SAXS/WAXS study on the formation of precursors and crystallization of silicalite

The formation of colloidal aggregates which can act as precursors in zeolite crystallization has been studied using small-angle scattering of X-rays (SAXS). In the same experiment the onset of crystallization and the increase of crystallinity were measured using wide-angle scattering of X-rays (WAXS). To perform in situ time-resolved combined SAXS/WAXS measurements high brilliance synchrotron radiation was applied. The influence of silica, template, and OH- concentrations on the synthesis of silicalite (MFI structure) from a clear solution was studied. The alkalinity of the synthesis mixture appeared to be decisive for the formation of colloidal aggregates. At relatively low alkalinity colloidal aggregates were formed prior to the onset of the crystallization. At higher alkalinities no colloidal aggregates could be observed at any time during the synthesis, suggesting that nucleation and crystal growth occurred directly from the solution.