Selectivity

Organic chemistry

Covalent bond

Membrane

Ion transporter

Hydrogen bond

Engineering

Catalysis

Composite material

Ionic bonding

Inorganic chemistry

Chemistry

Metallurgy

Materials science

Hydration energy

Covalent organic framework

Molecule

Divalent

Chemical engineering

Permeation

Biochemistry

Nanometre

Advanced Materials

Membranes of sub-2-nanometer channels show high ion transport rates, but it remains a great challenge to design such membranes with desirable ion selectivities for ion separation applications. Here, covalent organic framework (COF) membranes with a channel size of ≈1.4 nm and abundant hydrogen bonding sites, exhibiting efficient ion sieving properties are demonstrated. The COF membranes have high monovalent cation permeation rates of 0.1-0.2 mol m-2 h-1 and extremely low multivalent cation permeabilities, leading to high monovalent over divalent ion selectivities for K+ /Mg2+ of ≈765, Na+ /Mg2+ of ≈680, and Li+ /Mg2+ of ≈217. Experimental measurements and theoretical simulations reveal that the hydrogen bonding interaction between hydrated cations and the COF channel wall governs the high selectivity, and divalent cations transport through the channel needs to overcome higher energy barriers than monovalent cations. These findings provide an effective strategy for developing sub-2-nanometer sized membranes with specific interaction sites for high-efficiency ionic separation.

Efficient Ion Sieving in Covalent Organic Framework Membranes with Sub‐2‐Nanometer Channels