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.