Synthetic subnanochannels in porous aromatic frameworks accelerate selective water permeation in membrane desalination

Abstract

Creation of freeways in a membrane accelerates water transport for efficient desalination; however, the synthesis of target membrane materials is very challenging. In the present study, the molecular-design strategy is developed for the synthesis of porous aromatic framework (PAF) membranes with rigid and open networks to regulate water transport. Nitrile-based monomers in triangle shape are designed for the construction of PAF-98 and its sister frameworks, and these PAF membranes are formed through trifluoromethan-esulfonic acid catalysis. The rigidity and geometry of the monomers endow PAFs with unique structures of connected pores with edging size (4.5–6.7 Å) between water (2.8 Å) and hydrated sodium molecules (7.2 Å), enabling effective differentiation of water and salt. The fabricated membranes with high flexibility and continuity are ideal for water desalination, and reverse osmosis experiments demonstrate that the three PAF membranes are selective for permeation of water molecules over hydrated sodium cations. Unprecedented water flux of 547.1 L m−2 h−1 and high rejection degree of 90% are obtained on a representative membrane of PAF-98-E; and the desalination performance is superior to currently available membrane materials. Furthermore, PAF-98-E membrane is capable of rejecting various cations with high efficiency in real seawater, holding great promise in practical water desalination application.