Layer-by-Layer Synthesis of Covalent Organic Frameworks on Porous Substrates for Fast Molecular Separations

Covalent organic frameworks (COFs) are expected to provide exceptional permselectivity in molecular separations because of their stable, uniform, molecular-sized pore channels. To realize the superior performances of COF-based membranes, it remains strongly desired for the rapid and controllable growth of COFs on substrates under mild conditions. Herein, we report on the layer-by-layer (LbL) strategy for the rapid synthesis of imine-linked COFs on porous polymeric substrates in ethanol at room temperature. This strategy exploits the alternative availability of each COF monomer to endow a self-limiting nature to the reaction, thus resulting in conformal growth of COFs along the pore wall of the substrate. The grown COFs with thicknesses tunable by LbL cycles reduce the effective pore sizes, while the inherent ∼2 nm channels in COFs allow additional water to permeate. Thus-produced membranes exhibit significantly enhanced selectivity (>99% rejection to dyes) and unprecedentedly high water permeances, which are ∼3–20 times higher than other membranes with similar rejections. This work potentially presents a new and general methodology to prepare imine-based COF membranes for molecular separations.