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Lamellar Hexadecyltrimethylammonium Silicates Derived from Kanemite Tatsuo Kimura,† Daigo Itoh,† Nanae Okazaki,† Mizue Kaneda,‡ Yasuhiro Sakamoto,‡ Osamu Terasaki,‡,§ Yoshiyuki Sugahara,† and Kazuyuki Kuroda*,†,| Department of Applied Chemistry, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan, Department of Physics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan, CREST, JST, and Center for Interdisciplinary Research, Tohoku University, Sendai 980-8578, Japan, and Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda 2-8-26, Shinjuku-ku, Tokyo 169-0051, Japan Received March 3, 2000. In Final Form: June 30, 2000 Lamellar organoammonium silicates with variable silicon environments have been synthesized by the reaction of a layered polysilicate kanemite with aqueous solutions of hexadecyltrimethylammonium (C16TMA) chloride. While the structure of kanemite is composed of only Q3 silicate species ((SiO)3SiO), SiO4 units with both Q3 and Q4 ((SiO)4Si) environments were present in the silicate frameworks of the lamellar C16TMA silicates. The Q4 species mainly formed by intralayer condensation of the Q3 species in the individual silicate sheets in kanemite rather than by interlayer condensation between the adjacent sheets. The intralayer condensation can be suppressed by lowering the reaction temperature and/or shortening the reaction time, which results in the relative retention of the silicate framework of kanemite in the lamellar C16TMA silicates.
Introduction Various organoammonium cations including cationic surfactants are frequently used for the interlayer modification of inorganic layered materials in order to construct inorganic-organic nanostructures. Layered inorganicsurfactant intercalation compounds themselves have also been utilized as host materials for further intercalation and selective adsorption of functional molecules.1 Among a wide variety of inorganic layered materials, layered polysilicates such as magadiite, octosilicate, and kanemite are unique because the frameworks are composed of only tetrahedral SiO4 units and both hydrated alkali metal ions and silanol (Si-OH) groups are present in the interlayer region.2 Therefore, these layered polysilicates are quite attractive because the interlayer surfaces can be organically modified by ion exchange, adsorption, and direct derivatization such as silylation.3 Kanemite was discovered as a mineral by Johan et al.,4 and the chemical synthesis and the reactivity were reported by Beneke and Lagaly.5 The structure of kanemite has been considered as a layered silicate with single sheets †
Department of Applied Chemistry, Waseda University. Department of Physics, Graduate School of Science, Tohoku University. § CREST, JST, and Center for Interdisciplinary Research, Tohoku University. | Kagami Memorial Laboratory for Materials Science and Technology, Waseda University. ‡
(1) (a) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (b) Ogawa, M.; Kuroda, K. Bull. Chem. Soc. Jpn. 1997, 70, 2593 and references therein. (2) Lagaly, G. Adv. Colloid Interface Sci. 1979, 11, 105. (3) Ogawa, M.; Okutomo, S.; Kuroda, K. J. Am. Chem. Soc. 1998, 120, 7361. (4) Johan, Z.; Maglione, G. F. Bull. Soc. Fr. Mineral. Crystallogr. 1972, 95, 371. (5) Beneke, K.; Lagaly, G. Am. Miner. 1977, 62, 763. (6) Pant, A. K.; Cruickshank, D. W. J. Acta Crystallogr., Sect. B 1968, 24, 13.
