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Preparation and Characterization of Silica-Pillared Derivatives from Kanemite
Journal of Colloid and Interface Science 255, 171–176 (2002) doi:10.1006/jcis.2002.8502
Preparation and Characterization of Silica-Pillared Derivatives from Kanemite Sunao Toriya,1 Yukako Tamura, Takashi Takei, Masayoshi Fuji, Tohru Watanabe, and Masatoshi Chikazawa Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan Received February 21, 2002; accepted May 22, 2002
The silica-pillared derivatives from kanemite (NaHSi2 O5 · 3H2 O) were prepared by intercalation of dialkyldimethylammonium (DADMA) ion and pillaring with tetraethylorthosilicate. The formation of silica pillars between the silicate sheets was demonstrated by X-ray diffraction, 29 Si CP/MAS NMR, and TEM observation. The basal spacing depended on the chain length of DADMA. Nitrogen adsorption study showed that the specific surface area was enlarged over 1000 m2 g−1 by the pillaring and that the pore size was in the micropore region. Water and benzene adsorption isotherms revealed that the surface properties of the pillared derivatives show hydrophobic character. C 2002 Elsevier Science (USA) Key Words: kanemite; pillaring; pore; silica; water adsorption; layered silicate; hydroxyl group.
for pillaring by chemical reaction with the hydroxyl groups. It is possible to control pillaring positions and the amount of pillars by controlling the hydroxyl groups. We selected kanemite as a starting material for pillared silicates. Kanemite can also be intercalated by long-chain alkylammonium ions and has hydroxyl groups on the interlayer surface similar to other hydrous sodium silicates. An important feature of kanemite is that kanemite consists of a single layer of SiO4 teterahedra. It is expected that the interlayer surface is efficiently used and the value of the specific surface area is close to the limiting value of porous silicas. In this study, the silica-pillared derivatives from kanemite have been prepared and the process of pillaring has been considered. The pore structure and surface properties of these samples also have been investigated.
1. INTRODUCTION 2. EXPERIMENTAL
Hydrous sodium silicates such as magadiite (Na2 Si14 O29 · 11H2 O) (1), kenyaite (Na2 Si22 O45 · 10H2 O) (1), makatite (Na2 Si4 O9 · 5H2 O) (2), and kanemite (NaHSi2 O5 · 3H2 O) (3) have a layered structure of sheets that are composed of SiO4 tetrahedra. These layered silicates have been investigated as a starting compound for the preparation of microporous and mesoporous materials using interlayer space. It is well known that the mesoporous silica called FSM-16 (4, 5) has been prepared from kanemite. One of the ways to prepare porous materials from the layered silicates is to pillar with metal oxides between the silicate layers. Silica-pillared (6) and alumina-pillared (7) derivatives from magadiite have been reported. The pillared interlayered silicates show two-dimensional porous structure and are useful for various applications such as adsorbent, molecular sieves, catalysts, and supports. There have been many studies on pillared clay materials such as pillared montmorillonite, hectorite, and so on. On the other hand, there have been few studies about pillared interlayered silicates prepared from the layered silicates such as hydrous sodium silicates. A remarkable difference in structure between smectite clays and the layered silicates is the existence of hydroxyl groups in the interlayer surface of the layered silicates. These hydroxyl groups are effective sites 1 To whom correspondence should be addressed. Fax: +81-426-77-2821. E-mail: [email protected].
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2.1. Materials Beneke and Lagaly (8) summarized various preparation methods for kanemite. We employed sodium disilicate (δ-Na2 Si2 O5 ) as a raw material. Sodium disilicate (25 g) obtained by calcination of the powdered sodium silicate (Nippon Chemical Industrial, SiO2 /Na2 O = 2.2) at 700◦ C was dispersed in 250 cm3 of ion-exchange water and the mixture was stirred for 3 h. After filtration, wet kanemite was obtained. Two different alkyl chain lengths of dialkyldimethylammonium (DADMA) ion were used for intercalation. Wet kanemite was dispersed in 100 cm3 of ion-exchange water. Then 400 cm3 of dilauryldimethylammoniumbromide [(C12 H25 )2 N+ (CH3 )2 ]Br− (C12DADMA) or dimethydipalmitylammoniumbromide [(C16 H33 )2 N+ (CH3 )2 ]Br− (C16DADMA) solution (0.125 mol dm−3 ) was added. The mixture was heated at 70◦ C for 3 h under stirring with pH 9 adjusted by 2 mol dm−3 hydrochloric acid. After filtration, the DADMA– kanemite complex was washed with water. After drying, the DADMA–kanemite complex was mixed with tetraethylorthosilicate (TEOS) (kanemite : TEOS = 1 : 10 in molar ratio) and stirred at room temperature for 1, 6, and 24 h. After filtration, the sample was washed with ethanol. Dried sample was calcined at 600◦ C for 6 h in air and pillared derivatives from kanemite were obtained. 0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
