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Langmuir 2000, 16, 5506-5508
Modification of the Interlayer Surface of Kaolinite with Methoxy Groups Yoshihiko Komori,†,‡ Hiroyuki Enoto,† Ryoji Takenawa,† Shigenobu Hayashi,§ Yoshiyuki Sugahara,† and Kazuyuki Kuroda*,†,‡ Department of Applied Chemistry, School of Science and Engineering, Waseda University, Ohkubo-3, Shinjuku-ku, Tokyo 169-8555, Japan, Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo 169-0051, Japan, and National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received November 5, 1999. In Final Form: March 31, 2000
Introduction Intercalation reaction of inorganic layered materials has been well-known as a method for preparing inorganicorganic multilayer nanocomposites, which have drawn increasing attention in recent years.1-3 Kaolinite, a layered clay mineral (Al2Si2O5(OH)4), is a unique host material for organizing supramolecular hybrid systems because the interlayer region is sandwiched between hydroxyl groups of the AlO2(OH)4 sheets in one side and the oxide arrangements of the SiO4 sheets in the other. Guest molecules such as dimethyl sulfoxide (DMSO) are intercalated and aligned in one direction between the layers of kaolinite.4,5 Although the interlayer surface reactivity of kaolinite for intercalation is very low owing to its inherent hydrogen bondings between the layers, a guest displacement method has diversified the kind of guest species.6,7 We have recently reported that a kaolinitemethanol intercalation compound forms at room temperature 7 and that it is an excellent intermediate for displacement reactions with various kinds of organic species such as alkylamines,8 poly(vinylpyrrolidone),9 p-nitroaniline,10 and -caprolactam.11 Therefore, the elucidation of the structure of the intermediate, methanoltreated kaolinite, is significant for the understanding of the interlayer surface reactions of kaolinite. Tunney and Detellier have already reported that methoxylated kaolinite forms from kaolinite and methanol under severe conditions in the temperature range above 200 °C by using a kaolinite-DMSO or N-methylformamide (NMF) intercalation compound as an intermediate.12 However, there have been no reports on the nature of a †
Department of Applied Chemistry, Waseda University. Kagami Memorial Laboratory for Materials Science and Technology, Waseda University. § National Institute of Materials and Chemical Research. ‡
(1) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (2) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593. (3) Bruce, D. W.; O’Hare, D. Inorganic Materials, 2nd ed.; John Wiley & Sons: New York, 1996. (4) Thompson, J. G.; Cuff, C. Clays Clay Miner. 1985, 33, 490. (5) Hayashi, S. J. Phys. Chem. 1995, 99, 7120. (6) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Adam Hilger: London, 1974. (7) Komori, Y.; Sugahara, Y.; Kuroda, K. J. Mater. Res. 1998, 13, 930. (8) Komori, Y.; Sugahara, Y.; Kuroda, K. Appl. Clay Sci. 1999, 15, 241. (9) Komori, Y.; Sugahara, Y.; Kuroda, K. Chem. Mater. 1999, 11, 3. (10) Kuroda, K.; Hiraguri, K.; Komori, Y.; Sugahara, Y.; Mouri, H.; Uesu, Y. Chem. Commun. 1999, 22, 2253. (11) Komori, Y.; Matsumura, A.; Itagaki, T.; Sugahara, Y.; Kuroda, K. Clay Sci. 1999, 11, 47. (12) Tunney, J. J.; Detellier, C. J. Mater. Chem. 1996, 6, 1679.
