Adsorptive removal of nitrogen-containing compounds from liquid fuel using a Zr-based metal–organic framework composed of amine or ammonium sites: Synergistic contribution of the loaded sites and defects

Removal of nitrogen-containing compounds (NCCs) is important to effectively utilize fuel, especially microalgae-derived ones with high N contents. In this work, adsorptive removal of NCCs like quinoline (QUI) and indole (IND) from liquid fuel was carried out by using various metal–organic frameworks (MOF-808s) such as pristine MOF-808, aminated MOF-808s with defect, an aminated MOF-808, defective MOF-808s, and a defective MOF-808 with ammonium sites. The performance of the MOF-808 increased with increasing the content of both the –NH₂ group and defects; showing a very favorable or synergistic contribution of the two sites to the adsorption. Moreover, the protonation of the aminated MOF-808s with defect increased further QUI adsorption although this treatment decreased the IND adsorption. Calculations were applied to understand the adsorption over amino or ammonium sites; hydrogen bonding had a dominant role in the adsorption of QUI and IND over the aminated MOF-808s; whereas, acid-base interactions were important in the QUI adsorption over MOF-808 with ammonium sites. One adsorbent, P-M808-50(NH₂/D) (a protonated MOF-808 composed of ample –NH₂ groups and defects), with a very remarkable maximum capacity for QUI (769 mg/g, the second one, after UiO-66(Zr)-DHTZ) could be suggested as a potential adsorbent to remove NCCs (especially basic ones) from fuel, based on the huge adsorption capacity and ready recyclability via simple solvent treatment.