Theoretical and experimental studies of zeolite nanocrystal growth
The objective of this dissertation is to elucidate the mechanisms of zeolite nucleation and crystal growth under different conditions. Our motivation stems from the recent progress in the synthesis of highly permselective zeolite membranes and the desire to control membrane microstructure and performance. Zeolite synthesis through the formation of a gel phase is examined first, followed by a detailed study of silicalite-1 growth from apparent clear solution. Finally, the synthesis of a faujasite membrane and its permselective behavior are presented.
The experimentally observed maximum in the nucleation rate under constant supersaturation has been explained by treating gel dissolution and nucleation as interfacial phenomena. The possibility of a gel-to-zeolite transformation through association and rearrangements of extended precursors has been also investigated using continuum-time Monte Carlo simulations. This model has been used to study the morphology of zeolite L nanocrystals synthesized from an initial amorphous microporous gel.
The seeded growth of silicalite-1 from apparent clear solutions has been also examined by a combination of experimental and modeling techniques. Changes of the seed size have been monitored by in-situ Dynamic Light Scattering (DLS) experiments. The DLS results combined with other experimental observations from the literature indicate several possible growth precursors. By applying a diffusion model and allowing interparticle interactions verified by atomic force microscopy, it has been possible to explain silicalite growth with the rate-limiting step being the addition of subcolloidal particles with a diameter of ∼3 nm.
Electrophoretic mobility studies on silicalite-1 nanoparticles reveal information about the charging behavior of a crystal surface. Such measurements were performed over a wide range of pH and ionic strength. The results, analyzed using three different surface complexation models, indicate adsorption of tetrapropylammonium cation (TPA) on the crystal surface. These models in conjunction with the microscopic structure of the silicalite-1 crystallographic planes can qualitatively predict the anisotropic growth of silicalite-1 crystals.
Finally, the preparation and characterization of faujasite membranes is presented. The membranes are able of separating saturated from unsaturated hydrocarbon mixtures. Benzene/cyclohexane permeation data have been analyzed with the Stephan/Maxwell formulation, and their separation has been attributed to differences in the strength of adsorption of the two molecules.