Aminobenzoate-defected UiO-66(Zr)–NH2 frameworks: Scalable synthesis and characterizations for adsorptive denitrogenation from model fuel

Abstract

In this work, imperfection UiO-66-NH2 frameworks (AUiO-66-NH2) were engineered and scalably produced by a continuous flow method with the assistance of 4-aminobenzoate (ABA) modulators. ABA modulators competed with MOF ligands to bond to Zr4+ clusters under continuous flow conditions, producing AUiO-66-NH2 frameworks containing aminobenzoate linkers and ligand-missing defects. The average ligand deficiency per Zr6 unit of defective AUiO-66(Zr)–NH2 framework ranged from 0.75 ∼ 2.36 upon changing ABA/H2N-BDC molar ratio and reaction temperature, thereby adjusting their porosity and exposing more metal open sites of Zr4+. The batch removal of N-containing compounds (NCCs), including quinoline (QUN) and 2-methyl pyrrole (MPR) in n-octane indicated that the ABA-modulated UiO-66-NH2 samples had improved adsorption capability compared to the perfect MOF. Noticeably, the adsorption performances were further enhanced by protonating 2.5AUiO-66-NH2 to 2.5AUiO-66-NH3+ owing to the synergetic effects of acid-base interaction, H-bonds, cation-π interactions, and π-π stacking interactions between adsorbent and adsorbate. Density functional theory (DFT) calculations further revealed that the NCCs preferably accumulated onto the protonated AUiO-66-NH3+ rather than AUiO-66-NH2. The AUiO-66-NH3+ exhibited adsorbing capacities of ∼204 mg/g and ∼275 mg/g for MPR and QUN, respectively, and obeyed the pseudo-second-order kinetic model. Finally, the continuous fixed-bed adsorptions of these NCCs in model fuel over the AUiO-66-NH3+ pellets (3 mm × 6 mm) showed that 1.0 g of adsorbent can purify with ∼28 L and ∼49 L liquid fuel containing MPR and QUN with a concentration of 1 mg L−1. These state that combining continuous-flow synthesis and defect engineering of the UiO-66-NH2 could be a promising strategy to produce efficient adsorbent materials for upgrading liquid fuel.