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Article http://pubs.acs.org/journal/acsodf
Unique Design of Porous Organic Framework Showing Efficiency toward Removal of Toxicants Rajan Kumar and Raja Shunmugam* Polymer Research Centre, Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata (IISER K), Mohanpur 741246, West Bengal, India S Supporting Information *
ABSTRACT: This report deligates about the creation of porous polymeric organic framework (POF) from dialkynes and poly(alkyne) with their discovery as an efficient set of purifier. POF showed efficient physisorption for dyesfluorescein and rhodamine B. The material POF selectively released rhodamine B and not fluorescein. The material was recyclable over number of cycles during the adsorption−release cycle. Moreover, the thiolfunctionalized POF expectedly showed chemisorption for mercury. Therefore, the prime attractive cause for such a material is its ability to recycle as well as its thiol functionalization toward the removal of heavy metal such as mercury.
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INTRODUCTION Industrialization has shown remarkable impact on human society. Although there are number of positive impact, the adverse effects such as deadly impact of dyes and heavy metals on society cannot be ignored. Amphiphilic organic dyes are the prime water toxicants that industries eliminate. The major contributor of such toxicants is the textile industry. Their accumulation in the body organs provides strong root to diseases that may or may not be curable. Nausea, hypertension, cardiac lock, and so on are some of the usual symptoms that may rise due to such toxicants. Other than dyes, heavy metals are another subclass of toxicants. Their release from the industrial sector to drinking water bodies needs to be checked. They are fatal to the body organs. Heavy metals can interfere with enzymes, leading to the cessation of the synthesis of energy sources such as adenosine 5′-triphosphate and NADH within the body. The scarcity of energy causes inhibition of metabolism within the body. To avoid such circumstances, preventive measures are essential. The priority should be given to the growth of noble materials that can squeeze out toxicants with no harm to marine lives. For this to happen, nontoxicity and biodegradability are the prime factors that need to be taken into consideration while implementing the application of these materials. Again, the architectural design of the defined pore size can be crucial for specific toxicants. Moreover, controlled rigidity and insolubility along with stability in the aqueous phase are the desired properties for their development. Such a material can be very influential toward the removal of organic and amphiphilic dyes via physisorption. Chemisorption can be initiated with the thiol-based surface functionalization. A number of approaches have been implemented and new techniques are introduced from time to time to prevent the © 2017 American Chemical Society
disposal of toxicants to water bodies via sorption. Thiol−alkyne click reaction can initiate such a functionalization within the material. The click chemistry based porous polymeric organic framework (POF) so synthesized can prove to be a vital tool for adsorption of such toxicants.1−12 Covalent organic framework, so-called COF, is a two-dimensional framework that has been explored significantly since last two decades.13,14 However, the POF is yet to be explored. Like COF, the covalently bonded three-dimensional (3D) polymeric frameworks derived can prove to be very crucial for adsorption and catalysis. Rigidity, compressibility, swellability, and stability are the prime characteristics exhibited by a polymeric network. Easy handling is the other crucial factor. Very few reports are highlighted for the application of POF in adsorption. Moreover, porous polymeric framework has raised its impact in the recent time for their application. Here, we report the utility of a 3D POF for the adsorption of toxicant with and without functionalization.
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RESULTS AND DISCUSSION With the objective set to sense, trap, and remove toxicants, we are looking forward to grow POFs for the efficient removal of dye molecules, for example, fluorescein (D1) and rhodamine B (D2) along with the heavy metal mercury, via sorption process.15−25 The growth of POF was assigned via thiol− alkyne click chemistry. The chain length for dialkynes was tuned to check the efficiency as a trapper for toxicants. Poly(alkyne)26 was clicked to compare the efficiency with a Received: May 24, 2017 Accepted: July 18, 2017 Published: August 1, 2017 4100
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Scheme 1. Synthetic Scheme for the Crosslinked POF 3C by Thiol−Alkyne Click Reactiona
a
“Initiator (E)” denotes 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone.
