Si-MFI crystallization using a "dimer" and "trimer" of TPA studied with small-angle X-ray scattering

The formation and consumption of precursors and the crystallization of Si-MFI using bis(tripropylammonium) hexamethylene dihydroxide ("dimer" of tetrapropylammonium cation, TPA) and bis(tripropylammonium-N-N'-hexamethylene)-N ",N "-dipropylammonium trihydroxide ("trimer" of TPA) as structure-directing agents have been investigated in situ using simultaneous, time-resolved, SAXS and WAXS and using USAXS. The formation of 2.8-nm-sized primary units is observed upon dissolution of the silica source, which is in agreement with results from earlier studies on systems with the TPA cation as a structure-directing agent. Aggregation of these nanometer-scale primary units to 10-15-nm-sized particles is found to be an essential step in nucleation: of the zeolite. Crystal growth occurs via the addition of the primary units to the growing crystal. Although the size of the primary units for MFI is independent of the structure-directing agent used, the organic species does have a pronounced influence on the crystal growth step and, therefore, on the crystal growth rate, size, and morphology. The results presented here confirm a common mechanism proposed for organic-mediated crystallization of high-silica zeolites.