Porphyrinic metal-organic frameworks as separation membranes: from synthesis to advanced applications

Tetrakis(4-carboxyphenyl)porphyrin (TCPP) metal-organic frameworks (MOFs) represent a distinctive class of flexible, highly porous, crystalline, and compatible nanomaterials that have garnered substantial attention for the fabrication of high-performance advanced separation membranes. The TCPP ligand, when coordinated with various metal ions and nodes, such as Cu²⁺, Zn²⁺, Co²⁺, Cd²⁺, Al²⁺, Fe²⁺ and Zr⁴⁺, synthesize different M-TCPP MOFs with diverse porous nanostructures, exhibiting different nano-morphologies, including ultrathin nano-sheets, -fibers, -particles, and -rods. These nanostructures act as fundamental building blocks for the preparation of both porous and dense membranes. The accessible side functionalities (dangling carboxyl groups) of the TCPP MOFs significantly enhance the hydrophilicity and facilitate the formation of compatible interfaces with various polymers. This enhanced hydrophilicity, coupled with membrane stability in water and under acidic conditions, the presence of surface nanopores and the creation of continuous selective micro-/nano-transport channels (upon the assembly of nanostructures), substantially improves the membrane permeance and selectivity. These attributes make TCPP MOFs membranes suitable for a wide range of separation applications. This review comprehensively summarizes the synthesis strategies of TCPP MOFs, their nano-morphologies, properties, and their integration into state-of-the-art separation membranes, such as mixed matrix membranes (MMMs), thin-film composite (TFC) membranes, and thin-film nanocomposite (TFN) membranes. It examines their performance, discusses challenges, and explores potential solutions. Furthermore, the versatile separation capabilities of TCPP MOFs membranes, including gaseous, liquid, and ionic separation, as well as proton and metal ion conductivity are thoroughly analyzed in the light of mass transport theories and reported mechanisms. This review also delves into the photo-activity and photothermal effects of TCPP MOFs membranes, highlighting their implications for photocatalysis and membrane percolation. Finally, it outlines future directions and identifies potential opportunities for advancing TCPP MOFs membranes, aiming to elevate state-of-the-art separation technologies.