Metal-organic frameworks (MOFs); hybrid organic-inorganic (inorganic metal centers and organic linkers), crystalline, flexible, highly tunable pore structure materials (pore windows 3 Å to ~100 Å), offer a great variety of nanomaterials in membrane separation application . Although the structural diversity and highly tunable hybrid structure make a promising filler for aqueous separation, the majority of the MOF structures degraded even exposure to ambient moisture . Owing to the exploration of the water stable MOFs, many of them currently available for water included membrane applications, have been reviewed by . Among them, zeolitic imidazolate (Im) frameworks (ZIFs), a well-known subclass of MOFs, have received increasing…show more content… This promising method may address the TFN particle orientation challenge and may reduce expensive particle usage. Interestingly, at the optimum loading which is 0.005 w/v%, the filler particle size, whether 75 nm or 150 nm, has no significant effect on the membrane performance. However, due to the difference in the diffusion path of the particle and the number of the particle (8 times difference), there should be a difference. This may indicate the performance enhancement may be related to the polymer but not the particle incorporation. However, at higher loadings, the permeability increases with the particle size and decreases with the loading.
Another TFN synthesis method with ZIF-8 is introducing the particles into the aqueous casting solution with an anionic, water-soluble polymer, Poly(sodium 4-styrenesulfonate) (PSS) ●●. The purpose of the PSS is to modify the particle surface to well disperse the particles in the aqueous solution. Thus, the final membrane structure may have less particle aggregation. The method involves also the evaporation step to positioning the particle before the polymerization. The modified ZIF-8 TFN NF membranes demonstrated increased hydrophilicity, roughness, and negative charge and double water permeation compared to control membranes and slightly decreased in divalent ion retention ●●.
Another hydrostable MOF structure is hydrophilic