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2.2. Characterization X-ray powder diffractograms were measured with a Mac Science MXP21 diffractometer using CuK α radiation. 29 Si CP/ MAS NMR spectra were measured with a JEOL JNM-EX270 solid-state FT–NMR spectrometer operating at 53.6 MHz. Other operating conditions were a repetition time of 10 s, a contact time of 5 ms, and 4000 accumulations. IR spectra were measured with a Nicolet System 800 FT–IR spectrometer with a diffuse reflectance accessory (Collector, Spectra Tech Inc.) and a chamber for elevated temperature and reduced pressure. The measurement of IR spectra was carried out using a diffuse reflectance method after heating at 200◦ C for 2 h under reduced pressure. TEM observation was performed with a JEOL JEM-2000FX electron microscope. Measurement of the adsorption isotherms was performed with a glass apparatus provided with a capacitance manometer (MKS BARATORON 390HA). Nitrogen, water, and benzene were used as adsorptives. Nitrogen adsorption was measured at −196◦ C. Water and benzene adsorption measurements were performed at 20 and 10◦ C, respectively. 3. RESULTS AND DISCUSSION
3.1. Pillaring Process Structural change during preparation. Changes in the structures of the samples during preparation have been investigated in series by X-ray diffraction, 29 Si CP/MAS NMR, and TEM. Figure 1 shows one typical example of the X-ray diffraction patterns of the samples in each preparation step. The X-ray diffraction pattern of the starting material agreed with the JCPDS data of kanemite. The narrow basal spacing (1.01 nm) of kanemite is a hinderance to penetration of the reacting species for pillaring. In addition, interaction between silicate layers is strong because of hydrogen bonds with hydroxyl groups on silicate layers. It is necessary to construct a condition that will allow easy penetration of molecules between silicate layers. The intercalation of C12DADMA cations was used to expand the interlayer spacing of kanemite. By the intercalation of C12DADMA cations, the diffraction pattern of kanmite disappeared and a sharp peak corresponding to a basal spacing of 3.26 nm appeared. It means that a lamella structure of DADMA cations was produced between interlayers of kanemite. A basal spacing of 3.26 nm is slightly small compared to the thickness (4.30 nm) of two layers of C12DADMA cations, so the intercalated DADMA cations were adsorbed, tilting the carbon chain toward the interlayer surfaces. The mixture of DADMA–kanemite complexes and TEOS was allowed to increase in a basal spacing slightly, and the peak shape was broadened. It is suggested that the silicate sheets of the DADMA–kanemite complex were distorted by the growth of precursors of silica pillars. TEOS molecules were hydrolyzed by physisorbed water and condensed with the hydroxyl groups in kanemite layers. So, the precursors of silica pillars were fixed in kanemite layers. After calcination, the peak due to the basal
FIG. 1. X-ray diffraction patterns of (a) kanemite, (b) C12DADMA– kanemite complex, (c) a mixture of b and TEOS, and (d) the calcined product of c.
plane was further broadened; however, recognition of the peak indicates that the layered structure of the silicate was maintained. The basal spacing of the calcined sample is almost the same as that of DADMA–kanemite complex and the basal spacing of the sample prepared from the C12DADMA–kanemite derivative is smaller than that prepared from C16DADMA–kanemite. Consequently it is suggested that the height of pillars can be controlled by the alkyl chain length of DADMA. 29 Si CP/MAS NMR spectra of the samples in each preparation step are given in Fig. 2. The spectrum of kanemite showed the only single peak due to Q3 sites ((–Si–O–)3 Si(–O–H)) as expected from the structure of kanemite. By the intercalation of DADMA cations, the peak position of Q3 sites shifted by ca. 4 ppm. This peak shift is attributed to the change in the Si– O–Si bond angle (9) by the distortion of silicate sheets by the intercalation of DADMA cations. The distortion of the silicate sheets is compatible with the disappearance of the characteristic XRD patterns of kanemite by the intercalation of DADMA cations. The NMR spectrum of the DADMA–kanemite complex after mixing with TEOS showed a new peak due to Q4 sites ((–Si–O–)4 Si). This indicates the formation of the precursors of
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cess. It is expected that the silica pillars are fixed by the reaction with the interlayer hydroxyl groups. Before the reaction with TEOS, the interlayer hydroxyl groups of kanemite were eliminated by a reaction with chlorotrimethylsilane (CTMS). Consequently trimethylsilyl groups were substituted for the hydroxyl groups. Chlorotrimethylsilane was reacted with C16DADMA–kanemite complex. Trimethylsilylation was confirmed with FT–IR and 29 Si CP/MAS NMR spectroscopy. C16DADMA–kanemite complex modified with trimethylsilyl groups was mixed with TEOS for 24 h and then calcined at 600◦ C in air. The X-ray diffraction patterns of the calcined samples are shown in Fig. 4. The calcined sample prepared from the modified kanemite with trimethylsilyl groups did not exhibit the peak due to the basal plane. This suggests that the layered structure was destroyed by calcination. The adsorption and desorption isotherms of nitrogen on the samples are illustrated in Fig. 5. The amount of nitrogen adsorbed on the sample prepared
FIG. 2. 29 Si CP/MAS NMR spectra of (a) kanemite, (b) C12DADMA– kanemite complex, (c) a mixture of b and TEOS, and (d) the calcined product of c.
