The nucleation period for crystallization of colloidal TPA-silicalite-1 with varying silica source

The effect of varying silica source on the nucleation and crystallization o f TPA-silicalite-1 was investigated. A direct experimental method, involvin g a two-stage varying-temperature synthesis, was used to determine the nucl eation period for colloidal crystals of TPA-silicalite-1 with different sil ica sources, including tetraethoxysilane (TEOS) and amorphous silica (Ludox TM and Ludox LS). For syntheses performed at 60 degreesC with TEOS as sili ca source, the duration of the nucleation was about 72 h, and a very rapid increase in the crystal population occurred during the initial crystallizat ion time. However, with the amorphous silica sources (Ludox TM or Ludox LS) , the duration of the nucleation period was extended to about 120 h, and th e nucleation profile consisted of a self-accelerating nucleation rate at th e beginning of the nucleation period. The two-stage synthesis method could be used to determine the nucleation profile for the various silica sources. However, this technique overestimated the crystal concentration at the ear liest stage of nucleation with amorphous silica. The use of amorphous silic a gave rise to a broader crystal size distribution compared to that of TEOS . However, it was found that for both TEOS and amorphous silica the vast ma jority of the nucleation occurred during an induction period when little or no crystal growth was observed. In addition, Raman spectroscopy revealed s tructural differences between Ludox TM and Ludox LS which may account for d ifferences in the nucleation processes observed for these two amorphous sil icas. (C) 2000 Elsevier Science B.V. All rights reserved.