resorcinarene Cavitand Polymers for the Remediation of Halomethanes and 1,4-Dioxane Luke P. Skala, Anna Yang, Max Justin Klemes, Leilei Xiao, and William R. Dichtel J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06749 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Journal of the American Chemical Society

Resorcinarene Cavitand Polymers for the Remediation of Halomethanes and 1,4-Dioxane Luke P. Skala, Anna Yang, Max J. Klemes, Leilei Xiao, and William R. Dichtel* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208 USA Supporting Information Placeholder

ABSTRACT: Disinfection byproducts such as trihalomethanes are commonly found in drinking water. Trihalomethanes are formed upon chlorination of natural organic matter (NOM) found in many drinking water sources. Inspired by molecular CHCl3⊂cavitand host-guest complexes, we designed porous polymers comprised of resorcinarene receptors. These materials show higher affinity for halomethanes than a specialty activated carbon used for trihalomethane removal. The cavitand polymers show similar removal kinetics as activated carbon and have high capacity (49 mg g–1 of CHCl3). These materials maintain their performance in drinking water and can be thermally regenerated. Cavitand polymers also outperform commercial resins for 1,4-dioxane adsorption, which contaminates many water sources. These materials show promise for water treatment and demonstrate the value of using supramolecular receptors to design adsorbents for water purification.

Scheme 1. Design of Porous Cavitand Polymers R

OH

HO

OH

HO

HO

1

OH

N

Cl

Cl

O

O

N

O

O

Haloform Binding O

O

Prior Work

N

N

O

R

O

F

R

N

N

R

R

This Work

R N

N

K2CO3

OH

HO

N

R N

R

R

R

F N N

X F

O

F

N (TFIN) X = C CN (TFN)

K2CO3/DMSO

Haloform Removal

O

X

X

N

O

O

O

O

N

X O

O

X N

N (2) X = C CN (3)

Chlorination is effective and widely used to disinfect drinking water. However, free chlorine used in this process must remain throughout downstream distribution networks and reacts with natural organic matter.1 The resulting byproducts, such as halomethanes (CHCl3, CH2Cl2, etc.), are among the most common organic micropollutants present in drinking water.2 The prevalence and toxicity of these byproducts prompted their regulation in the United States through the Safe Water Drinking Act.3 The most abundant disinfection byproduct regulated by this legislation is CHCl3, which is a known carcinogen4–6 that affects fetal development,7,8 and causes other reproductive complications.9,10 Currently, the most effective approach to remove CHCl3 from drinking water is adsorption by activated carbons.11–16 However, these materials have relatively low affinity for trihalomethanes, especially CHCl3, in turn causing many municipalities to serve water near or above the US EPA limit of 80 µg L–1 total trihalomethane content.17,18

Integrating supramolecular receptors into porous polymer networks is an emerging strategy to remove organic micropollutants from water. We recently reported porous cyclodextrin polymers for removing organic pollutants,19–24 such as pharmaceutical agents and perfluorinated alkyl substances, but cyclodextrins are too large to bind halomethanes effectively. We hypothesized that incorporating other rigid molecular receptors would provide materials with improved uptake of halomethanes and other organic micropollutants. Additionally, these materials should be polymerized with relatively rigid comonomers to encourage the formation of permanently porous, high surface area materials. Resorcinarene cavitands (Scheme 1) are particularly attractive due to their host guest properties25,26 and rigid structures. These hosts, first synthesized by Cram and coworkers,27 bind various halomethanes28– 30 and 1,4-dioxane,31–33 another contaminant of emerging concern34–36 that has been detected in 22 percent of public water systems.37 Here we report permanently porous, high surface area cavitand-containing polymers that effectively remove halomethanes and 1,4-dioxane from drinking water.

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Figure 1. (a) Solid state 13C CP-MAS NMR spectra of 2 and 3, as well as solid state 13C CP-MAS NMR and solution 13C NMR spectra of MC-TFIN (4) and MC-TFN (5). The peak at 77 ppm in solution spectra corresponds to CDCl3. (b) Synthesis of model compounds 4 and 5. (c) N2 adsorption and desorption isotherms of 2 and 3 at 77 K (Adsorption:circles; desorption:triangles).

