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Preparation of Silica-Based Mesoporous Materials From Fluorosilicon Compounds: Gelation of H2SiF6in Ammonia Surfactant Solution
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
192, 156–161 (1997)
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Preparation of Silica-Based Mesoporous Materials From Fluorosilicon Compounds: Gelation of H2SiF6 in Ammonia Surfactant Solution Soon-Yong Jeong,* ,1 Jeong-Kwon Suh,* Jung-Min Lee,* and Oh-Yun Kwon† *Applied Chemistry & Engineering Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon, Korea 305-600; and †Department of Chemical Engineering, Yosu National Fisheries University, Yosu, Korea 550-749 Received February 3, 1997; accepted May 5, 1997
The silica-based mesoporous molecular sieves were synthesized from fluorosilicon compounds and Al, Ti with the hydrolysis reaction of H2SiF6 in an ammonia-surfactant mixed solution. Wellcrystalline mesoporous molecular sieves were obtained after several hours at the atmospheric conditions. The solid products were characterized by X-ray powder diffraction, nitrogen adsorption/ desorption, transmission electron microscopy and solid-state NMR spectroscopy. The solid products show high specific surface areas in the range of 627–1040 m2 /g, depending on the amount of Al and Ti. Also, they exhibit narrow pore size distributions in the range of 31–35 A˚ according to the addition of Al and Ti. 29Si MAS NMR indicate that all samples mainly contain Q 4 types sites. Also, the peak intensity of Q 3 sites decreases with increasing amount of aluminum. In 27Al MAS NMR spectra, all samples give intense lines from 4-coordinate aluminum. These results show that aluminum species are incorporated into the framework of Si-MMS. As the amount of Al increases, the intensity line from 6-coordinate aluminum increases. This result represents the increase of the fraction of amorphous aluminum species. q 1997 Academic Press Key Words: mesoporous material; fluorosilicon; gelation.
1. INTRODUCTION
Mesoporous materials have attracted great interest in relation to the utilization as catalysts and separation media for industrial applications. Recently, researchers at Mobil oil company (1, 2) introduced a new family of mesoporous molecular sieves. These materials were synthesized by hydrothermal reactions of aluminosilicate and silicate gel in the presence of quaternary ammonium surfactants. The silicate and aluminosilicate materials were designated as MCM41. They have regular arrays of a uniform channel size rang˚ according to the type of surfactant. ing from 20 to 100 A Hue et al. (3) showed that the synthetic approach can be generalized to non-silica-based types such as antimony oxide, tungsten oxide, iron oxide, and zinc oxide. Recently, Silva and Pastore (4) reported effects as the mineralizing 1
To whom correspondence should be addressed.
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agent of F 0 ion on the synthesis of mesoporous materials and several types of zeolite. On the other hand, Yanagisawa et al. (5) described the mesoporous silicate prepared from the layered silicate kanemite. These materials were prepared by a hydrothermal synthesis in the presence of tertiary ammonium halides such as dodecyltrimethylammonium chloride or cetyltriethylammonium. Also, Jeong et al. (6, 7) synthesized mesoporous materials having a pore size of 30– ˚ by calcining the resulting materials after TEOS-interca40 A lated layered silicates such as magadiite and kenyaite were gelled by an acid or base catalyst. These new mesoporous materials are expected to be useful for the catalytic reactions of high-molecular hydrocarbons because of higher diffusivities owing to their large pore diameters. In the synthesis of MCM-41 materials (1, 2, 8), the silica sources generally used are sodium silicate and Si-alkoxides. Especially, if the silica is used as starting material, the types of silica to be capable of using are limited because of solubility. Iler (9) reports that soluble silica, Si(OH)4 is produced by the dissolution of silica and ionized to (HO)3SiO 20 or (HO)2SiO 20 2 above pH 9. The dissolution of silica accompanied by ionization is a basic step to lead hydrolytic polycondensation with charged micelle template. Therefore, synthetic condition should be forced in dissolution of silica. Strong quaternary ammonium bases like tetramethylammonium hydroxide can dissolve silica at a temperature of 150– 2007C at elevated pressure, forming silicate salt. Considering this point, many researchers (1, 2, 9) make use of quaternary ammonium bases as silica dissolution media and alkali source. However, silica cannot be dissolved at atmospheric conditions, and easily soluble silica sources are limited. These problems in the use of conventional silica sources could be markedly improved by using fluorosilicon compounds such as H2SiF6 . H2SiF6 can be industrially obtained from silex and SiF4 as well as an industrial byproduct. However, there is no report in the literature on the synthesis of mesoporous materials using fluorosilicon compounds. The method of silica precipitation by NH4OH addition to H2SiF6 solution above pH 8 has mainly been used for manufacturing highly fluffy silica to be effective as pillars.
