Free-standing membranes based on 2D materials for selective separation

Two-dimensional (2D) materials show great potential as novel membrane materials due to their atomic thickness and periodic pore structure. Currently, free-standing membranes based on 2D materials open up new avenues for ultra-fast and highly selective separation. With the absence of porous substrates, free-standing membranes offer shortened transport paths for efficient mass transfer. The interfacial defects between the substrate and selective layer are eliminated to alleviate the internal membrane fouling, enabling the intact structure for precise separation. Hence, this review aims to outline the superiority of 2D material-based free-standing membranes for selective separation applications. Free-standing 2D material membranes composed of the most representative graphene-based materials, MXene, covalent organic framework (COF), metal organic framework (MOF), and hydrogen-bonded organic framework (HOF) are summarized with the discussion on the influence of substrate on their structural properties. The separation performance enhancement strategies in regard to the 2D material, membrane structure, and mechanical properties are examined. Finally, we propose several critical challenges and perspectives in terms of pore size control, mechanical strength improvement, understanding the underlying mass transfer mechanism, issues related to membrane fabrication optimization, scale production, and separation application versatility. This review will provide researchers with practical guidelines for advancing free-standing 2D material membranes for future selective separation applications.