Hyperbranched polymer wrapped UIO-66-NH2 as covalent intermediate layer to enhance polyamide membrane for Li+/Mg2+ separation and acid/alkaline stability

Herein, hyperbranched polyamide covalently wrapped UIO-66-NH₂ nanoparticles (PA-UIO-66-NH₂) were designed and synthesized via the in-situ self-condensation polymerization of 3,5-diaminobenzoic acid. Mixed matrix intermediate layer thus was prepared by doping it into the diazotization-coupling reaction, to tune the pore size and hydrophilic of the polysulfone (PSF) support. The interfacial polymerized polyamide (PA) nanofilm formed on modified PSF support exhibited optimized improved size exclusion and Donnan effect on separation of Li⁺/Mg²⁺. Experimental data showed that the Li⁺/Mg²⁺ separation factor (32.24 at Mg²⁺/Li⁺ ratio of 15.3) of the PA/m-PSF-2 membrane tuned with such intermediate layer reached 13.27 times that of the controlled PA/PSF-0, and surpassed some commercial PA (NF 270, DL, and DK) and literature reported membranes. Moreover, the related LiCl/water flux enhanced from 1.39 to 1.88 times compared with that of the PA/PSF-0 membrane, up to 244.25 kg m⁻² h⁻¹. Such mixed matrix intermediate layer can adsorb and isolate acidic or alkaline cleaning solutions, significantly reducing the impact of cleaning solutions on the rejection rate and water flux of the PA nanofilm during actual application. This work demonstrated that the mixed matrix covalent intermediate layer was an effective approach to construct the fine-tuned structural and robust PA nanofiltration (NF) composite membrane.