Framework-based nanofiltration membranes for liquid separation

Crystalline frameworks, primarily metal organic frameworks (MOFs) and covalent organic frameworks (COFs), have demonstrated great potential in membrane separation. These materials can effectively overcome the inherent limitations of conventional polymeric materials because of their tuneable porous structures and versatile functionalities. However, the controllable fabrication of framework-based membranes remains challenging, compromising their structural integrity, separation performance, and practical applicability in industrial processes. In this thesis, the controllable fabrication of different framework-based membranes for liquid separations is presented. First, MOF-303 membrane with a crystallographic preferred orientation (CPO) is fabricated on alumina hollow fibres (HFs) via a novel dual-source (DS) seeding method. The achieved (110) CPO provides straight permeation channels, ensuring high water permeance in nanofiltration and pervaporation. Additionally, the robust attachment between the substrate and the MOF membrane leads to high structural integrity over the long-term test. Second, a highly permeable loose nanofiltration (LNF) membrane is fabricated by combining nonsolvent-induced phase separation (NIPS) with the combined crystallization and diffusion (CCD) method. A homogeneous COF-polyethersulfone (PES) solution is prepared by dissolving PES in a single phase-solution containing synthesized DABA-TFP colloids. Dye-salt separation tests reveal a promising selectivity with an exceptional permeance. Finally, confined growth of a two-dimensional (2D) MOF, CuBDC, within the laminar structure of a porous graphene oxide (PGO) membrane is achieved via a contra diffusion approach. The oxygen-coating groups present on the PGO nanosheets, which are capable of coordinating with metal clusters, are adjusted by decreasing the flake size to control the loading of the MOFs. This controlled coordination significantly enhances the structural robustness of the composite membrane, ensuring the stabilization of the PGO membrane during desalination tests. Overall, these strategies effectively address key challenges associated with the fabrication and practical application of MOF/COF-based membranes from multiple perspectives, paving the way for their widespread application in advanced liquid separation technologies.