Langmuir 1997, 13, 4185-4186
Preparation of Silica-Lignin Xerogel
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Table 1. Surface Areas of Silica Xerogel and Silica-Lignin Xerogels
Jun’ichi Hayashi,* Tetsuo Shoji, Yuuki Watada, and Katsuhiko Muroyama
lignin weight ratio (lignin/Si)
surface area (m2/g)
Department of Chemical Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564, Japan
0 0.002 0.01 0.1
605.1 772.8 1074 900.3
Received January 23, 1997. In Final Form: April 18, 1997
Porous materials are widely used as adsorbents in various fields. The adsorption ability of an adsorbent mainly depends on its pore structure and its surface characteristic. Therefore how to control the pore structure and the surface characteristics is very important in producing a useful adsorbent. Silica xerogel is an important adsorbent, its surface area is several hundred square meters per gram, and the mesopore is well-developed. The surface nature is hydrophilic due to silanol function groups. Silica xerogel can be prepared from tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) by the sol-gel method.1 In the sol-gel method TEOS or TMOS is hydrolyzed, and the hydrolyzed product is condensed to silica sol and further condensed to gel. The pore structure and the surface characteristics of silica xerogel are expected to be modified when some substance is added during hydrolysis and condensation. Recently, many researchers have tried to incorporate organic polymer with silica by the sol-gel method, and various silica-organic composites have been prepared.2-7 The mechanical properties and thermal properties of silica-organic composites have been investigated, but not much has been investigated about the pore structure of the composites. We tried to prepare the modified silica xerogel pore structure using an organic compound as an additive. Lignin is a well-known natural polymer which constitutes the cell wall of plants. Lignin has hydroxyl functional groups and can be fixed in the silica xerogel matrix through condensation. As a result the pore structure of silica xerogel is expected to be modified by lignin. We call this composite material silica-lignin xerogel. We examined the influence of lignin weight content on the pore structure. The method to produce silica-lignin xerogel is as follows: tetraethoxysilane (TEOS) and ethanol were mixed in a flask, and lignin powder (30-50 µm) was added to the mixture. This mixture was stirred in order to form a homogenous suspension. The TEOS to methanol molar ratio was 1:8, and the lignin to silica weight ratio (lignin/ Si) was varied in the range 0-0.1. After 1 h of stirring, 0.1 N HCl was added to water to obtain a pH value of 4. This aqueous solution was then added to the suspension, and the suspension was further stirred to obtain the gel in the flask in an oil bath kept at 60 °C. Silica-lignin xerogel was prepared after drying the gel at 110 °C and further heat-treating it at 300 °C. * To whom correspondence should be addressed. Fax: +81-6388-8869. Telephone: +81-6-368-0913. E-mail: PXH02010@ niftyserve.or.jp. (1) Gesser, H. D.; Goswami, P. C. Chem. Rev. 1989, 89, 765. (2) Wang, S.; Xu, S.; Mark, J. E Rubber Chem. Technol. 1991, 64, 746. (3) Ravanaine, D.; Seminel, A.; Charbouillot, Y.; Vincens M. J. NonCryst. Solids 1986, 82, 210. (4) Saegusa, T. Macromol. Sci. Chem. 1991, A28, 817. (5) Morikawa, A.; Iyoku, Y.; Kamimoto, M.; Imai, Y. Polym. J. 1992, 24, 107. (6) Chujo, Y.; Matsuki, H.; Kure, S.; Saegusa, T.; Yazawa T. J. Chem. Soc., Chem. Commun. 1994, 635. (7) Liu, C.; Komarneni, S. J. Porous Mater. 1995, 1, 75.
S0743-7463(97)00072-3 CCC: $14.00
Figure 1. Adsorption and desorption isotherms on silica xerogel (a) and silica-lignin xerogel prepared with lignin ratios of 0.01 (b) and 0.1 (c).
