12184
J. Phys. Chem. B 2000, 104, 12184-12191
High Surface-Area Silica with Controlled Pore Size Prepared from Nanocomposite of Silica and Citric Acid Ryoji Takahashi,* Satoshi Sato, Toshiaki Sodesawa, Machiko Kawakita, and Katsuyuki Ogura Department of Materials Technology, Faculty of Engineering, Chiba UniVersity, Yayoi, Inage, Chiba, 263-8522, Japan ReceiVed: July 26, 2000
The pore formation process of silica gel from tetraethoxysilane in the presence of citric acid (CA) was investigated. The silica gel prepared by pyrolysis of a composite of CA and silica had high specific surface area, ca. 1000 m2 g-1 and its pore volume increased linearly with increasing CA content. In addition, the average pore size successively increased from the micropore range to the mesopore range of ca. 10 nm with the increase in the CA content. The silica and the CA-silica composite were amorphous without long-range ordering. In the composite, amorphous CA and polymeric silica gel are mixed on a nanometer scale without the aid of specific chemical bonding, except for weak hydrogen bonding. Because of the nature of the composite, the silica gel network becomes rigid through the additional formation of Si-O-Si bonds during heating, irrespective of the existence of CA. Therefore, the bulky structure is retained without shrinkage after elimination of CA, and silica gel with high specific surface area and pore volume results. Thus, CA is considered to provide mesopores as a template.
1. Introduction Recently, silicates with periodic 1-dimensional (1-d) mesopores and extra high specific surface area (ca. 1000 m2 g-1), named MCM-411,2 and FSM-16,3,4 have attracted much interest in the field of catalysis.5-7 They are prepared by calcining composites of silicate and a self-assembled surfactant in a hexagonal array. Because of a sharp pore size distribution in mesoscale and an oriented periodic pore structure, the silicates are considered to provide molecular recognition ability for large organic compounds for which zeolites with only micropores cannot be used due to their small pore size. Such molecular recognition is essential in the design of highly selective catalysis reactions. For application as a support for metal catalyst particles, however, filling of pores by metal particles arises as a problem due to restricted pathway for material transport in 1-d porous system. Hence, materials with a 3-d porous system, such as MCM-48 with cubic symmetry1 and conventional silica gel with mesopores randomly linked in 3-d space are also useful. On the other hand, pore structure control of silica gel by the sol-gel process has been also investigated widely.8-17 The pore size of silica gel can be easily enlarged to ca. 10-30 nm by immersing a wet gel in a basic solution.9-11 However, this prepared gel has relatively low specific surface area (4 in a usual sol-gel reaction, and silanol
12190 J. Phys. Chem. B, Vol. 104, No. 51, 2000 on the surface of silica is a weak acid. Hence, the silica gel network and carboxyl group cannot interact attractively. The 13C NMR result also indicates the absence of interaction between CA and silica gel network. In addition, the result that the carbon in CA-silica composite cannot be detected by CP technique suggests that the amorphous CA has an elastic nature. Thus, the CA-silica composite dried can be regarded as having a structure with a polymeric network of silica gel cross-linked in a sea of amorphous CA. With increasing CA content, the shrinkage ratio during drying becomes smaller and the proportion of amorphous CA in the composite increases. Incidently, it is speculated that a part of CA interacts with the surface of silanol with the molar ratio of CA/Si ) 0.130.14 as mentioned relating the TG-DTA results. The ratio of 0.13-0.14 is possibly determined by the structure of the silica gel network formed under acidic conditions, by number and distribution of remaining ethoxy groups and by the molecular size of CA. Taking into account the structural feature obtained from the spectroscopic measurement that part of the CA exits in the composites without forming intramolecular hydrogen bonding, particularly in larger proportion in composites with smaller x, it is suggested that interaction between CA and silica occurs by hydrogen bonding with silanol through hydroxyl group in CA instead of intramolecular hydrogen bonding. However, the hydrogen bonding is not so strong to prevent the condensation of silanol, because the condensation of silanol in the CA-silica composite proceeds up to 200 °C irrespective of the existence of CA. 4.3. Pore Formation in CA-Silica System. In the preparation of porous silica through sol-gel process using organic additives, the additives are expected to disperse in organicinorganic composites on a nanometer scale. By removing the additives, pores reflecting the molecular size and structure of organic additives are expected to be formed. However, the nanocomposite is not always obtained by simply mixing organic additives in sol-gel reaction. When a repulsive interaction arises between silica network and an organic additive, phase separation sometimes proceeds to provide domains with size typically as large as the micrometer scale.31-33 Similarly, an organic compound with low solubility in the reacting solution and with high crystallization tendency cannot be mixed on a nanometer scale in dried silica gel because aggregation of organic compounds by crystallization sometimes occurs during solgel reaction and drying. Therefore, organic-inorganic composites have been frequently prepared from alkoxides with organic ligand (RSi(OR′)3)12-14 or by using such a compound as PEG which has functional groups showing strong affinity with silica network.15-18 However, only microporus gels are obtained from most of these composites. In the PEG-silica system, for example, PEG is incorporated in silica gel network in molecular level forming large number of hydrogen bonds between silanols and ether oxygens in PEG.18 However, the strong interaction between silanol and PEG inhibits the bond formation of Si-O-Si during drying and heating, so that the silica network cannot become rigid in such composites. When PEG is eliminated, the silica gel shrinks and suppresses the space occupied with organic compounds by abrupt Si-O-Si bond formation, resulting in a microporous silica gel.18 One exception is the formation of mesoporous material with periodic pores such as MCM-41 from a composite of silica gel and surfactant.2 The organic-inorganic composite, a precursor of MCM-41, is formed with a hexagonal array of rod type micelles of surfactant and silica which surrounds each of micelle2. Upon heating, the silica gel network