Nucleophilic aromatic substitution of tetrafluoroisonicotinonitrile (TFIN) or tetrafluorophthalonitrile (TFN) with resorcinarene (1) provided cavitand containing polymers CP-TFIN (2) and CP-TFN (3), which contain fully fused cavitand subunits shown in Scheme 1, presumably along with partially fused or defective macrocycles. The polymerization was conducted at a molar feed ratio of 2:1 (linker:1) in anhydrous DMSO in the presence of K2CO3. Following preliminary optimization of the polymerization conditions, the reaction temperature was found to be important for obtaining materials with low residual fluorine content, indicating greater extent of diarylether formation. When prepared at 150 °C, polymers 2 and 3 exhibited a residual fluorine content of 3.45 and 3.19 weight percent respectively, both corresponding to 0.7 residual fluorines per crosslinker. In contrast, polymerizations conducted at 75 °C had 0.9 and 1.2 residual fluorines per crosslinker for 2 and 3, respectively (Table S3). Therefore, we conducted subsequent characterization and haloform removal experiments on materials polymerized at 150 °C. 2 and 3 had Brunauer Emmett Teller surface areas (SBET) of 1190 m2 g-1 and 1081 m2 g-1, respectively (Figure 1a). Thermogravimetric analysis of 2 and 3 demonstrated high thermal stability

up to 450 °C (Figure S8, S9), consistent with formation of diaryl ether-containing polymers. FTIR spectroscopy and solid-state cross-polarization magic angle spinning (CP-MAS) 13C NMR spectroscopy corresponded well to the expected structures of 2 and 3. Structural assignments were made by comparison to soluble model compounds prepared from 2,2′-methylenediphenol and either TFIN or TFN under similar conditions as the polymers to afford MC-TFIN (4) and MC-TFN (5) (Figure 1b). 13C CPMAS NMR spectroscopy of both model compounds were consistent with those of the corresponding polymers (Figure 1a). In the case of both polymers the most prominent resonances are present near 153 ppm, indicative of aryl C-O bond formation, as well as resonances between 136 ppm and 129 ppm, corresponding to aryl carbons on 1. Resonances near 110 ppm, 30 ppm, and 17 ppm correspond to the presence of the nitrile, methylene carbon, and methyl group, respectively. The major difference between the model compounds and their respective polymers is the resonance corresponding to the methyl group present in the polymers and absent in 4 and 5. Furthermore, FTIR spectroscopy shows C–O stretches at 1228 cm-1 and 1091 cm-1 for 2 as well as 1241 cm-1 and 1092 cm-1 for 3. C–H stretches were observed at 2973 cm-1 for both


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polymers. Nitrile stretches were observed at 2239 cm-1 for 3. The nitrile stretch could not be resolved in either the TFIN monomer or in 2.38 The spectra of 2 and 3 exhibited the disappearance of the O-H stretch from 1, suggesting the formation of aryl ether bonds (Figure S36, S38). Finally, combustion analysis of both polymers was in good agreement with expected values (Table S3). The low fluorine content of the polymers suggests high conversion of the SNAr reaction with residual content due to polymer termination and defects in the polymer network. 19F MAS NMR spectroscopy of both 2 and 3 provided further information about defects and terminations in the polymers. 2 demonstrated the presence of two peaks at -83.7 ppm and -136.4 ppm at a ratio of 2:1 (Figure S3-S5). These peaks suggest the presence of two main species: fluorines orthogonal to one another in pairs due to chain termination, as well as trisubstituted linkers caused by incomplete cavitand closure (See SI). The characterization of 2 and 3 suggests that they contain a large percentage of fully fused and partially fused cavitands. Even partially fused cavitands have useful host guest properties39–41 and may contribute to pollutant removal. Batch adsorption experiments probed the affinity of 2 and 3 for CHCl3 in water. Gas chromatography mass spectrometry (GCMS) of the headspace was used to quantify the residual CHCl3 concentration and calculate the amount removed by the adsorbent. A standard curve was constructed to demonstrate the detection limit as well as the linear

