NANOMETER-SCALE PRECURSORS IN THE CRYSTALLIZATION OF SI-TPA-MFI

The crystallization of Si-TPA-MFI from a clear synthesis mixture has b een 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 synthesi s mixture. Combined small- and wide-angle X-ray scattering (SAXS-WAXS) results show that for cases with relatively high alkalinity the cryst allization occurs while only 2.5 nm sized particles are present in the solution. In case of a lower alkalinity, additional 10 nm sized precu rsors are present, which act as a gel phase. These 10 nm sized particl es probably play a role in the nucleation process, although their pres ence is not indispensable, as is shown by their absence in the high al kalinity synthesis. Applying in situ ultra-small-angle X-ray scatterin g (USAXS), the size of the crystals could be monitored, and when their linear growth has finished, aggregation of the discrete crystals to s tructures larger than 6 mu m was found. (C) 1998 Elsevier Science B.V. All rights reserved.