Doctor of Philosophy
Dr. Yining Huang
Molecular sieves have been used extensively in industrial fields. A better understanding of their crystallization processes is very useful for controlling the structural properties in order to meet the industrial needs. This thesis focuses on examining the crystallization of several catalytically important molecular sieves by dry gel conversion (DGC) method which includes steam-assisted conversion (SAC) and vapor-phase transport (VPT), or hydrothermal synthesis (HTS) method or a combination of both.
For SAPO-34 templated by DEA, it crystallizes similarly under SAC and VPT conditions. It forms from the transformation of a semi-crystalline layered phase. When morpholine was used as the template, the crystallization pathways vary under different conditions. For DGC with HF, SAPO-34 in both triclinic and trigonal phases crystallizes from a highly crystalline prephase held by covalent bonds. For DGC without HF, SAPO-34 in pure trigonal form crystallizes from an amorphous phase held by weak nonbonding interactions. For VPT without HF, SAPO-34 forms from the transformation of an amorphous material and a dense phase. Moreover, all SAPO-34 samples show good catalytic properties.
For SAPO-37, its crystallization under HTS and DGC conditions follows somewhat similar pathway. It involves the formation of a precursor initially composed of only large cavities or both large cavities and quasi-sodalite cages. The size of a large cavity equals two directly connected supercages. Sodalite cages are formed at a later stage when increasing the crystallization time. They connect with the large cavities then forming SAPO-37 framework.
For zeolite LSX, it crystallizes from an amorphous aluminosilicate phase under HTS conditions. For ETS-10, a similar amorphous phase containing structure-building units of ETS-10 is formed under HTS and DGC conditions. Upon heating, the amorphous phase transforms to ETS-10 via a series of structural reorganization. Partially hydrated sodium cations are a crucial factor for the crystallization of ETS-10. For ETS-4, our work shows that small ETS-4 crystallites are formed quickly under HTS conditions. The crystallites are composed of layers, the distance between which is the same as the d-spacing of the (001) plane of ETS-4. Further heating results the formation of large ETS-4 crystals.
Zhang, Lu, "Investigations of the Crystallization Processes of Molecular Sieves" (2013). Electronic Thesis and Dissertation Repository. 1457.