(366i) Water Desalination through Zeolitic-Imidazolate Framework Membranes: A Molecular Simulation Study

Fresh water scarcity has become a major global concern due to the rapid growth in population, energy demand and industrialization. As over 95% of water on the Earth is seawater, there is considerable interest to desalinate seawater to supply freshwater. A handful of desalination techniques have been developed including reverse osmosis (RO) and multistage flash distillation (MSFD). Compared to thermal-based MSFD, RO is more energy efficient. In the present scenario, 42 million kg of potable water is produced daily by RO, which accounts for 60% of world desalination capacity. The capital cost of current RO technology is high and it needs to be improved prior to propagation. Aiming to develop efficient RO membranes, a large number of studies have been conducted. In the past decade, zeolitic-imidazolate frameworks (ZIFs) have emerged as a new class of nanoporous materials which possess exceptional chemical and thermal stability. In addition, their pore size and affinity are readily tunable, thus there is a large opportunity to explore ZIFs with improved desalination performance.

In this work, water desalination through ZIF-25, -71, -93, -96 and -97 membranes is investigated by molecular simulation. The ZIFs possess identical rho-topology, but differ in functional groups. Due to the presence of various functional groups, the aperture size and polarity differ in the five ZIFs. 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/(m².hr.bar), about one ~ two orders of magnitude higher compared to commercial RO membranes. Because of a larger aperture size d_a, ZIF-25, -71 and -96 exhibit much higher water flux than ZIF-93 and -97. The flux in ZIF-25, -71 and -96 is governed by the polarity of functional group rather than d_a. With hydrophobic â??CH3 group, ZIF-25 has the highest flux despite the smallest d_aamong ZIF-25, -71 and -96. With the highest flux, the life time of hydrogen bonding in ZIF-25 is shorter than in ZIF-71 and -96. Furthermore, water molecules undergo 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 governing water flux, and suggests that ZIF-25 might be an interesting reverse osmosis membrane for high-performance water desalination.