Adsorptive removal of nitrogen-containing compounds from a model fuel using a metal–organic framework having a free carboxylic acid group

Adsorptive removal of nitrogen-containing compounds from a model fuel was carried out by adsorbing indole (IND) and quinolone (QUI) over metal–organic frameworks (MOFs). UiO-66s, prototypes of MOFs, were applied as adsorbents with or without functionalization of free carboxylic groups. Even though the porosity of the functionalized UiO-66 (UiO-66-COOH) was lower than that of the pristine UiO-66, its adsorption capacity for neutral IND improved remarkably (103% and 28% based on surface area and weight, respectively) with functionalization. The enhanced performances of UiO-66-COOH could be explained by the formation of H-bond interactions between O (of –COOH) and H (of IND), which was supported by the differences in the adsorption of pyrrole vs. methylpyrrole. Curiously, the adsorbed amount (based on weight) of basic QUI decreased after the introduction of free –COOH on UiO-66, even though favorable acid–base interactions were expected. However, the amount of adsorbed QUI increased by 41% based on surface area. The adsorptive performances of UiO-66s for pyridine were very similar to those for QUI. Therefore, UiO-66-COOH might be also beneficial for the adsorption of basic NCCs, probably because of the existence of acid–base interactions (between N of NCCs and H of –COOH). However, the effect of free COOH on the adsorption of basic NCCs is generally lower than that of neutral NCCs having hydrogen atoms capable of acting as hydrogen bond donors. Based on the reusability results and the improved performances for IND and pyrrole adsorption, MOFs having free –COOH can be suggested as potential adsorbents to remove neutral NCCs, which are not easy to remove because of their lack of functionality.