with a greater density than on 1A and 2B. This would make poly(alkyne) more efficient as a multitasking agent. Further, the thermogravimetric analysis (TGA) for 1A, 2B, C′, and 3C were performed as shown in Figure S6, SI. The TGA curves confirmed the stability of compounds 1A, 2B, and 3C up to 300 °C. Further, to check their stability in water, the compounds were introduced to a buffer solution at pHs 5 and 9. The pictorial representation in Figure S7, SI, reveals their stability as the materials remained intact for over 1 week. The hydrophobic nature of 1A, 2B, and 3C inhibits the polar components to penetrate at different pH. This makes the system more stable and durable. Presence of free thiol (−SH) was confirmed from the Fourier transform infrared (FTIR) analysis. The FTIR curve reflected weak and sharp signals for the S−H stretching at 2550 cm−1, as shown in Figure 6c. Both solid-state 13C NMR and FTIR analyses confirmed the presence of free thiols in a partially reduced POF. The scanning electron microscopy (SEM) and cryo-SEM for 1A, 2B, and 3C were done to confirm the pore formation. Recyclization of the materials in terms of adsorption−release analysis was performed using UV−vis spectroscopy. The efficiency of the materials over the cycles was examined too. Raman spectroscopy was done to confirm the chemisorbed heavy metal ion mercury, Hg2+, on the thiol (−SH) functionalized surface. Metal−sulfur and carbon−sulfur (C−S) bonds were analyzed using micro-Raman spectroscopy. The photoirradiated thiol−alkyne clicked reaction mixture showed a transition from liquid to solid. The state conversion so confirmed from the pictorial representation of the expected POFs in Figure 2a was examined through SEM and cryo-SEM techniques. The snapshots from the SEM and cryo-SEM of dried solid materials, 1A, 2B, and 3C in Figures 2b−d and S8a−d, SI, confirm the formation of pores. As expected, the pores in 1A were small, enclosed due to compactness. Although 2B showed compact nature, the pores were comparatively larger than those in 1A. This indicates that switching carbon
pored network derived from dialkynes. On observation, the crosslinking via radical mechanism for poly(alkyne) was wellcontrolled as compared with dialkynes. The synthetic schemes for the pored networks 1A and 2B are shown in Scheme S1, Supporting Information (SI), whereas those for 3C is shown in Scheme 1. Scheme 2 represents the proposed mechanism for the synthesis of crosslinked polymers. The proposed free radical mechanism is the representative for thiol−alkyne C−S bond formation at single repeating unit only. The same mechanism takes place to other units of the polymers present within the system simultaneously. The condensed polymer 3 so synthesized for the growth of POF 3C was reprecipitated in cold methanol followed by dialysis in the dialysis tube of Mw = 3.5k and in solvent chloroform. The respective conversion rate for 1A, 2B, and 3C was approximately 0.4, 0.5, and 2.0 h, respectively. The reactions were time controlled. The synthesized dialkynes (A′ and B′) and poly(alkyne), C′ were characterized by 1H NMR spectroscopy (Figures S1−S3, SI). Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) analysis for white amorphous poly(alkyne) C′ was performed too. The MALDI-TOF spectrum for C′ is represented in Figure S5, SI. Partial reduction of the unsaturated clicked compound 3C was confirmed from the solid-state 13C NMR spectrum. The carbon signal present within the range 120−140 ppm corresponds to an alkene carbon, indicating the partial reduction of alkyne moieties (Figure S4, SI). Because the conversion was time controlled, we set the reaction time at 0.4 h under the photochamber and irradiated at wavelength of 365 nm to analyze the extent of polymerization. According to micro-Raman spectroscopic observation, the intensity of the C−S band varied. The spectra revealed the strongest signal for 1A followed by 2B and 3C (Figure 1). This reflects the crosslink polymerization via the C−S covalent bond formation, which was time controlled and the crosslink was highest for 1A and lowest for 3B. This shows that the thiol functionalization on 3C POF surface is possible 4101
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Scheme 2. Proposed Free Radical Mechanism via Thiol−Alkyne Click Reaction toward the Formation of Crosslinked Polymer Network (R = −CH2OC(O)CH2CH2SH)
POF derived from dialkyne implies that the alignment of thiol against alkyne influences the nature of the surface. The surface is different in the aspect that 3B has a greater percentage of roughness than 1A and 2B. This enhanced roughness in addition to the pores in 3C could be expected to show a greater role in adsorption. With pores confirmed, the POFs 1A, 2B, and 3C were made to swell in solvents of different polarity such as CHCl3 and dimethyl sulfoxide. Their swelling ratios were calculated in weight percent via eq S1, SI. The swelling ratios are tabulated in Table S1, SI. On observation, the swelled percentage for 3C was comparable to that for 2B but much greater than that observed for 1A. 1A and 2B break during swelling−deswelling process, whereas 3C retains its identity. Figure S9a clearly supports our observation. This was due to the rigidity and highly compact nature of 1A, and 2B. Rheometric analysis was possible for 3C, with the reaction being quenched before completion. As observed from Figure S9b, the partially clicked 3C could bear the oscillatory stress close to 150 with G′ > G″.27,28
Figure 1. Comparative Raman spectra to the formation of C−S bond during photoirradiated click crosslink polymerization of 1A, 2B, and 3C for time 0.4 h.
chain from C6 to C10 within the hydrocarbon backbone could control the formation of pores while preserving the compactness. The fact that the morphology of POF derived from poly(alkyne) was completely different from that observed for 4102
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Figure 2. (i): (a) Clicked products 1A, 2B, and 3C via photoirradiation. (ii): (b, c, d) SEM images for 1A, 2B, and 3C, respectively (scale bar, 2 μm). (iii): (e, g) and (f, h) Pictorial representations of dyes D1 and D2 before and after adsorption, respectively. (iv): UV spectra(i) and (ii) represent respective adsorption curves for D1 and D2 by POFs 1A, 2B, and 3C. Concentration for dyes D1 and D2 was 0.01 mM.
