Membrane-based separation is an emerging technology to separate different components. [1] Owing to not only the functions of separation, concentration, and purification, but also the properties of energy efficiency, environmental friendliness, high efficiency, simplicity, manufacturing scalability, small footprint, and ease of operation and control, this technology has been widely used in chemical industry, environmental protection, medicine, food, and desalination. [2,3] Membrane is regarded as a significant method to solve the far-reaching issues such as water shortages, environmental pollution, and energy crisis.Along with the rapid industrialization and sharp increase in population, the problems of water shortages, environmental pollution, and energy crisis become more and more severe and complex, leading to the higher technology requirement for membrane separation. Although the frequently used traditional polymer membrane featured with low cost, good mechanical strength, excellent flexibility, and ease of processing, the two key parameters of permeability and selectivity that are generally used to evaluate membrane separation performance are interinhibitive because of the well-known trade-off effect. [4] One of the main reasons for the trade-off effect is the wide distribution of pore size in polymeric membranes. Therefore, materials with uniform and well-ordered pores are considered as perfect candidates for fabricating highperformance separation membranes. Keeping this in perspective, many meaningful studies have been conducted for constructing homoporous membranes [5] with traditional polymers as starting materials. The obtained membranes have a pore size ranging from 5 to 50 nm and possess great potential for ultrafiltration (UF). [6] However, in some practical application systems such as gas separation, dyes removal, protein and drug recovery as well as desalination, the kinetic dimensions of components to be separated usually are less than 5 nm. Therefore, it is very important that a homoporous structure with a pore size of less than 5 nm is constructed to be used in membrane separation.Covalent organic frameworks (COFs) [7][8][9][10][11] are an emerging class of organic porous crystalline materials with pores that range from 0.5 to 5 nm, [12,13] which are entirely composed by light elements, such as C, H, O, N, B, and Si and are connected by strong covalent bonds. Owing to the unique nature of their well-ordered and tailorable pore channels, high and permanent porosity, excellent thermostability and chemical stability, and ease of functionalization, COF materials have been demonstrated promising potential in many applications, including separation, [14] catalysis, [15] sensor, [16] optoelectronics, [17] energy conversion, [18] and semiconductors, [19] among others. Particularly, these characteristics of COF materials perfectly meet the requirements for making advanced separation membrane. For example, the uniform aperture, high porosity, and excellent stability are beneficial
