Rapid preparation and mechanism investigation of covalent organic framework membranes by 3D printing based on electrostatic spraying

Covalent organic framework (COF) has great advantages in the field of membrane separation, but the efficient and convenient preparation of COF membranes is still a challenge. Herein, we propose a novel method with green solvent for the preparation of COF membranes based on electrostatic spraying, which can be successfully prepared within 10 min, forming the TpPa COF with a pore size of 1.8 nm. Molecular dynamics simulations demonstrate that electrostatic spraying enhances the reaction rate by lowering the energy barrier and increasing the movement of reacting monomers. Atomization of the solvent in electrostatic spraying causes the rapid volatilization of the solvent, resulting in supersaturation of the generated TpPa, precipitation and crystallization, and promotes the formation of TpPa COF structure. The optimal COF membrane for the separation of Congo red wastewater is prepared by adjusting the voltage, concentration, and spraying time, with selectivity of Na₂SO₄ for Congo Red attaining 118 and water permeance reaching 70.2 L m^-2 h^-1 bar^-1. This work not only elaborates the membrane preparation mechanism in the electrostatic spraying process, but also greatly shortens the time of COF membrane preparation compared to the traditional method (2–3 days), which contributes to the possibility for practical application of COF membranes.