kaolinite-methanol intercalation compound formed at room temperature. The present paper provides evidence for the modification of the interlayer surface with methoxy groups in kaolinite treated with methanol at room temperature. Although the intercalation of methanol has been achieved by a guest displacement reaction using a kaolinite-NMF intercalation compound as an intermediate,7 the structure of the product has not yet been elucidated. Because solid-state 2H NMR and 13C CP/MAS NMR are quite powerful for the study of methoxy-modified solid surfaces,13 the reaction products between kaolinite and methanol (CH3OH, CD3OD) at room temperature were characterized by NMR measurements. DTA analysis of this type of compound is not appropriate because of low organic content and a peak overlap with that due to dehydroxylation of kaolinite. Organic modification of the hydroxyl groups in the interlayer surface of kaolinite varies the properties of the interlayer environments of kaolinite. In addition to methanol,12 modifications with molecules such as ethylene glycol,14,15 amino alcohol,16 and phenylphosphonic acid 17 have been investigated. Therefore, the present paper further contributes to this field with more definitive data on organic modification of the interlayer surface of kaolinite. Experimental Section Kaolinite used in the present study was KGa-1, a well-ordered Georgia kaolinite obtained from the Source Clays Repository of Clay Minerals Society, U.S.A. 18 The method for intercalation of methanol was based on a previous report.7 A kaolinite-NMF intercalation compound (a basal spacing of 1.08 nm), which was prepared first as the intermediate, was added to methanol (CH3OH, CD3OD), and the reaction mixture was stirred for a day. After the wet sample was separated by centrifugation, it was added to fresh methanol and stirred for a day again. This procedure was repeated seven times to complete the reaction. After the reaction, a wet kaolinite-methanol intercalation compound was separated. The basal spacing of the kaolinitemethanol intercalation compound was 1.11 nm under wet conditions, as reported previously.7 The basal spacing decreased to 0.86 nm on drying in air. XRD patterns were obtained by using a Mac Science MXP3 diffractometer with monochromated Cu KR radiation. Solid-state 2H NMR spectra were recorded by a Bruker MSL400 spectrometer with Larmor frequency of 61.42 MHz. The quadrupole echo pulse sequence was used with a 90° pulse width of 2.5 µs, and the pulse interval between the two pulses was 15 µs. The frequency scales of the 2H spectra were expressed with respect to D2O. Solid-state 13C CP (cross-polarization)/MAS (magic-angle-spinning) NMR measurements were performed on Bruker ASX400 (a magnetic field of 9.4 T) and ASX200 (4.7 T) spectrometers with Larmor frequencies of 100.63 and 50.33 MHz, respectively. The spinning rate of the sample was 5 kHz. Chemical shifts were expressed with respect to neat tetramethylsilane. Infrared (IR) spectra of KBr pellets were recorded on a Perkin-Elmer FT-IR 1640 spectrometer. Thermogravimetric (TG) curves were measured on a Mac Science 2000S at a heating rate of 10 °C min-1 under a dry air flow. The amount of organic fractions was determined by CHN analysis (Perkin-Elmer PE-2400II). TG-MS (mass (13) Takahashi, S.; Nakato, T.; Hayashi, S.; Sugahara, Y.; Kuroda, K. Inorg. Chem. 1995, 34, 5065. (14) Tunney, J. J.; Detellier, C. Chem. Mater. 1993, 5, 747. (15) Tunney, J. J.; Detellier, C. Clays Clay Miner. 1994, 42, 552. (16) Tunney, J. J.; Detellier, C. Can. J. Chem. 1997, 75, 1766. (17) Guimara˜es, J. L.; Peralta-Zamora, P.; Wypych, F. J. Colloid Interface Sci. 1998, 206, 281. (18) Olphen, H. V.; Fripiat, J. J. Data Handbook for Clay Materials and Other Nonmetallic Minerals; Pergamon: Elmsford, NY, 1979.
10.1021/la991453o CCC: $19.00 © 2000 American Chemical Society Published on Web 05/12/2000
Notes
Langmuir, Vol. 16, No. 12, 2000 5507 Table 1. Fwhma (ppm) of the CH3O- Signal in the CP/MAS NMR Spectra compound methanol-treated kaolinite dimethylmalonic acid a
Figure 1. Solid-state 2H NMR spectrum of the methanol (CD3OD)-treated kaolinite (0.86 nm). spectrometry) analysis was performed on a combined Shimadzu TGA-50 and GCMS-QP1100EX under He atmosphere at a heating rate of 10 °C min-1
