Staggered-Stacking Two-Dimensional Covalent Organic Framework Membranes for Molecular and Ionic Sieving.

Two-dimensional covalent organic frameworks (2D COFs), a family of crystalline materials with abundant porous structures offering nanochannels for molecular transport, have enormous potential in the applications of separation, energy storage, and catalysis. However, 2D COFs remain limited by relatively large pore sizes (>1 nm) and weak interlayer interactions between 2D nanosheets, making it difficult to achieve efficient membranes to meet the selective sieving requirements for water molecules (0.3 nm) and hydrated salt ions (>0.7 nm). Here, we report a high-performance 2D COF membrane with narrowed channels (0.7 × 0.4 nm2) and excellent mechanical performance constructed by the staggered stacking of cationic and anionic 2D COF nanosheets for selectively sieving of water molecules and hydrated salt ions. The mechanical performance has been improved by two times than that of single-phase 2D COF membranes due to the enhanced interlayer interactions between nanosheets. The stacked 2D COF membranes exhibit significantly improved monovalent salt ions rejection ratio (up to 77.9%) compared with single-phase COF membranes (∼49.2%), while maintaining comparable water permeability. The design of stacked 2D COF membranes provides a potential strategy for constructing high-performance nanoporous membranes to achieve precise molecular and ionic sieving.