Kinetics of Crystal Growth in Synthesis of Silicalite-1

Previous studies have shown that (a) silicalite-1 or pure silica MFI zeolite containing tetrapropylammonium (TPA) ions can be synthesized from an apparently clear solution - 9.00TPAOH-25.0SiO2-480H2O-100.0EtOH; (b) pre-assembled primary particles (PPs) of about 3 nm diameter possessing the structural and compositional features of the large TPA-MFI crystals are already formed in the initial solution; and (c) upon heating, the nucleation and crystal growth proceed by an addition mechanism of the PPs. Our in situ calorimetric investigation reveal that the crystal growth at 95 C is associated with a first exothermic event and followed an endothermic event. The abrupt thermal switch coincides with a sharp rise in pH (12.6 to 13.3). The change from exothermic to endothermic is rather intriguing because it implies a strong entropy driving force, which may arise from the release of small molecular species (like H2O and/or OH-) from the eliminating interface to the liquid phase. During the exothermic period, the crystal yield is linearly proportional to the integral enthalpy. The size of the crystal increases linearly with crystallization time. The pH of the synthesis solution remains unchanged (~ 12.6). Thus the rate of the linear crystal growth during the exothermic period can be directly derived from the calorimetric curve. In this study, in situ calorimetric syntheses at various temperatures (75 -100 C) are examined. The activation energy for crystal growth is derived based on the calorimetric data. The results and their implication for zeolite synthesis mechanisms are further discussed.