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

A series of UiO-66 samples with various amino functional group ratios were prepared by modulating the proportion of terephthalic acid (H₂BDC) and aminoterephthalic acid (H₂BDC-NH₂) ligands, and the microstructure of the samples and dependence of methyl orange (MO) adsorption properties on the amino group content were investigated by X-ray diffraction, scanning electron microscopy, FTIR spectra, nitrogen adsorption, positron annihilation lifetime spectroscopy, and UV-vis spectra. The results showed that as the ratio of amino groups increased, the specific surface area and total porosity of the samples decreased, primarily due to decrement in the crystallinity as well as the bulky effect of amino groups in inherent pores. Interestingly, the amino-functionalized samples possessed considerable adsorption capacity of MO even in alkaline conditions due to the hydrogen bonding between the MO and -NH₂ groups. The adsorption kinetics, isotherms, and thermodynamics revealed that MOs' adsorption process in amino-functionalized UiO-66s was exothermic, obeying a Langmuir-type adsorption dominated by chemisorption. UiO-66-NH₂-0.4 (H₂BDC:H₂BDC-NH₂ = 2:3) exhibited the best adsorption performance, with a maximum adsorption capacity of 336.7 mg/g, and the adsorption capacity was slightly decreased with increasing salt concentration in solution. UiO-66-NH₂-0.4 could be easily regenerated by washing with a mixed solution of ethanol and water. The results demonstrated that although amino groups led to relatively less crystallinity and lower micropore volumes, the strong electrostatic attraction and hydrogen bonding between amino groups and MOs enhanced the adsorption capacity of MOs in amino-functionalized UiO-66s, in which MOs were adsorbed in two types of inherent pores, as shown by a significant decrement in positronium annihilation in them upon MO adsorption.