Superior selective adsorption of anionic organic dyes by MIL-101 analogs: Regulation of adsorption driving forces by free amino groups in pore channels

We herein report that the forces driving the adsorption of dyes onto MIL-101-NH 2 micro/nanoparticles, which are electrostatic , hydrogen bonding , and π-π stacking interactions, pore-volume, and steric hindrance , can be modulated by varying the molar ratio of terephthalic acid and 2-aminoterephthalic acid in the reaction system. The adsorption capacities of MIL-101-NH 2 -1 were determined to be 2967.1 mg g −1 for Congo red (CR), 461.7 mg g −1 for methyl orange (MO), and 259.8 mg g −1 for acid chrome blue K (AC), which were higher by 117.0%, 102.2%, and −19.6%, respectively, compared with those of pristine MIL-101. This adsorption behavior could be rationalized based on the molecular structure of the dye, its zeta potential , BET specific surface area, and the adsorption characteristics of the dye. The study revealed that free –NH 2 groups increase the hydrogen bonding and electrostatic interactions between MIL-101-NH 2 micro/nanoparticles and linear anionic dyes, thereby improving the adsorption performance for linear anionic dyes by overriding the effect of pore-volume. On the other hand, the increased steric hindrance between MIL-101-NH 2 micro/nanoparticles and a nonlinear anionic dye decreased the adsorption performance for the nonlinear dye, overriding the effects of hydrogen bonding, and electrostatic interactions. The adsorption uptakes of MIL-101-NH 2 -1 toward the linear anionic organic dyes (CR and MO) were found to be much higher than those of the other two MIL-101-NH 2 adsorbents investigated herein. However, the adsorption uptake of MIL-101-NH 2 -1 toward the nonlinear anionic organic dye (AC) was the smallest owing to different adsorption mechanisms. The adsorption performance of MIL-101-NH 2 is governed by the synergetic interplay between the hydrogen bonding, electrostatic and π–π stacking interactions, pore-volume, and the steric hindrance of the adsorbent.