Chinese Chemical Letters  2015, Vol.26 Issue (09): 1091-1095   PDF    
Thiacalix[4]arene 1, 2, 3-triazole-polyethylene glycol polymers: Synthesis and dye adsorption properties
Shu-Yun Zhua, Hong-Yu Guoa, Fa-Fu Yanga, b , Zu-Sheng Wanga    
a College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350007, China;
b Fujian Key Laboratory of Polymer Materials, Fujian Normal University, Fuzhou 350007, China
Abstract: By reacting alkynylthiacalix[4]arenes with polyethylene glycol azido compounds, a series of novel thiacalix[4]arene 1, 2, 3-triazole-polyethylene glycol netty polymerswere conveniently prepared in good yields. Their structures and morphologies were studied by 1H NMR, IR, and elemental analysis and SEM images. On average, approximately 28-31 thiacalixarene units exist in each polymeric molecule. These novel polymers possess excellent adsorption ability for both cationic and anionic dyes. The saturation adsorption capacity for Congo red reached 1.3-1.4 mmol/g. They exhibit high and stable adsorption ability in the scope of pH 5-9, and maintain good properties in five cycles.
Key words: Thiacalix[4]arene     Synthesis     Polymer     Dye adsorption    
1. Introduction

Dyes are widely applied in current industries such as dyestuffs, papers, textiles, leather, etc. However, these industries produce serious water pollutions [1,2]. Many conventional treatment methods, such as precipitation, extraction, adsorption, membrane filtration, and photodegradation, have been developed to remove the dyes from wastewaters [3,4,5,6,7]. Among them, the adsorption method has attracted much attention due to its advantages of selective separation and recovery of dyes. As a result, the search of effective absorbent for dyes is a crucial task to achieve this goal. Recently, supramolecular chemistry has provided a much-improved solution for the production of sophisticated molecules by anchoring functional groups oriented in such a way that they provided a suitable binding site for the dyes. For example, the synthesis and dye complexation ability of some calixarene-based derivatives and polymers was described by Yilmaz [8,9], Memnon [10], and Diao [11], respectively. Our group also described a series of calixarene derivatives and polymers with high binding affinity for organic dyes [12,13,14]. These results indicated that the structures of calixarene skeleton, the size of cavities and different functional groups influenced the dyes binding ability greatly. Comparing with normal calixarenes, thiacalix[4]arenes bridged by four sulfur atom, possess more structural flexibility, which might be favorable for adjusting its shape to match the structures of guests. On the other hand, the 1, 2, 3-triazole ring obtained conveniently by click chemistry [15,16,17,18], easily produced p-p stacking interactions with other aromatic systems, such as dyes. Lately, we synthesized several novel thiacalix[4]arene derivatives based on click chemistry and found their excellent binding capability for dyes for the first time [19]. Inspired by the effective dyes binding ability of thiacalix[4]arene triazole derivatives, we further prepared a series of novel thiacalix[4]arene 1, 2, 3-triazolepolyethylene glycol netty polymers and investigated the dyes adsorption ability. The obtained polymers exhibited outstanding adsorption ability for the tested dyes as expected.

2. Experimental

1H NMR spectra were recorded in CDCl3 on a Bruker-ARX 500 instrument at room temperature, using TMS as an internal standard. Elemental analyses were performed using a Carlo-Erba 1106 Elemental Analyzer. Osmometric molecular weight determinations were carried out using a Knauer vapor pressure osmometer at concentrations of ca. 10-3 mol/L (based on the polymeric units) in CHCl3 at 37˚ C using sucrose octaacetate for calibration. The quotients of vapor pressure signal over concentration were plotted against C. The Mn values were calculated by the inversely proportional to the intercept at [C] = 0. IR spectra were recorded on a Perkin-Elmer 1605 FTIR spectrometer as KBr pellets. UV-vis measurements were performed on a Varian UV-vis instrument. Surface morphology of polymers, which were scrunched and sieved with a diameter size of 500-1000 mm, was studied using SEM (JEOLJSM-6490LA) after the gold coating, operating at 10 kV. All solvents were purified by standard procedures before use. All other chemicals, except special instruction, were analytically pure and used without further purification. All reactions were carried out under nitrogen atmosphere. The azido compounds 2a-2c were easily obtained by a literature method [20]. Alkynylthiacalix[4]arenes were synthesized by reacting thiacalix[4]arenes with propargyl bromide according to the procedures reported in literature [19]. Polymers 4a-4c used in adsorption experiment were scrunched and sieved with a diameter size of 500-1000 mm.

