High Adsorption Affinity of Indole in Defective UiO-66 Metal-Organic Frameworks: A First-Principles Study

Herein the defect engineering is reported in zirconium 1,4-dicarboxybenzene metal-organic frameworks (UiO-66) both inducing new adsorption sites and enhancing the adsorption affinity of indole adsorbates. The result shows that high adsorption energies for indole are obtained at the deprotonated mu 3-O, followed by unsaturated Zr-cation sites in defective UiO-66 structures. The key interactions are the hydrogen bonding between the hydrogen donor NH group of indole with the negative charge oxygen atom of the induced mu 3-O site and the dative covalent bond between the nitrogen atom of indole with the unsaturated Zr-cation. For hydrogen bonding, the binding energies tend to increase with the increasing of defect concentrations and spread a broad range up to approximate to-2.00 eV. It is also shown that the vibrational modes of indole molecules are influenced by their adsorption sites. Accordingly, the NH stretching modes of the adsorbed indole are redshifted approximate to 100-300 cm-1 compared to its counterpart in the isolated indole. The strongest redshift is obtained for indole adsorbed at the mu 3-O site with three missing linkers. The occupation of water at the active sites may enhance or diminish the adsorption affinity of indole relying on the hydrogen bonding mechanism. High adsorption energies of INDOLE are found in the missing-linker UiO-66 structures compared to the ideal ones. The dative covalent and hydrogen bondings are the keys to those outstanding energetic performances arising from the binding of INDOLE to the induced unsaturated Zr-cation and dehydrogenated mu 3-O adsorption sites. The presence of water is favorable to intensify INDOLE binding for low linker-defect concentrations, but not for higher ones due to the shielding of active sites. image