Studies On Zeolite Formation Mechanism And Preparation And Application Of Porous Coatings

In the past half century, zeolites have played increasingly important roles as catalysts in petroleum refining, petrochemical and other chemical industries. Currently, there are 194 known zeolite frameworks, however, only a few of them have been widely used in industry. More and more zeolites with novel structures and compositions were highly desired not only in the traditional fields of catalysis, adsorption and ion-exchange, but also in the newly developed fields such as petroleum refining and intermediacy chemistry process. However, the elusive formation mechanism of zeolite limits the discovery of novel zeolites. Studies on the crystallization process and formation mechanism of zeolites are very important not only because of their theoretical significance but also due to practical values. Extensive efforts have been made to elucidate the crystallization mechanism of zeolites since 1950s. Several mechanisms were proposed based on the observations from specific synthesis conditions. However, no general mechanism was obtained to describe the formation of zeolites due to the complexity of hydrothermal chemical reactions. In this dissertation, zeolites MFI and FAU were selected as models to study the crystallization mechanism of zeolites.Room-temperature aging of zeolite precursor silica sol (with and without Al) was followed by SAXS and cryo-TEM. The results from all-silica system provide evidences supporting the recently proposed mechanism of evolution of nanoparticles followed by aggregative crystal growth while adding a new element. The new element, not included in the previously proposed model, is the formation of predominantly amorphous aggregates before MFI crystallization and points to the importance of intra-aggregate rearrangements in nucleation and growth. For the aluminosilicate clear sol, smaller nanoparticles were found in the initial clear sol. The particle size was slightly increasing with time but no larger particle was indentified throughout nearly 300-day aging process. The aluminosilicate nanoparticles have better colloidal stability compared with all-silica ones.The early stage of nucleation and crystal growth of FAU was investigated by using combined in situ/ex situ techniques such as Small Angle X-ray Scattering (SAXS), Nuclear Magnetic Resonance (NMR) and UV Raman Spectroscopy. The results show that ca. 2 nm particles exist in the clear solution and the composition and structure of those nanoparticles were evolved with time during the room-temperature aging. The crystal growth curve indicates that a large amount of nuclei exist in the 24-hour aged clear sol, while no change of the particle size was observed during the early 30-hour-aging period. In situ UV Raman and liquid 29Si NMR spectra show that only low-polymerized species exist in the liquid phase and those species do not change within the 4-day-aging process. From the FTIR spectra, we found that broaden bands at the characteristic regions do not show much structural changes with various aging times. All the aluminium atoms in the solid phase are 4-coordinated. The increasing fraction of Si(4Al) in the solid 29Si NMR spectra indicates that the solid phase was getting ordered and there was a chemical composition change during the aging period. The changes of the Si/Al ratio were confirmed by ICP elemental analysis. Based on the studies on both liquid phase and solid phase, we can conclude that i) nucleation happens in solid phase or solid-liquid interface; ii) the chemical evolution catalyzed by OH– anion makes the composition and structure of solid phase similar to that of zeolite; iii) aluminium is enriched onto the nanoparticles and the stability of nanoparticles decreases with time; iv) the aluminium enriched spots in the solid phase might be the starting points of nucleation.Besides the research on the fundamental formation mechanism, studies on preparation and applications of porous coatings are also main objects of this dissertation. Porous coatings have attracted much attention due to their usefulness as supporting media in tissue engineering, membranes in separation process, templates for inorganic growth, dielectric materials for electronic devices, and optical materials.A silver-ions exchanged zeolite (type LTA) coating on Ti alloy surface was successfully prepared for the first time. In vitro antimicrobial assessments indicates that the Ag-zeolite coating provides efficient antibacterial effect to inhibit the proliferation of bacteria both in suspension surrounding the materials and on the surfaces of the materials. Zeolite exhibits many advantages such as good biocompatibility, mechanical stability, easy manufacture and low cost when used as a Ag+-host surface coating material. Since Ag+ ions can be stoichiometrically exchanged into zeolite, the antibacterial activity and cytotoxicity can be finely tuned by optimizing the loading amount of Ag+ ions. These advantages may potentially benefit the application of this antibacterial coating in orthopedic implants.A benign and facile process for forming monodisperse, hollow porous silica shells of a novel cubic morphology is identified. Templated on monodisperse crystalline germania cores that can be removed via aqueous dissolution. The hollow shells and their core–shell precursors are amenable to assembly into highly porous, gap-free near-monolayer films through manual and evaporative assembly techniques.