silica pillars, which arise from the hydrolysis and polymerization of TEOS molecules. The calcination caused the shifts of the peak positions of Q3 and Q4 sites, which are responsible for the change in the Si–O–Si bond angle. Furthermore, the increase in the peak intensity of Q3 sites and the appearance of the peak of Q2 sites ((–Si–O–)2 Si(–O–H)2 ) were recognized. Formation of the hydroxyl groups is due to decomposition of the residual ethoxy groups in the precursors of the silica pillars and rehydroxylation by physisorbed water after calcination. Figure 3 shows the transmission electron micrographs of calcined C16DADMA–kanemite complex and calcined C16DADMA–kanemite complex mixed with TEOS. The platelike silicate sheets were not observed in calcined C16DADMA– kanemite complex and the layered structure was destroyed. On the other hand, calcined C16DADMA–kanemite complex mixed with TEOS showed the plate-like silicate sheets and the layered structure. It was demonstrated that the silica pillars are useful to maintain the layered structure of the derivatives from kanemite. This observation agrees with the previous X-ray diffraction data. Role of hydroxyl groups in pillaring. We have investigated the role of the interlayer hydroxyl groups in the pillaring pro-
FIG. 3. Transmission electron micrographs of (a) calcined C16DADMA– kanemite complex and (b) calcined C16DADMA–kanemite complex mixed with TEOS.
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3.2. Pore Structure and Surface Properties of Pillared Derivatives
FIG. 4. X-ray diffraction patterns of (a) calcined C12DADMA–kanemite complex mixed with TEOS and (b) calcined C12DADMA–CTMS modified kanemite complex mixed with TEOS.
from the modified kanemite was small (Fig. 5b) compared with the amount adsorbed on the sample prepared from the nonmodified kanemite (Fig. 5a). Decreases in the surface area and the pore volume of the sample prepared from the modified kanemite are due to the disappearance of interlayer spacing. The fixation of silica pillars on the silicate layers is difficult when the interlayer hydroxyl groups are eliminated. The importance of the interlayer hydroxyl groups for pillaring with TEOS molecules was confirmed.
Surface areas and pore size distributions. The adsorption and desorption isotherms of nitrogen on kanemite and the five pillared derivatives from kanemite (after calcination) are shown in Fig. 6. The surface areas, pore sizes, pore volumes, and basal spacings of the samples are summarized in Table 1. The specific surface areas and pore diameters were calculated from t plots (10) based on the nitrogen adsorption isotherms. The basal spacings were estimated from the XRD patterns. The pillared samples were prepared with a different carbon chain length of DADMA and mixing time with TEOS. The amount of nitrogen adsorbed on the pure kanemite sample (Fig. 6a) is very small compared to that adsorbed on the pillared samples. It means that nitrogen molecules cannot penetrate the interlayer spacing of kanemite. A remarkable hysteresis loop was observed in the adsorption and desorption isotherms of the sample that was mixed with TEOS for 1 h (Fig. 6d). This sample was expected to have mesopores because of the appearance of the hysteresis loop; however, the t curve of this sample did not suggest distinct capillary condensation phenomenon. Though the size of the mesopore calculated from the desorption branch of the isotherm was 3.2 nm, it is expected that the pore volume of mesopores is small. In the initial stage of the reaction with TEOS molecules, the precursors of silica pillars do not grow sufficiently. The small surface area of the sample that was mixed with TEOS for 1 h is attributed to the destruction of the part of interlayer spacing by calcination. The types of isotherms changed from type IV isotherm to type I isotherm (IUPAC classification (11)) with an increase in the mixing time with TEOS. The hysteresis loops disappeared in the samples mixed with TEOS for 6 and 24 h. The specific surface areas exhibited high values, over 1000 m2 g−1 , and depended on the mixing time with TEOS. The pore sizes that represent the distance among pillars were in the range of diameters from 0.8 to 1.2 nm, mainly micropores.
TABLE 1 Specific Surface Area and Pore Analysis of Kanemite and Its Pillared Derivatives Carbon number of DADMA
Mixing time with TEOS (h)
Specific surface area (m2 g−1 )
Pore diametera (nm)
Pore volumeb (cm3 g−1 )
Basal spacingc (nm)
(Kanemite) 12 12 16 16 16
— 6 24 1 6 24
15 1169 1281 562 898 1099
— 0.80 0.80 1.20e 1.16 0.78
— 0.54 0.45 0.46 0.53 0.47
1.01 2.93 3.22 —d 3.89 4.11
a
Estimated from t plot. Estimated from amount of nitrogen adsorbed at P/P0 = 1. c Calculated from XRD pattern. d Not observed. e This sample also has a small amount of mesopores. b
FIG. 5. Adsorption and desorption isotherms of nitrogen on (a) calcined C12DADMA–kanemite complex mixed with TEOS and (b) calcined C12DADMA–CTMS modified kanemite complex mixed with TEOS.
