| Membrane science and technology which is a new type of technology of separation and purification for various chemicals that we encounter in our daily activities has caused an enormous transformation in the separation area. Membrane science and technology has a large of advantages, such as energy-saving, efficiency, environmental protection and automation control. Inorganic membranes are characterized by long life, good mechanical property, thermal and chemical stability, which adapt to the needs of the modern industrial applications.With the development of coordination chemistry, zeolitic imidazolate frameworks (ZIFs) have become the other inorganic microporous membrane materials in addition to the zeolite membranes and carbon membranes. ZIFs are a class of inorganic-organic hybrid crystalline materials and have unique properties, such as structural diversity, construct or tailor frameworks with desired property. ZIFs have showed excellent performance in the gas storage, separation, catalytic, which makes ZIFs become a new generation of membrane materials.ZIF-78was selected as the research subject in this paper, and different preparation methods, such as the traditional solvothermal synthesis method, dynamic crystallization method and step by step crystallization method were used to synthesize ZIF-78crystals. Then the suitable ZIF-78crystals were selected to modify the surface of the carriers and be used as seeds. ZIF-78membranes were synthesized by secondary growth. The as-synthesized ZIF-78membranes were characterized by SEM, XRD and gas permeation tests. The main research results obtained are presented as follows:(1) ZIF-78crystals were synthesized by traditional rotation-static solvothermal synthesis method with the molar composition of Zn(NO3)2·6H2O:2-nIm:5-nbIm:DMF=1:5:5:646, and the effects of synthesis conditions on the ZIF-78crystals were studied. The results showed that the morphology of as-synthesized ZIF-78crystals was hexagonal prism, and the particle size was2μm and the size distribution was narrow when the condition was rotation-80℃,0.5h and static crystallization-90℃,24h.(2) Different morphologies and sizes of the ZIF-78crystals were synthesized by three dynamic crystallization methods including the direct rotation method, two-step variable temperature method and adding deprotonation agent in the synthesis solution with the molar composition of Zn(NO3)2·6H2O:2-nIm:5-nbIm:DMF=4:5:5:904. The results showed that the morphology of ZIF-78crystals synthesized was irregular and the particle size distribution was wide with the direct rotation method; The products were the hexagonal prismatic crystals with the size of800nm～2μm by the two-step variable temperature method when the condition was rotation-110℃,12h,140℃,3h, and the crystal morphology had turned into the plate shape when the ligand concentration doubled and the DMF solvent was replaced by a double solvents with the volume ratio of DMF:methanol=1:1; The hexagonal prismatic crystals with2～4μm were synthesized when the triethylamine was added to the synthesis solution and the synthesis solution was stirred for24h at room temperature.(3) ZIF-78crystals were synthesized by stepwise crystallization method of rotation-static crystallization-filter-static crystallization with the molar composition of Zn(NO3)2·6H2O:2-nIm:5-nbIm:DMF=1:5:5:646, and the effects of rotation and static crystallization times on morphologies and sizes of ZIF-78crystals were studied. The results showed that the hexagonal prismatic crystals with the size of2μm were obtained when the synthesis condition was rotation-110℃,30min, stactic crystallization-130℃,12h, filter, static crystallization-100℃,24h. When the concentration of the synthesis solution was lower, the particle size of ZIF-78reduced to400nm.(4) The ZIF-78membranes were obtained by secondary growth with the molar composition of Zn(NO3)2·6H2O:2-nIm:5-nbIm:DMF:methanol=4:5:5:452:452. The effects of some parameters including the concentration of PEG in seeding solution, the solvent, crystallization temperature and time on the ZIF-78membranes were investigated. Compact and continuous ZIF-78membranes were synthesized successfully under the condition that included4wt%of PEG concentration in seeding solution, DMF:methanol=1:1,100℃and24-72h. Single gas permeation showed that the permeance of H2was8.55×10-1mol·m-2·s-1·Pa-1and the ideal separation factor of H2/CO2was10.02, which surpassed the Knudsen constant. |
PDF Full Text Request