Results and Discussion Immobility of the methoxy groups in the methanoltreated kaolinite was proved by the 2H NMR spectrum. Figure 1 shows the 2H NMR spectrum of methanol (CD3OD)-treated kaolinite after drying (basal spacing of 0.86 nm). The spectrum shows a Pake doublet pattern with a quadrupole coupling constant (QCC) of 50 kHz and an asymmetric factor (η) of zero. The values of QCC and η represent typical CD3 groups reorienting rapidly only about their three-fold symmetry (C3) axis,19,20 indicating that the C-O axis of CD3O- is fixed. Thus, the doublet is assignable to CD3O groups bound to the layers of kaolinite. A similar pattern has been reported for methoxy species bound to layered perovskite.13 The Pake doublet was also observed in the spectrum of the wet methanoltreated kaolinite (1.11 nm). Namely, methoxy modification proceeds during stirring of the mixture of kaolinite and methanol at room temperature. The 2H NMR spectrum in Figure 1 also shows a large singlet at 0 kHz, indicating mobile molecules are present in the dry methanol-treated kaolinite (0.86 nm). The absence of methanol was confirmed by TG-MS data, which showed no signals due to methanol in the temperature range of desorption. Thus, the singlet is ascribed to mobile molecules such as HDO and D2O, which are presumably formed by the reaction between -OH and CD3OD and H-D exchange.20 The location of methoxy species was examined by 13C CP/MAS NMR. If methoxy species are located on the AlO2(OH)4 octahedra, Al-O-C bonds are formed. The dipoledipole interaction between 13C and 27Al spins is not averaged out by MAS since 27Al spins are quadrupolar.21 The residual dipolar broadening is inversely proportional to the square of the magnetic field if the line width is expressed in parts per million units. To examine the presence of the 13C-27Al residual dipolar broadening, 13C CP/MAS NMR spectra of the methanol (CH3OH)-treated (19) Eckman, R. R.; Vega, A. J. J. Phys. Chem. 1986, 90, 4679. (20) Luz, Z.; Vega, A. J. J. Phys. Chem. 1987, 91, 374. (21) Harris, R. K.; Olivieri, A. C. Prog. Nucl. Magn. Reson. Spectrosc. 1992, 24, 435.
fwhm (4.7 T) fwhm (9.4 T) 2.06 0.35
1.90 0.37
13C
∆b 0.16 -0.02
Full width at half-maximum. b Fwhm (4.7T) - fwhm (9.4 T).
kaolinite were measured under the different fields at 9.4 and 4.7 T. Table 1 lists the line widths. In the spectrum of the methanol-treated kaolinite (0.86 nm) at 9.4 T, the CH3O- signal was observed at 50.5 ppm with a line width of 1.90 ppm. The line width of the signal increased to 2.06 ppm when the field was changed to 4.7 T. The line width caused by the inhomogeneous magnetic field was suppressed to a value less than 0.01 ppm by careful shimming. For comparison, the spectra of dimethylmalonic acid ((CH3)2C(CO2H)2) were measured, and the line widths of the methyl signal at the two fields agreed with each other. The similar residual dipolar broadening has been analyzed for Si-O-Al bonds in kaolinite by 29Si CP/MAS NMR, where the difference in the line width measured under the fields of 9.4 and 4.7 T was 0.25 ppm.22 In the methanol (CH3OH)-treated kaolinite only 27Al spins are quadrupolar. Thus, the difference in the CH3O- line width is considered to originate from the residual dipolar interaction between 13C and 27Al spins. Namely, the formation of Al-O-C bonds is strongly indicated. The amount of methoxy groups was estimated by CHN analysis of the methanol (CH3OH)-treated kaolinite (0.86 nm). On the basis of the carbon content of 2.0 mass %, the number of methoxy groups (x) per Al2Si2O5(OH)4-x is calculated to be 0.36. On the other hand, the nitrogen content was less than 0.3 mass %, indicating that most of the NMF molecules were deintercalated. The presence of water molecules and hydrogen bonding of OH groups of kaolinite were analyzed by IR, as shown in Figure 2. To monitor the proceeding of the guest displacement reaction between NMF and methanol (CH3OH), the IR spectrum of the kaolinite-NMF intercalation compound after five cycles of methanol treatment was also measured. In the IR spectrum of the kaolinite-NMF intercalation compound (Figure 2b), the band due to the carbonyl stretching appears at 1682 cm-1. After five cycles of methanol treatment (Figure 2c), the intensity of the band at 1682 cm-1 is decreased and the band at 1657 cm-1 is clearly detected. After seven cycles of methanol treatment (Figure 2d), the band at 1682 cm-1 almost disappears and a broad band at around 1657 cm-1 remains. Because most NMF molecules were deintercalated in the sample (as indicated by CHN analysis), the band at 1657 cm-1 is assigned to H-O-H bending, indicating that some amount of water is present in the methanol-treated kaolinite (0.86 nm). In the OH stretching region, kaolinite shows four OH-stretching bands (Figure 2a), and three of them at 3693, 3670, and 3650 cm-1 are perturbed by guest species. By the intercalation of NMF molecules, the band at 3675 cm-1 appears. After five cycles of methanol treatment, this band is diminished and a band at around 3540 cm-1 is detected. After seven cycles of methanol treatment, the band shifts to lower wavenumber at around 3519 cm-1. The band at around 3540 cm-1 is characteristic for hydrated kaolinite,8,23 indicating that the hydrogen bonds between OH groups of kaolinite and water molecules are present although the band position varies with the reaction conditions. On the other hand, a shoulder band at ca. 3720 (22) Hayashi, S.; Ueda, T.; Hayamizu, K.; Akiba, E. J. Phys. Chem. 1992, 96, 10922. (23) Tunney, J. J.; Detellier, C. Clays Clay Miner. 1994, 42, 473.