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FIG. 6. Adsorption and desorption isotherms of nitrogen on (a) kanemite, (b), (c) calcined C12DADMA–kanemite complex mixed with TEOS for 6 and 24 h, respectively, and (d), (e), (f) calcined C16DADMA–kanemite complex mixed with TEOS for 1, 6, and 24 h, respectively.
The basal spacing was controlled by the length of the alkyl groups of the DADMA molecules and the mixing time with TEOS. Surface hydroxyl groups. The IR spectra of the surface hydroxyl groups of the pillared derivatives are shown in Fig. 7. These samples have different mixing times (6 and 24 h) with TEOS. The two kinds of surface hydroxyl groups were observed in both the samples. The sharp band at 3740 cm−1 and the broad band around 3600 cm−1 are attributed to free and hydrogenbonded hydroxyl groups, respectively. In both the samples, the
peak intensity due to the free hydroxyl groups is stronger than that due to the hydrogen-bonded hydroxyl groups. Most of the hydrogen-bonded hydroxyl groups were removed by the calcination at 600◦ C, so the observed hydrogen-bonded hydroxyl groups were formed by the rehydroxylation with physisorbed water after calcination. Two kinds of hydrogen-bonded hydroxyl groups were observed in the sample mixed with TEOS for 24 h. One was observed around 3540 cm−1 , which was also observed in the sample mixed for 6 h. The other observed at the low frequency region (3303 cm−1 ) formed strong hydrogen bonds and was removed by outgassing at 400◦ C.
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The substitution of trimethylsilyl groups for the free hydroxyl groups by the reaction with chlorotrimethylsilane was confirmed. It was demonstrated that these hydroxyl groups are effective sites for chemical modifications. The concentration of the free hydroxyl groups calculated from the amount of trimethylsilyl groups was 0.84 OH nm−2 . Adsorption properties. Figure 8 shows the adsorption isotherms of water and benzene vapor on the pillared derivatives. The water adsorption isotherm initially exhibited the type III isotherm, which indicates hydrophobicity. Generally many pillared clays show hydrophobicity. The sudden increase in the amount adsorbed at the middle relative pressure is attributed to pore filling of water molecules rather than capillary condensation. The origin of hydrophobicity is responsible for the small amount of hydroxyl groups (0.84 OH nm−2 ) compared to an average concentration of hydroxyl groups on amorphous silicas (4.9 OH nm−2 ) (12), and to an abundance of free hydroxyl groups compared with hydrogen-bonded hydroxyl groups (13). On the other hand, the benzene adsorption isotherm showed the type I isotherm which is characteristic of micropore filling. Consequently, adsorption properties of pillared derivatives from kanemite show hydrophobic character and are expected to adsorb hydrophobic molecules strongly. These materials are useful in recovering organic compounds from wastewater.
FIG. 8. Adsorption isotherms of water and benzene vapor on calcined C16DADMA–kanemite complex mixed with TEOS for 6 h.
4. CONCLUSIONS
The silica-pillared derivatives from kanemite were prepared. It was demonstrated that the mixture of DADMA–kanemite complex and TEOS is effective for formation of silica pillars. The pillared derivatives showed high specific surface area over 1000 m2 g−1 and consisted of micropores. The alkyl chain length of DADMA molecules and the mixing time with TEOS affect pore size and basal spacing. The high specific surface area and the hydrophobic character of pillared derivatives make them appropriate for use as adsorbents for organic compounds. ACKNOWLEDGEMENT The support of this research by the Grant-in-Aid for Scientific Research (C) (No. 12650826) from Japan Society for the Promotion of Science is gratefully acknowledged.
REFERENCES
FIG. 7. IR spectra of calcined C16DADMA–kanemite complex mixed with TEOS for (a) 6 h and (b) 24 h.
