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.