Poly(bis(1-methylpiperazin-1-ium-amide) Nanofilm Composite Membrane with Nanochannel‑Enabled Microporous Structure and Enhanced Steric Hindrance for Magnesium/Lithium Separation

Efficient lithium/magnesium (Li⁺/Mg²⁺) separation attainment is fundamental to the extraction of lithium from brine by nanofiltration membrane separation process, which is essential for resource recovery and a circular water economy. However, for poly(piperazine-amide) nanofilm composite membranes, the higher electronegativity affects the Mg²⁺ rejection and consequently Li⁺/Mg²⁺ separation performance. Manipulating the positive charge density and pore size regulation of the nanofiltration membranes are determinative of the Li⁺/Mg²⁺ separation performance improvement. Here, a new monomer 1,1′-(hexane-1,6-diyl)bis(1-methylpiperazin-1-ium) bromide containing bis-quaternary ammonium cations is employed as a molecular building block to fabricate polyamide nanofilms via interfacial polymerization. The dual quaternary ammoniums and the rod-shaped conformation of the monomer confer enhanced electropositivity, steric hindrance, loosely packed microporous network structure (pore diameter∼0.8–1.35 nm), and high free volume. The resultant membrane exhibits high water permeance of 28.34 L m⁻² h⁻¹ bar⁻¹ with good Li⁺/Mg²⁺ selectivity of up to 76.9. In addition, the membrane also exhibits chlorine stability performance owing to the lack of the chlorine sensitive −NH groups in the formed tertiary amide structures. Computational insights on the structural properties, nanofilm formation, and transmembrane water and ion transport behaviors are provided. This study offers insightful theoretical and technological concepts to design and construct membrane materials for energy-efficient separations.