Dihydrotetrazine-Functionalized\nZirconium-Based Metal–Organic\nFrameworks for High-Capacity Oil Denitrogenation

High structural stability, dual organic–inorganic\nnature,\nand tunability in chemical functionality are promising characteristics\nof zirconium-based metal–organic frameworks (Zr-MOFs). These\nproperties assist Zr-MOFs in extending their applications in various\nfields, especially adsorptive removal of pollutants. In this work,\ntwo well-known Zr-MOFs (UiO-66(Zr) and MIL-140(Zr) with the formula\nZr<sub>6</sub>O<sub>4</sub>(OH)<sub>4</sub>(BDC)<sub>6</sub>, H<sub>2</sub>BDC is benzene 1,4-dicarboxylic acid) were synthesized and\ndecorated with a dihydrotetrazine functional group through postsynthesis\nlinker exchange (PSLE). Two dihydrotetrazine (DHTZ)-functionalized\nframeworks, UiO-66(Zr)-DHTZ and MIL-140(Zr)-DHTZ, were applied for\nthe removal of quinoline (Qui) and indole (Ind) from the model oil.\nThe results of adsorption experiments at room temperature display\nthat these functionalized Zr-MOFs have significantly improved removal\ncapacities for Qui (875% for UiO-66(Zr)-DHTZ and 303% for MIL-140(Zr)-DHTZ)\nand Ind (722% for UiO-66(Zr)-DHTZ and 257% for MIL-140(Zr)-DHTZ).\nMechanistic studies based on X-ray photoelectron (XPS) and Fourier-transform\ninfrared (FT-IR) spectroscopies reveal that there is a specific kind\nof host–guest interaction between dihydrotetrazine and nitrogen-containing\ncompounds (NCCs). UiO-66(Zr)-DHTZ adsorbs 1426 mg·g<sup>–1</sup> Qui and 1176 mg·g<sup>–1</sup> Ind, while MIL-140(Zr)-DHTZ\nadsorbs 619 mg·g<sup>–1</sup> Qui and 511 mg·g<sup>–1</sup> Ind. The lower adsorption capacities of MIL-140(Zr)-DHTZ\ncompared to UiO-66(Zr)-DHTZ are related to its lower surface area\n(783 m<sup>2</sup>·g<sup>–1</sup> versus 330 m<sup>2</sup>·g<sup>–1</sup>). The recyclability of the frameworks\ngoes up to five cycles without any significant decrease in the removal\ncapacity. These results indicate that dihydrotetrazine-functionalized\nZr-MOFs are highly stable platforms with superior adsorption capacity\ncompared to basic and neutral NCCs.