Carboxylic-acid-functionalized UiO-66-NH₂: A promising adsorbent for both aqueous- and non-aqueous-phase adsorptions

UiO-66 functionalized with –NH– and –COOH groups was obtained by postsynthetic modification of aminated UiO-66. In order to introduce uncoordinated carboxyl groups into metal–organic frameworks (MOFs), UiO-66-NH₂ was treated with oxalyl chloride to form UiO-66 functionalized with –NH– and –COOH groups (UiO-66-NH-CO-COOH). The successful functionalization of the MOF with uncoordinated carboxyl group was confirmed using some characterization techniques. The obtained MOFs were applied in liquid-phase adsorptions for both water and a model fuel. The UiO-66-NH-CO-COOH, consisting of dual functional groups (–NH– and –COOH), showed better performance than pristine UiO-66, UiO-66-NH₂, and commercial activated carbon (AC) in the removal of several organics, e.g. triclosan (TCS) from water, and indole (IND) and quinoline (QUI) from model fuel. The UiO-66-NH-CO-COOH adsorbed ∼2.4 and 5.5 times more TCS and IND, respectively, than AC and its adsorption performances were comparable with other reported adsorbents. The maximum adsorption capacity of UiO-66-NH-CO-COOH for IND was 402 mg·g⁻¹, which was the highest among all reported UiO-66 MOFs. The remarkable adsorption efficiency of the MOF for both aqueous and non-aqueous phases was explained in terms of H-bonding: UiO-66-NH-CO-COOH and adsorbates act as H-bond acceptor and H-bond donor, respectively. Moreover, UiO-66-NH-CO-COOH can be regenerated by simple solvent washing and can be reused several times for the removal of the studied adsorbates. Therefore, UiO-66-NH-CO-COOH having both –NH– and –COOH groups is recommended as a versatile adsorbent for both aqueous- and non-aqueous-phase adsorptions.