Orientation-Dependent Anisotropic Desalination by Assembled Zeolite Nanotube Membranes

Porous nanomaterials have shown great promise in many desalination applications. Zeolite nanotubes, featuring abundant but inhomogeneous nanopores on their surface, have been recently synthesized in experiments; however, their capacity for desalination is not yet understood. In this work, we use molecular dynamics simulations to investigate the capability of assembled zeolite nanotube membranes to perform in desalination applications due to their inherent multiscale porous properties. Two different membrane assemblies are examined to determine the effect of membrane orientation on desalination performance. Interestingly, we find that zeolite nanotube membranes present anisotropic desalination behavior, which is directly dependent on the assembled orientation of the zeolite nanotubes. Specifically, directing the transport through the axial channels of the nanotubes results in a water permeability of 59.8 L/cm²/day/MPa and 88% ion rejection. However, when the membrane is rotated 90° and the flow is directed perpendicular to the tube axis, the permeability drops to 22.3 L/cm²/day/MPa, but 100% ion rejection is achieved. This difference is attributed to the multiscale pore dimensions of the zeolite nanotube; that is, they possess large pores (a diameter of 3 nm) along the axial channel direction, but smaller pores (a diameter of 0.25 nm) along the direction perpendicular to the tube axis. The ion rejection capabilities are further verified by quantifying the free energy barriers to transport obtained via umbrella sampling simulations. Therefore, our findings demonstrate the orientation-dependent, anisotropic desalination performance in assembled zeolite nanotube membranes for the first time, which could be useful in designing future advanced desalination membranes.