Pore Characteristics\nand Dye Adsorption Mechanism\nof Functionalized UiO-66s with Various Ratios of Amino Groups

A series of UiO-66 samples with various amino functional\ngroup\nratios were prepared by modulating the proportion of terephthalic\nacid (H<sub>2</sub>BDC) and aminoterephthalic acid (H<sub>2</sub>BDC-NH<sub>2</sub>) ligands, and the microstructure of the samples and dependence\nof methyl orange (MO) adsorption properties on the amino group content\nwere investigated by X-ray diffraction, scanning electron microscopy,\nFTIR spectra, nitrogen adsorption, positron annihilation lifetime\nspectroscopy, and UV–vis spectra. The results showed that as\nthe ratio of amino groups increased, the specific surface area and\ntotal porosity of the samples decreased, primarily due to decrement\nin the crystallinity as well as the bulky effect of amino groups in\ninherent pores. Interestingly, the amino-functionalized samples possessed\nconsiderable adsorption capacity of MO even in alkaline conditions\ndue to the hydrogen bonding between the MO and −NH<sub>2</sub> groups. The adsorption kinetics, isotherms, and thermodynamics revealed\nthat MOs’ adsorption process in amino-functionalized UiO-66s\nwas exothermic, obeying a Langmuir-type adsorption dominated by chemisorption.\nUiO-66-NH<sub>2</sub>-0.4 (H<sub>2</sub>BDC:H<sub>2</sub>BDC-NH<sub>2</sub> = 2:3) exhibited the best adsorption performance, with a\nmaximum adsorption capacity of 336.7 mg/g, and the adsorption capacity\nwas slightly decreased with increasing salt concentration in solution.\nUiO-66-NH<sub>2</sub>-0.4 could be easily regenerated by washing with\na mixed solution of ethanol and water. The results demonstrated that\nalthough amino groups led to relatively less crystallinity and lower\nmicropore volumes, the strong electrostatic attraction and hydrogen\nbonding between amino groups and MOs enhanced the adsorption capacity\nof MOs in amino-functionalized UiO-66s, in which MOs were adsorbed\nin two types of inherent pores, as shown by a significant decrement\nin positronium annihilation in them upon MO adsorption.