Synthesis of titanium silicalite-1 using amorphous SiO2 as silicon source

Titanium-containing silicalite-1 (TS-1) has been synthesized with amorphous silica as silicon source. Two routes have been followed which differ not only in the recipe itself but also in the nature of the titanium alkoxide. Both routes led to the formation of large TS-1 crystals, contrasting with the very small particles usually obtained with tetraethyl orthosilicate. The maximum titanium incorporation did not depend on the synthesis method and was about 2 Ti/unit cell. When samples were synthesized with high Ti concentrations in the gel, the excess of unreacted titanium remained in the solution, and extralattice oxide species were never observed. For comparison, the same experiments performed with colloidal silica led to very irregular and inhomogeneous batches. The crystal size could be decreased significantly by stirring the gel or preceding the crystallization by an aging period at room temperature. In that case, pure TS-1 crystals of about 0.8 mu m in size could be obtained after a 3-h crystallization. Critical parameters like the alkalinity or the water content greatly influenced the crystallization process and thus, the morphology of TS-1 crystals. Relatively small crystals could be obtained with concentrated gels, the dilution leading to a considerable increase in the particle size.