Preparation of Silica-Lignin Xerogel Jun’ichi Hayashi,* Tetsuo Shoji, Yuuki Watada, and Katsuhiko Muroyama Department of Chemical Engineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564, Japan 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 hy- drophilic due to silanol function groups. Silica xerogel can be prepared from tetraethoxysilane (TEOS) or tetra- methoxysilane (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. 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 method 8 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 mono- lithic 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 middle- pressure 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 silica- lignin 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. * To whom correspondence should be addressed. Fax: +81-6- 388-8869. 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. Non- Cryst. 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. (8) Dollimore, D.; Heal, G. R. J. Appl. Chem. 1964, 14, 109. (9) Mc Bain, J. W. J. Am. Chem. Soc. 1935, 57, 699. Table 1. Surface Areas of Silica Xerogel and Silica-Lignin Xerogels lignin weight ratio (lignin/Si) surface area (m 2 /g) 0 605.1 0.002 772.8 0.01 1074 0.1 900.3 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). 4185 Langmuir 1997, 13, 4185-4186 S0743-7463(97)00072-3 CCC: $14.00 © 1997 American Chemical Society