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Iler (9) suggests that OH 0 ions could act as the improving agent in the hydrolytic polycondensation of SiF 20 6 , like a role of OH 0 and F 0 in polymerization of Si(OH)4 . Especially, in the presence of cationic surfactant micelles, the gelation of SiF 20 6 by NH4OH was proceeded rapidly even at pH 6. This fact is an interesting clue on the synthesis of mesoporous marterials. We now report a simple method for the synthesis of silicabased mesoporous molecular sieves (MMS) from fluorosilicon compounds. The principle is based on the hydrolysis reaction of H2SiF6 in the ammonia cationic micelle template solution. This method is supposed to be markedly profitable in the industrial aspect because H2SiF6 could be industrially obtained from silex and SiF4 as well as an industrial byproduct. Also, the synthetic process could be simplified because products are manufactured for several hours at atmospheric conditions. 2. EXPERIMENTAL
(1) Silica-Based Mesoporous Molecular Sieve (Si-MMS) The synthesis procedures were as follows: 20g of H2SiF6 solution (10 wt % SiO2 ) was prepared by gradually dissolving 2 g of SiO2 (grade 62, 60–200 mesh, Aldrich, U.S.A.) to 18 g of 25% hydrofluoric acid (48%, Merck, Germany) (pH of solution: 1.5). 4 wt % of CTAB (cetyltrimethylammonium bromide); (99%, BDH, England) surfactant solution was also prepared by dissolving CTAB in distilled water. NH4OH (28 wt %, Merck, Germany) was added to 20g H2SiF6 solution stirred at 40–507C until pH 6 was reached. The solution did not show any sign of gelation. This solution was added at once to 130 g of CTAB solution at 507C and stirred vigorously during 5 min. A white gel was produced abruptly within 10 s. NH4OH was added until the pH reached 7–8. The gel solution was aged for 5 h in a dryer at 707C. The resulting solid product was recovered by filtration, washed 5 times with distilled water, and dried at 607C. The dried sample was calcined at 6007C for 4 h to remove organic compounds. (2) Al-Containing Silica-Based Mesoporous Molecular Sieve (Si-Al-MMS) Various amounts of Al(NO3 )3r9H2O (0.13g, 0.52g, and 0.91g) (98%, Aldrich, USA) were added to 20 g of H2SiF6 solution (10 wt % SiO2 ), respectively, and dissolved perfectly, and the solutions exhibited between pH 1.4 and 2.0, depending on the amount of Al(NO3 )3r9H2O. Ammoniasurfactant mixed solution was also prepared by adding 30 g of NH4OH to the solution containing 100 g of CTAB solution (4 wt %), and this solution exhibited pH 11.7. H2SiF6 solutions containing Al(NO3 )3r9H2O were added at once to 130 g of ammonia–surfactant mixed solution, respectively, and stirred vigorously during 5 min at 40–507C, and the pH of
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solutions were between 7.0 and 7.5, depending on the amount of Al(NO3 )3r9H2O. A white gel was formed abruptly within 10 s and aged for 5 h in a dryer at 707C. The following procedures were the same as for the preparation of Si-MMS. The solid product was filtered, washed, dried, and calcined at 6007C. (3) Ti-Containing Silica-Based Mesoporous Molecular Sieve, Si-Ti-MMS Ti-containing mesoporous molecular sieve was prepared by a similar procedure of Si-MMS. Various amounts of titanium isopropoxide, Ti(OC3H7 )4 (0.1, 0.4, and 0.7 g); (97%, Aldrich, USA) were added to 20 g of H2SiF6 solution respectively, and dissolved perfectly. The pH of these solutions exhibited values of 1.5 and 2.1, depending on the amount of Ti(OC3H7 )4 . These solutions were added at once to 130 g of the prepared ammonia–surfactant mixed solution respectively, and stirred vigorously during 2 min at 607C and then pH was between 7.3 and 8.0, depending on the amount of Ti(OC3H7 )4 . A white gel was formed abruptly within 10 s and aged for 5 h in dryer at 707C. The following procedures were the same as the preparation of Si-MMS. The solid product was filtered, washed, dried, and calcined. Basal spacings of samples were determined from the 00l X-ray diffraction using a Rigaku diffractometer equipped with CuKa radiation (35 kV, 15 mA). Nitrogen adsorption/ desorption isotherms were determined by Micromeritics ASAP 2000 at 77K. All samples were degassed at 3007C under vacuum for 4 h. The specific surface area was determined by the BET equation. The pore size distributions of samples were determined by the Horva´th-Kawazoe equation (11). 29Si MAS-NMR and 27Al MAS-NMR were performed on a Bru¨ker AM 300 solid-state NMR spectrophotometer. For transition electron micrographs (TEM) analysis, samples were dispersed ultrasonically in ethanol, and a drop of the suspension was deposited on a carbon coated copper grid. Micrographs were obtained on a Philips CM-20 electron microscope. The chemical compositions of samples were determined by an energy dispersive X-ray spectrometer (Link System AN10000-85S). EDS analyses were carried out using electron beam with the sample of carbon-coated pellet. 3. RESULTS AND DISCUSSION
Silica-based mesoporous molecular sieves were synthesized from fluorosilicon and compounds containing Al and Ti with the hydrolysis reaction of H2SiF6 in cetyltrimethylammonium bromide solution. NH4OH was added to H2SiF6 solution in order to promote the hydrolysis reaction of 20 SiF 20 6 . Also, it was confirmed that the gelation of SiF 6 by NH4OH proceeded rapidly even at pH 6 in the presence of cationic surfactant micelle.