Figure 2. Solid bars: removal of CHCl3 (100 µg L-1) from nanopure water by 2, 3, and FA600 (30 mg L-1) after 24 hours. Striped bars: removal of CHCl3 (32 µg L-1) from drinking water by 2, 3, and FA600 (30 mg L-1). The residual CHCl3 concentrations were determined through headspace-GCMS (see supporting information). Error bars represent standard deviation of triplicate measurements. Inset: photograph of the drinking water source (Northwestern Technological Institute, November 2018).

relationship between peak area and CHCl3 concentration (Figure S19, S20). A commercial activated carbon (FILTRASORB 600, FA600, Calgon Carbon) was used to benchmark the adsorption performance of 2 and 3. FA600 is a specialty activated carbon specifically marketed for trihalomethane removal in drinking water. Each polymer sample, as well as FA600, was sieved to a particle size of 45-90 µm and stirred for 24 hours in nanopure H2O spiked with 100 µg L-1 of CHCl3. 2 removed over 85% of the CHCl3, leaving a residual CHCl3 concentration below 15 µg L-1. Both FA600 and 3 removed only 51% and 35% under the same conditions (Figure 2). To test realistic drinking water samples, we measured the CHCl3 concentration of drinking water taken from a Northwestern University drinking fountain to be 32 µg L-1 and used 2, 3, and FA600 (30 mg L-1) to remove it. None of the adsorbents demonstrated a loss in performance relative to experiments performed in nanopure water (Figure 2) indicating the ability of 2 and 3 to remediate pollutants in complex mixtures. Among the two polymers, removal was dependent on the polymerization conditions, with both temperature and monomer concentration playing critical roles. Polymers formed at 150 °C removed more CHCl3 than those formed at 75 °C. We attribute this difference to higher reaction temperatures resulting in a greater abundance of fused cavitand binding sites. Furthermore, the best CHCl3 removal was obtained from polymerizations conducted at 57.5 mM of 1. Higher polymerization concentrations provided inferior CHCl3 uptake, whereas polymerizations conducted at lower concentrations did not form insoluble polymer networks. Finally, the presence of excess K2CO3 had no effect on polymer performance. From these studies we were able to find optimal polymerization conditions (Table S2). 2, 3, and FA600 (100 mg L-1) showed similar CHCl3 (100 µg L-1) removal rates, each reaching equilibrium after approximately 40 minutes (Figure S12). Isotherms of 2 and 3 were constructed at polymer loadings of 100 mg L-1. This data was fit to the Langmuir model, which suggested CHCl3 binding capacities of 49 mg g-1 and 45 mg g-1 for 2 and 3, respectively (Figure S13-S14). Additional isotherms were constructed at lower polymer loadings (25 mg L-1) to measure the affinity of the adsorbents at environmentally relevant concentrations, which provided Langmuir affinity coefficients (KL) of 5.49･ 104 and 3.70･105 for FA600 and 2, respectively (Figure S15). The higher KL value for 2 is consistent with its higher CHCl3 removal performance. 3 did not provide an informative isotherm at these polymer loadings because of its insignificant CHCl3 removal. In addition to its promising removal characteristics, 2 was also thermally regenerated and reused. To assess an appropriate temperature for thermal regeneration, the polymer was loaded with CHCl3 vapor. Thermogravimetric analysis



Figure 3. Batch adsorption of CH2Cl2, CHBrCl2, CHBr2Cl, CHBr3, (80 µg L-1) and CCl4 (200 µg L-1) with 2, 3, and FA600 (30 mg L-1). Samples were stirred for 24 hours.

