Strategic Secondary Ligand Selection for Enhanced Pore-Type Construction and Water Purification Capacity in Zeolitic Imidazolate Frameworks

Abstract

Selective ligand removal (SeLiRe) is a powerful strategy for constructing novel pore-type and ligand-defective structures in metal–organic frameworks (MOFs), but few studies have focused on the effect of secondary ligands with different functional groups on this process. We synthesized versions of zeolitic imidazolate framework-8 with six different secondary ligands and comprehensively investigated their pore-type structures after SeLiRe treatment. Their pore volume, size, and distribution are closely related to the respective organic functional groups on the secondary ligands. NH₂-functionalized ligands tend to form larger domains and have weaker Zn–N_β covalent bonds, which facilitate the removal process and the construction of larger cavities. Among the six secondary ligands, 5-bromo-1H-benzo[d]imidazol-2-amine exhibits the composite pore-type structure with hierarchical micro- and mesopores, achieving the highest methylene blue adsorption capacity of 28.1 mg g^–1. Compared to traditional sodalite-type ZIFs, this results in a 53-fold increase in water pollutant adsorption. This work highlights the crucial role of the secondary ligand in the SeLiRe strategy and provides valuable insights for designing other hierarchical porous hybrid structures.