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TABLE 1 Chemical Compositions of Gel Mixtures and Calcined Samples Si-Ti-MMS-95
Si-Ti-MMS-25
Si-Ti-MMS-13
Si-Al-MMS-150
Si-Al-MMS-50
Si-Al-MMS-25
93.8
23.4
13.4
—
—
—
95
25
13
—
—
—
95.2
23.8
13.6
50
25
Si/Ti atomic ratio of gel mixture (calculated) Si/Ti atomic ratio of calcined sample (EDS analysis) Si/Al atomic ratio of gel mixture (calculated) Si/Al atomic ratio of calcined sample (EDS analysis)
—
—
—
—
—
—
150
The chemical compositions of the gel mixtures and the calcined products are shown in Table 1. The Si/Ti ratios of the calcined products are similar to those of the gel mixtures. The results indicate that the amount of titanium does not reduce in the course of gelation, filtration and calcination. However, the Si/Al ratios of the calcined products are above those in the gel mixtures, indicating a relative loss of aluminum in the course of gelation, filtration, and calcination. The X-ray diffraction pattern of Si-MMS is shown in Fig. 1a. The relatively well-defined pattern is typical for MCM41 materials as described by Kresge et al. (1) The first four XRD peaks of Si-MMS can be indexed on a hexagonal ˚ (hexagonal with ao Å lattice qwith pore diameter of ca. 47A 2d100 / 3). The X-ray diffraction patterns of Si-Al-MMS with the variation of the molar ratios of Si/Al from 25 to
150 are given in Fig. 1b–d. All aluminosilicate samples give poorer quality XRD patterns than pure siliceous MMS. The intensity of the first peak decreases even at very low levels of aluminum incorporation, and other peaks overlap and greatly decrease. It is therefore clear that the structure of the aluminosilicate MMS is less uniform than that of purely siliceous MMS. Figure 2 represents the X-ray diffraction patterns of Si-Ti-MMS with the variation of the molar ratio of Si/Ti from 13 to 95. The XRD pattern of Si-Ti-MMS with Si/Ti Å 95 consists of the first four peaks with which a pattern similar to Si-MMS indicate a hexagonal lattice. However, XRD patterns of Si-Ti-MMS with Si/Ti õ 25 decrease strongly in intensity and overlap. The results indicate that incorporation of a small amount (Si/Ti ú 95) of Ti does not affect the framework of silica-based mesoporous molecular sieves.
FIG. 1. X-ray powder diffraction patterns of Si-MMS and Si-Al-MMS with variation of Si/Al molar ratio.
FIG. 2. X-ray powder diffraction patterns of Si-Ti-MMS with the variation of Si/Ti molar ratio.
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FIG. 4. N2 adsorption isotherms of Si-MMS, Si-Ti-MMS-95, and SiAl-MMS-150.
FIG. 3. Transmission Electron Micrographs of (A) Si-MMS and (B) Si-Ti-MMS-95.
Transmission electron micrographs (Fig. 3) of Si-MMS and Si-Ti-MMS-95 reveal a regular characteristic array of ˚ in diameter. The pores observed uniform channels ca. 30 A by TEM showed a regular hexagonal arrangement like in a nest of hornets. Figure 4 shows the N2 adsorption isotherms of Si-MMS, Si-Ti-MMS-95, and Si-Al-MMS-150. The adsorptions in Si-MMS, Si-Ti-MMS-95, and Si-Al-MMS-150 at low relative pressure, P/P0 , stem from the monolayer adsorption of N2 on the walls of the mesopores and do not represent the presence of any micropores. Different from amorphous silica and zeolite, the step of inflection in the adsorption isotherms of Si-MMS, Si-Ti-MMS-95, and SiAl-MMS-150 reflects the filling of the mesopore system. The isotherms for mesoporous molecular sieves exhibit a sharp inflection characteristic for capillary condensation within uniform pores, where the P/Po position of the inflection point is related to the diameter of the pores (9, 10, 12). Figure 5 shows the pore size distributions of Si-MMS, SiTi-MMS-95, and Si-Al-MMS-150, using the Horva´th–Kawazoe equation (11) with the adsorption isotherm. Si-MMS ˚ with show a sudden increase in mesopore volume near 31 A narrower pore size distribution. The value is almost same as
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the diameter of pore measured by TEM. These results are in general agreement with those of Kresge et al. (1). From the value (ao ) calculated from XRD peak, the thickness of the pore walls of a hexagonal lattice can be calculated as ˚ . Si-Ti-MMS-95 and Si-Al-MMS-150 represent a ca. 16 A ˚ of sudden increase in mesopore volume near 31 and 35 A pore diameter, respectively. Si-MMS also shows a stronger
FIG. 5. Horva´th-Kawazoe pore size distributions for Si-MMS, Si-TiMMS-95, and Si-Al-MMS-150.
