Protonated MIL-125-NH₂: Remarkable Adsorbent for the Removal of Quinoline and Indole from Liquid Fuel

The removal of nitrogen-containing compounds (NCCs) from fossil fuels prior to combustion is currently of particular importance, and so we investigated an adsorptive method using metal-organic frameworks (MOFs) for the removal of indole (IND) and quinoline (QUI), which are two of the main NCCs present in fossil fuels. We herein employed an amino (-NH₂)-functionalized MIL-125 (MIL-125-NH₂) MOF, which was further modified by protonation (P-MIL-125-NH₂). These modified MOFs exhibited extraordinary performance in the adsorption of both IND (as representative neutral NCC) and QUI (as representative basic NCC). These MOFs were one of the most efficient adsorbents for the removal of NCCs. For example, P-MIL-125-NH₂ showed the highest adsorption capacity for QUI among ever reported adsorbent. The improved adsorption of IND was explained by H-bonding and cation-Ï€ interactions for MIL-125-NH₂ and P-MIL-125-NH₂, respectively, while the mechanisms for QUI were H-bonding and acid-base interactions, respectively. This is a rare phenomenon for a single material (especially not with very high porosity) to exhibit such remarkable performances in the adsorption of both basic QUI and neutral IND. The adsorption results obtained using regenerated MIL-125-NH₂ and P-MIL-125-NH₂ also showed that these materials can be used several times without any severe degradation.