Computational design of 2D functional covalent-organic framework membranes for water desalination

A series of 2D functional covalent-organic framework (COF) membranes are computationally designed from an experimentally synthesized COF (TpPa-1) for water desalination. The COFs consist of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa) (TpPa-X) with various functional groups, namely TpPa-AM2, -AMC2NH2, -OC3OH, -OC4H9, -AMCOOH, -OBn and -AM3. From molecular dynamics simulations, water flux through the TpPa-X membranes is found to increase with increasing aperture size; however, it is also significantly affected by membrane functionality. In the presence of hydrophilic functional groups, TpPa-AMC2NH2, -OC3OH and -AMCOOH exhibit higher water flux than their hydrophobic counterparts of similar aperture size. This is attributed to the preferential interaction between water and hydrophilic groups, thus resulting in a larger water density at the aperture of the hydrophilic membranes. The TpPa-X membranes show exceptionally high water permeance ranging from 1216 to 3375 kg m(-2) h(-1) bar(-1), approximately three orders of magnitude higher than commercial reverse osmosis membranes. Except TpPa-AMCOOH, the membranes have salt rejection over 98%. This computational study provides quantitative understanding of the key factors governing water permeance and suggests that TpPa-X membranes might be interesting for water desalination.