Interface‐Confined Catalytic Synthesis of Anisotropic Covalent Organic Framework Nanofilm for Ultrafast Molecular Sieving

Covalent organic frameworks (COFs) have emerged as prominent membrane materials for efficiently fractionating organic molecules and ions due to their unique pore structure. However, the fabrication of free-standing COF nanofilms with high crystallinity remains an arduous undertaking, and feasible methods that can enable precise control over the film microstructure are barely reported. This work conceives an exquisite interface-confined catalytic strategy to prepare Tp-BD(OH)₂ COF nanofilm with an anisotropic structure analogously to conventional polymeric membranes. Experimental data and molecular simulations reveal that the hydroxyl groups on the framework substantially capture and anchor the acid catalyst through hydrogen bonding interactions at the incipient stage of interfacial polycondensation, instigating confined catalysis and self-termination reaction at the interface. The distinctive asymmetric structure endows the Tp-BD(OH)₂ COF nanofilm with a record-breaking pure water permeance of 525.3 L m^-2 h^-1 bar^-1 and unprecedented dye/salt selectivity of 648.6, surpassing other reported COF films and state-of-the-art nanofiltration membranes, as well as enduring structural durability and chemical stability. The implemented interface-confined catalysis strategy opens up a new avenue for regulating the COF nanofilm microstructure and holds broad prospects for the rational design of high-performance membranes for sustainable water purification and treatment.