TPA-silicalite crystallization from homogeneous solution: Kinetics and mechanism of nucleation and growth

The formation and growth of tetrapropylammonium-templated crystal nuclei of silicalite from a clear homogeneous solution was recorded in situ as a function of time at temperatures between 90 and 115 degrees C using small-angle X-ray scattering (SAXS), The kinetics of the nucleation process was studied and give an apparent activation energy of 70 kJ mol(-1). A small-angle neutron scattering (SANS) contrast variation study confirms that the nuclei contain the tetrapropylammonium template in the expected stoichiometry of the completed crystal, Infrared spectroscopy provides evidence that the silicalite framework structure is present in the nuclei. Our SAXS and SANS data both show that a cylindrical form factor gave the best fit to the measured scattering functions for the particles that developed from nucleation to the end of the induction period, Dynamic light scattering measurements were used to follow the crystal growth in the 100-6000 Angstrom size domain, beyond the range of the SAXS measurements. Scanning electron microscopy was also used to determine crystal morphology and particle sizes of sedimented crystals and freeze-dried solutions. We have proposed a detailed model for the nucleation and crystallization processes wherein cylindrical primary nuclei, 2 x 2 unit cells in Gross section, form very quickly upon heating and then assemble end-to-end along the crystal c axis into 330 Angstrom long primary crystallites during an extended induction period, followed by the aggregation of the primary crystallites into polycrystalline ellipsoids of length 6000 Angstrom.