For the constant weight of material (60 mg) as well as for the constant concentration of dyes D1 and D2 (0.01 mM), the adsorption analysis for dyes D1 and D2 was performed using POFs. The pictorial representation in Figure 2e−h reflects the adsorption of both dyes per day. The adsorption of D1 was 100% by all POFs as shown in Figure 2i. Figure 2j shows that the adsorption for D2 was 100% in 3C among all of the POFs. The decrease in trends observed for the adsorption of D2 as revealed by the UV curve was in the order 3C > 2B > 1A. The same trend was observed for the concentration 0.1 mM against 90 mg POF/mL. The pictorial representation is shown in Figure S11, SI. The observations were noted after 24 h. Further, the materials 1A, 2B, and 3C were examined to check their optimum limit for the adsorption of dyes D1 and D2. The amount required for the adsorption of D1 at concentration 0.01 mM/mL solution was 60 mg of 1A, 50 mg of 2B, and 40 mg of 3C (Figure 3a). For the adsorption of D2, the amount needed was 80 mg of 1A, 50 mg of 2B, and 30 mg of 3C (Figure 3b). The kinetics of the adsorption of the dyes was performed too. The kinetic studies for the adsorption of the dyes were done with UV spectroscopic techniques (Figures 4 and5). The amount of materials for adsorption kinetic analysis was fixed at 60 mg/mL. The concentration of the dye solutions was 0.01 mM. Figure 4a−c represents the UV spectra where the intensity of dye D1 decreases with time. The comparative curve in Figure 4d reveals that the rate of adsorption by clicked crosslinked polymer materials was in the order (2B and 3C) > 1A. 2B and 3C adsorbed fluorescein within 6 h, whereas 1A took 18 h for complete adsorption. Similarly, the adsorption kinetics for dye D2 showed that the adsorption increased with time. The adsorption rate was highest for 3C and lowest for 1A (Figure 5a−c). The comparative plot in Figure 5d reveals that the
Figure 3. (a, b) Bar diagrams for the plot of residual dye (in percent) remaining after optimum intake of D1 and D2, respectively, against specific amount of materials taken into consideration. The concentration was fixed at 0.01 mM, and the volume of the solution considered was 1 mL.
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Figure 4. (a−c) UV adsorption spectra of fluorescein by POFs 1A, 2B, and 3C, respectively, with time. (d) Comparative flow of adsorption with time (h). Concentration of dye D1 was 0.01 mM.
Figure 5. (a−c) UV adsorption spectra of rhodamine B by POFs 1A, 2B, and 3C, respectively, with time. (d) Comparative flow of adsorption with time (h). Concentration of dye D2 was 0.01 mM. 4104
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Figure 6. (a, b) Respective bar diagrams for adsorption and release over cycles. (c) FTIR spectrum for the presence of free thiols on POF (3C). (d) Pictorial data for a change in the color and texture of thiol-functionalized POF in the presence of Hg(II) in aqueous phase. (e) SEM images for a trapped toxicant mercury, Hg(II). The concentration of the aqueous solution was 0.01 mM. The amount of 1A, 2B, and 3C taken was 60 mg/mL.
adsorption for 3C completed within 12 h. The complete adsorption for 2B took 36 h, but 48 h was not sufficient for 1A. Later, the material was tested for its recyclability and reusability. The release of the adsorbed dyes D1 and D2 was undertaken in solvent methanol. The release was insignificant for D1. For D2, the trend for release was similar to that observed for adsorption. Further, the recyclization of POFs was done for a number of cycles. The efficiency of the release by an individual POF was examined. Their relative efficiency over the cycles was compared too. Equations S2 and S3, SI, were used to calculate the respective adsorption and release by POFs. Figure 6a,b reveals the fact that the poly(alkyne)-derived POF, 3C, showed dominance in adsorption−release compared with 1A and 2B. This confirmed the fact that the greater roughness on 3C stimulates greater adsorption than in smooth pored POFs 1A and 2B.29−31 Moreover, with the flexibility of the polymer network film taken into account, the thiol (−SH) functionalization was performed via change in equivalence of C′ (1 equiv) and D (1.2 equiv). The reaction was quenched after 2 h with methanol and exposure to atmospheric air. The material so prepared was left to swell and washed with chloroform. The pictorial data for the film with unsaturated and free thiol is shown in Figure 7a. The film with unsaturation was nonsticky and dry, whereas that with free thiol was sticky. Also, the sticky nature increased with an increase in the equivalence of D. Figure 6c confirms the presence of POF embedded with thiols (−SH). The thiolembedded POF (3C′) was dipped in water contaminated with mercury (Hg2+) at the concentration 0.01 mM. We observed the characteristic texture change for POFs when allowed to stand for overnight as shown in Figure 6d. The texture for the functionalized POF in a mercury-free and mercury-contaminated aqueous phase changed to white opaque from a transparent one. When dried, the same opaque texture was
Figure 7. (a) Pictorial representation of the films with unsaturated and free thiols. (b) Raman spectroscopic curves for 3C′ + Hg(II).
