Molecular Recognition of 4-Nonylphenol on a Layered Silicate

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Molecular Recognition of 4-Nonylphenol on a Layered Silicate Modified with Organic Functionalities Yusuke Ide,† Shota Iwasaki,‡ and Makoto Ogawa*,†,‡ † ‡

Department of Earth Sciences, Waseda University, 1-6-1 Nishiwaseda Shinjuku-ku, Tokyo 169-8050 Japan Graduate school of Creative Science and Engineering, Waseda University, 1-6-1 Nishiwaseda Shinjuku-ku, Tokyo 169-8050, Japan ABSTRACT: A layered alkali silicate, octosilicate (Na2Si8O17), modified with the controlled amount and ratio of octadecyl and phenyl groups was synthesized to control the spatial distribution of the two functional groups and then achieve the effective and selective adsorption of 4-nonylphenol from aqueous solution. Octosilicates modified with the varied amount and ratio of the attached octadecyl/phenyl groups (0.7/0.7, 0.4/0.4, 0.3/0.3, and 0.4/1.0 groups per Si8O17 unit, respectively) were prepared by the reaction of the dodecylammonium-exchanged octosilicate with a controlled amount of octadecyltrichlorosilane and phenyltrichlorosilane sequentially. The adsorption of 4-nonylphenol from water on the four silylated octosilicates was investigated to find that the adsorption isotherm for the silylated octosilicate bearing the surface coverage with octadecyl/phenyl groups of 0.7/0.7 groups per Si8O17 unit was H-type, while the other silylated octosilicates gave S-type adsorption isotherms. The silylated octosilicate having surface coverage with octadecyl/phenyl groups of 0.7/0.7 groups per Si8O17 unit selectively adsorbed 4-nonylphenol from aqueous mixture of 4-butylphenol, 4-hexylphenol, and 4-nonylphenol.

1. INTRODUCTION Layered inorganic solids have widely been investigated as adsorbents because of the advantages such as the large surface area derived from layered structures composed of ultrathin sheets (so-called “nanosheets”), chemical stability if compared to organic counterparts, and material’s diversity.1 The organic modification through ion exchange2 and grafting3 is a way to control the adsorption characteristics. Though there are several examples to achieve the selective adsorption of target ions and molecules on the organically modified layered solids,4-6 the ability to specifically bind a particular species is still a topic of interest to achieve enzymatic reactions. For this goal, the modification with different kinds of organic functional units is a versatile way; however, two different functionalities tend to segregate, which is often observed for the adsorption of two cationic species in smectite clays.7 The organic derivative of a layered titanate, modified with phenyltrimethoxysilane and octadecyltrimethoxysilane, selectively and effectively adsorbs 4-nonylphenol from aqueous mixture with nonane and phenol.8 This phenomenon was explained as a result of the cooperative interactions between alkyl and phenyl groups of 4-nonylphenol and the spatially arranged two functional units on the titanate sheets. The amount of the attached silyl group on a layered alkali silicate, which possibly correlates to the spatial distribution of the silyl group on the silicate sheets,6a-6c,8,9 is controlled by changing the added amount of the corresponding silane coupling reagent.10 These results motivate us to study more precise molecular recognition r 2011 American Chemical Society

of 4-nonylphenol by controlling the spatial distribution of alkyl and phenyl groups on various layered solids. In this study, we examined the adsorption of 4-nonylphenol from an aqueous mixture of 4-butylphenol, 4-hexylphenol, and 4-nonylphenol on a layered alkali silicate, octosilicate,11 modified with the tuned amount and ratio of alkyl and phenyl groups. Because of the structural similarity, it seems to be more difficult to separate 4-nonylphenol from analogous alkylphenols than from nonane and phenol, which we have examined in the previous study.8 4-Substituted alkylphenols are one of the suspected environmental endocrine disruptors. Among the alkylphenols, 4-nonylphenol exists in environmental water at relatively high concentration;12 therefore, it is worth investigating to effectively concentrate it from dilute aqueous mixtures.

