| Chemical separation process plays a significant role in chemical industry.In recent years, membrane-based separation is attractive due to its low cost,high efficiency, energy saving, simple equipment, safe and simple manipulation, and eco-friendliness. The development of advanced membrane materials is the key and has become a hot research topic up to now.Meanwhile, nanoporous materials like metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted wide research attention during the development of membrane materials due to their highly designable and tunable functionalities as well as the well compatibility with other materials. Thus, these MOFs and COFs have been excellent candidates as membrane materials.In this work, we make full use of polymer as matrix, or graphene oxide(GO) nanosheets as assistants to fabricate novel MOF based composite membranes or COF ultrathin membranes for liquid or gas separation. We conducted some characterizations such as scanning electron microscopy,X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy to demonstrate relevant information, and further discussed the separation mechanism. The ideas proposed in this dissertation will provide valuable insights for the development of novel membrane materials and intensification of chemical separation processes. The main creative research contents and findings are here.1. Two kinds of novel mixed matrix membranes (MMMs), ZIF-7/PDMS and ZIF-71/PDMS, were prepared by adding two MOFs that were simply prepared at room temperature into polymer matrix. These membranes were used to recover acetone from aqueous solution and separate dimethyl carbonate-methanol azeotrope through pervaporation (PV) separation technology. The experimental results demonstrated that hybrid membranes,combining the advantages of both polymer and MOFs, achieved the enhanced separation performance than that of pure polymeric membrane and the synergistic effect that called "1+1>2” .2. Novel MOF@GO membranes were prepared by intercalating superhydrophilic nanosized MOFs into the GO membranes, thanks to the advantage of its facile preparation resulting from the rich functional groups,through a novel pressure-assisted self-assembly (PASA) filtration technique instead of traditional vacuum filtration method. The water molecule fast transport channels were constructed by intercalated MOFs, and as a result that the MOF@GO membranes comparing with GO membrane have significant enhancement of separation performance including flux and separation factor for ethyl acetate/water mixtures (98/2, w/w) through PV process, which are also superior to the reported other kinds of membranes. This nanosized MOF-intercalating strategy could be also extended to other laminated membranes, providing valuable references in designing and developing of advanced membranes for strengthening separation through nanostructure manipulation of the nanomaterials.3. A GO-assisted layer-by-layer restacking method was firstly adopted to fabricate COF ultrathin membranes for gas separation. We restacked the exfoliated two dimensional COF nanosheets with the assistance of the abundant functional groups in GO nanosheets, acting as "adhesives" to improve the binding interactions, to prepare continuous and dense ultrathin membranes. Membranes with different thickness (100 nm, 210 nm, and 290 nm) could be tuned by simple vacuum filtration with different amounts of nanosheets, which could form narrow interlayer passages with different lengths. A simple method, adjusting the ratio of the two kinds of nanosheets,was also proposed to fabricate membranes with an emphasis on different features: larger selectivity, higher permeance or a balance between them.These ultrathin membranes displayed excellent separation performance toward H2-CO2 system, which all surpass the Robeson’s 2008 upper bound.These methods provided valuable references to fabricate other laminated membranes with controllable separation performance.4. A polymer sealing strategy that using a semi-continuous polymer thin coating layer to seal the frequent inter-crystal defects of MOF membrane was proposed. Based on this, a novel ZIF-9@Matrimid membrane, differing from traditional MMM whose performance was mainly dominated by polymer, was prepared. Since the non-selective gaps between the ZIF-9 crystals are sealed by the polymer coating layer, the gas molecules can penneate through both the ZIF-9 micro-pores (0.29 nm) and polymer instead of the non-selective defects.The gases would diffuse preferentially through ZIF-9 part due to the low permeability of polymer, and thus could make full use of the separation effect of ZIF-9 to achieve high separation performance. Consequently, the ZIF-9@Matrimid membrane has a selectivity of 30.1 for H2-C02 system at 298 K and 2.0 bar of pressure difference, which has an enhancement by 123%comparing with that of ZIF-9 membrane. Furthermore, this facile and versatile polymer sealing strategy can be extended to fabricate other advanced functional membranes, providing potential for intensifying chemical separation processes. |
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