retained in 3C′ when dipped in mercury-contaminated water. The transparency was regained for 3C′ when dipped in mercury-free water. The retained and changed textures are represented pictorially in Figure 3d. The morphology for the trapped soft acids (Hg2+) onto the surface was examined from SEM as well as a microscope. On observation, the morphology of the POF was changed as shown in Figures 6e and S10, SI. Raman spectroscopy was performed to confirm C−S and Hg− S bond formation. Figure 7b supports the finding. The respective Raman shifts were observed at 321 and 678 cm−1. The mechanism of trapping was based on the hard and soft acids and bases principle proposed by Prof. Pearson. The salts, [Hg2+,2Cl−], a combination of soft acid−hard base, were 4105
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dissolved in water. As expected, in the presence of thiol, we assumed from the evidence, there was an exchange of (hard, soft) and (soft, hard) ion pairs to (soft, soft) and (hard, hard) ion pairs, which was entropically driven.32−34
ORCID
CONCLUSIONS We have developed a class of neutral and thiol-functionalized POFs. The photoclicked system was not only effective but also cost efficient. The POF derived from alkyne polymer showed maximum efficiency for the removal of phenolphthalein-derived amphiphilic dye molecules. The thiol-functionalized POF system responded positively when tested for mercury. The toxicants were removed efficiently via sorption. With these, POF network has the potential to dominate the industrial sector with its application as the remover and purifier of the waste and harmful chemicals generated. The work in our group is in progress toward growth of material with improved efficiency, compressibility, and solvent selectivity from organic to aqueous and vice versa in terms of swellability.
Notes
Raja Shunmugam: 0000-0002-0221-127X Author Contributions
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The project is an outcome of effort from R.S. and R.K. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.K. thanks DBT for research fellowship. R.S. thanks DBT and DST for research projects. Authors thank IISER-Kolkata for the infrastructure facility.
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MATERIALS AND METHODS Materials. Sebacoyl chloride, 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone, thionyl chloride, and penterthritoltetrakis (3-mercaptopropionate)tetrathiol were supplied by Sigma-Aldrich. Adipic acid, propargyl alcohol, and 2-butyne1,4-diol were supplied by Spectrochem Pvt. Ltd. Chloroform, anhydrous sodium sulfate, methanol, and triethylamine were supplied by Merck. Methods. NMR spectroscopy was carried out on a Bruker 500 MHz using CDCl3 as a solvent. The NMR spectra of the solutions in CDCl3 were calibrated to tetramethylsilane as the internal standard (δH 0.00). MALDI-TOF experiment was carried out on a Q-Tof Micro YA263 high-resolution (Waters Corporation) mass spectrometer. The IR spectrum was obtained from the IR PerkinElmer spectrometer at a nominal resolution of 2 cm−1. Micro-Raman spectrum was obtained from Raman Spectrometer, Horiba Jobin Yvon, HR 800 double grating. Field emission SEM and cryo-SEM analyses were performed on a Zeiss microscope; SUPRA 55VP-Field Emission Scanning Electron Microscope. The rheological measurement was carried out on rheometer AR 2000 (TA instrument) using a steel parallel plate at 25 °C with a 1.0 mm gap spacing for gel samples. TGAs were carried out using PerkinElmer thermogravimetric analyzer at a heating rate 10 °C/min. The temperature range was 50−600 °C. UV−vis absorption measurements were carried out on PerkinElmer Lambda-35 UV−vis spectrometer.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00661. Synthetic schemes; synthetic procedures; 1H NMR for A′, B′, and C′; 13C solid-state NMR for C′; MALDITOF for C′; cryo-SEM images for POFs; swellability in weight percent for POFs; rheometric curve for 3C; microscopic images for (3C′ + Hg2+); pictorial data for dye adsorption by POFs at high concentration (PDF)
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