such as δ-Na2Si2O56 and KHSi2O5.7 Indeed, the 29Si MAS (magic angle spinning) NMR (nuclear magnetic resonance) spectrum of kanemite exhibits only one peak due to Q3 silicate species ((SiO)3SiONa and (SiO)3SiOH) at -97 ppm. Recently, the structure has been defined in detail by Gies et al.8 The single silicate sheets are wrinkled regularly, and ion-exchangeable Na+ ions are present in the interlayer space.8,9 The mineral is a very interesting layered polysilicate from the viewpoint of the development of ordered mesoporous silicas. The formation of a mesoporous silica derived from kanemite was originally discovered by Yanagisawa et al.10 Inagaki et al. have reported the synthesis of hexagonal mesoporous silicas (designated as FSM-16) by optimizing the synthetic conditions.11 These materials are differentiated because of the difference in the synthetic conditions and the structures. During the formation of kanemite-surfactant mesophases as the precursors for mesoporous silicas, the environment of tetrahedral SiO4 units in kanemite sheets is varied by the formation of three-dimensional (3-D) silicate network; SiO4 species with both Q3 ((SiO)3SiO) and Q4 ((SiO)4Si) environments are detected in the silicate frameworks.10,11 Almost all the researches to date have been focused on the formation of 3-D silicate frameworks. Lamellar mesophases synthesized by the reaction of kanemite with (7) Le Bihan, M.-T.; Kalt, A.; Wey, R. Bull. Soc. Fr. Mineral. Crystallogr. 1971, 94, 15. (8) (a) Gies, H.; Marler, B.; Vortmann, S.; Oberhagemann, U.; Bayat, P.; Krink, K.; Rius, J.; Wolf, I.; Fyfe, C. Microporous Mesoporous Mater. 1998, 21, 183. (b) Vortmann, S.; Rius, J.; Marler, B.; Gies, H. Eur. J. Mineral. 1999, 11, 125. (9) Garvie, L. A. J.; Devouard, B.; Groy, T. L.; Camara, F.; Buseck, P. R. Am. Mineral. 1999, 84, 1170. (10) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (11) (a) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (b) Inagaki, S.; Koiwai, A.; Suzuki, N.; Fukushima, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1996, 69, 1449.
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surfactants have not been investigated, though the presence of a layered intermediate and the transformation have been inferred by in situ XRD study on the formation of FSM-16.12 In this paper, we report that lamellar C16TMA silicates are prepared from kanemite under various conditions and that the mesophase silicates have Q4 silicate species caused by intralayer condensation. Lamellar mesophases have already been prepared by the reaction of kanemite with CnTMA cations in the presence of n-alkyl alcohols or with dialkyldimethylammonium cations, and the presence of Q4 silicate species was observed in the silicate frameworks.13 However, detailed studies on the formation of Q4 species have not been performed yet. In addition, the present study will contribute to the understanding on previously reported interlayer condensation of adjacent silicate sheets of acid-treated H2Si2O5-III.14 Experimental Section 1. Preparation of a Layered Polysilicate Kanemite. Sodium hydroxide was added to high-quality water glass (SiO2/ Na2O ) 3.32, Nissan Kagaku Kogyo Co.) to prepare a precursor solution with the Na/Si ratio of 1.0, and the aqueous solution was stirred at room temperature. The solution was dried and heated at 750 °C for 1 h in air, which results in the formation of δ-Na2Si2O5 after cooling and milling. δ-Na2Si2O5 was dispersed in 20 mL of distilled water, and the suspension was stirred at room temperature for 30 min. The resultant product (kanemite, Na1.14H0.86Si2O5‚xH2O) was centrifuged and air-dried. 2. Reactions of Kanemite with C16TMA Surfactant. A 1 g amount of kanemite was added to 20, 50, 100, or 200 mL of an aqueous solution of 0.1 M hexadecyltrimethylammonium chloride (C16H33(CH3)3NCl, C16TMACl, Tokyo Kasei Kogyo Co.), where C16TMA/Si molar ratios were 0.2, 0.5, 1.0, and 2.0, respectively. After the samples were stirred at 70 °C for 2 days, the pH values of the suspensions were 11.4, 11.3, 10.4, and 10.9, respectively. Then, the products were separated by centrifugation and airdried. To examine the effect of the reaction temperature, 1 g of kanemite was dispersed in 200 mL of an aqueous solution of 0.1 M C16TMACl (C16TMA/Si ) 2.0) and stirred at room temperature, 50 °C, 70 °C, or 90 °C for 2 days. The products were centrifuged and air-dried. 3. Characterization. Powder X-ray diffraction (XRD) patterns were obtained by using a Mac Science MXP3 diffractometer with monochromated Cu KR radiation and a Rigaku RAD-1B diffractometer with monochromated Fe KR radiation. Solid-state 29Si MAS NMR measurements were performed on a JEOL GSX400 spectrometer at a spinning rate of 5 kHz and a resonance frequency of 79.30 MHz with a 45° pulse length of 4.1 µs and a recycle time of 60 s. The chemical shift was expressed with respect to tetramethylsilane. A transmission electron micrograph (TEM) was taken by a JEM-3010 electron microscope, operated at 300 kV. The Na/Si ratio in kanemite was measured by ICP (Jarrell Ash ICAP575 Mark II), and the amounts of organic fractions were determined by CHN analysis (Perkin-Elmer PE-2400II).
