Evaluation of the stability of β-ketoenamine-Type covalent organic framework membranes in organic solvent

Abstract

Covalent organic framework (COF) materials hold significant promise for membrane technology due to their precisely defined porous structures and tunable functionalities. However, the stability of COF membranes in organic solvents, critical for long-term operation, has seldom been studied and remains unclear. This study synthesizes three β-ketoenamine type COF membranes with different amounts of sulfonate groups via vacuum-assisted assembly and systematically evaluates their stability in four typical organic solvents (alcohol, ketone, ester, and alkane), respectively. We investigated the time-dependent membrane morphologies, thickness swelling, and long-term organic solvent nanofiltration (OSN) performance within a period of up to 180 days. Although the COF membranes suffer from a certain degree of structural change and permeance decline during the initial testing period, they can finally achieve quite stable separation performance. The effects of solvent type and the number of -SO₃H groups on membrane stability are systematically discussed. Based on the evaluation methods adopted in this study, the prospects of COF membranes for long-time application in organic solvent can be highlighted, but some critical problems are also raised. We also investigate the swelling behavior under harsh conditions (e.g. elevated temperature, strong acids, and alkalis) to accelerate the swelling process, which is found to enable fast and effective evaluation of the long-term stability.