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TABLE 2 Physical Properties of Silica-Based Mesoporous Molecular Sieves (5% Error Limits) Si-MMS BET surface area (m2/g) ˚ ) at Pore diameter (A maximum peak ˚) Basal spacings (A
1040
Si-Ti-MMS-95 877
31.2 40.7
Si-Ti-MMS-25 768
31.4 41.2
31.3 42.2
Si-Ti-MMS-13 627 28.3 43.0
Si-Al-MMS-150 813 34.7 46.8
Si-Al-MMS-50 827 30.0 43.0
Si-Al-MMS-25 801 30.9 43.9
peak than Si-Ti-MMS-95 and Si-Al-MMS-150. Table 2 represents the specific surface areas and pore diameters of several samples. The BET specific surface areas of Si-MMS, Si-Ti-MMS-95, and Si-Al-MMS-150 were 1041, 877, and 813 m2 /g with exceptionally high sorption capacity. The results indicate that the incorporation of AI or Ti decreases the specific surface area. The specific surface area decreases with increasing the content of Ti. As the content of Al increases, the specific surface area is almost constant within the experimental precision. The broad 29Si MAS NMR spectra of Si-MMS are illustrated in Fig. 6A. Si-MMS mainly consists of Q 4 (SiO4 ) type sites, and a low-intensity line from Q 3 (HOSiO3 ) exists. The
Q 4 region shows resonances at 0109.5 ppm, and the Q 3 region shows a resonance at 0101.0 ppm. Fig. 6B–D represents the 29Si MAS NMR spectra of Si-Ti-MMS. All spectra exhibit Q 3 sites of low-intensity line with main Q 4 type sites. The resonance assigned to a Q 2 site ((HO)2SiO2 ) appears obviously in the samples of Si-Ti-MMS-25 and Si-Ti-MMS13. Figure 7 illustrates the 29Si MAS NMR spectra of SiAl-MMS. Q 4 sites are dominant at higher Si/Al ratio and the breadth of Q 4 peak increases with increasing Al content. Q 3 sites appear with weak intensity, while Q 3 peaks disappear completely at lower Si/Al ratios. The explanation is that the breadth of the resonance has been attributed to the wide range of chemical environments in the second coordination sphere of silicon by Si-O-Al bonding (13). Figure 8 illustrates 27Al MAS NMR spectra of Si-Al-MMS over a wide range of Si/Al ratios from 25 to 150. The samples prepared with aluminum nitrate exhibit two reso-
FIG. 6. 29Si MAS NMR of Si-MMS and Si-Ti-MMS with the variation of Si/Ti molar ratio. (A) Si-MMS; (B) Si-Ti-MMS-95; (C) Si-Ti-MMS25; (D) Si-Ti-MMS-13.
FIG. 7. 29Si MAS NMR of Si-Al-MMS with the variation of Si/Al molar ratio. (A) Si-Al-MMS-150; (B) Si-Al-MMS-50; (C) Si-AlMMS-25.
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tions. Fluorosilicon compounds can be prepared by addition to HF with any type of silica. The principle is based on the hydrolysis reaction of H2SiF6 in an ammonium micelle template solution. The silca-based mesoporous materials were successfully synthesized by the gelation of fluorosilicon compound into ammonia–surfactant mixed solution. It was found that well-crystalline mesoporous molecular sieves were obtained after a reaction time of several hours at atmospheric conditions. The solid products show high specific surface areas in the range of 627–1040 m2 /g, depending on the amount of Al and Ti. Also, they exhibit narrow pore ˚ . From 29Si MAS size distributions in the range of 31–35 A NMR spectra, all samples mainly consist of Q 4 (SiO4 ) type sites. From 27Al MAS NMR spectra, it was confirmed that aluminum species of all samples incorporated into the framework of Si-MMS. As the amount of Al increases, amorphous phase of aluminum species increases. From these results, it was expected that the synthesis of mesoporous materials using the fluorosilicon compound is more profitable because fluorosilicon compound such as H2SiF6 could be industrially obtained from silex and SiF4 as well as industrial byproduct. FIG. 8. 27Al MAS NMR of Si-Al-MMS with the variation of Si/Al molar ratio. (A) Si-Al-MMS-150; (B) Si-Al-MMS-50; (C) Si-Al-MMS-25.
nances: (a) at ca. 50 ppm, assigned to tetrahedral aluminum species incorporated into the framework of Si-MMS, and (b) at ca. 0 ppm, assigned to octahedral aluminum species of amorphous phases. All samples give an intense line at ca. 50 ppm from 4-coordinate aluminum. This result shows that aluminum species incorporated into the framework of SiMMS. As the amount of Al increases, the intensity line from 6-coordinate aluminum (ca. 0 ppm) increases. This result represents that the fraction of the amorphous phase of aluminum species increases as the amount of aluminum increases. From experimental results, the relationship of NMR results with specific surface area with increasing the Al content was not found. On the other hand, the addition of Ti gives rise to the decrease of the specific surface area due to the collapse of pore. These results are closely related with the appearance of a Q 2 site ((HO)2SiO2 ) on NMR spectra, reflecting the looseness of pore structure. 4. CONCLUSIONS
Silica-based mesoporous molecular sieves were successfully synthesized from fluorosilicons at atmospheric condi-
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REFERENCES 1. Kresge, C. T., Leonowicz, M. E., Roth, W. T., Vartuli, J. C., and Beck, J. S., Nature 359, 710 (1992). 2. Beck, J. S., Varturi, 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., J. B. Higgins J. B., and Schlenker, J. L., J. Am. Chem. Soc. 114, 10834 (1992). 3. Huo, Q., Margolese, D. I., Ciesla, U. P., Feng, P., Gier, T. E., Sieger, P., Leon, R., Petroff, P. M., Schu¨th F., and Stucky, G. D., Nature 368, 317 (1994). 4. Silva F. H. P., and Pastore H. O., Chem. Commun., 833 (1996). 5. Yanagisawa, T, Shimizu, T., Kazayuki, K., and Kato, C., Bull. Chem. Soc. Jpn. 63, 988 (1990). 6. Jeong, S. Y., Kwon, O. Y., Suh, J. K., Jin, H., and Lee, J. M., J. Colloid Interface Sci. 175, 253 (1995). 7. Jeong, S. Y., Kwon, O. Y., Suh, J. K., Jin, H., and Lee, J. M., J. Colloid Interface Sci. 180, 269 (1996). 8. Vartuli, J. C., Kresge, C. T., Leonowicz, M. E., Chu, A. S., McCullen, S. B., Johnson, I. D., and Sheppard, E. W., Chem. Mater. 6, 2070 (1994). 9. Iler, R. K., ‘‘The Chemistry of Silica.’’ Academic Press, London, 1990. 10. Chen, C. Y., Li, H. X., and Davis, M. E., Microporous Mater. 2, 17 (1993). 11. Horva´th, G., and Kawazoe, K., J. Chem. Eng. Jpn. 16, 470 (1983). 12. Gregg, S. J., and Sing, K. S. W., ‘‘Adsorption, Surface Area, Porosity,’’ 2nd ed. Academic Press Inc., London, 1982. 13. Brinker, C. J., and Scherer, G. W., ‘‘Sol–Gel Science The Physics and Chemistry of Sol–Gel Processing.’’ Academic Press, London, 1990.
