Highly water-selective membranes based on hollow covalent organic frameworks with fast transport pathways

Covalent organic frameworks (COFs) are attractive candidates for membrane separation owing to their well-ordered channels and facilely tailored functionalities. However, the development of COF-based membranes remains in its infancy. To establish novel hierarchical structures in COFs, in this study, a facile template-directed approach is exploited for constructing hollow COF nanospheres. An imine-linked COF TpBD, derived from 1,3,5-triformylphloroglucinol (Tp) and benzidine (BD), was selected as the building block due to its rich hydrophilic groups and high stability. The synthesized hollow TpBD (H-TpBD) nanospheres were introduced into sodium alginate (SA) matrices to fabricate water-selective hybrid membranes. The H-TpBD nanospheres confer the membranes rapid diffusion pathways and abundant interacting sites for water molecules, thus fast water-selective permeation through the membranes was achieved. The hybrid membranes exhibit the optimal performance with permeation flux of 2170 g m−2 h−1 and separation factor of 2099 when used for dehydrating 90 wt% ethanol aqueous solution at 76 °C. The effects of hollow structures of H-TpBD on membrane performance were investigated and the molecular transport mechanisms through the membranes were elucidated. Besides, the favorable compatibility between SA and H-TpBD endows the hybrid membranes with long-term stability.