Covalent-organic
frameworks (COFs) are ordered crystalline materials
covalently formed by building blocks of light elements. There has
been increasing interest in the development of COF membranes for chemical
separation. In this study, seven two-dimensional (2D) COF membranes
are computationally designed with different functional groups and
aperture sizes. By 245 sets of molecular simulations, the seven ultrathin
COF membranes are investigated for organic solvent nanofiltration
(OSN) of seven solvents (acetonitrile, acetone, methanol, ethanol,
isopropyl alcohol, methyl ethyl ketone, and n-hexane)
and four solutes (2,5-furandiamine, paracetamol, α-methylstyrene
dimer, and Nile red). The solvent fluxes through the COF membranes
are revealed to be governed by the aperture size and membrane functionality,
as well as solvent properties. In general, the larger the aperture
size, the higher is the flux. For membranes with comparable aperture
size, the hydrophobic one exhibits higher fluxes than the hydrophilic
counterpart for all the solvents except n-hexane.
To elucidate this trend, solvent structures near the membranes are
analyzed and the potentials of mean force for solvent permeation are
evaluated. The solvent permeances through hydrophobic and hydrophilic
membranes are correlated respectively with two different combinations
of solvent properties. The solute rejection is found to depend on
a complex interplay among solute size and polarity, solvent viscosity,
solute–solvent interaction, aperture size and membrane functionality.
In the presence of solutes, solvent permeances are reduced by approximately
10%. From the bottom-up, this comprehensive computational study provides
quantitative insights into solvent permeation and solute rejection
in the COF membranes, unravels the key governing factors, and would
facilitate the development of new membranes for high-performance OSN.