The liquid-liquid extraction studies were performed following the classic Pedersen’s procedure [20]. A 10 mL of 2.0 × 10-5 mol/L aqueous solution of the dye of interest (pH 7, adjusted by a solution of HCl or NaOH) and 10 mL of 1.0 × 10-3 mol/L solution of carrier in CH2Cl2 were vigorously agitated in a stoppered glass tube with a mechanical shaker for 2 min. Then the mixture was stirred magnetically in a thermostated water-bath at 25˚ C for 1 h, and was finally left standing for an additional 30 min. The concentration of the dye remaining in the aqueous phase was subsequently determined by UV-vis analyses. Blank experiments showed that the dye extraction was less than 2% in the absence of extractant. The percent extraction (E%) was calculated as the follows: E% = 100(Ao - A)/Ao, where Ao and A were initial and final concentrations of the dyes before and after the extraction, respectively. Each experiment was repeated three times. The dye concentration in the receiving phase was reported as the mean of the measurements and the relative standard deviation from the mean was less than 5%. The approach of changing the volume of the dye was employed to examine the saturated adsorption capacity of polymer 4a-4c [21]: [2TD$DIF]The polymer (10 mg) was added to the solutions of the dye (1.0 × 10-3 mol/L), whose volumes were 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, and 60 mL, respectively. Then the mixture was stirred for 24 h at 25˚ C and was filtered. The concentration in the filtrate was determined by UV spectra. The adsorption capacity was calculated as Q = (C - C*)V/W, where Q was the adsorption capacity. C was the initial concentration before adsorption. C* was the concentration after adsorption. V was the volume of solution and W was the mass of the polymer. The adsorption experiments at different pH values were adjusted by adding a solution of HCl or NaOH to attain corresponding pH values.

Synthesis of thiacalix[4]arene 1, 2, 3-triazole-polyethylene glycol netty polymers 4a-4c: Under nitrogen atmosphere, a mixture of compound 3 (0.32 g, 0.4 mmol), compound 2a (0.14 g, 0.9 mmol), CuSO4 (0.40 g, 1.6 mmol), and sodium ascorbate (0.79 g, 4 mmol) were stirred in dry DMF (10 mL). The mixture was heated at 90 ℃ for 15 h until compound 3 completely vanished by TLC analysis. After cooling, the reaction system was diluted with CHCl3 (30 mL) and washed with water (3 × 20 mL). The organic phase was separated and dried over MgSO4. After evaporating off the solvent to dryness, the powder was washed subsequently by 10 mL of acetone and 10 mL of methanol to remove unreacted and small intramolecular-bridging materials. The residue was then dried in vacuum. 0.38 g of polymer 4a was obtained as light yellow powders. Using the same procedure substituting 2a with compound 2b or 2c, polymer 4b (0.41 g), and 4c (0.43 g) were obtained as light yellow powders, respectively. Polymer 4a: 1H NMR (500 MHz, CDCl3): δ 1.13 (bs, 36H, But), 3.86-5.14 (m, 24H, OCH2 and NCH2), 7.01 (bs, 4H, N-CH(55C)), 7.19-7.33 (m, 8H, ArH). Polymer 4b: 1H NMR (500 MHz, CDCl3): δ 1.14 (bs, 36H, But), 3.90-5.17 (m, 32H, OCH2, NCH2), 6.99 (bs, 4H, N-CH(55C)), 7.17-7.36 (m, 8H, ArH). Polymer 4c: 1H NMR (500 MHz, CDCl3): δ 1.15 (bs, 36H, But), 3.89-5.20 (m, 40H, OCH2, NCH2), 7.03 (bs, 4H, N-CH(55C)), 7.15-7.35 (m, 8H, ArH). The elemental analysis, IR spectrum, SEM, and Osmometric Mn of polymers 4a-4c are shown in Table 1, Figs. 1 and 2, respectively.

