Porous metal–organic molecular cage: a promising candidate to highly improve the nanofiltration performance of thin film nanocomposite membranes

Abstract

Porous materials offer an attractive pathway to enhance the nanofiltration performance of thin film nanocomposite (TFN) membranes. Widely studied porous materials, such as zeolite, and metal–organic frameworks suffer from interfacial defects and poor dispersion in a polymer matrix, which leads to less than optimal performance of the TFN membranes. Porous metal–organic molecular cages are an exciting new family of porous materials that could ameliorate these issues by their better dispersing ability and intimate integration within the polymer matrix. By using the interfacial polymerization (IP) process, we synthesized a novel polyamide (PA) TFN membrane with highly improved nanofiltration performance using porous iron–organic molecular cage (Fe-cage) as the filler. Owing to the porous tetrahedral structure and polar –SO₃Na group of the Fe-cage, the as-synthesized PA/Fe-cage TFN membranes at 1.0 wt% Fe-cage concentration in the aqueous solution showed water flux up to 26.33 L m⁻² h⁻¹, which was 3 times more than that of neat PA thin film composite (TFC) membranes, while simultaneously increasing the rejection ratio to 89.3% for Na₂SO₄ and 96% for Congo red. Further, the addition of Fe-cages induced a significant improvement in the antifouling property of the membranes, which could be attributed to both increased hydrophilicity and smooth surface microstructure of the membranes. These results provide a starting point for the high-performance TFN NF membranes by utilizing porous metal–organic molecular cages.