2. EXPERIMENTAL SECTION 2.1. Reagents. Octosilicate (Na2Si8O17 3 nH2O) was synthesized by the reported methods:13 SiO2 (special grade-silica gel, Wako Chemicals), NaOH, and distilled water were mixed in a molar ratio of SiO2:NaOH:H2O = 4:1:25.8. The mixture was sealed in a Teflon-lined autoclave and hydrothermally treated at 100 °C for 9 days. The product was separated by centrifugation (3500 rpm, 10 min) and washed with dilute aqueous solution of NaOH (pH = 10.0) and dried at 40 °C for 2 days. Hexadecyltrimethylammonium (C16TMA) chloride (98%) was Received: November 7, 2010 Revised: January 13, 2011 Published: February 16, 2011 2522

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Figure 1. XRD patterns of (a) Na-octosilicate, (b) C16TMA-oct, (c) C16TMA-oct reacted with 0.7 groups per Si8O17 of C18TCS, and (d) C18TCS0.7-PTCS0.7-oct. purchased from Tokyo Chemical Industry Co., Ltd. Octadecyltrichlorosilane (C18TCS, 85%) and phenyltrichlorosilane (PTCS, 98%) were purchased from Tokyo Chemical Industry Co., Ltd. 4-Nonylphenol (C9Ph, 97%), 4-hexylphenol (C6Ph, 98.5%), and 4-butylphenol (C4Ph, 98%) were obtained from Kanto Chemical Co., Inc., and Tokyo Chemical Industry Co., Ltd. All the chemicals were used without further purification. 2.2. Preparation of Adsorbents. To immobilize octadecyl and phenyl groups in an interlayer space of octosilicate, the hexadecyltrimethylammonium-exchanged form of the silicate (abbreviated as C16TMA-oct) was first synthesized,13 and C16TMA-oct was then reacted with C18TCS and PTCS sequentially, as reported for the preparation of a layered titanate modified with phenyl/octadecyl groups8 and glycidyl/octadecyl groups.14 The silylation of C16TMAoct was conducted in a manner similar to that we have developed for the silylation of another layered alkali silicate, magadiite.10 C16TMA-oct (0.20 g), which was synthesized by the reaction between octosilicate and an aqueous solution of C16TMA chloride,13 was dispersed in a solution of C18TCS in toluene (20 mL), and the mixture was heated at 60 °C for 12 h. After removal of the solvent from the mixture by evaporation, the resulting solid was dispersed in a solution of PTCS in toluene (20 mL), and the mixture was heated at 60 °C for 12 h. The product was separated by removing the solvent from the mixture and washed with a mixture of 0.1 mol L-1 HCl aqueous solution and ethanol (1:1 in volume). In order to control the amount of the attached silyl groups, the amount of the added silane coupling reagents were varied. Before the silylation, C16TMA-oct (or that reacted with C18TCS) was degassed for 3 h in a groove box, and then it was mixed with toluene and C18TCS (or PTCS) in the N2-purged groove box. The silylated derivatives thus obtained were abbreviated as C18TCSx-PTCSy-octosilicate, where x and y denoted the amounts of the attached C18TCS and PTCS (groups per Si8O17), respectively. 2.3. Adsorption Tests. A 0.55 mg portion of the adsorbent was added to 100 mL of an aqueous solution of C9Ph or mixed aqueous solution of C4Ph, C6Ph, and C9Ph in a glass vessel, and the mixture was shaken for 2 days at room temperature. After the mixture was separated by centrifugation (25 000 rpm, 10 min), the amount of the adsorbed substrates on the adsorbent was determined on the basis of the remaining amount in the supernatant by photoluminescence spectroscopy and high-performance liquid chromatography for single and trinal solutes systems, respectively. 2.4. Characterization. XRD patterns of solid products were recorded on a Rigaku RAD IB powder diffractometer equipped with monochromatic Cu KR radiation operated at 20 mA and 40 kV. Infrared spectra of KBr disks were recorded on a Shimadzu FT-8200 Fouriertransform infrared spectrophotometer at a resolution of 2.0 cm-1.

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Figure 2. IR spectra of (a) C18TCS0.7-PTCS0.7-oct, (b) C18TCS0.4-PTCS0.4-oct, (c) C18TCS0.3-PTCS0.3-oct, and (d) C18TCS0.4PTCS1.0-oct.