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CS974974
Preparation of Silica-Based Mesoporous Materials From Fluorosilicon Compounds: Gelation of H2SiF6 in Ammonia Surfactant Solution Soon-Yong Jeong,* ,1 Jeong-Kwon Suh,* Jung-Min Lee,* and Oh-Yun Kwon† *Applied Chemistry & Engineering Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon, Korea 305-600; and †Department of Chemical Engineering, Yosu National Fisheries University, Yosu, Korea 550-749 Received February 3, 1997; accepted May 5, 1997
The silica-based mesoporous molecular sieves were synthesized from fluorosilicon compounds and Al, Ti with the hydrolysis reaction of H2SiF6 in an ammonia-surfactant mixed solution. Wellcrystalline mesoporous molecular sieves were obtained after several hours at the atmospheric conditions. The solid products were characterized by X-ray powder diffraction, nitrogen adsorption/ desorption, transmission electron microscopy and solid-state NMR spectroscopy. The solid products show high specific surface areas in the range of 627–1040 m2 /g, depending on the amount of Al and Ti. Also, they exhibit narrow pore size distributions in the range of 31–35 A˚ according to the addition of Al and Ti. 29Si MAS NMR indicate that all samples mainly contain Q 4 types sites. Also, the peak intensity of Q 3 sites decreases with increasing amount of aluminum. In 27Al MAS NMR spectra, all samples give intense lines from 4-coordinate aluminum. These results show that aluminum species are incorporated into the framework of Si-MMS. As the amount of Al increases, the intensity line from 6-coordinate aluminum increases. This result represents the increase of the fraction of amorphous aluminum species. q 1997 Academic Press Key Words: mesoporous material; fluorosilicon; gelation.
1. INTRODUCTION
Mesoporous materials have attracted great interest in relation to the utilization as catalysts and separation media for industrial applications. Recently, researchers at Mobil oil company (1, 2) introduced a new family of mesoporous molecular sieves. These materials were synthesized by hydrothermal reactions of aluminosilicate and silicate gel in the presence of quaternary ammonium surfactants. The silicate and aluminosilicate materials were designated as MCM41. They have regular arrays of a uniform channel size rang˚ according to the type of surfactant. ing from 20 to 100 A Hue et al. (3) showed that the synthetic approach can be generalized to non-silica-based types such as antimony oxide, tungsten oxide, iron oxide, and zinc oxide. Recently, Silva and Pastore (4) reported effects as the mineralizing 1
To whom correspondence should be addressed.
156
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agent of F 0 ion on the synthesis of mesoporous materials and several types of zeolite. On the other hand, Yanagisawa et al. (5) described the mesoporous silicate prepared from the layered silicate kanemite. These materials were prepared by a hydrothermal synthesis in the presence of tertiary ammonium halides such as dodecyltrimethylammonium chloride or cetyltriethylammonium. Also, Jeong et al. (6, 7) synthesized mesoporous materials having a pore size of 30– ˚ by calcining the resulting materials after TEOS-interca40 A lated layered silicates such as magadiite and kenyaite were gelled by an acid or base catalyst. These new mesoporous materials are expected to be useful for the catalytic reactions of high-molecular hydrocarbons because of higher diffusivities owing to their large pore diameters. In the synthesis of MCM-41 materials (1, 2, 8), the silica sources generally used are sodium silicate and Si-alkoxides. Especially, if the silica is used as starting material, the types of silica to be capable of using are limited because of solubility. Iler (9) reports that soluble silica, Si(OH)4 is produced by the dissolution of silica and ionized to (HO)3SiO 20 or (HO)2SiO 20 2 above pH 9. The dissolution of silica accompanied by ionization is a basic step to lead hydrolytic polycondensation with charged micelle template. Therefore, synthetic condition should be forced in dissolution of silica. Strong quaternary ammonium bases like tetramethylammonium hydroxide can dissolve silica at a temperature of 150– 2007C at elevated pressure, forming silicate salt. Considering this point, many researchers (1, 2, 9) make use of quaternary ammonium bases as silica dissolution media and alkali source. However, silica cannot be dissolved at atmospheric conditions, and easily soluble silica sources are limited. These problems in the use of conventional silica sources could be markedly improved by using fluorosilicon compounds such as H2SiF6 . H2SiF6 can be industrially obtained from silex and SiF4 as well as an industrial byproduct. However, there is no report in the literature on the synthesis of mesoporous materials using fluorosilicon compounds. The method of silica precipitation by NH4OH addition to H2SiF6 solution above pH 8 has mainly been used for manufacturing highly fluffy silica to be effective as pillars.
