Synthesis of silicalite-1 from organo-silicic gels

Well crystallized silicalite-1 has been obtained from three sources of amorphous silica, namely, rice hull ashes, commercial Davisil, and a fume silica from Aldrich. The silicas were first dissolved in glycerol according to a recently described reaction. This reaction transforms rapidly and efficiently large surface area silicates into poly-alkoxide gels. It can be schematized as an etherification of an alcohol function of glycerol by the weakly acid surface silanol groups. The facile hydrolysis of the alkoxide permits the preparation of relatively pure and reactive silica, keeping the mesoporous character of the parent starting material. We insist on the mesoporous character of the solids obtained upon hydrolyzing the organo-silicic gel because we believe the gel plays a role of template in the secondary synthesis of mesoporous structures. The hydrolysis is carried out in presence of a structure directing agent, namely tetra-propylammonium hydroxide, TPAOH. After aging, the residue is dried and calcined. The first advantage of using the organo-silicic gel is probably related to the high degree of depolymerization of silica, witness by the C/Si ratio. The second one, more subtle to define, is to provide an intermediate silica with hydrophilic a hydrophobic regions, interfering differently with the surfactant. After calcination at 500 °C, well crystallized silicalite-1 is obtained. The texture of the starting silica influences the textural characteristics of the final silicalite-1. Left: silicalite-1 structure. Center: correlation between pore volume of silicalite-1 and pore volume of the starting silica. Right: large crystal of silicalite-1 obtained from rice hull ash (SLR).