Membrane distillation

Engineering

Distillation

Materials science

Chromatography

Fouling

Biochemistry

Chemical engineering

Organic chemistry

Membrane

Desalination

Permeance

Composite material

Chemistry

Wetting

Covalent bond

Permeation

Nature Materials

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%.

Hydrophilicity gradient in covalent organic frameworks for membrane distillation