The structure-property relationships that determine the selectivity and permeate flux through a membrane have lacked systematic investigation on the molecular level for Covalent Organic Frameworks, especially for complex liquid mixtures such as dye molecules, proteins and ionic liquids. Ultra-thin membranes are highly desirable, ideally with nanometer thickness and ordered nanopores that offer high separation performance while maintaining high strength. In pursuit of this goal, the past decade has seen an explosion of interest in two-dimensional (2D) materials as building blocks for molecular sieving membranes, which started with the realization of the extraordinary properties of graphene and extended to other 2D materials such as transition metal dichalcogenides and exfoliated zeolites. Compared with conventional porous three-dimensional (3D) membranes, such as 3D covalent organic frameworks (COFs)and metal organic frameworks (MOFs), these 2D materials possess transformative properties, including a large and chemically active external surface area, chemically tailored pores, and atomic thickness.  The ultrathin structure and potential for tunability of pore size and interlayer distances of porous 2D materials enable the resultant membranes to achieve high flux while maintaining high selectivity, owing to their inherent high selectivity based on size, charge or other molecular properties. In addition, unlike 3D-MOF materials, the 2D structure of these materials allow them to naturally stack into a 2D structure with no large gaps between grains, unlike most MOF membranes. 

There has also been a great deal of activity using graphene oxide as a size-selective membrane material. However, its selectivity is limited allowing only small molecular species to squeeze between the layers creating a tortuous path that limits their flux. As such, the biggest current challenge in the field of separation science is to design and construct membrane materials via a bottom-up approach that allows the separation of the species from solutions based on size, charge, or both by either designing or modifying the membrane materials’ building blocks. Recently, MOF nanosheets composed of carboxylate ligands have been investigated as promising building blocks for 2D molecular sieving membranes because of their structural diversity and pore geometry. However, structural deterioration and morphological damage of MOF nanosheets during the membrane preparation and separation have hindered their application. We have fabricated membranes from novel nanoporous, two-dimensional covalent organic framework (2D-COF) materials that have unprecedented charge and size-selectivity due to their high degree of crystalline order and their synthetic flexibility for nanopore functionalization. Straightforward synthetic modifications allow control and tuning of both pore size and the number of charged functional groups within the pores. Therefore, by extending our preliminary studies where we have demonstrated precise size-selectivity on a series of cations.  We propose to systematically investigate fundamental separation problems, especially related to the complex microenvironments at the liquid-membrane interface resulting from the solute-solvent-COF interaction. Specifically, cation and anion solutes with various sizes, with their counterion fixed, will be utilized in a plethora of solvents, carefully selected based on size, polarity and dipole moment, for a series of COF membranes, whose size and charge will be systematically varied. In addition, we will probe the fundamental limits of flux and selectivity of these 2D-COF membranes by measuring the selectivity and ion fluxes through individual sheets of both single and multiple 2D-COF layers by using scanning electrochemical cell microscopy (SECCM).