Water desalination of a new three-dimensional covalent organic framework: a molecular dynamics simulation study

Preparing a nanoporous membrane with high density and ordered pore sizes which allows high water permeability and salt rejection rate is the key to realize highly efficient desalination. However, preparing a nanoporous membrane with high density and order pore sizes is still extremely hard due to the limitation of experimental techniques. Recently, a 3D covalent organic framework (3D-COF) material named as the 3D-OH-COF with good crystallinity and large specific surface areas has been synthesized. Based on the structural features of the 3D-OH-COF, we speculate that it may be a good candidate for the desalination application derived from its high-density sub-nanometer pore. In this work, using molecular dynamics simulations, the possibility of the 3D-OH-COF for desalination application was explored, the influence of membrane thickness on its desalination performance was also studied, and the detailed structure and dynamics of ions and water transport in the channel of the 3D-OH-COF was discussed. The results show that the rectangular channel structure and charged H atoms are responsible for the excellent salt rejection rate (100%) and high water flux (41.44 Lit cm^-2 day^-1 MPa^-1), respectively. Furthermore, the water flux is three orders of magnitude higher than that of the commercial reverse osmosis membrane and is four times higher than that of the theoretically reported monolayer nanoporous MoS₂ membrane. It is also about 28% higher than that of the recently reported 2D-CAP membrane. This work theoretically confirms that the 3D-OH-COF is a promising membrane material for desalination applications and the underlying molecular mechanisms are clarified.