Evidence of solid-solid transformations during the TS-1 crystallization from amorphous wetness impregnated SiO 2TiO 2 xerogels

Abstract

The crystallization mechanism of TS-1 has been investigated when this Ti-containing material is obtained from amorphous SiO 2TiO 2 solids impregnated with TPAOH solutions followed by thermal treatment under autogenous pressure. Samples with different degrees of crystallinity have been prepared and characterized by elemental analysis, X-ray diffraction (XRD), infrared (IR) and diffuse reflectance ultraviolet visible (DR UV-Vis) spectroscopies, scanning and transmission electron microscopy (SEM and TEM), n-hexane adsorption and catalytic tests of n-hexane oxyfunctionalization. In contrast to the classical mechanism proposed to describe the zeolite synthesis through a mediated solution process, in the system studied here the TS-1 crystallization is mainly governed by solid-solid transformations, and a true crystal growth step is not observed. The major role of the solution is to provide the TPA + species necessary to complete the solid crystallization and to favour the migration of the secondary particles. During the first stages of the synthesis, a small amount of the raw material is dissolved yielding soluble Ti-rich species ( Si Ti = 5 ), which are no longer involved in the crystallization. Simultaneously, the starting polymeric SiO 2TiO 2 cogel is converted into a particulate amorphous material formed by a tight packing of primary particles with sizes around 50 nm. In a subsequent step, the latter are aggregated leading to secondary particles that become independent of the cogel as they reach a critical diameter of 0.5–0.7 μm. Once individualized, the secondary particles undergo a zeolitization process, being gradually transformed into TS-1 crystals of approximately the same size. In addition, the coordination of Ti sites become tetrahedral as the crystallization progresses due to the higher connectivity between Ti and Si atoms existing in the TS-1 network compared to the raw amorphous xerogel.