Postsynthetic modification strategies to improve polycrystalline metal-organic framework membranes

A wide variety of metal organic frameworks (MOFs) created by periodic combinations between organic ligands and metal ions or metal-oxo clusters have paved the way for the development of energy-efficient membrane-based separations that potentially can serve as feasible substitutes for thermal counterparts. Although significant progress has been made in the synthesis of polycrystalline MOF membranes over the last decade, only a limited number of MOFs have been utilized for relevant research. The lack of a clear solution for intercrystalline defects, non-selective diffusion pathways in polycrystalline membranes, is most likely a deciding factor behind the delay. Postsynthetic modifications (PSMs) are considered as newly emerging strategies for offering polycrystalline MOF membrane diversity by utilizing existing membranes as a platform and improving their separation functions via physical and/or chemical treatments and therefore, neither designing new MOFs nor customizing membrane synthesis techniques for targeted MOFs are required. In this minireview, eight subclasses ((1) covalent tethering, (2) intercrystalline defect plugging, (3) intracrystalline defect healing, (4) inclusion of functional materials in pores, (5) stiffening, (6) ligand exchange, (7) amorphization, and (8) MOF to MOF transformation) of PSM strategies, that have been applied to polycrystalline MOF membranes, are summarized, and challenges and future directions are discussed.