Results 1. Synthesis of Lamellar C16TMA Silicates. The XRD patterns of the products which were prepared under the different C16TMA/Si molar ratios at 70 °C for 2 days are shown in Figure 1. When the C16TMA/Si molar ratio is 0.2, all the XRD peaks in low-2θ angles are assignable to a hexagonal phase with a lattice constant of 4.7 nm (d100 ) 4.1 nm) (Figure 1a), which is consistent with the synthesis of a precursor of FSM-16.11 By the increase in (12) (a) O’Brien, S.; Francis, R. J.; Price, S. J.; O’Hare, D.; Clark, S. M.; Okazaki, N.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1995, 2423. (b) O’Brien, S.; Francis, R. J.; Fogg, A.; O’Hare, D.; Okazaki, N.; Kuroda, K. Chem. Mater. 1999, 11, 1822. (13) Inagaki, S.; Yamada, Y.; Fukushima, Y.; Kuroda, K. Stud. Surf. Sci. Catal. 1994, 92, 143. (14) Deng, Z. Q.; Lambert, J. F.; Fripiat, J. J. Chem. Mater. 1989, 1, 375.
Figure 1. XRD patterns of C16TMA-silicate phases obtained at 70 °C under the reaction conditions with C16TMA/Si molar ratios of (a) 0.2, (b) 0.5, (c) 1.0, and (d) 2.0. The peaks marked by filled (b) and open circles (O) are assignable to hexagonal and lamellar phases, respectively.
Figure 2. 29Si MAS NMR spectra of (a) kanemite and C16TMA-silicate phases synthesized by using C16TMA/Si molar ratios of (b) 0.2 and (c) 2.0.
the C16TMA/Si ratio, the XRD patterns of the products changed. In each XRD pattern (Figure 1b-d), the peak at the d-spacing of 3.2 nm and the peaks of higher order diffractions were observed. The layered nature of the products was proved by the increase in the d-spacings to 4.6 nm by further intercalation of n-decyl alcohol to the organoammonium-silicate materials. In addition, the structures collapsed upon calcination at 700 °C for 6 h. The 29Si MAS NMR spectrum of the products (C16TMA/ Si ) 0.2) showed the presence of SiO4 species of both Q3 and Q4 environments observed at around -101 and -111 ppm (Figure 2b). The NMR profile has been observed in many cases as an evidence of the formation of a 3-D silicate network of the hexagonal structure.11 In the present study, however, Q4 silicate species were detected in the spectrum
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Figure 3. XRD patterns of lamellar C16TMA silicates (C16TMA/Si ) 2.0): (a) L-RT; (b) L-50; (c) L-70; (d) L-90.
