Selective and adsorptive removal of anionic dyes and CO₂ with azolium-based metal-organic frameworks

The positively charged azolium moieties make imidazolium linker an ideal linker for the construction of cationic metal-organic frameworks because the ligand induces cationic environments in the frameworks. Therefore, we employed two imidazolium ligands, 1,3-bis(4-carboxyphenyl)imidazolium chloride (H₂L⁺Cl¯) and 1,3-bis(3,5-dicarboxyphenyl)imidazolium chloride (H₄L⁺Cl¯), to synthesize two nickel azolium-based MOFs, 1 and 2. The as-synthesis MOFs were characterized by PXRD, TGA, FE-SEM, HR-TEM, FTIR and BET measurements. By applying 1 and 2 in liquid phase adsorption of charged molecules of dyes, they successfully exhibit remarkable efficiency for adsorptive removal of anionic dyes, Methyl orange (MO), Congo red (CR), and Orange II sodium salt (OS), from aqueous solution. The framework proves efficient in photocatalytic degradation of anionic dye. Furthermore, in the gaseous phase adsorption, 1 and 2 selectively adsorb CO₂ over CH₄ due to the higher quadrupole moment of CO₂. Overall, the results show that azolium-based MOFs have potential applications for adsorptive removal of charged organic contaminants from both aqueous and gaseous environment.