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Zeolite nanocrystals and hierarchical porous structures: synthesis, characterization and formation mechanism
thesisposted on 22.03.2017, 01:26 by Huang, Yi
Significant effort has been made to synthesize zeolite nanocrystals with controllable crystal sizes and high yield. A number of synthetic strategies involving optimization of hydrothermal conditions and/or composition of synthesis solutions (or gels), and phase transformation have been developed so far. However, controlled synthesis of zeolite nanocrystals still remains one of the difficult tasks in many systems due to their very complex crystallization mechanisms. Especially, in a zeolite synthesis system (clear solution or gel) without using a structure-directing agent (SDA), only a limited success has been achieved in controlling crystal sizes. In this dissertation, a novel synthesis method combining space confined synthesis and templating approach has been developed to prepare zeolite nanocrystals with controlled particle size distributions. This synthetic strategy involves the following steps: selection of colloidal silica nanoparticles; formation of silica-mesoporous carbon composite by in situ polymerization of furfuryl alcohol (FA) in the presence of triblock copolymer-Pluronic P123 and colloidal silica nanoparticles and subsequent carbonization; confined space conversion of silica nanoparticles into zeolite nanocrystals; and sample collection via carbon burn-off. This novel synthesis method allows one to control nanocrystal size and size distribution simply by choosing colloidal silica nanoparticles with a desired particle size distribution. Zeolites with hierarchical porous structures and designed shapes are in high demand for new technological as well as traditional applications (for instance as catalysts or highly selective adsorbents). In this study, hierarchical porous particles (150-600 nm) aggregated by primary NaY zeolite nanocrystals (20-80 nm) have been synthesized without using a secondary template via a newly developed three-stage temperature controlled strategy. The mesoporosity of synthesized particles, possessing a globular crystal-like morphology, can be tuned in the range of 30.8-57.6% by simply varying water contents in the synthesis gels. The mechanistic study of zeolite NaY growth in this organic-free system suggested that the formation mechanism that governed the crystallization process was similar to that reported by Valtchev and Bozhilov, i.e., fast spontaneous aggregation of nanoparticles around a crystallization center, followed by crystallization and agglomeration of larger crystals around these centers. This approach to the preparation of hierarchical porous zeolite particles can be applied to a number of zeolite materials. Core-shell/hollow zeolite NaP particles and macroporous NaP monoliths with designed shapes (such as cylinder, rectangular-prism and donut shapes) have been fabricated by combing the ceramic gel-casting technique and in situ vapour phase transport (VPT) method. It has been found that the employment of aged zeolite gel as the starting material is a prerequisite for the successful formation of NaP zeolite structures. However, by incorporating different amounts of silica nanoparticles in the gel-casting process, one can control the mechanical strength and Si/Al ratio of the resulting zeolitic monoliths. Dispersible and individual hollow NaP zeolite particles with relatively lower Si/Al ratios have been produced in the absence of colloidal silica. Careful analyses of the samples collected after a series of crystallization times revealed that the growth of the core-shell/hollow NaP structures possibly followed a surface-to-core formation process. The fabrication of macroporous zeolite monoliths with functional magnetic Fe3O4 has been presented, demonstrating the feasibility of this synthetic approach for zeolite functionalization. It is well known that the addition of organic additives has an effect on zeolite nucleation and crystallization. The role of ethanol in sodalite crystallization was systematically studied in 3.09Na2O: 1.00Al2O3: 1.08SiO2: 45.5H2O: nC2H6O (nethanol = 0, 3.6, 7.2, 14.5, 29 and 43.5) systems. It has been found that the introduction of ethanol into the gel system significantly facilitated phase transformation from LTA to a denser SOD phase. Moreover, the increase in the amount of ethanol in gel systems has led to the morphological evolution of as-synthesized sodalites from clusters of micro-sized discs to nanoplates assembled thread-ball-like particles, and then to globular polyhedral nanocrystals aggregated core-shell/hollow structures. Importantly, in the system with high ethanol contents (nethanol ≥ 14.5), where core-shell/hollow sodalites were formed, extraordinary phenomena including multiple surface nucleation and crystallization followed by surface-to-core growth, have been observed. Supported by experimental evidence obtained in this work, surface nucleation and crystallization is probably a common process, in particular when an appropriate amount of alcohol is involved in zeolite synthesis.