Alkaline-resistant covalent organic framework membranes for selective hydroxide ion separation

Alkali holds a pivotal role in the chemical industry, however, the substantial discharge of alkaline wastewater causes a severe waste of resources and environmental pollution. Conventional polymer membranes have ether bonded backbones and form randomly assembled nanochannels, suffering from a weak alkali resistance and a low hydroxide ion selectivity. Covalent organic frameworks (COFs), known for their robust framework structure, highly ordered channels, and tunable channel chemistry, have been considered as the competent candidate for alkaline-resistant membranes for selective hydroxide ion separation. Herein, we fabricate a β-ketoenamine COF (TpTAPB) membrane with a high alkaline resistance via the interfacial growth strategy. After post-sulfonation, the negatively charged channels of sulfonated TpTAPB membrane allows OH- with smaller size and lower charges to pass through while repulsing WO42- ions, achieving a selectivity of OH-/WO42- up to ∼ 60 and surpassing commercial and most reported polymeric membranes. This work expands the application scope of COF membranes in selective alkali separation.