Nanometer scale precursors in the crystallization of Si-TPA-MFI

The crystallization of Si-TPA-MFI from a clear synthesis mixture has been studied in situ using X-ray scattering. Utilizing a combination of scattering techniques and high brilliance synchrotron X-ray radiation, we were able to study a unique range of length scales (four decades), which covers the scattering from all species present in the synthesis mixture. Combined small- and wide-angle X-ray scattering (SAXS-WAXS) results show that for cases with relatively high alkalinity the crystallization occurs while only 2.5 nm sized particles are present in the solution. In case of a lower alkalinity, additional 10 nm sized precursors are present, which act as a gel phase. These 10 nm sized particles probably play a role in the nucleation process, although their presence is not indispensable, as is shown by their absence in the high alkalinity synthesis. Applying in situ ultra-small-angle X-ray scattering (USAXS), the size of the crystals could be monitored, and when their linear growth has finished, aggregation of the discrete crystals to structures larger than 6 mu m was found. (C) 1998 Elsevier Science B.V. All rights reserved