Adsorptive Removal of Indole and Quinoline from Model Fuel over Various UiO-66s: Quantitative Contributions of H-Bonding and Acid-Base Interactions to Adsorption

Nitrogen-containing compounds (NCCs) such as indole (IND) and quinolone (QUI) in a model fuel were adsorbed over pristine and variously functionalized metal-organic frameworks (MOFs) (here, UiO-66 and -NH₂, -NH₃⁺, -COOH, -COONa, -OH, -SO₃H functionalized UiO-66s) to quantitatively understand the interactions between the adsorbates (IND and QUI) and UiO-66s. The adsorbed quantity of IND and QUI increased linearly with increasing number of H-acceptors and H-donors (for H-bond), respectively, on UiO-66s (excluding one MOF for each adsorption), confirming the importance of H-bonding in the adsorption. UiO-66-NH₃⁺ and UiO-66-NH₂ showed a deviated trend in the IND and QUI adsorption, respectively; this might be explained by cation-π interactions and base-base repulsion, respectively. Moreover, the QUI adsorption increased linearly with increasing number of acidic sites on the MOFs (excluding basic ones), also suggesting the importance of acid-base interactions. Finally, UiO-66-NH₃⁺ showed the highest adsorption for both IND and QUI among the studied MOFs, suggesting that introducing an ammonium group on MOFs can be one way to develop a competitive adsorbent for the adsorptive denitrogenation of fuels.