Figure 3. XRD patterns of (a) C18TCS0.7-PTCS0.7-oct, (b) C18TCS0.4-PTCS0.4-oct, (c) C18TCS0.3-PTCS0.3-oct, and (d) C18TCS0.4PTCS1.0-oct. Thermogravimetric and differential thermal analysis (TG-DTA) curves were recorded on a Rigaku TG8120 at a heating rate of 10 °C min-1 under air flow using R-Al2O3 as the standard material. Photoluminescence spectra were recorded on a Hitachi F-4500 fluorospectrophotometer. High-performance liquid chromatography was performed on a Shimadzu SCL-10Avp equipped with a UV detector.

3. RESULTS AND DISCUSSION 3.1. Preparation of Adsorbents. Figure 1 shows the X-ray diffraction patterns of C18TCS0.7-PTCS0.7-oct together with those of pristine octosilicate and C16TMA-oct. Upon the successive reactions of C16TMA-oct with C18TCS and PTCS followed by washing, the basal spacing of C16TMA-oct changed from 2.7 to 3.6 nm and then to 2.6 nm. In the infrared spectrum of C18TCS0.7-PTCS0.7-oct (Figure 2a), the absorption bands due to alkyl group such as C-H stretching vibrations together with those of PTCS such as Si-C stretching vibration were observed at around 2900 and 1430 cm-1, respectively. Similar results on the X-ray diffraction patterns (Figure 3) and infrared spectra (Figure 2) were obtained for the other silylated products with the different amount and ratio of the added silane coupling reagents, showing that the two silyl groups were immobilized in the silylated octosilicates. The C16TMA was completely removed from the silylated products during the reactions, as evidenced by the absence of the adsorption band due to C-N stretching 2523

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vibration at 1487 cm-1 in the infrared spectra of the products. The X-ray diffraction peaks of the silylated octosilicates were much broader than those of the pristine octosilicate (Figures 1 and 3), indicating the distortion of the original structure such as the shift of the neighboring silicate layers and loss of the periodic stacking ordering of the layers upon the silylation.15 The attachment of two different functionalities often resulted in “segregation” as observed for the adsorption of two cationic species in a smectite clay.7 In the present systems, judging from the single X-ray diffraction patterns of the silylated products (Figure 3), the two functionalities, octadecyl and phenyl groups, are located in a same interlayer and distributed homogeneously (Scheme 1) as reported previously for the layered titanates modified with ocatadecyltrimethoxysilane/ phenyltrimethoxysilane8 and octadecyltrimethoxysilane/glycidylpropyltrimethoxysilane.14 The amount of the attached octadecyl and phenyl groups in C18TCS-PTCS-octs was determined by subtracting the Si/Si8O17 molar ratio of C18TCS-reacted C16TMA-octs (after removal of C16TMA) from that of C18TCSPTCS-octs, and the results are summarized in Table 1. The

amount and the ratio of the attached octadecyl and phenyl groups were successfully controlled by changing the added amount of the silane coupling reagents.10 From the composition of the silylated octosilicates, the distance between the adjacent silyl groups on the silicate sheets was able be calculated, which is shown in Table 1. The spatial distribution of one or two kinds of the attached functional groups in the silylated layered materials has been suggested to control their adsorption characteristics.6a-6c,8,9,14 For example, in the study on the silylated layered titanate with octadecyl and phenyl groups,8 the closely arranged octadecyl and phenyl groups in an interlayer space of the titanate played an important role on the selective and effective adsorption of aqueous C9Ph. Accordingly, the reaction of aqueous C9Ph with the presently synthesized four silylated octosilicates is worth conducting to probe the spatial distribution of octadecyl and phenyl groups on the silicate sheets as well as to optimize selective and effective binding of C9Ph, which is a known contaminant of water. 3.2. Adsorption of 4-Alkylphenols. Figure 4 shows the adsorption isotherms of C9Ph on the four silylated octosilicates from water. The isotherm of C9Ph for C18TCS0.7-PTCS0.7-oct was type H according to Giles classification, showing strong adsorbent-adsorbate interactions.16 On the other hand, the other silylated octosilicates exhibited S-type isotherms for C9Ph, indicating relatively weak adsorbent-adsorbate interactions.16 The adsorption capacity of 4-nonylphenol for C18TCS0.7-PTCS0.7-oct was estimated to ca. 3.8 mmol g-1, which was equivalent to 1.5 groups per an attached silyl group.

