Fast Desalination by Multilayered Covalent Organic Framework (COF) Nanosheets

Abstract

Covalent organic frameworks (COFs) are penetrated with uniform and ordered nanopores, implying their great potential in molecular/ion separations. As an imine-linked, stable COF, TpPa-1 is receiving tremendous interest for molecular sieving membranes. Theoretically, atomically thin TpPa-1 monolayers exhibit extremely high water permeance but unfortunately no rejection to ions because of its large pore size (∼1.58 nm). The COF monolayers tend to stack to form laminated multilayers, but how this stacking influences water transport and ion rejections remains unknown. Herein, we investigate the transport behavior of water and salt ions through multilayered TpPa-1 COFs by nonequilibrium molecular dynamics simulations. By analyzing both the interfacial and interior resistance for water transport, we reveal that with rising stacking number of COF multilayers exhibit increasing ion rejections at the expense of water permeance. More importantly, stacking in the offset eclipsed fashion significantly reduces the equivalent pore size of COF multilayers to 0.89 nm, and ion rejection is correspondingly increased. Remarkably, 25 COF monolayers stacked in this fashion give 100% MgCl2 rejection, whereas water permeance remains 1 to 2 orders of magnitude higher than that of commercial nanofiltration membranes. This work demonstrates the rational design of fast membranes for desalination by tailoring stacking number and fashion of the COF monolayers.

Nanofluidic Behaviors of Water and Ions in Covalent Triazine Framework (CTF) Multilayers

“…The pressure and temperature were modulated by the Nosé–Hoover barostat and thermostat, respectively. Other details of the NEMD simulations are in consistent with our previous work . The numbers of atoms in our simulations are varying due to the presence of several numbers of CTF layers.…”

“…Compared to TpPa‐1, which is an extensively studied, imine‐linked COF, CTF monolayers exhibit lower water permeance due to their smaller intrinsic pore sizes. However, smaller pore sizes will result in better ion rejection.…”

“…Following our previous work, we define the total resistance of membranes as where J is PWF, m s −1 , Δ P is the pressure drop across the membrane, Pa, and R is the total resistance, Pa s m −1 . Our previous works demonstrated that the total resistance ( R total ) is composed of the interfacial resistance ( R interfacial ) and the interior resistance ( R interior ) where R interior is related to the membrane thickness while R interfacial is independent of thickness. According to Sampson' equation, the R interfacial can be described as where r is the pore radius, µ = 0.91 × 10 −3 N s m −2 is the dynamic viscosity for SPC/E water and C is the loss coefficient that is a constant related to the physical and chemical properties of CTF in this work.…”

“…In addition to the contribution of pore size and porosity, there is another component of R interior , which is originated from the stacking structure of COFs. It was discovered in our previous work that the layer‐by‐layer stacking structure resulted in additional R interior due to the “rough” pore walls consisted of layered positions and interlayer gaps. The CTF multilayers have the similar stacking structure as well, therefore, the water density profile along the axis of pores shows a regularly jagged curve corresponding to the positions of CTF monolayers ( Figure ).…”

“…Therefore, CTF multilayers composed of multiple monolayers is a more realistic choice to build CTF‐based membranes. We previously demonstrated that, in the case of an imine‐linked COF, TpPa‐1, the number and fashion of stacking COF monolayers significantly influenced the effective pore sizes and consequently the permselectivity of the COF multilayers . Such an effect is expected to take place in multilayered CTFs, but an in‐depth, systematic investigation is required as CTFs are qualitatively different from imine‐linked COFs in terms of both chemical nature and pore structure.…”