| dc.description.abstract | Separations of light olefin/paraffin, e.g., ethylene/ethane and propylene/propane, are crucial in the petrochemical industries. However, conventional separation processes, e.g., cryo-distillation, require tremendous amounts of energy and high capital cost. Membrane-based separations are a promising alternative to conventional separation processing, however, because of the limited separation capability of current membrane materials, such as polymers, the development of advanced membrane materials is highly desirable.
Zeolitic-imidazolate frameworks (ZIFs) are nanoporous crystalline materials with well-defined ultramicropores (< 5 Å), thereby attractive for membrane-based gas separations. A prototypical ZIF-8 have shown unprecedented propylene/propane separation capabilities because of its effective aperture size of ca. 4 Å, which is in between the sizes of propylene and propane. However, some challenges are impeding their commercial applications; slow batch processes, expensive and fragile ceramic supports, long-term stability, lack of general processing methods. A significant improvement in the productivity (i.e., throughput) of polycrystalline ZIF-8 membranes is required to overcome the current challenges of membrane processing. Therefore, the membrane productivity can be enhanced by increasing membrane area and substantially reducing membrane thickness.
In this dissertation, advanced membrane processing techniques toward high flux ZIF-8 membranes for their practical propylene/propane separation are proposed. Two unique membrane fabrication methods, in situ counter diffusion method and microwave-seeding and secondary growth method, previously developed from our group were chosen and thoroughly studied in the overall study. Firstly, the separation performances of ZIF-8 membranes depend on synthesis method, for propylene/propane separation, are studied. Defect-density of ZIF-8 powders and membranes were characterized with a series of characterizations. Post-synthetic treatments effectively stabilized less stable membranes. Secondly, high propylene-throughput ZIF-8 membrane was prepared by a novel approach for propylene/propane separation. Propylene-selective layer thickness was systematically reduced via post-synthetic linker exchange of ZIF-8 membranes. The resulting linker-exchanged ZIF-8 membranes exhibited significant increase in propylene permeance by four times. Lastly, scalable ZIF-8 membrane processing method on polymeric hollow fiber substrates is introduced. The unique microwave-assisted seeding prepared was capable of ultrathin ZIF-8 membranes formation. Propylene/propane separation performance of resulting ZIF-8 membranes was performed. | en |
