High Surface-Area Silica with Controlled Pore Size Prepared from Nanocomposite of Silica and Citric Acid

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 m 2 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.