Water Desalination through Zeolitic Imidazolate Framework Membranes: Significant Role of Functional Groups.

A molecular simulation study is reported for water desalination through five zeolitic imidazolate framework (ZIF) membranes, namely ZIF-25, -71, -93, -96, and -97. The five ZIFs possess identical rho-topology but differ in functional groups. The rejection of salt (NaCl) is found to be around 97% in ZIF-25, and 100% in the other four ZIFs. The permeance ranges from 27 to 710 kg/(m(2)·h·bar), about one∼two orders of magnitude higher compared with commercial reverse osmosis membranes. Due to a larger aperture size da, ZIF-25, -71, and -96 exhibit a much higher water flux than ZIF-93 and -97; however, the flux in ZIF-25, -71, and -96 is governed by the polarity of functional group rather than da. With the hydrophobic CH3 group, ZIF-25 has the highest flux despite the smallest da among ZIF-25, -71, and -96. The lifetime of hydrogen bonding in ZIF-25 is shorter than that in ZIF-71 and -96. Furthermore, water molecules undergo a fast flushing motion in ZIF-25, but frequent jumping in ZIF-96 and particularly in ZIF-97. An Arrhenius-type relationship is found between water flux in ZIF-25 and temperature, and the activation energy is predicted to be 6.5 kJ/mol. This simulation study provides a microscopic insight into water desalination in a series of ZIFs, reveals the key factors (aperture size and polarity of functional group) governing water flux, and suggests that ZIF-25 might be an interesting reverse osmosis membrane for high-performance water desalination.