Scheme 1. Distribution of Octadecylsilyl and Phenylsilyl Groups on (a) C18TCS0.7-PTCS0.7-oct, (b) C18TCS0.4PTCS0.4-oct, (c) C18TCS0.3-PTCS0.3-oct, and (d) C18TCS0.4PTCS1.0-oct (Scale Bars: 0.5 nm)

Figure 4. Adsorption isotherms of C9Ph from water on (O) C18TCS0.7-PTCS0.7-oct, (2) C18TCS0.4-PTCS0.4-oct, (9) C18TCS0.3PTCS0.3-oct, and ([) C18TCS0.4-PTCS1.0-oct.

Table 1. Compositions of the Silylated Octosilicates added silanes/groups per Si8O17

attached silanes/groups per Si8O17a

adsorbent

C18TCS

PTCS

C18TCS

PTCS

distance between adjacent silyl groups/nmb

C18TCS0.7-PTCS0.7-oct

0.75

0.75

0.74

0.74

0.60

C18TCS0.4-PTCS0.4-oct C18TCS0.3-PTCS0.3-oct

0.40 0.25

0.40 0.25

0.38 0.25

0.37 0.28

0.84 0.92


0.50

1.0

0.48

0.98

0.60

Calculated on the basis of the mass loss due to the oxidative decomposition of the attached C18TCS or PTCS in the temperature range of 280-800 °C and the residual mass at 800 °C. The decomposed silyl group was supposed to be CH3(CH2)17(OH)- or (C6H5)(OH)-, and all the residual mass at 800 °C was assumed to be the SiO2 content of octosilicate. b Calculated as (ab/x)1/2, where ab (0.73 0.73 = 0.53 nm2) and x are the basal area per unit cell of octosilicate11b and the attached amount of the two silyl group per unit cell of octosilicate, respectively. a

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Langmuir This value means that almost all the adsorbed C9Ph are located between the adjacent octadecyl and phenyl groups on the silicate sheets when the adsorption is saturated (Scheme 2), confirming clearly cooperative interactions of the attached octadecyl and phenyl groups (and silanol groups derived from the hydrolysis of chlorides) with the two functionalities (and hydroxyl group) of C9Ph.8 For the other silylated octosilicates, octadecyl and phenyl groups are not closely packed on the silicate sheets; accordingly, C9Ph does not interact with the two functionalities. The amount of the immobilized octadecyl and phenyl groups in C18TCS0.7PTCS0.7-oct was tuned to be equivalent to that in the previously reported organic derivative of a layered titanate modified with Scheme 2. Location of the Adsorbed C9Ph on C18TCS0.7PTCS0.7-oct (Scale Bar: 0.5 nm)

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phenyltrimethoxysilane and octadecyltrimethoxysilane (abbreviated as PTMS-C18TMS-TLO).8 We expected that the maximum adsorbed amount of C9Ph per 1 g of C18TCS0.7-PTCS0.7-oct was larger than that of PTMS-C18TMS-TLO because silica is lighter than titania. However, the maximum adsorbed amount of C9Ph was similar between the two adsorbents. This result suggests that the adsorption of C9Ph was not controlled only by the spatial distribution of octadecyl and phenyl groups on the oxide sheets. The variation in the swelling ability of the adsorbents was another possible factor. On the basis of the above results, we expected that C18TCS0.7PTCS0.7-oct effectively concentrated C9Ph from an aqueous mixture. The reaction of C18TCS0.7-PTCS0.7-oct with an aqueous mixture containing similar molecules, C9Ph, C6Ph, and C4Ph, which seems to be difficult to separate if compared with that containing C9Ph, phenol, and nonane, which we have previously examined,8 was done using the other silylated derivatives as comparison. Figure 5 shows the adsorption isotherms of C9Ph, C6Ph, and C4Ph on the four silylated octosilicates from an aqueous mixture containing the three solutes at the same concentration. Taking a fact that only C18TCS0.7-PTCS0.7-oct exhibited H-type adsorption of C9Ph from water into consideration (Figure 4), it was unexpected that the all silylated octosilicates showed H-type adsorption isotherms for C9Ph. It is difficult to explain clearly the phenomenon; the difference in the solubility of C9Ph between pure water and the aqueous mixture of C6Ph and C4Ph may be concerned. On the other hand, the 4-alkylphenols other than C9Ph showed different adsorption behaviors with the kind of the adsorbents, and only C18TCS0.7-PTCS0.7-oct selectively adsorbed C9Ph (Figure 5).