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Iler (9) suggests that OH 0 ions could act as the improving agent in the hydrolytic polycondensation of SiF 20 6 , like a role of OH 0 and F 0 in polymerization of Si(OH)4 . Especially, in the presence of cationic surfactant micelles, the gelation of SiF 20 6 by NH4OH was proceeded rapidly even at pH 6. This fact is an interesting clue on the synthesis of mesoporous marterials. We now report a simple method for the synthesis of silicabased mesoporous molecular sieves (MMS) from fluorosilicon compounds. The principle is based on the hydrolysis reaction of H2SiF6 in the ammonia cationic micelle template solution. This method is supposed to be markedly profitable in the industrial aspect because H2SiF6 could be industrially obtained from silex and SiF4 as well as an industrial byproduct. Also, the synthetic process could be simplified because products are manufactured for several hours at atmospheric conditions. 2. EXPERIMENTAL
(1) Silica-Based Mesoporous Molecular Sieve (Si-MMS) The synthesis procedures were as follows: 20g of H2SiF6 solution (10 wt % SiO2 ) was prepared by gradually dissolving 2 g of SiO2 (grade 62, 60–200 mesh, Aldrich, U.S.A.) to 18 g of 25% hydrofluoric acid (48%, Merck, Germany) (pH of solution: 1.5). 4 wt % of CTAB (cetyltrimethylammonium bromide); (99%, BDH, England) surfactant solution was also prepared by dissolving CTAB in distilled water. NH4OH (28 wt %, Merck, Germany) was added to 20g H2SiF6 solution stirred at 40–507C until pH 6 was reached. The solution did not show any sign of gelation. This solution was added at once to 130 g of CTAB solution at 507C and stirred vigorously during 5 min. A white gel was produced abruptly within 10 s. NH4OH was added until the pH reached 7–8. The gel solution was aged for 5 h in a dryer at 707C. The resulting solid product was recovered by filtration, washed 5 times with distilled water, and dried at 607C. The dried sample was calcined at 6007C for 4 h to remove organic compounds. (2) Al-Containing Silica-Based Mesoporous Molecular Sieve (Si-Al-MMS) Various amounts of Al(NO3 )3r9H2O (0.13g, 0.52g, and 0.91g) (98%, Aldrich, USA) were added to 20 g of H2SiF6 solution (10 wt % SiO2 ), respectively, and dissolved perfectly, and the solutions exhibited between pH 1.4 and 2.0, depending on the amount of Al(NO3 )3r9H2O. Ammoniasurfactant mixed solution was also prepared by adding 30 g of NH4OH to the solution containing 100 g of CTAB solution (4 wt %), and this solution exhibited pH 11.7. H2SiF6 solutions containing Al(NO3 )3r9H2O were added at once to 130 g of ammonia–surfactant mixed solution, respectively, and stirred vigorously during 5 min at 40–507C, and the pH of
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solutions were between 7.0 and 7.5, depending on the amount of Al(NO3 )3r9H2O. A white gel was formed abruptly within 10 s and aged for 5 h in a dryer at 707C. The following procedures were the same as for the preparation of Si-MMS. The solid product was filtered, washed, dried, and calcined at 6007C. (3) Ti-Containing Silica-Based Mesoporous Molecular Sieve, Si-Ti-MMS Ti-containing mesoporous molecular sieve was prepared by a similar procedure of Si-MMS. Various amounts of titanium isopropoxide, Ti(OC3H7 )4 (0.1, 0.4, and 0.7 g); (97%, Aldrich, USA) were added to 20 g of H2SiF6 solution respectively, and dissolved perfectly. The pH of these solutions exhibited values of 1.5 and 2.1, depending on the amount of Ti(OC3H7 )4 . These solutions were added at once to 130 g of the prepared ammonia–surfactant mixed solution respectively, and stirred vigorously during 2 min at 607C and then pH was between 7.3 and 8.0, depending on the amount of Ti(OC3H7 )4 . A white gel was formed abruptly within 10 s and aged for 5 h in dryer at 707C. The following procedures were the same as the preparation of Si-MMS. The solid product was filtered, washed, dried, and calcined. Basal spacings of samples were determined from the 00l X-ray diffraction using a Rigaku diffractometer equipped with CuKa radiation (35 kV, 15 mA). Nitrogen adsorption/ desorption isotherms were determined by Micromeritics ASAP 2000 at 77K. All samples were degassed at 3007C under vacuum for 4 h. The specific surface area was determined by the BET equation. The pore size distributions of samples were determined by the Horva´th-Kawazoe equation (11). 29Si MAS-NMR and 27Al MAS-NMR were performed on a Bru¨ker AM 300 solid-state NMR spectrophotometer. For transition electron micrographs (TEM) analysis, samples were dispersed ultrasonically in ethanol, and a drop of the suspension was deposited on a carbon coated copper grid. Micrographs were obtained on a Philips CM-20 electron microscope. The chemical compositions of samples were determined by an energy dispersive X-ray spectrometer (Link System AN10000-85S). EDS analyses were carried out using electron beam with the sample of carbon-coated pellet. 3. RESULTS AND DISCUSSION
Silica-based mesoporous molecular sieves were synthesized from fluorosilicon and compounds containing Al and Ti with the hydrolysis reaction of H2SiF6 in cetyltrimethylammonium bromide solution. NH4OH was added to H2SiF6 solution in order to promote the hydrolysis reaction of 20 SiF 20 6 . Also, it was confirmed that the gelation of SiF 6 by NH4OH proceeded rapidly even at pH 6 in the presence of cationic surfactant micelle.
