Insights into the mixing effect in organic solvent nanofiltration from simulations on covalent organic frameworks (COFs)

Low-energy organic solvent separation is crucial for the petrochemical industry. Covalent organic framework (COF) membranes hold immense promise for rapid separation of organic solvents. Understanding of the transport mechanisms of solvents is critical for COF-membrane design. In this study, molecular dynamics simulations are employed to investigate the separation behaviors of three most extensively used solvents (methanol, ethanol, and acetone) and their mixtures on COF membranes. It is found that despite high ideal selectivity, real selectivity approaches to 1, primarily due to in-pore transport effects, notably enhanced flow rates of ethanol in mixed-solvent cases. Subsequently, mixing effect is explored from molecular analysis. It is revealed that no clear evidence of preferential adsorption for methanol or acetone based on density distribution results. Calculation of self-diffusion coefficients indicates that the mobility of ethanol is evidently promoted in the mixed-solvent conditions, caused by the attraction between ethanol molecules is reduced because of the coexistence of other solvent molecules. The unexpectedly promoted permeance of ethanol in mixed-solvent cases consequently lowers the selectivity. Results in this work highlight the solvent–solvent interactions as crucial factors influencing separation performance and provide knowledge of membrane design for highly efficient separations of organic solvents.