Ameliorating Adsorption Performance for Poisonous Methylene Blue through the Amino Functionalization of Metal–Organic Frameworks

Amino (-NH2)-functionalized metal-organic frameworks (MOFs) are widely applied to improve the properties of materials owing to the rich host-guest chemical properties of amino groups. In this work, the amino-functionalization strategy was thus employed to improve the sorption performance of methylene blue (MB). The introduction of -NH2 groups in AOBTC-Zn did not reduce the pore size of the framework but rather modulated and optimized the host-guest interactions of MOFs. The MB+ sorption result was significantly improved by the NH2-functionalized NH2-AOBTC-Zn. The results showed that the maximum sorption capacity of NH2-AOBTC-Zn is much higher (1623 mg/g) than that of AOBTC-Zn (204 mg/g), which was comparable with that of MIL-68(Al) (1666 mg/g). The adsorption kinetics and isothermal models indicated that the MB+ sorption processes of both MOFs were consistent with the Langmuir isothermal and pseudo-second-order kinetic models. The single-group and multicomponent sorption experiments showed that the sorption behavior was the result of π-π interaction, electrostatic interaction, hydrogen bonding interaction, and pore size interaction. In particular, NH2-AOBTC-Zn exhibits a higher adsorption capacity than AOBTC-Zn due to the additional hydrogen bonding interactions it provided. These may guide the design of porous MOFs with side group modification for liquid phase sorption/separation.