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TABLE 1 Chemical Compositions of Gel Mixtures and Calcined Samples Si-Ti-MMS-95
Si-Ti-MMS-25
Si-Ti-MMS-13
Si-Al-MMS-150
Si-Al-MMS-50
Si-Al-MMS-25
93.8
23.4
13.4
—
—
—
95
25
13
—
—
—
95.2
23.8
13.6
50
25
Si/Ti atomic ratio of gel mixture (calculated) Si/Ti atomic ratio of calcined sample (EDS analysis) Si/Al atomic ratio of gel mixture (calculated) Si/Al atomic ratio of calcined sample (EDS analysis)
—
—
—
—
—
—
150
The chemical compositions of the gel mixtures and the calcined products are shown in Table 1. The Si/Ti ratios of the calcined products are similar to those of the gel mixtures. The results indicate that the amount of titanium does not reduce in the course of gelation, filtration and calcination. However, the Si/Al ratios of the calcined products are above those in the gel mixtures, indicating a relative loss of aluminum in the course of gelation, filtration, and calcination. The X-ray diffraction pattern of Si-MMS is shown in Fig. 1a. The relatively well-defined pattern is typical for MCM41 materials as described by Kresge et al. (1) The first four XRD peaks of Si-MMS can be indexed on a hexagonal ˚ (hexagonal with ao Å lattice qwith pore diameter of ca. 47A 2d100 / 3). The X-ray diffraction patterns of Si-Al-MMS with the variation of the molar ratios of Si/Al from 25 to
150 are given in Fig. 1b–d. All aluminosilicate samples give poorer quality XRD patterns than pure siliceous MMS. The intensity of the first peak decreases even at very low levels of aluminum incorporation, and other peaks overlap and greatly decrease. It is therefore clear that the structure of the aluminosilicate MMS is less uniform than that of purely siliceous MMS. Figure 2 represents the X-ray diffraction patterns of Si-Ti-MMS with the variation of the molar ratio of Si/Ti from 13 to 95. The XRD pattern of Si-Ti-MMS with Si/Ti Å 95 consists of the first four peaks with which a pattern similar to Si-MMS indicate a hexagonal lattice. However, XRD patterns of Si-Ti-MMS with Si/Ti õ 25 decrease strongly in intensity and overlap. The results indicate that incorporation of a small amount (Si/Ti ú 95) of Ti does not affect the framework of silica-based mesoporous molecular sieves.
FIG. 1. X-ray powder diffraction patterns of Si-MMS and Si-Al-MMS with variation of Si/Al molar ratio.
FIG. 2. X-ray powder diffraction patterns of Si-Ti-MMS with the variation of Si/Ti molar ratio.
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FIG. 4. N2 adsorption isotherms of Si-MMS, Si-Ti-MMS-95, and SiAl-MMS-150.
FIG. 3. Transmission Electron Micrographs of (A) Si-MMS and (B) Si-Ti-MMS-95.
Transmission electron micrographs (Fig. 3) of Si-MMS and Si-Ti-MMS-95 reveal a regular characteristic array of ˚ in diameter. The pores observed uniform channels ca. 30 A by TEM showed a regular hexagonal arrangement like in a nest of hornets. Figure 4 shows the N2 adsorption isotherms of Si-MMS, Si-Ti-MMS-95, and Si-Al-MMS-150. The adsorptions in Si-MMS, Si-Ti-MMS-95, and Si-Al-MMS-150 at low relative pressure, P/P0 , stem from the monolayer adsorption of N2 on the walls of the mesopores and do not represent the presence of any micropores. Different from amorphous silica and zeolite, the step of inflection in the adsorption isotherms of Si-MMS, Si-Ti-MMS-95, and SiAl-MMS-150 reflects the filling of the mesopore system. The isotherms for mesoporous molecular sieves exhibit a sharp inflection characteristic for capillary condensation within uniform pores, where the P/Po position of the inflection point is related to the diameter of the pores (9, 10, 12). Figure 5 shows the pore size distributions of Si-MMS, SiTi-MMS-95, and Si-Al-MMS-150, using the Horva´th–Kawazoe equation (11) with the adsorption isotherm. Si-MMS ˚ with show a sudden increase in mesopore volume near 31 A narrower pore size distribution. The value is almost same as
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the diameter of pore measured by TEM. These results are in general agreement with those of Kresge et al. (1). From the value (ao ) calculated from XRD peak, the thickness of the pore walls of a hexagonal lattice can be calculated as ˚ . Si-Ti-MMS-95 and Si-Al-MMS-150 represent a ca. 16 A ˚ of sudden increase in mesopore volume near 31 and 35 A pore diameter, respectively. Si-MMS also shows a stronger
FIG. 5. Horva´th-Kawazoe pore size distributions for Si-MMS, Si-TiMMS-95, and Si-Al-MMS-150.