1. Eugster, H. P., Science 157, 1177 (1967). 2. Sheppard, R. A., Gude, A. J., and Hay, R. L., Am. Mineral. 55, 358 (1970). 3. Johan, Z., and Maglione, G. F., Bull. Soc. Fr. Mineral. Cristallogr. 95, 371 (1972). 4. Yanagisawa, T., Shimizu, T., Kuroda, K., and Kato, C., Bull. Chem. Soc. Jpn. 63, 988 (1990). 5. Inagaki, S., Fukushima, Y., and Kuroda, K., J. Chem. Soc. Chem. Commun. 680, (1993). 6. Dailey, J. S., and Pinnavaia, T. J., Chem. Mater. 4, 855 (1992). 7. Wong, S. T., and Cheng, S., Chem. Mater. 5, 770 (1993). 8. Beneke, K., and Lagaly, G., Am. Mineral. 62, 763 (1977). 9. Smith, J. V., and Blackwell, C. S., Nature 303, 223 (1983). 10. Lippens, B. C., and de Boer, J. H., J. Catal. 4, 319 (1965). 11. Sing, K. S. W., Everett, D. H., Haul, R. A. W., Moscou, L., Pierotti, R. A., Rouqu´erol, J., and Siemieniewska, T., Pure Appl. Chem. 57, 603 (1985). 12. Zhuravlev, L. T., Langmuir 3, 316 (1987). 13. Hair, M. L., and Hertl, W., J. Phys. Chem. 73, 4269 (1992).
Preparation and Characterization of Silica-Pillared Derivatives from Kanemite Sunao Toriya,1 Yukako Tamura, Takashi Takei, Masayoshi Fuji, Tohru Watanabe, and Masatoshi Chikazawa Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan Received February 21, 2002; accepted May 22, 2002
The silica-pillared derivatives from kanemite (NaHSi2 O5 · 3H2 O) were prepared by intercalation of dialkyldimethylammonium (DADMA) ion and pillaring with tetraethylorthosilicate. The formation of silica pillars between the silicate sheets was demonstrated by X-ray diffraction, 29 Si CP/MAS NMR, and TEM observation. The basal spacing depended on the chain length of DADMA. Nitrogen adsorption study showed that the specific surface area was enlarged over 1000 m2 g−1 by the pillaring and that the pore size was in the micropore region. Water and benzene adsorption isotherms revealed that the surface properties of the pillared derivatives show hydrophobic character. C 2002 Elsevier Science (USA) Key Words: kanemite; pillaring; pore; silica; water adsorption; layered silicate; hydroxyl group.
for pillaring by chemical reaction with the hydroxyl groups. It is possible to control pillaring positions and the amount of pillars by controlling the hydroxyl groups. We selected kanemite as a starting material for pillared silicates. Kanemite can also be intercalated by long-chain alkylammonium ions and has hydroxyl groups on the interlayer surface similar to other hydrous sodium silicates. An important feature of kanemite is that kanemite consists of a single layer of SiO4 teterahedra. It is expected that the interlayer surface is efficiently used and the value of the specific surface area is close to the limiting value of porous silicas. In this study, the silica-pillared derivatives from kanemite have been prepared and the process of pillaring has been considered. The pore structure and surface properties of these samples also have been investigated.
1. INTRODUCTION 2. EXPERIMENTAL
Hydrous sodium silicates such as magadiite (Na2 Si14 O29 · 11H2 O) (1), kenyaite (Na2 Si22 O45 · 10H2 O) (1), makatite (Na2 Si4 O9 · 5H2 O) (2), and kanemite (NaHSi2 O5 · 3H2 O) (3) have a layered structure of sheets that are composed of SiO4 tetrahedra. These layered silicates have been investigated as a starting compound for the preparation of microporous and mesoporous materials using interlayer space. It is well known that the mesoporous silica called FSM-16 (4, 5) has been prepared from kanemite. One of the ways to prepare porous materials from the layered silicates is to pillar with metal oxides between the silicate layers. Silica-pillared (6) and alumina-pillared (7) derivatives from magadiite have been reported. The pillared interlayered silicates show two-dimensional porous structure and are useful for various applications such as adsorbent, molecular sieves, catalysts, and supports. There have been many studies on pillared clay materials such as pillared montmorillonite, hectorite, and so on. On the other hand, there have been few studies about pillared interlayered silicates prepared from the layered silicates such as hydrous sodium silicates. A remarkable difference in structure between smectite clays and the layered silicates is the existence of hydroxyl groups in the interlayer surface of the layered silicates. These hydroxyl groups are effective sites 1 To whom correspondence should be addressed. Fax: +81-426-77-2821. E-mail: [email protected].
171
2.1. Materials Beneke and Lagaly (8) summarized various preparation methods for kanemite. We employed sodium disilicate (δ-Na2 Si2 O5 ) as a raw material. Sodium disilicate (25 g) obtained by calcination of the powdered sodium silicate (Nippon Chemical Industrial, SiO2 /Na2 O = 2.2) at 700◦ C was dispersed in 250 cm3 of ion-exchange water and the mixture was stirred for 3 h. After filtration, wet kanemite was obtained. Two different alkyl chain lengths of dialkyldimethylammonium (DADMA) ion were used for intercalation. Wet kanemite was dispersed in 100 cm3 of ion-exchange water. Then 400 cm3 of dilauryldimethylammoniumbromide [(C12 H25 )2 N+ (CH3 )2 ]Br− (C12DADMA) or dimethydipalmitylammoniumbromide [(C16 H33 )2 N+ (CH3 )2 ]Br− (C16DADMA) solution (0.125 mol dm−3 ) was added. The mixture was heated at 70◦ C for 3 h under stirring with pH 9 adjusted by 2 mol dm−3 hydrochloric acid. After filtration, the DADMA– kanemite complex was washed with water. After drying, the DADMA–kanemite complex was mixed with tetraethylorthosilicate (TEOS) (kanemite : TEOS = 1 : 10 in molar ratio) and stirred at room temperature for 1, 6, and 24 h. After filtration, the sample was washed with ethanol. Dried sample was calcined at 600◦ C for 6 h in air and pillared derivatives from kanemite were obtained. 0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
172
TORIYA ET AL.
