(369b) Molecular Insight into Seawater Pervaporation through Zeolitic Imidazolate Framework Membranes

The scarcity of freshwater has escalated as a global concern due to increasing population, energy demand and industrialization. Currently, there is considerable interest to desalinate seawater, which constitutes over 95% of water on the Earth. Among several desalination techniques, multistage flash distillation (MSFD) and reverse osmosis (RO) have been majorly utilized. However, phase transition occurs in energy intensive thermal-based MSFD; and RO needs pressurization and is associated with easy fouling. Recently, pervaporation (PV) has emerged as a relatively new economically viable separation technique. As a unique integration of membrane permeation and evaporation, PV offers several advantages such as low energy consumption, high separation capability and easy scaling-up. To date, most PV membranes still suffer from low mass transfer (flux or permeability) compared to graphene and carbon nanotube membranes. Therefore, increasing interest is to explore new PV membranes that exhibit superior performance for water desalination. With large surface areas and high porosities, zeolitic imidazolate frameworks (ZIFs) have emerged as a new class of porous materials that could be used as membranes for pervaporation.

In this study, an atomistic simulation study is reported for seawater pervaporation through five zeolitic imidazolate framework (ZIF) membranes including ZIF-8, -93, -95, -97 and -100. Salt rejection in the five ZIFs is predicted to be 100% due to the sieving of small apertures. The hierarchy of water flux is as ZIF-100 >> -8 > -95 > -93 > -97. With the largest aperture, ZIF-100 possesses the highest water permeability of 5 ´ 10^-4 kg×m/(m²×hr×bar), which is substantially higher compared to commercial reverse osmosis membranes, as well as zeolite and graphene oxide pervaporation membranes. In ZIF-8, -93, -95 and -97 with similar aperture size, water flux is governed by framework hydrophobicity/hydrophilicity; in hydrophobic ZIF-8 and -95, water flux is higher than in hydrophilic ZIF-93 and -97. Furthermore, water molecules in ZIF-93 move slowly and remain in the membrane for a long time, but undergo to-and-fro motion in ZIF-100. The lifetime of hydrogen bonds in ZIF-93 is found to be longer than in ZIF-100. This simulation study quantitatively elucidates the dynamic and structural properties of water in ZIFs, identifies the key governing factors and suggests ZIF-100 is an intriguing membrane for seawater pervaporation.