Effect of the silica source on the formation of nanosized silicalite-1: an in situ dynamic light scattering study

The formation of nanosized MFI-type zeolites from precursor mixtures containing different silica sources, i.e., tetraethylorthosilicate (TEOS), colloidal silica (Ludox LS 30) and fumed silica (Cab-O-Sil) was studied. In situ dynamic light scattering (DLS) was used to investigate the evolution of the colloidal entities in the systems under investigation. The DLS data showed that the size of the primary species in the precursor mixtures is strongly dependent on the type of silica source employed. The precursor particles formed in the TEOS-containing solution aged for 30 h at room temperature are much smaller (similar to2 nm radius) than those prepared from Ludox LS 30 (50 nm) and Cab-O-Sil (similar to50 nm). After heating of the aged mixtures at 90 degreesC the amorphous precursor colloids are completely transformed into crystalline silicalite-1 after 10 h from the TEOS-containing mixture and 15 h when colloidal and fumed silica were used. The mean hydrodynamic radii of the silicalite-1 nanocrystals are 15, 25 and 50 nm for the samples synthesized with TEOS, Cab-O-Sil and Ludox LS 30, respectively. The size of the amorphous particles formed in the precursor mixture affect the ultimate size of the silicalite-1 nanocrystals synthesized with the different silica sources. The silicalite-1 crystallized from the three precursor mixtures was characterized by X-ray diffraction, scanning electron microscope, thermal analyses and N-2 adsorption measurements. (C) 2002 Elsevier Science Inc. All rights reserved.