2.2. Characterization X-ray powder diffractograms were measured with a Mac Science MXP21 diffractometer using CuK α radiation. 29 Si CP/ MAS NMR spectra were measured with a JEOL JNM-EX270 solid-state FT–NMR spectrometer operating at 53.6 MHz. Other operating conditions were a repetition time of 10 s, a contact time of 5 ms, and 4000 accumulations. IR spectra were measured with a Nicolet System 800 FT–IR spectrometer with a diffuse reflectance accessory (Collector, Spectra Tech Inc.) and a chamber for elevated temperature and reduced pressure. The measurement of IR spectra was carried out using a diffuse reflectance method after heating at 200◦ C for 2 h under reduced pressure. TEM observation was performed with a JEOL JEM-2000FX electron microscope. Measurement of the adsorption isotherms was performed with a glass apparatus provided with a capacitance manometer (MKS BARATORON 390HA). Nitrogen, water, and benzene were used as adsorptives. Nitrogen adsorption was measured at −196◦ C. Water and benzene adsorption measurements were performed at 20 and 10◦ C, respectively. 3. RESULTS AND DISCUSSION
3.1. Pillaring Process Structural change during preparation. Changes in the structures of the samples during preparation have been investigated in series by X-ray diffraction, 29 Si CP/MAS NMR, and TEM. Figure 1 shows one typical example of the X-ray diffraction patterns of the samples in each preparation step. The X-ray diffraction pattern of the starting material agreed with the JCPDS data of kanemite. The narrow basal spacing (1.01 nm) of kanemite is a hinderance to penetration of the reacting species for pillaring. In addition, interaction between silicate layers is strong because of hydrogen bonds with hydroxyl groups on silicate layers. It is necessary to construct a condition that will allow easy penetration of molecules between silicate layers. The intercalation of C12DADMA cations was used to expand the interlayer spacing of kanemite. By the intercalation of C12DADMA cations, the diffraction pattern of kanmite disappeared and a sharp peak corresponding to a basal spacing of 3.26 nm appeared. It means that a lamella structure of DADMA cations was produced between interlayers of kanemite. A basal spacing of 3.26 nm is slightly small compared to the thickness (4.30 nm) of two layers of C12DADMA cations, so the intercalated DADMA cations were adsorbed, tilting the carbon chain toward the interlayer surfaces. The mixture of DADMA–kanemite complexes and TEOS was allowed to increase in a basal spacing slightly, and the peak shape was broadened. It is suggested that the silicate sheets of the DADMA–kanemite complex were distorted by the growth of precursors of silica pillars. TEOS molecules were hydrolyzed by physisorbed water and condensed with the hydroxyl groups in kanemite layers. So, the precursors of silica pillars were fixed in kanemite layers. After calcination, the peak due to the basal
FIG. 1. X-ray diffraction patterns of (a) kanemite, (b) C12DADMA– kanemite complex, (c) a mixture of b and TEOS, and (d) the calcined product of c.
plane was further broadened; however, recognition of the peak indicates that the layered structure of the silicate was maintained. The basal spacing of the calcined sample is almost the same as that of DADMA–kanemite complex and the basal spacing of the sample prepared from the C12DADMA–kanemite derivative is smaller than that prepared from C16DADMA–kanemite. Consequently it is suggested that the height of pillars can be controlled by the alkyl chain length of DADMA. 29 Si CP/MAS NMR spectra of the samples in each preparation step are given in Fig. 2. The spectrum of kanemite showed the only single peak due to Q3 sites ((–Si–O–)3 Si(–O–H)) as expected from the structure of kanemite. By the intercalation of DADMA cations, the peak position of Q3 sites shifted by ca. 4 ppm. This peak shift is attributed to the change in the Si– O–Si bond angle (9) by the distortion of silicate sheets by the intercalation of DADMA cations. The distortion of the silicate sheets is compatible with the disappearance of the characteristic XRD patterns of kanemite by the intercalation of DADMA cations. The NMR spectrum of the DADMA–kanemite complex after mixing with TEOS showed a new peak due to Q4 sites ((–Si–O–)4 Si). This indicates the formation of the precursors of
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cess. It is expected that the silica pillars are fixed by the reaction with the interlayer hydroxyl groups. Before the reaction with TEOS, the interlayer hydroxyl groups of kanemite were eliminated by a reaction with chlorotrimethylsilane (CTMS). Consequently trimethylsilyl groups were substituted for the hydroxyl groups. Chlorotrimethylsilane was reacted with C16DADMA–kanemite complex. Trimethylsilylation was confirmed with FT–IR and 29 Si CP/MAS NMR spectroscopy. C16DADMA–kanemite complex modified with trimethylsilyl groups was mixed with TEOS for 24 h and then calcined at 600◦ C in air. The X-ray diffraction patterns of the calcined samples are shown in Fig. 4. The calcined sample prepared from the modified kanemite with trimethylsilyl groups did not exhibit the peak due to the basal plane. This suggests that the layered structure was destroyed by calcination. The adsorption and desorption isotherms of nitrogen on the samples are illustrated in Fig. 5. The amount of nitrogen adsorbed on the sample prepared
FIG. 2. 29 Si CP/MAS NMR spectra of (a) kanemite, (b) C12DADMA– kanemite complex, (c) a mixture of b and TEOS, and (d) the calcined product of c.