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Figure 2. IR spectra (KBr method) of (a) kaolinite, (b) kaolinite-NMF intercalation compound, (c) kaolinite-NMF intercalation compound treated with methanol (CH3OH) five times, and (d) fully methanol-treated kaolinite (treated seven times).
cm-1 appears in the methanol-treated kaolinite (Figure 2c,d), which may be assigned to isolated hydroxyl groups without hydrogen bonds whose formation is blocked by methoxy groups. The content of water molecules was estimated by TG. The TG curve of methanol (CH3OH)-treated product (0.86 nm) showed mainly two steps of mass losses: a gradual mass loss in the temperature range from room temperature to ca. 400 °C and a large one mainly due to dehydroxylation of kaolinite from ca. 400 to 700 °C. To separate each mass loss, the sample was maintained at 350 °C for 2 h on the heating cycle. As a result, the mass loss before 350 °C was 4.8 mass % and the loss after 350 °C was 12.6 mass %. Because methoxy groups are stable
Notes
up to 350 °C 12 and no signals due to methanol (or methoxy groups) were detected by TG-MS, the former mass loss corresponds mainly to adsorbed water. On the basis of CHN and TG results, the chemical formula of the methanol-treated kaolinite (0.86 nm) was Al2Si2O5(OH)3.64(OCH3)0.36‚0.58H2O. Because the intermediate, the kaolinite-NMF intercalation compound, contains no H2O in the interlayer space,24 the source of H2O is considered to be H2O in ambient air as well as H2O generated by the reaction between methanol molecules and hydroxyl groups of kaolinite. The basal spacing of the methanol-treated kaolinite after drying was 0.86 nm, which is larger than that of kaolinite (0.72 nm) and the methoxy kaolinite (0.82 nm).12 Similar values for the basal spacing have been reported for a hydrated kaolinite (0.84-0.86 nm). 8,25-27 Furthermore, when the methanol-treated kaolinite (0.86 nm) was heated at 150 °C for 1.5 h under reduced pressure, the basal spacing decreased to 0.82 nm without loss of the methoxy groups. These results indicate that the basal spacing of 0.86 nm was affected by the water molecules. As for the wet methanol-treated kaolinite (1.11 nm), water molecules and methanol molecules in addition to grafted methoxy groups are suggested to expand the interlayer space. However, a quantitative estimation of the amount of methanol was difficult because of spontaneous deintercalation. In summary, methanol-treated kaolinite prepared at room temperature was characterized and methoxy modification on the AlO2(OH)4 layers of kaolinite is confirmed. Although methanol molecules are deintercalated spontaneously, methoxy groups and water molecules remain in the interlayer space of kaolinite after drying in air. Acknowledgment. K.K. is thankful for the financial support by a Grant-in Aid for Scientific Research by the Ministry of Education, Science, and Culture of the Japanese Government. Supporting Information Available: XRD patterns of kaolinite and methanol-treated kaolinite and solid-state 13C CP/ MAS NMR spectrum of methanol-treated kaolinite (2 pages). This material is available free of charge via the Internet at http://pubs.acs.org. LA991453O (24) Adams, J. M. Clays Clay Miner. 1978, 26, 169. (25) Costanzo, P. M.; Giese, R. F.; Lipsicas, M. Clays Clay Miner. 1984, 32, 419. (26) Raythatha, R.; Lipsicas, M. Clays Clay Miner. 1985, 33, 333. (27) Costanzo, P. M.; Giese, R. F. Clays Clay Miner. 1990, 38, 160.
