Zhou Wei, Wei Mingjie, Zhang Xin, Xu Fang, Wang Yong
State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, and College of Chemical Engineering , Nanjing Tech University , Nanjing 211816 , Jiangsu , P. R. China.
ACS Appl Mater Interfaces. 2019 May 8;11(18):16847-16854. doi: 10.1021/acsami.9b01883. Epub 2019 Apr 24.
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% MgCl 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.
共价有机框架（COFs）具有均匀且有序的纳米孔，这意味着它们在分子/离子分离方面具有巨大潜力。作为一种亚胺连接的稳定COF，TpPa-1在分子筛膜方面受到了极大关注。理论上，原子级薄的TpPa-1单层膜表现出极高的水渗透率，但遗憾的是由于其较大的孔径（约1.58纳米），对离子没有截留作用。COF单层膜倾向于堆叠形成层状多层膜，但这种堆叠如何影响水传输和离子截留仍不清楚。在此，我们通过非平衡分子动力学模拟研究了水和盐离子在多层TpPa-1 COFs中的传输行为。通过分析水传输的界面阻力和内部阻力，我们发现随着COF多层膜堆叠数的增加，离子截留率增加，但水渗透率降低。更重要的是，以错位重叠方式堆叠可将COF多层膜的等效孔径显著减小至0.89纳米，离子截留率相应提高。值得注意的是，以这种方式堆叠25个COF单层膜可实现100%的MgCl截留率，而水渗透率仍比商业纳滤膜高1至2个数量级。这项工作展示了通过调整COF单层膜的堆叠数和堆叠方式来合理设计用于海水淡化的快速膜。