Figure 5. Adsorption isotherms of (b) C9Ph, (]) C6Ph, and (0) C4Ph from their aqueous mixture on (a) C18TCS0.7-PTCS0.7-oct, (b) C18TCS0.4PTCS0.4-oct, (c) C18TCS0.3-PTCS0.3-oct, and (d) C18TCS0.4-PTCS1.0-oct. 2525


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Table 2. Calculated Partition Coefficients (log Ksa) and Octanol-Water Partition Coefficients of C9Ph, C6Ph, and C4Ph C9Ph a

C6Ph

C4Ph


6.31

4.80


6.14a

6.07,b 5.15b


a

6.37c

5.17d

a

d

4.66d

e

3.31e

C18TCS0.4-PTCS1.0-oct octanol-water partition coefficient

6.78 6.35

e

5.76

5.94 4.52

a

Only four points at lower equilibrium concentration were used. b Each two points at lower and higher equilibrium concentration was used. c Only three points at lower equilibrium concentration were used. d Only two points at higher equilibrium concentration were used. e According to ref 19.

This result is worth noting as a merit of the present system since the separation of C9Ph from C6Ph is difficult.17 The adsorption isotherms of C6Ph and C9Ph from the aqueous solutions on a octylsilyl group-grafted mesoporous silica is both H-type.17a The difference in the adsorption characteristics of the four silylated octosilicates was discussed on the basis of the interlayer structures (Table 1 and Scheme 1) as well as pseudo-partition coefficients of the 4-alkylphenols between the silylated octosilicate and aqueous phase, log Ksa (Table 2), which was calculated from the following equation:18 log Ksa = log 100K/OM, where K and OM are the slope of adsorption isotherms if adsorption amount (mg kg-1) is plotted against equilibrium concentration (mg L-1) and organic matter contents (%) in adsorbents, respectively. On C18TCS0.7-PTCS0.7-oct, the 4-alkylphenols other than C9Ph were scarcely adsorbed (Figure 5a). Since octadecyl and phenyl groups closely and homogeneously arranged on the entire silicate sheets in the material (Table 1 and Scheme 1), C9Ph is thought to be preferentially adsorbed to let the interlayer space more hydrophobic, permitting C6Ph to be intercalated by partitioning between the interlayer space and the aqueous phase. The partition coefficient of C6Ph (4.5) is similar to log Ksa (4.8) of C6Ph (Table 2). C18TCS0.4-PTCS0.4 gave isotherms similar to that observed for C18TCS0.7-PTCS0.7-oct, while C6Ph was adsorbed from relatively low concentration (∼0.006 mmol L-1) solution (Figure 5b). Both C6Ph and C9Ph exhibited H-type adsorption on C18TCS0.3PTCS0.3. These results were explained as follows: the surface coverage of C18TCS0.4-PTCS0.4 and C18TCS0.3-PTCS0.3 were relatively low (Table 1 and Scheme 1), so that both C9Ph and C6Ph can interact with the attached octadecyl or phenyl group on the silicate sheets. Our previous study on the adsorption of C9Ph from water using a layered titanate modified with octadecyl and phenyl groups and those modified with one of the two functionalities (their surface coverage was half compared to that of the octadecyl and phenyl group-modified titanate) demonstrated that all the three adsorbents exhibited H-type isotherms, while the titanate modified with the two functional units adsorbed C9Ph more effectively than the others.8 The surface coverage of C18TCS0.3-PTCS0.3-oct was enough low to uptake both C9Ph and C6Ph effectively. The log Ksa values of C9Ph and C6Ph for C18TCS0.3-PTCS0.3-oct are highest among those of the present silylated octosilicates (Table 2), suggesting that both adsorption and partitioning played an important role in the result. On the other hand, C6Ph was hardly adsorbed on C18TCS0.4PTCS1.0 from relatively low concentration solution (Figure 5d).

