Micrqporous Silica Gels from Alkylsilicate-Water Two Phase Hydrolysis
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INTRODUCTION Nano-particle production using sol-gel processing represents an attractive approach for fabricating ceramic materials possessing a pore size close to the molecular level ( about 2 nm in diameter) [1-7]. These type of materials find applications in gas-phase separation membranes, adsorbents, catalysts or catalyst supports. In the particulate sol-gel process, the diameter of the resulting pores in the xerogels and resulting ceramics is a function of the primary particle size in the precursor sols, the configuration and the packing geometry of the particles, and the thermal history of the sample during subsequent sintering steps. The ideal pore structure can be obtained only when one controls all steps in this process. It is generally accepted that in the sol-gel process for silica synthesis from alkyl silicates, spherical colloids are formed under base catalyzed systems having higher water:alkyl silicate ratios [8,9]. Mono dispersed silica particles in the colloidal range made from alkyl silicates have been produced by Stbber et al [10]. A two step hydrolysis process was also reported in preparing silica gels [111. The gels obtained from acid and base catalyzed silica polymer species were found to differ. This is caused by differences in the hydrolysis and condensation rates. Regardless of whether one chooses the polymeric or particulate route, both routes traditionally have the common attribute that hydrolysis and condensation reactions are carried out under homogeneous conditions[ 10- 14]. All alkyl silicates are immiscible with water. Therefore, alcohol is typically used as a co-solvent to homogenize the two phase mixture. However, two exceptions to this situation exist in the literature. Schmidt and coworkers have manufactured silica coatings at the Si(0C 2 H 5 )4 -H 2 0 phase boundary on a porous support [ 15]. Additionally, in a glass synthesis experiment, Avnir et al have noticed that an alcohol co-solvent was not necessary in their Si(OCH 3 )4 -H 2 0 system [16]. They noted that sufficient methanol was released from the hydrolysis to form a homogeneous sol. In this study, we attempt to prepare microporous silica gels through a new aqueous particulate sot route. In this case, the sol-gel process is started from the immiscible state. No alcohol is added. The hydrolysis reaction is carried out at the interfacial boundary of alkyl silicate droplets in water. In this system, rapid hydrolysis generates an oversaturated solution of silicic acid or ionized monomer species in aqueous solution. The current study seeks to investigate the 855 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
parameters which influence the size of the primary particles produced in this alkylsilicate:water two phase reaction system. In the final analysis, the results are consistent with classic nucleation theory. EXPERIMENTAL The reagents used in this work include tetramethyl orthosilicate (TMOS), tetraethyl orthosilicate (TEOS), and tetrapropyl orthosilicate (TPOS), all 98%; ammonia (30%) and nitric acid
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