TS-2 SYNTHESIS FROM WETNESS-IMPREGNATED SIO2-TIO2 XEROGELS

TS-2 has been synthesized from SiO2-TiO2 xerogels by wetness impregnat ion with TBAOH solutions and subsequent autoclaving at 170 degrees 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 advanta ges 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 th e samples obtained by i.r., DR u.v.-vis, and n-hexane oxyfunctionaliza tion measurements confirms that the Ti species are located in tetrahed ral positions of the MEL framework, whereas extraframework Ti is not d etected. t.g., n-hexane and N-2 adsorption, and TEM measurements revea l the presence of a relatively high external surface and a secondary m esoporosity in these samples. This mesoporosity is originated by the a ssembly of small crystallites (0.03 mu m width) to yield zeolite macro particles (1 mu m). TS-2 prepared by wetness impregnation exhibits cat alytic properties for the oxyfunctionalization of n-hexane with H2O2 s imilar to those of the conventional TS-2. The TS-2 crystallization und er these conditions takes place through a nonconventional mechanism go verned mainly by solid-solid transformations, leading to a reordering of the amorphous starting solid into the TS-2 crystalline structure. ( C) Elsevier Science Inc. 1997.