indicated that the CHCl3 desorbed below 125 °C (Figure S16). 2 (1 g L-1) was exposed to 7 mg L-1 of CHCl3 to achieve a removal of 96%. After being regenerated 3 times, 2 still achieved a removal of 91% at 7 mg L-1 of CHCl3 (Figure S17). These results indicate that 2 can be regenerated under mild thermal conditions and reused several times without significant performance loss. The known affinity of cavitands for other guests, such as bromoform and 1,4-dioxane, inspired further investigation into pollutants beyond CHCl3. For example, 2, 3, and FA600 were challenged with a series of halomethane pollutants. Solutions containing CH2Cl2, CHBrCl2, CHBr2Cl, CHBr3, (80 µg L-1) and CCl4 (200 µg L-1) were stirred for 24 hours with 30 mg L-1 of adsorbent. CCl4 removal was conducted at higher concentrations because it had a higher limit of quantification in our headspace-GCMS assay. 2 demonstrated superior or equal removal of all of the halomethanes when compared to FA600 and 3 (Figure 3). Moreover, 2, 3, FA600 and Ambersorb 560 (Dow Chemical Company), a commercial resin for 1,4-dioxane removal, were challenged with 1,4-dioxane (100 µg L-1) and were stirred for 24 hours with 500 mg L-1 of adsorbent. 2 removed 86% of the 1,4-dioxane while Ambersorb 560, FA600, and 3 removed 51%, 47%, and 33% respectively (Figure 4). The polymer loadings of 2 were dropped to 100 mg L-1 and still resulted in similar performance to Ambersorb 560 at five-fold lower polymer loading. 1,4-dioxane removal studies were also conducted in tap water which revealed no loss in removal performance for 2 and minor variations for Ambersorb 560 and FA600 (Figure S18). These removal studies suggest the potential for 2 as a useful adsorbent for both halomethane and 1,4-dioxane removal. In conclusion, we have incorporated resorcinarene cavitand receptors into a porous, high surface area polymer adsorbent. The synthesis and characterization of model compounds helped elucidate the cavitand structure of these polymers.

Figure 4. Batch removal of 1,4-dioxane (100 µg L-1) from nanopure water by 2, 3, Ambersorb 560, FA600 (500 mg L-1) and 2 (100mg L-1) after 24 hours. The residual 1,4-dioxane concentration was determined through GCMS (see supporting information). Error bars represent standard deviation of triplicate measurements.

Polymer 2 outperforms activated carbon and a commercial resin in terms of their affinity for halomethanes and 1,4-dioxane. Moreover, these materials have fast removal kinetics and high capacity. Polymers synthesized with the TFIN linker consistently demonstrated a higher removal percentage for all the micropollutants tested. We speculate this difference in performance is due to 3 being less likely to form cavitand pockets due to steric constraints. However, further investigation is being conducted to understand this difference in performance. Finally, the polymer’s performance was unchanged when challenged to remove CHCl3 or 1,4-dioxane in municipal tap water. These results demonstrate that cavitand polymers are a promising new class of porous organic materials for remediation of pervasive and challenging-to-remove micropollutants. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, characterization, additional CHCl3 removal experiments. (PDF)

AUTHOR INFORMATION Corresponding Author

*[email protected] ORCID

Max Klemes: 0000-0002-4481-890X William R. Dichtel: 0000-0002-3635-6119 Notes


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Northwestern University has filed patent applications related to this work. W. R. D. owns equity and/or stock options in CycloPure, Inc., which is commercializing related novel adsorbents.

ACKNOWLEDGMENT We thank Dr. Riqiang Fu for conducting 19F MAS NMR experiments at the National High Magnetic Field Lab (NHMFL) supported by the NSF Cooperative agreement No. DMR-1644779 and the State of Florida. We also thank Ioannina Castano for useful discussions on electron microscopy characterization. L.P.S. is supported by the NSF Graduate Research Fellowship under grant DGE-1842165. This research made use of the EPIC facility of NUANCE and IMSERC at Northwestern University, which have received support from the NSF (CHE-1048773); Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the MRSEC program (NSF DMR1720139) at the Materials Research Center, the Keck Foundation, the State of Illinois, and International Institute for Nanotechnology (IIN).

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Resorcinarene Cavitand Polymers for the Remediation of

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