of the lamellar phase (Figure 2c, C16TMA/Si ) 2.0). The Q4 silicate species are not present in the original kanemite (Figure 2a), meaning that the lamellar C16TMA silicates is not simply composed of alternating C16TMA ions and silicate sheets of kanemite. 2. Lamellar C16TMA Silicates with Various Silicon Environments. The XRD patterns of the lamellar C16TMA silicates (C16TMA/Si ) 2.0) prepared at room temperature (L-RT), 50 °C (L-50), 70 °C (L-70), and 90 °C (L-90) are shown in Figure 3. The peak at the d-spacing larger than 3 nm and the peaks of higher order diffractions were observed for each product. Even when the reactions were conducted at various temperatures, the structural ordering did not change remarkably. The d-spacing became smaller with the decrease in the reaction temperatures (L-RT, 3.05 nm; L-50, 3.16 nm; L-70, 3.20 nm; L-90, 3.35 nm). The mass percents of the organic fractions in L-RT, L-50, L-70, and L-90 were 57.2, 56.1, 53.1, and 52.6 (C16TMA/Si ) 0.28, 0.27, 0.24, and 0.24), respectively. Although the basal spacings of n-alkylammonium intercalated compounds generally tend to be larger with the increase in the contents of organic fractions,15 the tendency was not found for the lamellar C16TMA silicates derived from kanemite. The 29Si MAS NMR spectra of L-RT, L-50, L-70, and L-90 (C16TMA/Si ) 2.0) are shown in Figure 4. The spectrum of L-RT showed that several Q3 peaks were mainly observed in the range from -95 to -105 ppm and that a small Q4 peak centered at around -110 ppm was detected (Figure 4a). This Q4 peak did not increase remarkably in the spectrum of the product after stirring for 50 days. The change of the 29Si MAS NMR profile is related to the variation in both the interaction of the silicate sheets with C16TMA ions and the bonding angle among tetrahedral SiO4 units in the silicate sheets owing to the difference in ionic radii between Na+ and the headgroup of C16TMA ions.16 The TEM image of L-RT (Figure 5) showed clear striped patterns with a repeated (15) Weiss, A. Angew. Chem. 1963, 75, 113. (16) Liebau, F. Structural Chemistry of Silicates; Springer-Verlag: Berlin, Heidelberg, Germany, 1985.
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Figure 4. 29Si MAS NMR spectra of (a) L-RT, (b) L-50, (c) L-70, and (d) L-90.
Figure 5. TEM image of L-RT.
distance of ca. 3.0 nm. The observed thickness of silicate layers was estimated to be 0.6 nm, being in agreement with that of the original kanemite.8 These results indicate that single silicate sheets in kanemite were almost retained by the synthesis at room temperature in the present system. The peak intensity due to Q4 silicate species increased and the Q3 peaks were broadened (Figure 4b-d) with the increase in the reaction temperature. The Q4/(Q3 + Q4) ratio of L-90 reached to 0.59. The XRD profiles in higher 2θ angles (15-30°) were also varied with the reaction temperature (Figure 3). Three peaks with the d-spacings of 0.42, 0.37, and 0.35 nm were observed distinctly although the broad peak centered at around 21° (0.42 nm) seemed to be overlapped by several peaks. The peak intensity at the d-spacing of 0.35 nm increased gradually against that of 0.37 nm. Discussion 1. Formation of the Q4 Silicate Species. Depending on the synthesis temperatures, silicate frameworks in the lamellar C16TMA silicates derived from kanemite involve SiO4 species with Q4 environments in addition to Q3 silicate
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Table 1. Summary of the Reaction of Kanemite with CnTMA Surfactants 0.1 M CnTMA/mLa
CnTMA/Si molar ratio
Beneke et al.5 Yanagisawa et al.10
100 100
1.0 1.0
Inagaki et al.11 Vartuli et al.17 Chen et al.18
20 100 150
0.2 1.0 1.5
a
pH 8.5 8-9 12.3 9.0-9.5
x value in NaxH2-xSi2 O5
structure (d value/alkyl chain length)
0.8 1.0
lamellar (3.1 nm/C12) lamellar (3.1 nm/C12) and hexagonal (3.7 nm/C12) hexagonal (4.0 nm/C16) lamellar (3.4 nm/C16) lamellar (3.3 nm/C16) and hexagonal (4.7 nm/C16)
1.6 0.4 0.8
The value corresponds to 1.0 g of kanemite.