The geometric hindrance due to the attached phenyl groups suppressed the uptake of C6Ph. The adsorption of C9Ph on C18TCS0.4-PTCS1.0 was preferential; accordingly, the uptake of C6Ph was competed with that of C9Ph to be suppressed. Once the interlayer space was further modified with alkyl groups by C9Ph adsorption, C6Ph was permitted to intercalate (relatively high equilibrium concentration in Figure 5d). C4Ph was adsorbed only on C18TCS0.3-PTCS0.3-oct and C18TCS0.4PTCS1.0-oct (Figure 5c,d), which was thought to be concerned with the variation in the solubility of C4Ph between pure water and the aqueous mixture of C9Ph and C6Ph.

4. CONCLUSIONS We have successfully synthesized the silylated derivatives of a layered alkali silicate, octosilicate (Na2Si8O17), bearing the tuned amount and ratio, which correlated to the spatial distribution, of the attached octadecyl/phenyl groups (0.7/0.7, 0.4/0.4, 0.3/0.3, and 0.4/1.0 groups per Si8O17, respectively). The silylated octosilicate, where the same ratios of octadecyl and phenyl groups were densely packed (0.7/0.7 groups per Si8O17 unit), showed H-type adsorption for 4-nonylphenol in water; on the other hand, the other derivatives showed S-type adsorption. The effective concentration of 4-nonylphenol from the aqueous mixture with 4-hexylphenol and 4-butylphenol was possible only on the silylated octosilicate bearing 0.7/0.7 groups per Si8O17 unit of octadecyl/phenyl groups. These results suggested that the control of the spatial distribution of two kinds of organic functional units in layered solids is a versatile way to precisely design molecular recognition abilities for a wide variety of organic compounds. ’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]; Tel þ81-3-5286-1511; Fax þ81-33207-4950.

’ ACKNOWLEDGMENT This work was partially supported by the Global COE Program of MEXT “Center for Practical Chemical Wisdom”. Waseda University also supported us financially as special research projects (2009B-077 and 2009B-370). ’ REFERENCES (1) Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds. Handbook of Layered Materials; Marcel Dekker: New York, 2004. (2) (a) Weiss, A. Angew. Chem., Int. Ed. Engl. 1980, 20, 850–860. (b) Lagaly, G.; Beneke, K. Colloid Polym. Sci. 1991, 269, 1198–1211. (c) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593–2618. (3) (a) Ruiz-Hitzky, E.; Rojo, J. M. Nature 1980, 287, 28–30. (b) Ide, Y.; Ogawa, M. Chem. Commun. 2003, 1262–1263. (4) (a) Barrer, R. M. Clays Clay Miner. 1989, 37, 385–395. (b) Okada, T.; Matsutomo, T.; Ogawa, M. J. Phys. Chem. C 2010, 114, 539–545. (5) (a) Johnson, J. W.; Jacobson, A. J.; Butler, W. M.; Rosenthal, S. E.; Brody, J. F.; Lewandowski, J. T. J. Am. Chem. Soc. 1989, 111, 381– 383. (b) Yamanaka, S.; Yamasaka, K.; Hattori, M. J. Inclusion Phenom. 1984, 2, 297–304. (6) (a) Ogawa, M.; Okutomo, S.; Kuroda, K. J. Am. Chem. Soc. 1998, 120, 7361–7362. (b) Fujita, I.; Kuroda, K.; Ogawa, M. Chem. Mater. 2003, 15, 3134–3141. (c) Fujita, I.; Kuroda, K.; Ogawa, M. Chem. Mater. 2005, 17, 3717–3722. (d) Mochizuki, D.; Kowata, S.; Kuroda, K. Chem. 2526


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Molecular Recognition of 4-Nonylphenol on a Layered Silicate

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