Browsing by Subject "Faujasite"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Nucleation and Growth of Faujasite Zeolite Nanostructures(2015-12) Khaleel, MaryamZeolites 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.Item Synthesis of Faujasite (FAU) Zeolites Applied for Acid Catalysis(2022-08) Li, XinyuFaujasite (FAU) zeolites are extensively used in petrochemical refining with more than 50% of global gasoline supplies produced using fluid catalytic cracking, which primarily relies on FAU-based catalytic formulations. In this thesis, we accomplished control of Al siting and accessibility, manipulation/control of FAU crystal habit, and template-free synthesis of low Al content FAU materials. FAU materials circumscribe protons in two different void environments with potential for shape selectivity being greatest in sodalite cages (< 4.5 Å) that are inaccessible for reaction in pristine FAU materials, because these protons can only be accessed via six-membered ring openings of diameter < 3Å. Here, we prove that FAU zeolites (with Si/Al ratio of ca. 1.7) undergo mild dealumination at moderate ion exchange conditions (0.01 to 0.6 M of NH4NO3 solutions) resulting in protons circumscribed by sodalite cages becoming accessible for reaction without conspicuous changes to bulk crystallinity. Protons in sodalite cages show higher rate constants of propane dehydrogenation and cracking than protons in supercages plausibly due to confinement effects being more prominent in smaller voids. Low-silica FAU zeolites (namely zeolite X, Si/Al = 1~1.5) hitherto find very limited or no catalytic application as solid acids because of their limited stability under conditions required to introduce acid sites in these materials. We report synthetic protocols based on moderate ion exchange (0.01 M of NH4NO3 solution) that enable only Na+ circumscribed by large pore 12-membered ring supercages to be removed in low-silica FAU materials but allows occluded alkali cations within small-pore sodalite cages to remain unperturbed, and the latter stabilize the framework of low-silica FAU materials for H+-exchange. We report that H+-containing zeolite X samples catalyze protolytic reactions at temperatures ~800 K enabling their application as solid acid catalysts. In order to explore new recipes of FAU zeolites prepared via organic-free routes, we demonstrate the use of Machine Learning to organize and extend procedures for crystallizing zeolites to accomplish desirable structural characteristics and achieve improved catalytic activity. Through ML, synthesis conditions were identified to enhance the Si/Al ratio of high purity FAU zeolite to the hitherto highest level (i.e., Si/Al = 3.5) achieved via direct (not seeded), and organic structure-directing-agent-free synthesis from sodium aluminosilicate sols. The analysis of the ML algorithms’ results offers the insight that reduced Na2O content is key to formulating FAU materials with high Si/Al ratio. An acid catalyst prepared by partial ion exchange of the new high-Si/Al-ratio FAU (Si/Al = 3.5) exhibits improved proton reactivity in propane cracking and dehydrogenation compared to the catalyst prepared from the previously reported highest Si/Al ratio (Si/Al = 2.8).