Fine-Tuning Dye Adsorption Capacity of UiO-66 by Mixed-Ligand Approach

Mixed ligand synthetic approach offers an alternative to engineer a specific character in metal-organic framework (MOFs) materials. Herein, we synthesized and characterized a well-known prototype zirconium based-MOF, so called UiO-66 and its mixed ligand derivatives UiO-66-xATA, where x is mole fraction (0.5, 0.75 and 1.0) and ATA is 2-animoterephthalate. The aim of the study is to observe whether the dye adsorption capacity can be tuned/enhanced by the ATA ligand substitution into the framework. We found that, at room temperature, UiO-66-0.75ATA shows the highest adsorption capacity toward various dye solution including methylene blue (MB), indigo carmine (IC) and congo red (CR). The optimum adsorption conditions in all four materials were found to be in a common trend where their adsorption capacities can be increased with decreasing pH and adsorbent dose, increasing of IC concentration, contact time and temperature. Pseudo-second order kinetics model fits best with their adsorption data, where UiO-66-ATA has the fastest rate of adsorption. Langmuir and Freundlich isotherms were found to best describe adsorption behaviour in ATA-containing UiO-66 and UiO-66 respectively, where adsorption processes were found to be physisorption. Confirming by thermodynamic studies, the adsorption in all four materials occurred spontaneously, driven by entropy. Computational studies showed ligand to metal charge transfer where the distribution of electron densities was varied with the amount of functionalized ligand. Adsorption mechanism is proposed as a synergistic interplay between electrostatic interaction and hydrogen bonding. The findings in this work broaden potential strategy to fine-tune the dye adsorption capacity in MOF materials.