silica pillars, which arise from the hydrolysis and polymerization of TEOS molecules. The calcination caused the shifts of the peak positions of Q3 and Q4 sites, which are responsible for the change in the Si–O–Si bond angle. Furthermore, the increase in the peak intensity of Q3 sites and the appearance of the peak of Q2 sites ((–Si–O–)2 Si(–O–H)2 ) were recognized. Formation of the hydroxyl groups is due to decomposition of the residual ethoxy groups in the precursors of the silica pillars and rehydroxylation by physisorbed water after calcination. Figure 3 shows the transmission electron micrographs of calcined C16DADMA–kanemite complex and calcined C16DADMA–kanemite complex mixed with TEOS. The platelike silicate sheets were not observed in calcined C16DADMA– kanemite complex and the layered structure was destroyed. On the other hand, calcined C16DADMA–kanemite complex mixed with TEOS showed the plate-like silicate sheets and the layered structure. It was demonstrated that the silica pillars are useful to maintain the layered structure of the derivatives from kanemite. This observation agrees with the previous X-ray diffraction data. Role of hydroxyl groups in pillaring. We have investigated the role of the interlayer hydroxyl groups in the pillaring pro-
FIG. 3. Transmission electron micrographs of (a) calcined C16DADMA– kanemite complex and (b) calcined C16DADMA–kanemite complex mixed with TEOS.
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3.2. Pore Structure and Surface Properties of Pillared Derivatives
FIG. 4. X-ray diffraction patterns of (a) calcined C12DADMA–kanemite complex mixed with TEOS and (b) calcined C12DADMA–CTMS modified kanemite complex mixed with TEOS.
from the modified kanemite was small (Fig. 5b) compared with the amount adsorbed on the sample prepared from the nonmodified kanemite (Fig. 5a). Decreases in the surface area and the pore volume of the sample prepared from the modified kanemite are due to the disappearance of interlayer spacing. The fixation of silica pillars on the silicate layers is difficult when the interlayer hydroxyl groups are eliminated. The importance of the interlayer hydroxyl groups for pillaring with TEOS molecules was confirmed.
Surface areas and pore size distributions. The adsorption and desorption isotherms of nitrogen on kanemite and the five pillared derivatives from kanemite (after calcination) are shown in Fig. 6. The surface areas, pore sizes, pore volumes, and basal spacings of the samples are summarized in Table 1. The specific surface areas and pore diameters were calculated from t plots (10) based on the nitrogen adsorption isotherms. The basal spacings were estimated from the XRD patterns. The pillared samples were prepared with a different carbon chain length of DADMA and mixing time with TEOS. The amount of nitrogen adsorbed on the pure kanemite sample (Fig. 6a) is very small compared to that adsorbed on the pillared samples. It means that nitrogen molecules cannot penetrate the interlayer spacing of kanemite. A remarkable hysteresis loop was observed in the adsorption and desorption isotherms of the sample that was mixed with TEOS for 1 h (Fig. 6d). This sample was expected to have mesopores because of the appearance of the hysteresis loop; however, the t curve of this sample did not suggest distinct capillary condensation phenomenon. Though the size of the mesopore calculated from the desorption branch of the isotherm was 3.2 nm, it is expected that the pore volume of mesopores is small. In the initial stage of the reaction with TEOS molecules, the precursors of silica pillars do not grow sufficiently. The small surface area of the sample that was mixed with TEOS for 1 h is attributed to the destruction of the part of interlayer spacing by calcination. The types of isotherms changed from type IV isotherm to type I isotherm (IUPAC classification (11)) with an increase in the mixing time with TEOS. The hysteresis loops disappeared in the samples mixed with TEOS for 6 and 24 h. The specific surface areas exhibited high values, over 1000 m2 g−1 , and depended on the mixing time with TEOS. The pore sizes that represent the distance among pillars were in the range of diameters from 0.8 to 1.2 nm, mainly micropores.
