In situ growing ZIF-L particles by cross-substrate counter diffusion: Breaking the trade-off between NF permeation and Li+/Mg2+ selectivity

Polyamide/polyethylene (PA/PE) nanofiltration (NF) membranes based on polyethyleneimine (PEI) had demonstrated excellent Li + /Mg 2+ selectivity, but showing trade-off between their permeance and Li + /Mg 2+ selectivity. In this work, a novel cross-substrate counter diffusion (CSCD) method was introduced to grow and secure ZIF-L particles in the PA layers. Specifically, rather than directly contacting the PA layers containing Zn(II) cation (PA/Zn) with an aqueous solution of 2-methylimidazole (2-MIM) ligand from the top sides, 2-MIM ligand molecules were diffused across the PE substrate from the bottom sides to react with Zn(II) secured on the top sides of PA layers. Compared with the top-down method, the bottom-up approach had yield PA/ZIF-B NF membranes with higher surface roughness, larger average pore size and narrower pore size distribution. Notably, the pure water permeance of PA/ZIF-B membrane was increased by 2.6-folds to 15.1 L·m −2 ·h −1 ·bar −1 at a high Li + /Mg 2+ separation factor (S Li/Mg ) of 16.5, breaking the trade-off performance upper bound between water permeance and Li + /Mg 2+ separating selectivity and outperforming most of the reported TFN NF membranes.