Alkyl-Engineered Hydrophobic Channels in Covalent Organic Frameworks toward Fast Organic Solvent Nanofiltration

Covalent organic frameworks (COFs) with regular channels and tailored functionalities hold great promise for building molecular sieving membranes. However, the development of fast and precise molecular transport channels is still considered a great challenge because of the difficulty in accurately controlling the internal structure and chemistry. Herein, we rationally designed and prepared a series of alkyl-engineered COF membranes at a molecular scale for promoting molecular separation within organic solvent systems. The alkyl groups of different lengths annexed on the pore wall could systematically and effectively fine-tune the pore sizes from 3.8 down to 2.6 nm, thus regulating the molecular transport behavior. Besides, the pore hydrophobicity could also be effectively adjusted through alkyl engineering, in which higher hydrophobic pores favor higher permeability of organic solvents. The resulting membrane shows a remarkable methanol permeance of up to 66.1 L·m–2·h–1·bar–1, along with a notable molecular weight cutoff down to 405 Da. Moreover, the regular channels can discriminate between molecules with nearly identical molecular sizes. This work paves a promising avenue for the exploration and manipulation of COF pore channels, providing a scientific base for the design of COF membranes with highly efficient organic solvent nanofiltration.