Silicalite-1 macrostructures - preparation and structural features

Zeolite macrostructures in the form of silicalite-1 spheres were prepared using anion exchange resin beads as shape directing macrotemplates. The resin was removed after the synthesis by combustion leaving solid spherical particles identical in shape and size to the original resin beads. The formation of the silicalite-1 spheres was studied by Raman spectroscopy, X-ray diffraction,, scanning electron microscopy and nitrogen adsorption measurements. Samples prepared for different times of treatment and at two different temperatures, 100 degrees C and 165 degrees C, were investigated. The spheres obtained for short treatment times consist of amorphous silica, which after longer hydrothermal treatment in the presence of TPA cations is partially or entirely transformed into an MFI structure. A single treatment at 100 degrees C resulted in hard silicalite-1 spheres of low crystallinity, whereas fully crystalline spheres could be obtained by a treatment at the higher temperature, However, the spheres prepared at 165 degrees C have inferior mechanical properties and can even loose their shape at certain conditions. A better control of both hardness and crystallinity was achieved by using a two-step synthesis procedure, where a treatment at 100 degrees C was followed by a treatment at 165 degrees C. (C) 2000 Elsevier Science B.V. All rights reserved.