Hydrophilicity gradient in covalent organic frameworks for membrane distillation

Desalination can help to alleviate the fresh-water crisis facing the world. Thermally driven membrane distillation is a promising way to purify water from a variety of saline and polluted sources by utilizing low-grade heat. However, membrane distillation membranes suffer from limited permeance and wetting owing to the lack of precise structural control. Here, we report a strategy to fabricate membrane distillation membranes composed of vertically aligned channels with a hydrophilicity gradient by engineering defects in covalent organic framework films by the removal of imine bonds. Such functional variation in individual channels enables a selective water transport pathway and a precise liquid–vapour phase change interface. In addition to having anti-fouling and anti-wetting capability, the covalent organic framework membrane on a supporting layer shows a flux of 600 l m –2 h –1 with 85 °C feed at 16 kPa absolute pressure, which is nearly triple that of the state-of-the-art membrane distillation membrane for desalination. Our results may promote the development of gradient membranes for molecular sieving. Membrane distillation can use low-grade heat for salt water desalination, but the materials used can often suffer from limited permeance. Here, a strategy is proposed to construct a pore surface and size functionality gradient in a covalent organic framework, enabling a flux of 600 l m –2 h –1 with NaCl rejection of 99.99%.