Polyethyleneimine-Templated ZIF-8 Nanoparticles Impart the Nanofiltration Membrane with High Mg2+/Li+ Separation Performance.

MOFs-modified nanofiltration (NF) membranes have been gained a lot of attention due to their favorable permeability and ion separation performance. Nevertheless, the prevailing preparation techniques are afflicted by the incompatibility of MOFs with polymers and the facile loss of MOFs. In this work, polyethyleneimine (PEI)-templated ZIF-8 (PEI-ZIF-8) was synthesized and incorporated into the PEI aqueous solution, then interfacial polymerized with trimesoyl chloride (TMC) to obtain the PEI-ZIF-8 modified polyamide NF membrane. This PEI modified strategy could endow the ZIF-8 nanoparticles with positively charged properties to avoid the aggregation and increase the interfacial compatibility with the polyamide. Meanwhile, the appropriate pore size of ZIF-8 (3.4 Å), which is between the hydration sheath surrounding of Li⁺ (2 Å) and Mg²⁺ (4.2 Å) impart the membrane with precise Mg²⁺/Li⁺ separation ability. The optimal PEI-ZIF-8-TMC membrane exhibits a permeance of 9 L/h m²bar and a Mg²⁺/Li⁺ separation factor (SF) of 19, both of which surpass the performance of the pure PEI-TMC membrane, which has a permeance of 4 L/h m²bar and a Mg²⁺/Li⁺ separation factor of 11. Meanwhile, the membrane exhibited excellent long-term stability of 85 h. This novel approach to preparing MOFs-modified NF membrane represents a promising avenue for the separation of lithium and magnesium.