Table 1
Elemental analysis (%) and molecular weight of polymers 4a-4c.

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Fig. 1.The infrared spectra of compound 3 and polymers 4a-4c.

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Fig. 2.The SEM images of polymers 4a-4c.
3. Results and discussion

The synthetic route is shown in Scheme 1. Alkynylthiacalix[4]- arene 3 and azido compounds 2a-2c were prepared by the reported methods [19,20]. Then by reacting compound 3 with azides 2a-2c in DMF at 90 ℃ using copper(II) sulfate and sodium ascorbate as catalysts, polymers 4a-4c were obtained in good yields. In order to ensure the complete reaction of four alkynyl groups in compound 3, an excess of azido compounds 2a-2c were used. Also, only 10 mL of DMF was used as solvent to ensure the formation of polymer and avoid the intramolecularly bridged compound 3. Moreover, unreacted materials and byproducts were subsequently washed out by a small amount of acetone and methanol in the treating procedures. After that, no intramolecular products were observed on TLC, indicating the formation of polymers 4a-4c as shown in Scheme 1. Since compound 3 and azides 2a-2c possess four and two reactive groups, respectively, it was reasonable that polymers 4a-4c were a tridimensional netty "4 + 200" crosslinking polymers. The structures and morphologies of polymers 4a-4c were investigated by 1H NMR spectra, elemental analysis, IR spectra and SEM images. The signals of 1H NMR of polymers 4a-4c were assigned for the corresponding protons, comparing to the ideal polymeric reaction of compound 3 and azides 2a-2c in an exact molar ratio of 1:2. The molecular weight of polymers 4a-4c and elemental analysis data are illustrated in Table 1. The elemental analysis data were also approximately in accordance with the calculated data obtained by the hypothesis that one molecule of compound 3 reacted with two molecules of compounds 2a-2c exactly. The Mn of polymers 4a-4c after calculationwere 36, 688, 37, 045, and 41, 127, respectively, indicating that on average approximately 28-31 calixarene units exist in each polymer molecule.

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Scheme 1.The synthetic route of polymers 4a-4c.

The structures of polymers 4a-4c were also confirmed by IR spectra as shown in Fig. 2. It can be seen that the strong absorption peaks at 3306 and 2127 cm-1 for alkynyl groups in compound 3 disappeared completely in the IR spectra of polymers 4a-4c. On the other hand, the strong absorption peaks for triazole groups appeared at 1626 cm-1 in the IR spectra of polymers 4a-4c. These IR spectra certainly supported that almost all of the alkynyl groups of compound 3 were transferred to the triazole rings, which were also in agreement with the results of elemental analysis. Moreover, it was reasonable to deduce that these polymers might prefer to three-dimensional polymers than linear polymers due to compound 3 possessed four alkynyl groups in 1, 3-alternate conformation [19]. The surface morphologies of polymers 4a-4c were also investigated by scanning electron microscopy (SEM) as shown in Fig. 3. All of them exhibited loose porous and netty morphologies, which were favorable for binding guests.

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Fig. 3.Saturated adsorption curves of 4a-4c for CR.

