Ion Rejection in Covalent Organic Frameworks: Revealing the Overlooked Effect of In-Pore Transport

Covalent organic frameworks (COFs), possessing highly ordered and intrinsic nanopores with high density and tunable sizes, are expected to find important applications in ion separation and desalination. The design of COF membranes with outstanding permselectivity requires understanding the ion rejection behaviors of COF multilayers. However, the ion rejection mechanism of COF multilayers remains to be elucidated because it may significantly differ from that of conventional polyamide membranes. Herein, we use nonequilibrium molecular dynamics simulations to investigate the ion transport through multilayers of TpHZ, which is a stable, imine-linked COF. Surprisingly, we find that the rejection to NaCl is determined by its rejection to Na+ rather than to Cl- although hydrated Cl- is bigger than hydrated Na+. Inside the channels of the TpHZ multilayers, Na+ ions transport evidently slower than water molecules, implying that the in-pore transport effect instead of the commonly thought pore-entrance sieving effect governs ion rejection. The in-pore transport effect of Na+ is mainly due to the hydration of Na+ with pore wall and stronger capability of the hydrated Na+ ions to form hydrogen bonds with pore wall, both of which are primarily originated from the polarity of the COF. This work reveals the significant role of in-pore transport in ion rejection, which is overlooked in commonly used polyamide desalination membranes, and develops a universal equation capable of describing ion rejections of all membranes, especially those that contain structures of nanopores or nanochannels by considering both the effects of pore-entrance sieving and in-pore transport.