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Development of zeolitic imidazolate framework composite membranes and adsorbents for gas separations
thesisposted on 2017-02-16, 02:47 authored by Li, Lunxi
Gas separations by selective transport through membranes and adsorbents are two of the fastest growing branches of separation technologies. However, both the existing materials and fabrication techniques are inadequate to fully exploit the application opportunities on industrial scale. Metal organic frameworks (MOFs) are a class of porous crystalline materials, consisting of metal ions linked together by organic bridging ligands. Zeolitic imidazolate frameworks (ZIFs), a subfamily of MOFs, with exceptional thermal and chemical stability, are very promising for gas separations. The basic building blocks of ZIFs are metal or metal clusters bridged through the nitrogen atom of imidazolate ligands or their derivatives. ZIFs have been widely studied for gas separations because of their regular pores, low density, and large surface area. In this thesis, ZIF-8 was synthesized as both membranes and adsorbents for gas separations. ZIF-8 membrane was developed to separate N2 from H2, because its porous structure allows access of guest molecules. A new seeding layer synthesis method was introduced to grow ZIF-8 membrane on a flat alumina substrate by hydrothermal synthesis. The seeding layer was rooted in the substrate by melting the organic linker, and then the membrane grew on the seeded substrate to form a continuous membrane. The membrane performance showed a H2/N2 selectivity of 5.7, which was over the Knudsen selectivity, indicating that a compact and continuous membrane was successfully fabricated by our method. ZIF-8 can also be used as an adsorbent material for CO2 sorption due to its large surface area. To make particles more easily collected and handled, ZIF-8 particles were incorporated into polysulfone (PS) sphere for gas adsorption. ZIF-8/PS composite spheres were fabricated in a one-step method via phase inversion spinning. CO2 sorption analysis indicated the ZIF-8/PS spheres have similar properties compared with ZIF-8 powder. Considering the difficulty in growth of defect-free ZIF membrane on porous substrate, ZIF crystals were incorporated into polymers to form mixed matrix membranes (MMMs). ZIF-11, having a similar porous structure to ZIF-8, was used as a filler in this study. A MMM with PBI as polymer matrix and ZIF-11 as filler was fabricated using a solution casting method. The effects of the membrane solution concentration and the evaporating temperature were investigated. The results showed that the H2 permeability increased about four times on the ZIF-11/PBI composite membrane compared with the pure PBI membrane. In addition, ZIF membranes can also grow on ceramic hollow fiber substrates for H2/N2 selectivity. Hollow fiber substrates have the advantages of high surface-to-volume ratio, a high packing density and good processability over flat substrate. A hollow fiber supported ZIF-8 membrane was prepared in diluted aqueous solutions. The fabricated membrane had a thickness around 1 μm, and the gas permeation test revealed the membrane was compact and continuous.