Recent advances in developing mixed matrix membranes based on covalent organic frameworks

Mixed matrix membranes (MMMs) combining the virtues of porous fillers with polymeric matrices have been widely investigated due to their potential to overcome the permeability-selectivity trade-off. However, the compatibility issues at the organic/inorganic interface have barred their large-scale applications. With the rapid advances in chemistry and material science, recent research has turned to covalent organic frameworks (COFs) with regular pore structure, high specific surface area, and good compatibility with polymer matrix as promising fillers to improve the separation performance of MMMs. This review summarizes the research progresses on COFbased MMMs for gas and liquid separations, including various COF linkages, properties of COFs, and the selection principle of COF materials for target applications. Moreover, according to the geometric symmetry of building blocks and the different dimensions of the covalently connected frameworks, COFs fillers are categorized into two types: (1) 2D COFs; (2) 3D COFs. The applications of COF-based MMMs are subsequently reviewed, focusing on the effects of COF fillers on the performance of mixed matrix membranes in gas and liquid separations. Finally, the prospects and challenges of COF-based MMMs in industrial applications are briefly summarized to guide the future design of high-performance COF-based MMMs.