Covalent organic framework membrane with sub-nano pores for efficient desalination

Covalent organic frameworks (COFs) have emerged as advanced membrane materials for molecular separations, but it is rarely used for desalination due to the mismatch between the COFs pore size (1−5 nm) and the diameter of hydrated ions (<1 nm). Herein, we designed and fabricated an imide-linked TbTG COF membrane (COFM) with intrinsic sub-nano pores. The TbTG nanosheets were prepared by interfacial polymerization, and then self-assembled into COFMs on the porous PAN substrate. The sub-nano pore size (∼0.4 nm by simulation) conferred excellent size sieving capacity to the TbTG COFM with the molecular weight cut-off of only 180 Da, which was the lowest among COFMs ever reported. Moreover, the positive charge on the skeleton of TbTG COFM greatly fortified the Donnan effect. The TbTG COFM exhibits excellent salt rejections of 98.7% to Na2SO4, 99.1% to MgSO4, 99.5% to MgCl2 and 93.3% to NaCl, respectively. The TbTG COFM also maintained high salt rejections under both high salt concentration up to 10000 ppm and high pH value of 9.4, showing great potential in seawater desalination applications.