| MOFs (Metal Organic Frameworks) are a class of noble organic-inorganic hybrid porous materials consisting of metal ions or metal ion clusters and bridging organic ligands. ZIFs (Zeolitic Imidazolate Frameworks), as an important subclass of MOFs, have the features of really high surface area, specific pore structure and excellent thermal and chemical stability, showing great potential in membrane fabrication. Thus far, research on ZIFs membrane preparation has attracted intensive attension and there are a lot of papers on ZIF-8 membrane preparation. However, the research mainly focused on forming membrane on disc supports and dispersing ZIFs crystals into polymers in mixed matrix membranes. And the current synthesis method always needs complex modification of supports or addition of adhesive. As the disc supports used are less porous and the membranes obtained are generally thick, the resistance of gas permeation increases, resulting in the low permeance. Furthermore, the alumina disc supports are not suitable to make membrane modules.This limits the large-scale industrialization applications of ZIF-8 membranes. To solve the problems above, in this paper we select alumina hollow fiber tubes as supports, which are of high porosity, large surface area and suitable for industrial applications, and study on the preparation of ZIF-8 membranes on alumina hollow fiber supports.A traditional dip-coating seeding approach is used to seed homemade alumina hollow fiber supports and continuous ZIF-8 membranes are successfully prepared via secondary growth method. Influences of seed size, synthesis time and synthetic composition on membrane preparation are studied. As a result, owing to the presence of ZIF-8 crystal dissolution phenomenon, for nano-sized seed crystals, as the surface/volume ratio is much higher than large seed crystals, their dissolution rates are much higher than large crystals. Therefore, no dense ZIF-8 membranes can be obtained. On the contrary, the dissolution rates of large seed crystals with lower surface/volume ratios are much slower. Therefore, the dissolution speed is lower than growth and more ZIF-8 seeds are remained to act as the nucleation sites, resulting in the dense and well-intergrown ZIF-8 membranes with a thickness of 7 μm in a short synthesis time of 2.0 h. The gas permeation test indicates that the H2 permeance is up to 6.07×10-6 mol·Pa-1·s-1·m-2, with a H2/N2 ideal separation factor of 10.52, which is much higher than the Knudsen separation factor. The formation process of ZIF-8 membranes is studied by adjusting the synthesis time of nano-sized seeds and micro-sized seeds and the existence of crystal dissolution phenomenon is confirmed. Compared with membranes prepared by small and large size seeds in different synthetic compositions, it is proved that the crystal dissolution phenomenon is both present in the two different synthesis solutions with HCOONa.The combination of hot treatment of supports with pre-crystallization of synthetic solution is applied to the preparation of ZIF-8 membranes on alumina hollow fibers via in situ growth method for the first time. The detailed procedures are as follows:(1) the alumina hollow fiber support is treated in 150℃ oven for at least 4 h; (2) the synthetic solution of ZnCl2:meIm:HCOONa:CH3OH=1:1.5:1:250 is pre-crystallized at 100℃ for a period of time; (3) when the synthetic solution is cooled to room temperature, the hot-treated support is placed into the pre-crystallized solution rapidly and crystallized at 100℃ for another period of time. Effects of pre-crystallization time, synthesis time and hot treatment of support on the membrane preparation are studied. Results show that, under the condition of pre-crystalliztion of 1.5 h and crystallization time of 4.0 h, ZIF-8 membranes with a thickness of 6 μm are achieved with the H2 permeance of 1.38×10-5 mol·Pa-1·s-1·m-2 and H2/N2 ideal separation factor of 5.27, higher than the Knudsen separation factor of 3.78. |
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