Figure 7. Variation in C16TMA/Si ratios as a function of Q4/ (Q3 + Q4) ratios in L-RT, L-50, L-70, and L-90.
Figure 6. (a) Structural unit of kanemite (after Gies et al.8), (b) typical powder XRD pattern of kanemite, and (c) schematic model of intralayer condensation.
species (Figure 4). The structural change of the silicate frameworks has never been found for organoammonium intercalation compounds of other layered silicates.1 Accordingly, the formation of the Q4 silicate species is related to the unique structure of kanemite. On the basis of the 29 Si MAS NMR results, the organic contents, and the swelling behavior, the intralayer condensation shown in Figure 6 is unequivocally plausible because the individual silicate sheets of kanemite are composed of SiO4 tetrahedra and wrinkled regularly and the adjacent Si-OH groups are alternatively confronting each other.8,9 The precise location of the condensed sites cannot be identified owing to ill-defined XRD peaks of the lamellar C16TMA silicates. Considering two possible reactions (Figure 6), the silicate layers are able to be slightly thickened after the intralayer condensations. Such intralayer condensation leads to the formation of Q4 silicate species (Figure 4) and then the d-spacing increases with the reaction temperature (Figure 3), possibly reflecting the change of the XRD profiles in higher 2θ angles. As shown in Figure 7, the C16TMA/Si ratio decreased with the increase in the Q4/(Q3 + Q4) ratio. The charge density of the silicate framework, which is reflected as the C16TMA/Si ratio here, is varied by intralayer condensation. 2. Formation of CnTMA Silicate Mesophases Derived from Kanemite. The reactions of kanemite with CnTMA surfactants reported so far5,10,11,17,18 are summarized in Table 1, though the reaction conditions such
as the amount of surfactant solutions and the composition of kanemite are not identical. Such conditions lead to different pH values of the reaction mixtures during the syntheses; the concentration of Na+ is varied. The pH value intensely affects the behavior of silicate species in the reaction mixtures.16 Table 1 shows the tendency that layered materials and hexagonal mesophases can be formed under lower and higher pH conditions, respectively. The flexibility of the silicate framework in kanemite becomes lower by intralayer condensation, and the lamellar phases occur more easily. In the present system, the lamellar silicates were prepared by using quite a high C16TMA/Si ratio (2.0) even at relatively high pH (ca. 11). On the basis of the insight on “intralayer condensation” described here, the structural variety of silicate-organic mesostructures derived from kanemite is restricted to the variation in the silicon environments. In fact, a cubic mesophase silicate has not been obtained by the reaction of kanemite with CnTMA surfactants (Figure 1). This is contrast to the synthesis of various mesostructured silicates including cubic phases which are achieved by the reaction of soluble silicate species with CnTMA surfactants.19,20 Conclusions Lamellar CnTMA silicates with various silicate frameworks were synthesized by the reaction of layered poly(17) Vartuli, J. C.; Kresge, C. T.; Leonowicz, M. E.; Chu, A. S.; McCullen, S. B.; Johnson, I. D.; Sheppard, E. W. Chem. Mater. 1994, 6, 2070. (18) Chen, C.-Y.; Xiao, S.-Q.; Davis, M. E. Microporous Mater. 1995, 4, 1. (19) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (20) Fyfe, C. A.; Fu, G. J. Am. Chem. Soc. 1995, 117, 9709.
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silicate kanemite and CnTMA ions. The silicate frameworks with different Q4/Q3 ratios were induced by intralayer condensation of the silicate sheets, and the unique structure of kanemite results in the condensation. This finding widens the reaction system of kanemite with surfactants, which contribute to novel materials design toward both the preparation of layered materials with
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new silicate frameworks and the charge density control of layered silicates. Acknowledgment. This work is supported by a Grantin-Aid for the Scientific Research by the Ministry of Education, Science, and Culture of the Japanese Government and CREST, Japan Science and Technology Corp. LA000325T
