Simultaneous adsorption of anionic and cationic species on UiO-66 and MIL-125 MOFs: Mechanisms & selectivity

• Published in: Microporous and mesoporous materials Vol. 368; p. 113010
• Main Authors: Palhares, Hugo G., Andrade, Pedro H.M., Moissette, Alain, Volkringer, Christophe, Loiseau, Thierry, Houmard, Manuel, Nunes, Eduardo H.M.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15.03.2024; Elsevier
• Series: Microporous and Mesoporous Materials
• Subjects: Chemical Sciences; Inorganic chemistry; Metal-organic frameworks; MIL-125; Organic dyes; Simultaneous adsorption; UiO-66; Simultaneous adsorption; MIL-125; UiO-66; Metal-organic frameworks; Organic dyes
• ISSN: 1387-1811; 1873-3093
• DOI: 10.1016/j.micromeso.2024.113010

Metal-organic frameworks (MOFs) have attractive properties, including regular pore structure, high specific surface area, tunable surface properties, and abundant active sites. Such properties have boosted their recent development in many fields, including water treatment. In this study, MOFs composed of tetravalent metals (Ti4+ and Zr4+) and dicarboxylate linker (terephthalate – BDC) were modified using surface functionalization and/or pore structure through defect engineering to enhance adsorption properties. It has been shown that the Ti-containing materials (MIL-125) show a greater affinity for cationic species, while the Zr-containing samples (UiO-66) have a higher affinity for anionic species. Surface charge and electrostatic interactions could account for such a difference in adsorption properties. Zeta potential measurements showed that MIL-125 has a more negative surface charge for both non-functionalized and amino-functionalized versions, which favors the interaction with cationic molecules. On the other hand, UiO-66 has a more positive surface, which facilitates interactions with anionic molecules. Within each MOF family, the non-functionalized structures emerged as the best-performing materials. Specifically, MIL-125(H) and UiO-66(H)_5eq, with 5eq denoting the amount of H2O added during synthesis, showed superior performance. Furthermore, by performing simultaneous adsorption experiments with anionic and cationic contaminants, it was found that the adsorption of the anionic dye acid orange 7 (AO7) onto the UiO-66 framework is primarily based on specific interactions between the sulfonic group (-SO3-) within the AO7 molecule and the inorganic core Zr6O4(OH)4 of UiO-66. The interaction between MIL-125 and the cationic dye methylene blue (MB) was found to be more delocalized, based on electrostatic and π-π interactions between the benzene domains. Finally, except for MIL-125(H), all the tested MOFs showed high stability in water and in the adsorption process. The higher stability of UiO-66 over MIL-125 materials can be attributed to steric effects and lower metal electronegativity. Compared to MIL-125(H), the higher stability of MIL-125(NH2) is probably related to the formation of intramolecular hydrogen bonds. [Display omitted] •Optimized crystalline and porous structure of UiO-66(H) and UiO-66(NH2).•Higher affinity of UiO-66 for anionic species and MIL-125 for cationic species.•Verification of different adsorption mechanisms of MB on MIL-125 and AO7 on UiO-66.•Higher water stability of UiO-66 family due to steric hindrance and electronic effects.