Since the 1, 2, 3-triazole-modified thiacalix[4]arenes exhibited an excellent extraction ability for dyes [19], it is interesting to investigate the adsorption ability of novel polymers 4a-4c for dyes. Table 2 shows the adsorption percentages of polymers 4a-4c for a series of normal dyes including Alizarin green (AG), Orange I (OI), Neutral red (NR), Congo red (CR), Orange G (OG), Crystal violet (CV), Victoria blue B (VB), and Methylene blue (MB). All polymers exhibited good adsorption percentages for the eight tested dyes. The adsorption percentages for NR and CR were as high as above 90%. The highest adsorption percentage reached 96% for CR. These data were excellent comparing with the previous results of calixarene resins or polymers [8,9,10,11]. Moreover, one can see that the adsorption percentages for both anionic dyes (AG, CR, OG, and OI) were similar to that of cationic dyes (MB, VB, CV, and NR). These results might indicate that the adsorption ability of novel polymers was influenced by not only the dipole, electrostatic, and hydrogen bonding interactions, which were usually observed in adsorption, but also the π-π stacking interactions of the triazole ring with the aromatic system of dyes, and flexible cavities of thiacalix[4]arene skeleton for dye complexation. Due to the π-π stacking interactions and flexible cavities were minimally influenced by the bridging chains, thus, polymers 4a-4c showed similar adsorption ability for both anionic and cationic dyes. The percentages of polymer 4b were slightly higher than that of polymer 4a and 4c. This phenomena might suggest that the different polyethylene glycol chains in polymers 4a-4c produced some influences on the adsorption ability and the length of triethylene glycol chain was favorable for constructing the suitable cavities for binding dyes.

Table 2
dyes adsorption percentages of polymers 4a-4c.

Congo Red was chosen as a representative to study the saturated adsorption capacity of polymers 4a-4c. The results are shown in Fig. 3. The saturation adsorption capacity was as high as 1.3-1.4 mmol/g, which was outstanding comparing with the other absorbents for dyes [22,23]. On the other hand, the similar saturated adsorption capability of polymers 4a-4c with different bridging polyethylene glycol chains also supported the above proposed adsorption mechanism. Fig. 4 shows the adsorption percentages of polymers 4a-4c for CR under different pH values of 5-9. The adsorption percentages changed in the range of 78%-96%, which were fairly stable comparing with the other absorbents [22,23], indicating the good applicability in a wide pH range. The reused property of polymer 4c for CR was also investigated at pH 7. After adsorption for CR, the polymer 4c was desorbed by 10% HCl and 10% NaOH, subsequently washed adequately by distilled water and dried by vacuum. Then the polymer was reused for dye desorbed. The adsorption percentages in five cycles were 95.4%, 87.6%, 82.7%, 80.6%, 78.3%, respectively. The gradual decrease of adsorption percentages might be attributed to the strong binding action in some complexation cavities, which was difficult to be desorbed in acid or base. The adsorption percentage was as high as 78.3% after five cycles, suggesting their good reused property.

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Fig. 4.The effect of pH values on adsorption percentages.
4. Conclusion

In summary, series of novel thiacalix[4]arene 1, 2, 3-triazolepolyethylene glycol netty polymers were synthesized by reacting alkynylthiacalix[4]arene with polyethylene glycol azido compounds in good yields. Their structures were characterized by 1H NMR, IR, and elemental analysis. Their SEM images showed the loosed porous and netty architectures. The Mn of these polymers suggested average approximately 28-31 thiacalixarene units in each polymer molecule. The adsorption experiments for dyes indicated that they possess excellent adsorption abilities for both cationic and anionic dyes. The saturation adsorption capacities for CR attain 1.3-1.4 mmol/g. The adsorption abilities keep high and fairly stable level at pH 5-9. They also exhibit good reused property in five times’ cycle. They in-deep thermodynamic and kinetic behaviors in dyes adsorption, which were important factors in practical application, will be investigated in the following work.

Acknowledgments

Financial support from the National Natural Science Foundation of China (No: 21406036), Fujian Natural Science Foundation of China (No. 2014J01034), Project of Fujian provincial department of education (Nos. JA11044, JA10056, JB13011) and the Program for Innovative Research Team in Science and Technology in Fujian Province University were greatly acknowledged.

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