2D nanosheets seeding layer modulated covalent organic framework membranes for efficient desalination

Covalent organic frameworks (COFs) are emergent ideal membrane materials, owing to their pre-designable and well-defined pore structures. However, COFs typically exist as solid powder and are hard to dissolve in most solvent, which renders COFs poor processability in membrane formation. In this study, TpPa-SO₃H/PAN COF membranes (COFMs) were prepared via counter diffusion method, which is modulated by the pre-assembled two-dimensional (2D) crystalline TpPa-SO₃H nanosheets as seeding layer. The TpPa-SO₃H seeding layer regulated the diffusion-reaction behavior of monomers and induced the confined growth of TpPa-SO₃H COFs, which is favorable for eliminating the intergranular defects inside the TpPa-SO₃H/PAN COFMs. The thickness of TpPa-SO₃H seeding layer can be controlled by the amount of TpPa-SO₃H nanosheets. The charge of the channel wall inside the membrane can fortify the electrostatic repulsion mechanism and facilitate efficient desalination. Under the optimal conditions, the Na₂SO₄ rejection rate of TpPa-SO₃H/PAN COFMs can reach 97.4%. Unlike conventional polyamide membranes whose desalination performance usually decreases with the increase of salt concentration, the TpPa-SO₃H/PAN COFMs preserved superior desalination performance toward high salt concentration up to 10,000 ppm.