Adsorption of Nitrogen-Containing Compounds from Model Fuel over Sulfonated Metal–Organic Framework: Contribution of Hydrogen-Bonding and Acid–Base Interactions in Adsorption

Adsorptive denitrogenation (ADN) was carried out by adsorption of indole (IND) and quinoline (QUI) over metal–organic frameworks (MOFs) including acidic UiO-66—SO 3 H for the first time. The adsorbed amount of IND increased with increasing content of —SO 3 H in UiO-66. The favorable effect of the —SO 3 H group on the adsorptive removal of IND could be explained by hydrogen bonding between the O of —SO 3 H and the H of IND, which was firmly supported by the adsorption of pyrrole and methylpyrrole and by theoretical calculations. The application of an —SO 3 H group in the adsorptive removal of neutral IND is meaningful since neutral nitrogen-containing compounds are not easy to remove and since UiO-66—SO 3 H is reusable after simple washing with ethanol. The expected increase in QUI adsorption (due to acid–base interaction) with acidic —SO 3 H was observed when QUI was present at low concentrations (<∼400 ppmw). This favorable contribution of acidic —SO 3 H to the adsorption of basic QUI was also supported by calculations for the adsorption of one QUI molecule on the —SO 3 H group of UiO-66. Interestingly, the adsorbed amount of QUI decreased with increasing content of —SO 3 H in UiO-66 when the QUI concentration was high (initial concentration of 1000 ppmw). One of the reasons for the negative effect of acidic —SO 3 H on QUI adsorption might be the presence of only one H atom in —SO 3 H or steric hindrance (due to decreased pore space), although detailed works are needed to support this.