Synthesis of zeolite nanocrystals and their application for mixed matrix membranes
thesisposted on 15.01.2017, 23:09 by Li, Dan
Zeolites are a class of microporous solids with well-defined crystalline structures. Due to their ability to distinguish molecules on the basis of size and shape, zeolites are often referred to as molecular sieves. There has been considerable interest in the synthesis of zeolite nanocrystals (or nanozeolites), because they can serve as a model system for fundamental understanding of zeolite nucleation and growth mechanisms, as seeds for secondary growth of zeolite films and membranes, as building blocks for construction of hierarchical porous nanostructures, and for preparation of mixed matrix membranes (MMMs). Presently, in the synthesis of zeolite crystals, it is well-known that the addition of organic additives or polymers has an effect on the zeolite nucleation and crystallization. Despite several synthetic strategies of using some polymers (e.g. polyacrylamide and methylcellulose) have been developed to grow zeolites, there has been no research on the synthesis with crosslinked chitosan hydrogels or uncrosslinked chitosan polymers, which is one primary goal of this thesis. Therefore, chitosan (crosslinked or uncrosslinked polymers) was introduced into zeolite crystallization process. The zeolite crystal sizes were significantly affected by formulation of silica-containing crosslinked hydrogels and alkaline solution, and by aging and heating conditions. Importantly, a novel method of using hydrogen peroxide solution was developed to remove crosslinked hydrogels after zeolite synthesis, which was considered as an effective way for removal of hydrogels. The resultant zeolite nanocrystals were readily redispersed in deionized water and some other solvents, and therefore they may be useful for some applications, e.g. in the fabrication of zeolite-polymer mixed matrix membranes (MMMs) and hierarchical porous zeolitic structures. In this thesis, the effect of uncrosslinked chitosan hydrogels on zeolite nucleation and crystallization was also studied. Cubic zeolite with a single crystalline shell and an amorphous core was prepared for the first time by in-situ crystallization of sodium aluminosilicate gel inside the chitosan polymer networks. The TEM characterization further revealed that this formation process of cube-like or rectangular core-shell structures involved particle aggregation and surface-to-core crystallization induced by chitosan networks. It is expected that this work would provide a new model system for understanding and studying complex zeolite nucleation and growth mechanisms. To date, the research into efficient separation of hydrogen has been driven by its potential as an essential component of future energy economies. Despite some materials emerging for this purpose, it is believed that there should be plenty of room to develop mixed matrix membranes (MMMs) with an addition of inorganic particles, such as zeolites, for hydrogen separation or purification. In order to reduce the phase separation between organic and inorganic phases, organic functionalization is suggested as an effective way, which has been applied in my study. Sodalite, whose framework consists of a six-membered ring aperture, was selected as inorganic fillers in MMMs. To functionalize sodalite nanocrystals, organic functional groups were successfully attached to sodalite nanocrystals by the newly developed method – the direct transformation of organic-functionalized silicalite nanocrystals. Organic-functionalized sodalite nanocrystals were incorporated into polyimide membranes to form sodalite MMMs for hydrogen separation. Characterization by SEM showed that zeolite can be well distributed with polyimide phase, as confirmed by the FTIR spectroscopy and XRD results. TG results revealed the temperatures for corresponding major mass loss increased with the increasing inorganic content of MMMs. This was attributed to the interaction between the amino moieties from inorganic nanoparticles and polymer matrix, which restricted the movement of the main chains. The gas permeation results exhibited the significantly improved hydrogen separation property.