Covalent organic framework membranes modified by end-capping monomers for organic solvent nanofiltration

Covalent organic frameworks (COFs) with excellent solvent stability, high porosity, and well-designed pore size, are competitive candidates for organic solvent nanofiltration (OSN). To manipulate their structures, pre-synthetic and post-synthetic modifications are commonly used. Nevertheless, pre-synthetic modifications may alter the stacking fashion of COF nanosheets; post-synthetic modifications are constrained by limited pre-functionalities and can hardly achieve complete conversion. In contrast, defect engineering is a facile method to modify the structure of COFs through defects generated by the breakage of linkages or the usage of end-capping molecules. In this study, we adopted defect engineering to improve the OSN performance of TpPa-COF membranes constructed of 2,4,6-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa) monomers with end-capping molecules—aniline (An). The membrane synthesized with 30 percent of An molecules (TpPaAn-30/HPAN membrane) maintained a continuous COF layer with a thickness of ∼20 nm but lower density or more free channels, compared to the control membrane. The optimized TpPaAn-30/HPAN membrane had improved permeance for various organic solvents (e.g., 31.8 L m-2 h-1 bar-1 for methanol, fourfold of the control membrane) but maintained rejection towards methyl blue (>90%). It also allowed the passage of rhodamine B but blocked methyl blue when filtrating a mixed-dye methanolic solution. Herein, the usage of end-capping monomers is proven to be an efficient method to introduce transport channels and improve the separation performance of COF membranes.