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—SO3H for the first time. The adsorbed amount of IND increased with increasing content of —SO3H in UiO-66. The favorable effect of the —SO3H group on the adsorptive removal of IND could be explained by hydrogen bonding between the O of —SO3H and the H of IND, which was firmly supported by the adsorption of pyrrole and methylpyrrole and by theoretical calculations. The application of an —SO3H group in the adsorptive removal of neutral IND is meaningful since neutral nitrogen-containing compounds are not easy to remove and since UiO-66—SO3H is reusable after simple washing with ethanol. The expected increase in QUI adsorption (due to acid–base interaction) with acidic —SO3H was observed when QUI was present at low concentrations (<∼400 ppmw). This favorable contribution of acidic —SO3H to the adsorption of basic QUI was also supported by calculations for the adsorption of one QUI molecule on the —SO3H group of UiO-66. Interestingly, the adsorbed amount of QUI decreased with increasing content of —SO3H in UiO-66 when the QUI concentration was high (initial concentration of 1000 ppmw). One of the reasons for the negative effect of acidic —SO3H on QUI adsorption might be the presence of only one H atom in —SO3H or steric hindrance (due to decreased pore space), although detailed works are needed to support this.