Nitrogen adsorption and desorption isotherms were measured at 77 K in order to characterize the pore structure of silica-lignin xerogel by BELSORP 28 (BEL Japan Inc.). The surface area was analyzed by the BET (Brunauer-Emmett-Teller) method, and the pore volume distribution was analyzed by and Dollimore-Heal method8 using desorption isotherm data. In Dollimore-Heal method, the pore shape is assumed to be cylindrical and the measured desorption is assumed to be made up of both condensed liquid lost from pores and adsorbate lost from multilayers of adsorbed molecules. The surface areas of silica-lignin xerogel and silica xerogel are summarized in Table 1. The surface area of silica-lignin xerogel is greater than that of silica xerogel. We measured the surface area of lignin carbonized at 300 °C and found that the surface area was almost zero. Therefore, it is clear that the increase of surface area of silica-lignin xerogel is not attributed to the carbonization of lignin. Figure 1 shows the nitrogen adsorption and desorption isotherms on the silica xerogel and the silica-lignin xerogels prepared for various lignin ratios. It is clearly found that the shapes of monolithic isotherms were quite different from each other. In the isotherm of the monolithic silica xerogel, the amount adsorbed increases rapidly near the relative pressure of 1.0, indicating that there is large mesopore volume. When the lignin ratio is 0.01, the amount adsorbed increases in the low- and middlepressure range, while the adsorption levels off near the relative pressure of 1.0. This indicates a decrease in the mesopore volume. At a lignin ratio of 0.1, there is no increase in the amount adsorbed above the relative pressure of 0.5, showing that there is little mesopore volume. In the silica xerogel and silica-lignin xerogel with a lignin ratio of 0.01, a hysteresis loop is clearly found in the adsorption-desorption isotherm, but in the silicalignin xerogel with a lignin ratio of 0.1, the hysteresis loop cannot be found. On the basis of the shape of the hysteresis loop in the isotherm,9 there are constrictions, so-called bottlenecks, in the pore structure of the silica xerogel and in the silica-lignin xerogel with the lignin ratio 0.01. (8) Dollimore, D.; Heal, G. R. J. Appl. Chem. 1964, 14, 109. (9) Mc Bain, J. W. J. Am. Chem. Soc. 1935, 57, 699.
© 1997 American Chemical Society
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Figure 2. Pore volume distributions of silica xerogel and silicalignin xerogel prepared with various lignin ratios.
Figure 2 shows the pore volume distributions of the silica xerogel and the silica-lignin xerogel prepared with various lignin ratios. The pore size distribution shifts to smaller radii and the peak in the distribution curve becomes sharper as the lignin ratio increases from 0 to 0.01. At a lignin ratio of 0.1, the pore volume seems to be very small. However, the surface area of silica-lignin xerogel is larger than that of silica xerogel, and the pore size is so small that it cannot be exactly analyzed by the Dollimore-Heal method. The silica xerogel has a weak cross-linked structure, a large pore radius, a large mesopore volume, and few constrictions in the pore, since the xerogel is prepared
Notes
using acid as catalyst and the xerogel structure is linearly polymerized.10 By addition of lignin, the lignin is chemically fixed in the silica xerogel matrix, and the added lignin is bridged between the linear silica xerogel structures. The pore size becomes narrow near the cross-linked point, and then the bottleneck structure occurs. Since there are more cross-linked points in the silica-lignin xerogel are in the silica xerogel, the hysteresis is found definitely in the adsorption and desorption isotherm of silica-lignin xerogel with the lignin ratio 0.01, as shown in Figure 1b. In the case of the lignin ratio 0.1, there are many cross-linked points and then the pore becomes narrow all over. Therefore the pore size becomes very small, leading to little mesopore volume and no hysteresis in the isotherm. Thus it is obvious that the pore structure of silica xerogel can be modified by addition of lignin and that the silicalignin xerogels with various pore structures can be prepared by changing the lignin ratio. Acknowledgment. The donation of lignin from Professor A. P. Watkinson (University of British Columbia) is greatly appreciated. LA970072C (10) Keefer, K. K. In Better Ceramics through Chemistry; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; North Holland: New York, 1984; p 15.