TABLE 1 Specific Surface Area and Pore Analysis of Kanemite and Its Pillared Derivatives Carbon number of DADMA
Mixing time with TEOS (h)
Specific surface area (m2 g−1 )
Pore diametera (nm)
Pore volumeb (cm3 g−1 )
Basal spacingc (nm)
(Kanemite) 12 12 16 16 16
— 6 24 1 6 24
15 1169 1281 562 898 1099
— 0.80 0.80 1.20e 1.16 0.78
— 0.54 0.45 0.46 0.53 0.47
1.01 2.93 3.22 —d 3.89 4.11
a
Estimated from t plot. Estimated from amount of nitrogen adsorbed at P/P0 = 1. c Calculated from XRD pattern. d Not observed. e This sample also has a small amount of mesopores. b
FIG. 5. Adsorption and desorption isotherms of nitrogen on (a) calcined C12DADMA–kanemite complex mixed with TEOS and (b) calcined C12DADMA–CTMS modified kanemite complex mixed with TEOS.
SILICA-PILLARED DERIVATIVES FROM KANEMITE
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FIG. 6. Adsorption and desorption isotherms of nitrogen on (a) kanemite, (b), (c) calcined C12DADMA–kanemite complex mixed with TEOS for 6 and 24 h, respectively, and (d), (e), (f) calcined C16DADMA–kanemite complex mixed with TEOS for 1, 6, and 24 h, respectively.
The basal spacing was controlled by the length of the alkyl groups of the DADMA molecules and the mixing time with TEOS. Surface hydroxyl groups. The IR spectra of the surface hydroxyl groups of the pillared derivatives are shown in Fig. 7. These samples have different mixing times (6 and 24 h) with TEOS. The two kinds of surface hydroxyl groups were observed in both the samples. The sharp band at 3740 cm−1 and the broad band around 3600 cm−1 are attributed to free and hydrogenbonded hydroxyl groups, respectively. In both the samples, the
peak intensity due to the free hydroxyl groups is stronger than that due to the hydrogen-bonded hydroxyl groups. Most of the hydrogen-bonded hydroxyl groups were removed by the calcination at 600◦ C, so the observed hydrogen-bonded hydroxyl groups were formed by the rehydroxylation with physisorbed water after calcination. Two kinds of hydrogen-bonded hydroxyl groups were observed in the sample mixed with TEOS for 24 h. One was observed around 3540 cm−1 , which was also observed in the sample mixed for 6 h. The other observed at the low frequency region (3303 cm−1 ) formed strong hydrogen bonds and was removed by outgassing at 400◦ C.
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The substitution of trimethylsilyl groups for the free hydroxyl groups by the reaction with chlorotrimethylsilane was confirmed. It was demonstrated that these hydroxyl groups are effective sites for chemical modifications. The concentration of the free hydroxyl groups calculated from the amount of trimethylsilyl groups was 0.84 OH nm−2 . Adsorption properties. Figure 8 shows the adsorption isotherms of water and benzene vapor on the pillared derivatives. The water adsorption isotherm initially exhibited the type III isotherm, which indicates hydrophobicity. Generally many pillared clays show hydrophobicity. The sudden increase in the amount adsorbed at the middle relative pressure is attributed to pore filling of water molecules rather than capillary condensation. The origin of hydrophobicity is responsible for the small amount of hydroxyl groups (0.84 OH nm−2 ) compared to an average concentration of hydroxyl groups on amorphous silicas (4.9 OH nm−2 ) (12), and to an abundance of free hydroxyl groups compared with hydrogen-bonded hydroxyl groups (13). On the other hand, the benzene adsorption isotherm showed the type I isotherm which is characteristic of micropore filling. Consequently, adsorption properties of pillared derivatives from kanemite show hydrophobic character and are expected to adsorb hydrophobic molecules strongly. These materials are useful in recovering organic compounds from wastewater.
FIG. 8. Adsorption isotherms of water and benzene vapor on calcined C16DADMA–kanemite complex mixed with TEOS for 6 h.
4. CONCLUSIONS
The silica-pillared derivatives from kanemite were prepared. It was demonstrated that the mixture of DADMA–kanemite complex and TEOS is effective for formation of silica pillars. The pillared derivatives showed high specific surface area over 1000 m2 g−1 and consisted of micropores. The alkyl chain length of DADMA molecules and the mixing time with TEOS affect pore size and basal spacing. The high specific surface area and the hydrophobic character of pillared derivatives make them appropriate for use as adsorbents for organic compounds. ACKNOWLEDGEMENT The support of this research by the Grant-in-Aid for Scientific Research (C) (No. 12650826) from Japan Society for the Promotion of Science is gratefully acknowledged.
REFERENCES
FIG. 7. IR spectra of calcined C16DADMA–kanemite complex mixed with TEOS for (a) 6 h and (b) 24 h.
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