Tuning the size and ion affinity of channels within guanidinium-based covalent organic framework membranes for efficient Li+/Mg2+ separation

Covalent organic framework (COF) emerges as a promising membrane material for ion separation, such as Li + extraction from brines. However, the single-functionalization of most COF channels hinders further improvements in separation efficiency. Herein, the angstrom size, positive charge and Li + affinitive sites have been integrated in the channels of triaminoguanidine (TG)-based COF membranes. Aldehydes with different lengths of oxygen-containing side chains were employed to precisely tune the size and ion affinity of the channels. Precisely engineered cationic channels with moderate pore size and Li + affinitive sites enable Mg 2+ rejection through synergistic size exclusion and Donnan exclusion, while simultaneously accelerating Li + permeation via affinitive interactions. The optimized membrane, TG-1, achieves a remarkable Li + /Mg 2+ selectivity of 423 with a Li + permeation rate of 34.02 mmol m -2 h -1 . Additionally, TG-1 is able to perform well in practical conditions including high Mg 2+ /Li + mass ratio, high temperature, long operation time and also in various mono-/divalent metals separations. This study offers insights in the construction of multi-functional membrane channels for efficient ion separations.