Metal-Organic Frameworks as a Subnanometer Platform for Ion- Ion Selectivity

CONSPECTUS: Selective ion transport in nanoporous membranes has become a research hot spot in the fields of both nanofluidics and membrane separation because of its wide potential applications, such as ion separation, energy harvesting and conversion, and ion sensing. Developing nanofluidic devices or membranes with nanoconfined to subnanoconfined space is the core part of nanofluidic studies because the materials used to construct nanofluidic devices determine the exploration potential of the ion-transport performance. In addition, tuning the macroscopic structure of devices, such as asymmetric or heterogeneous structures, in terms of both aperture size and charge density can impart multiple nanofluidic functions. Metal- organic frameworks (MOFs) are a promising class of crystalline porous materials with desirable structures for constructing controllable microscopic nanofluidic devices as an alternative to other materials, such as Si/SiNx, polymers, and two dimensional (2D) layered materials. (i) MOFs have high porosity with aperture sizes ranging from the angstrom to nanometer scale and can provide abundant pathways for ion transport. (ii) Water-stable MOFs can maintain channel stability without severe swelling issues. (iii) The huge diversity of MOFs in terms of aperture size, channel dimension, and chemical environment allows for the systematic investigation of one individual effect on selective ion transport by excluding other influencing factors to a great extent. (iv) Most MOFs have well-ordered molecular structures with the arrangement of each atom being known. This favors the high reliability of theoretical modeling to simulate nanofluidic transport and then unveil mechanisms for selective ion transport. In this Account, we discuss recent progress in the selective ion transport of MOF-based subnanochannel (MOFSNC) membranes by the rational design and fabrication of high-quality SNC membranes, mainly based on our group's work. First, we refer to ion-ion selectivity as anion-anion or cation-cation selectivity and cation-anion or anion-cation selectivity while excluding the water molecule-ion selectivity in desalination and water purification. Second, we highlight the advantages of MOF structures as SNC platforms for the investigation of subnanometer confined ion transport. Third, we discuss the rational design of MOFSNCs for ultimate ion selectivity by molecular engineering and membrane structural engineering of MOFSNCs with the biomimetic strategy. Finally, we conclude this Account with a perspective on the future opportunities and major challenges in MOFSNC membranes, such as their practical applications in ion separation and nanofluidics.