Nucleation and Growth of Faujasite Zeolite Nanostructures

Abstract

Zeolites are porous materials with 3-dimensional crystalline frameworks made of silicon and aluminum atoms linked through oxygen atoms. Zeolite frameworks have cages or channels of molecular dimensions that give them superior properties in separation, adsorption, ion exchange, and catalytic applications. However, diffusion limitations of bulky molecules in the zeolite pores can lead to a reduction in activity, selectivity and catalyst lifetime. This can be alleviated by modifying the zeolite crystal morphology or size (reducing the diffusion path length) or by introducing larger pores to improve diffusion (hierarchical zeolites). However, most of the procedures reported to create hierarchical zeolites are not well understood, and so in many cases, the properties cannot be precisely controlled. Moreover, they mostly utilize expensive and unsafe additives and so cannot be commercialized. This dissertation focuses on developing a better understanding of the growth of hierarchical Faujasite zeolite morphologies (one of the most widely used zeolites in industry). This may allow the design and engineering of hierarchical zeolites from inorganic routes. In chapter 2, a structural study using transmission electron microscopy imaging and diffraction of house-of-card-like nanosheet assembly of Faujasite sheets was undertaken, and it was demonstrated that there is a direct link between polytypism and the repetitive branching mechanism leading to hierarchical structures. In chapter 3, the effects of synthesis conditions on the FAU/EMT content and the size of nanocrystals, formed from inorganic aluminosilicate sols, were investigated using high resolution transmission electron microscopy imaging and comparison of experimental X-ray diffraction patterns with simulations. Findings demonstrated that it is possible to combine the effects of pre- and post-nucleation sol composition to steer crystal size and crystal structure, respectively. With a better understanding of the evolution of sol structure and the nucleation of zeolites at the early stages, it may be possible to control particle size and shape, and the intergrowth of zeolite polymorphs in crystals. In chapter 4, further insight was acquired by cryogenic transmission electron microscopy and small angle X-ray scattering studies on representative precursor sols (aged and crystallized at ambient temperature). Results confirmed precursor nanoparticle evolution and aggregation, and emphasized the importance of solution phase composition at both pre- and post-nucleation stages of aggregative crystal growth.