Microporous Alumino-Silicate Gels Synthesized by Sol-Gel Method
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ABSTRACT Microporous alumino-silicate gels were prepared by the sol-gel method. The Al cations were incorporated into these systems by mixing an aluminum nitrate solution into silica water based sols under controlled pH conditions. The micro structure as well as the form of the products are influenced by the amount of Al additives. Al cations have significant effects on the properties of silica sols and gels. The alumino-silicate gel has micro porosity with surface area as high as 300 m2/g even after calcining at 900 oC for one hour. Strong negative charge is found on these alumino-silicate surfaces in aqueous solution indicating the Al cations have been substituted isomorphically into the Si sites in the framework of silica.
INTRODUCTION Alumino-silicates have several attractive material properties. They are stable at high temperatures, they resist corrosion and they are mechanically tough. In addition, natural or synthesized zeolites and some amorphous alumino-silicates have a porous structure which is molecular level in size, uniform in size distribution and high in surface area. This micro structure makes them useful in the area of adsorption, separation of gases and in catalysis [1-3]. In addition, different compositions of these materials result in a variety of surface structures and properties. These interesting properties can be tailored according to the potential application. Solgel synthetic routes have been recognized as effective methods for synthesizing varied aluminosilicate materials [3-61. For this reason the sol-gel process can be used to produce thin films of
these materials, and this method is recognized as a useful technique in fabricating ceramic membranes. In this later case, one can more effectively utilize the controlled pore structure in the applications of separation. In this paper, we report on the synthesis of alumino-silicate materials in which we have incorporated Al ions into the silica particle surface. First, we take advantage of the fact that silica gels having pore diameters smaller than 10 A can be synthesized by the sol-gel method. This pore size does not increase during heating. Second, the freshly generated surface of amorphous silica particles in the sol or xerogel state contains highly reactive silanol groups. This allows us to modify this silica by adsorbing Al cationic species on its surface. The adsorption of cations, or ion exchange in oxide systems has long been used as a technique in the field of catalyst preparation [7-11]. By incorporating Al cations as a surface component we can improve significantly on the acidity of amorphous silica. This is useful in cracking processes [12]. We believe that this technique is even more effective when used to modify freshly prepared, nano sized silica systems.
EXPERIMENTAL The alumino-silicate gels were prepared by introducing Al(N0 3 )3 solution into aqueous silica sols. The silica sols used in this work were prepared from tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS) (both used as received from Aldrich C
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