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

Yang, Hao, Cheng, Xiaopo, Cheng, Xuanxuan, Pan, Fusheng, Wu, Hong, Liu, Guanhua, Song, Yimeng, Cao, Xingzhong, Jiang, Zhongyi
Journal of membrane science 2018 v.565 pp. 331-341
aqueous solutions, artificial membranes, ethanol, hydrophilicity, nanospheres, sodium alginate
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.