Insight into Ion Transport in Discrete Frameworks of Porous Organic Cage Membranes

Porous organic cages (POCs) of discrete frameworks are promising candidates for separation membranes of high ion permeability and selectivity. Here, we investigate the ion transport properties of POC (i.e., CC1, CC3, and CC19) membranes with different cage arrangements, channel wettabilities, and surface charges. Compared to CC1, the window-to-window alignment of CC3 membranes leads to continuous channels with the window-cavity structure and guarantees a high ion permeability and ion selectivity. In comparison with CC19, the hydrophobic channels of CC3 membranes suppress the hydrogen-bonding interaction between hydrated ions and the channel wall, facilitating fast ion transport. Tuning the surface charge of CC3 membranes from the negative to positive state, the mono-/divalent cation selectivity increases obviously due to the enhanced electrostatic repulsion toward divalent cations. This work reveals ion transport behaviors in POCs with various channel configurations and surface chemistries, offering guidelines for the construction of efficient ion separation membranes.