TS-2 synthesis from wetness-impregnated SiO 2-TiO 2 xerogels

Abstract

TS-2 has been synthesized from SiO2-TiO2 xerogels by wetness impregnation with TBAOH solutions and subsequent autoclaving at 170°C. The preparation of the raw materials through sol-gel methods leads to a stabilization of the Ti atoms by dispersion in isolated positions of the amorphous SiO2 network, which prevents the formation of undesired TiO2 phases during both gel and zeolite synthesis. Additional advantages of this method compared to the conventional TS-2 preparations are the use of lower amounts of TBAOH and smaller reactor volumes as well as the higher zeolite yields that are obtained. Characterization of the samples obtained by i.r., DR u.v.-vis, and n-hexane oxyfunctionalization measurements confirms that the Ti species are located in tetrahedral positions of the MEL framework, whereas extraframework Ti is not detected, t.g., n-hexane and N2 adsorption, and TEM measurements reveal the presence of a relatively high external surface and a secondary mesoporosity in these samples. This mesoporosity is originated by the assembly of small crystallites (0.03 μm width) to yield zeolite macroparticles (1 μm). TS-2 prepared by wetness impregnation exhibits catalytic properties for the oxyfunctionalization of n-hexane with H2O2 similar to those of the conventional TS-2. The TS-2 crystallization under these conditions takes place through a nonconventional mechanism governed mainly by solid-solid transformations, leading to a reordering of the amorphous starting solid into the TS-2 crystalline structure.