(647d) Molecular Dynamics Simulation of Water Desalination Via Functionalized Covalent Organic Framework Membranes

The scarcity of fresh water has become a great concern with a rapidly growing global water gap. As an energy-efficient desalination technique, reverse osmosis (RO) accounts for more than half of the existing desalination capacity. In order to screen suitable nanoporous membrane materials, and to investigate the effects of pore size and functionality, we design a series of single layer covalent organic framework (COF) membranes based on an experimentally synthesized COF structure TpPa-1. They consist of 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa) with various functional groups, namely TpPa-AM2, TpPa-AMC₂NH₂, TpPa-OC₃OH, TpPa-OC₄H₉, TpPa-AMCOOH, TpPa-OBn and TpPa-AM3. The molecular dynamics (MD) simulation results indicate that water permeances in these COF membranes are affected by both pore size and functionality. Water traverse across the pore in AMC₂NH₂, TpPa-OC₃OH and TpPa-AMCOOH is enhanced, due to hydrophilic functional groups pointing inward the pores. When the polarity of functional groups is similar, the pore size becomes a dominant factor exhibiting a positive correlation with water permeance. Nevertheless, all the COF membranes show exceptionally high water permeance ranging from 1216 to 3375 kg/(m²·h·bar), more than three orders of magnitude higher than commercial RO membranes. Furthermore, water density distribution within the membrane pores are found to be highly dependent on hydrophilicity of the pores, resulting in higher water density for hydrophilic membranes. The salt rejection is calculated to be 95.8% in TpPa-AMCOOH, and above 98.3% in other TpPa membranes. Among all the TpPa membranes designed, TpPa-OC₃OH has high water permeance (2816 kg/(m²·h·bar)) with excellent salt rejection (99.4%). This simulation study provides a microscopic insight into the key factors governing water desalination, and suggests that TpPa-OC₃OH might be a potential candidate for high performance water desalination.