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TABLE 2 Physical Properties of Silica-Based Mesoporous Molecular Sieves (5% Error Limits) Si-MMS BET surface area (m2/g) ˚ ) at Pore diameter (A maximum peak ˚) Basal spacings (A
1040
Si-Ti-MMS-95 877
31.2 40.7
Si-Ti-MMS-25 768
31.4 41.2
31.3 42.2
Si-Ti-MMS-13 627 28.3 43.0
Si-Al-MMS-150 813 34.7 46.8
Si-Al-MMS-50 827 30.0 43.0
Si-Al-MMS-25 801 30.9 43.9
peak than Si-Ti-MMS-95 and Si-Al-MMS-150. Table 2 represents the specific surface areas and pore diameters of several samples. The BET specific surface areas of Si-MMS, Si-Ti-MMS-95, and Si-Al-MMS-150 were 1041, 877, and 813 m2 /g with exceptionally high sorption capacity. The results indicate that the incorporation of AI or Ti decreases the specific surface area. The specific surface area decreases with increasing the content of Ti. As the content of Al increases, the specific surface area is almost constant within the experimental precision. The broad 29Si MAS NMR spectra of Si-MMS are illustrated in Fig. 6A. Si-MMS mainly consists of Q 4 (SiO4 ) type sites, and a low-intensity line from Q 3 (HOSiO3 ) exists. The
Q 4 region shows resonances at 0109.5 ppm, and the Q 3 region shows a resonance at 0101.0 ppm. Fig. 6B–D represents the 29Si MAS NMR spectra of Si-Ti-MMS. All spectra exhibit Q 3 sites of low-intensity line with main Q 4 type sites. The resonance assigned to a Q 2 site ((HO)2SiO2 ) appears obviously in the samples of Si-Ti-MMS-25 and Si-Ti-MMS13. Figure 7 illustrates the 29Si MAS NMR spectra of SiAl-MMS. Q 4 sites are dominant at higher Si/Al ratio and the breadth of Q 4 peak increases with increasing Al content. Q 3 sites appear with weak intensity, while Q 3 peaks disappear completely at lower Si/Al ratios. The explanation is that the breadth of the resonance has been attributed to the wide range of chemical environments in the second coordination sphere of silicon by Si-O-Al bonding (13). Figure 8 illustrates 27Al MAS NMR spectra of Si-Al-MMS over a wide range of Si/Al ratios from 25 to 150. The samples prepared with aluminum nitrate exhibit two reso-
FIG. 6. 29Si MAS NMR of Si-MMS and Si-Ti-MMS with the variation of Si/Ti molar ratio. (A) Si-MMS; (B) Si-Ti-MMS-95; (C) Si-Ti-MMS25; (D) Si-Ti-MMS-13.
FIG. 7. 29Si MAS NMR of Si-Al-MMS with the variation of Si/Al molar ratio. (A) Si-Al-MMS-150; (B) Si-Al-MMS-50; (C) Si-AlMMS-25.
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tions. Fluorosilicon compounds can be prepared by addition to HF with any type of silica. The principle is based on the hydrolysis reaction of H2SiF6 in an ammonium micelle template solution. The silca-based mesoporous materials were successfully synthesized by the gelation of fluorosilicon compound into ammonia–surfactant mixed solution. It was found that well-crystalline mesoporous molecular sieves were obtained after a reaction time of several hours at atmospheric conditions. The solid products show high specific surface areas in the range of 627–1040 m2 /g, depending on the amount of Al and Ti. Also, they exhibit narrow pore ˚ . From 29Si MAS size distributions in the range of 31–35 A NMR spectra, all samples mainly consist of Q 4 (SiO4 ) type sites. From 27Al MAS NMR spectra, it was confirmed that aluminum species of all samples incorporated into the framework of Si-MMS. As the amount of Al increases, amorphous phase of aluminum species increases. From these results, it was expected that the synthesis of mesoporous materials using the fluorosilicon compound is more profitable because fluorosilicon compound such as H2SiF6 could be industrially obtained from silex and SiF4 as well as industrial byproduct. FIG. 8. 27Al MAS NMR of Si-Al-MMS with the variation of Si/Al molar ratio. (A) Si-Al-MMS-150; (B) Si-Al-MMS-50; (C) Si-Al-MMS-25.
nances: (a) at ca. 50 ppm, assigned to tetrahedral aluminum species incorporated into the framework of Si-MMS, and (b) at ca. 0 ppm, assigned to octahedral aluminum species of amorphous phases. All samples give an intense line at ca. 50 ppm from 4-coordinate aluminum. This result shows that aluminum species incorporated into the framework of SiMMS. As the amount of Al increases, the intensity line from 6-coordinate aluminum (ca. 0 ppm) increases. This result represents that the fraction of the amorphous phase of aluminum species increases as the amount of aluminum increases. From experimental results, the relationship of NMR results with specific surface area with increasing the Al content was not found. On the other hand, the addition of Ti gives rise to the decrease of the specific surface area due to the collapse of pore. These results are closely related with the appearance of a Q 2 site ((HO)2SiO2 ) on NMR spectra, reflecting the looseness of pore structure. 4. CONCLUSIONS
Silica-based mesoporous molecular sieves were successfully synthesized from fluorosilicons at atmospheric condi-
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