MIL-101(Fe)-derived porous amorphous materials for efficient Congo red adsorption

In this study, MIL-101(Fe)-derived porous amorphous materials were prepared using 2-methylimidazole (2-MelM) as a competitive ligand, and their adsorption performance for Congo red (CR) was investigated. The characterization of the porous amorphous materials was carried out using various techniques, such as SEM, XRD, BET, FT-IR, and XPS. The effects of the mass ratio of MIL-101(Fe) to 2-MelM and the amorphization time on material properties were studied, and the influences of dye concentration, pH, and temperature on adsorption performance were evaluated. The results showed that amorphization enhanced the adsorption performance. The sample prepared with a mass ratio of MIL-101(Fe) : 2-MelM = 1 : 16 and an amorphization time of 10 min displayed the highest adsorption capacity of 7078 mg g^-1 under conditions of pH 7, an adsorption time of 24 h, a temperature of 45 °C, and an initial concentration of 600 mg L^-1, representing a 47% improvement compared to MIL-101(Fe). Monolayer adsorption was confirmed by fitting the adsorption isotherm using the Langmuir equation, and the saturated adsorption capacity was estimated to be 7095 mg g^-1. This value was remarkably high compared with most reported values in the literature. Reuse experiments indicated high stability of the materials. Various characterizations revealed that hydrogen bonding, electrostatic interactions, and π-π interactions contributed to the adsorption process. This study demonstrated that the amorphization of MIL-101(Fe) efficiently enhanced the CR adsorption capacity.