Takahashi et al. becomes rigid, and remains as a honeycomb type skeleton after the elimination of surfactant. Thus, pores reflecting the size and shape of a self-assembled structure of surfactant are formed. The molecular size of CA is substantially smaller than the pore size in the resultant gel, and CA cannot form a selfassembled structure such as a micelle. Therefore, the pore formation mechanism as well as the structure of the CA-silica composite may differ from those in MCM-41. Furthermore, the pore formation mechanism in CA-silica system is also different from that of other organic-inorganic systems such as PEGsilica, although both CA-silica and PEG-silica systems form amorphous nanocomposites. The CA-silica composite can be regarded as having a structure with a polymeric network of silica gel cross-linked in a sea of amorphous CA. Here, a CA per 7-8 Si atoms forms hydrogen bonding with surface silanol, and the residual CA is free from the interaction with silanol. Since the strength of hydrogen bonding between silanol and the hydroxyl group in CA is not so strong to restrict the condensation of silanol during heating, the silica gel network can form additional Si-O-Si bonds despite the existence of CA. Thus, the silica gel gains rigidity high enough to maintain the bulky structure up to 200 °C, and the space occupied with free CA remains as pores after the decomposition of free CA. The CA forming hydrogen bonding with silanol contributes to the increase in the specific surface area, while all CA contributes to the increase in the pore size and pore volume. Therefore, the specific surface area of the mesoporous amorphous silica is constant at ca. 9001000 m2 g-1 over the CA/Si ratio of 0.13-0.14, while the pore volume increases linearly with the increase in x. At x > 0.8, part of the space occupied with CA collapsed upon heating. However, the small change in the Q3/Q4 ratio in the 29Si NMR spectra of the sample with x ) 1 in heating between 200 and 500 °C suggests that the resulting silica gel gains a semi-stable structure and can support the porous structure. The pore formation mechanism in CA-silica system thus speculated is similar to that in super critical drying of wet silica gel34-37 rather than those in the other organic-inorganic composite systems. In supercritical drying, the bulky structure of wet silica gel can be dried without shrinkage because no surface tension arises during drying. Thus, the silica network gains rigidity by condensation under supercritical conditions. In the CA-silica system, the bulky structure of wet silica gel can be also maintained in the dried composite in which the space among gel network is filled with amorphous CA. The CA filling the open space inhibits the shrinkage of the silica gel network, while it does not inhibit the condensation of silanol. Thus, a highly open structure is obtained by CA incorporation as well as supercritical drying. It is speculated that organic compounds with the following characteristics have pore formation ability like CA: (1) high solubility in solvent such as water and alcohol, (2) low ability to crystallze during drying, (3) without functional groups which show strong attractive interaction with silica, such as polyoxyethylene units and amino groups, and (4) high vaporization or decomposition temperature. Characteristics 1 and 2 ensure the formation of organic-inorganic composites mixed in nanometer scale, and those of 3 and 4 are necessary to proceed the condensation reaction of silanol in the organic-inorganic composite during heating to be rigid enough to support a bulky gel structure by itself before the elimination of the organic additives.
High Surface-Area Silica with Controlled Pore Size 5. Conclusion An amorphous porous silica gel was prepared by pyrolysis of a composite of citric acid (CA) and silica derived by solgel reaction of tetraethoxysilane. The silica gel had high specific surface area, ca. 1000 m2 g-1, and its pore volume increased linearly with increasing the CA content. The pore volume agreed with the space occupied with CA in the CA-silica composite, suggesting that CA acts as a template for the pore formation. Although the pore size distribution of the silica was broader than that of MCM-41, the average pore size was successively controlled from micropore range to ca. 10 nm by the increase in the CA content. The amorphous silica as well as the CAsilica composite was amorphous without distinct structural units. NMR, IR, XRD, and TG-DTA results suggest that CA and silica are mixed on the nanometer scale in the composite without specific chemical bonding except for the weak hydrogen bonding. Because of the nature of the composite, the silica gel network can form additional Si-O-Si bonds which are rigid during heating irrespective of the existence of CA. Therefore, a bulky structure is retained after calcination without shrinkage, and a silica gel with high specific surface area and pore volume results. Thus, CA is concluded to provide mesoporous silica as a template. Acknowledgment. The present work was partially supported by following grants from the Japan Society for the Promotion of Science: Grant-in-Aid for Encouragement of Young Scientists (11750580), Grant-in Aid for Scientific Research C (12650075), and “Research for the Future” Program (JSPS-RFTF96P00304). References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) 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. (3) Inagaki, S.; Fukushima Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (4) Inagaki, S.; Koiwai, A.; Suzuki, N.; Fukushima, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1996, 69, 1449. (5) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. (6) Mehnert, C. P.; Ying, J. Y. Chem. Commun. 1997, 2215.
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