Ion separation using membrane separation technology is of great importance in the fields including energy conversion and storage, environmental detection and resource reuse. Membranes with sub-2-nanometer channels have high ion transport rates but they fail to sieve different ions effectively due to the lack of hydrogen bonding sites.
Covalent organic framework (COF) membranes with highly ordered one-dimensional nanochannels, uniform pore size and abundant hydrogen bond sites show broad application prospects in ion separation.
In a study published in Advanced Materials, a research team led by Prof. XU Tongwen from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences constructed a novel COF membrane with sub-2-nanometer channels and abundant hydrogen bonding sites. The COF membrane shows high monovalent cation permeation rate and low multivalent cation permeation rate.
Researchers synthesized a 20nm membrane (TpBDMe2) through interfacial growth strategy. Based on the infrared characterization of TpBDMe2 under N2 and air atmosphere, they found that hydrogen bonding sites in the channel can form hydrogen bond interactions with water molecules.
Concentration gradient diffusion test showed that TpBDMe2 has a much higher permeation rate for monovalent ions than that for multivalent ions. The ion selectivity of TpBDMe2 is significantly higher than that of the reported sub-2-nm channel membranes.
Besides, researchers investigated the ion separation mechanism of COF membrane. Through density functional theory (DFT) calculations, they found that hydrated ions interact with hydrogen bonds through the channel, and multivalent ions have stronger hydrogen bondings, resulting in slower transportation rate than monovalent ions.
This study provides theoretical basis for ion transportation mechanism in sub-2-nanometer channels as well as the design and manipulation of ion separation membranes. Based on this study, researchers has developed the preparation technology with proprietary intellectual property rights and they have prepared the membrane in pilot plant tests.