The first study of the adsorptive removal of dye using MOFs was reported by Haque et al. [107] in 2010. They used two highly porous Cr-based MOFs, namely MIL-101-Cr and MIL-53-Cr (MIL: Material of Institute Lavoisier) for the adsorptive removal of methyl orange (MO), a harmful anionic dye, from aqueous solutions. It was observed that the MOF performances were much better than that of activated carbon (AC). The adsorption capacity and adsorption kinetic constant of MIL-101-Cr are greater than those of MIL-53-Cr, showing the importance of porosity and pore size for adsorption. However, much better adsorption was observed after functionalizing MIL-101-Cr by grafting with ED and protonated ED (PED), even though the porosity and pore size decreased slightly after grafting. MO usually exists in the sulfate form, therefore it has a strong electrostatic interactions with a positively charged adsorbent. The positive charge distribution increases in the order MIL-101-Cr < ED-MIL-101-Cr < PED-MIL-101-Cr, as shown in Fig. 1a. In contrast, the positive charge on PED-MIL-101-Cr decreases with increasing solution pH, because of deprotonation of the protonated adsorbent. Therefore, the increases in the adsorption capacity and kinetic constant with modification (MIL-101-Cr < ED-MIL-101-Cr < PED-MIL-101-Cr) or increasing acidity may be explained by the increased positive charge on the adsorbent, especially for PED-MIL-101 at low pH. Haque et al. also studied the adsorptive removals of methylene blue (MB, cationic dye) and MO from aqueous solutions over an NH2-functionalized MOF, NH2-MIL-101-Al [109]. The maximum adsorption capacity of MB over NH2-MIL-101-Al at 30 °C is 762 ± 12 mg/g which is higher than those of other MOFs and most other materials. In contrast, lower adsorption capacities were observed in adsorption of the same dye on the analogous non-NH2-functionalized framework (MIL-101-Al, 195 mg/g), suggesting electrostatic interactions between the amino groups of the MOF and the cationic MB.
The first study of the adsorptive removal of dye using MOFs was reported by Haque et al. [107] in 2010. They used two highly porous Cr-based MOFs, namely MIL-101-Cr and MIL-53-Cr (MIL: Material of Institute Lavoisier) for the adsorptive removal of methyl orange (MO), a harmful anionic dye, from aqueous solutions. It was observed that the MOF performances were much better than that of activated carbon (AC). The adsorption capacity and adsorption kinetic constant of MIL-101-Cr are greater than those of MIL-53-Cr, showing the importance of porosity and pore size for adsorption. However, much better adsorption was observed after functionalizing MIL-101-Cr by grafting with ED and protonated ED (PED), even though the porosity and pore size decreased slightly after grafting. MO usually exists in the sulfate form, therefore it has a strong electrostatic interactions with a positively charged adsorbent. The positive charge distribution increases in the order MIL-101-Cr < ED-MIL-101-Cr < PED-MIL-101-Cr, as shown in Fig. 1a. In contrast, the positive charge on PED-MIL-101-Cr decreases with increasing solution pH, because of deprotonation of the protonated adsorbent. Therefore, the increases in the adsorption capacity and kinetic constant with modification (MIL-101-Cr < ED-MIL-101-Cr < PED-MIL-101-Cr) or increasing acidity may be explained by the increased positive charge on the adsorbent, especially for PED-MIL-101 at low pH. Haque et al. also studied the adsorptive removals of methylene blue (MB, cationic dye) and MO from aqueous solutions over an NH2-functionalized MOF, NH2-MIL-101-Al [109]. The maximum adsorption capacity of MB over NH2-MIL-101-Al at 30 °C is 762 ± 12 mg/g which is higher than those of other MOFs and most other materials. In contrast, lower adsorption capacities were observed in adsorption of the same dye on the analogous non-NH2-functionalized framework (MIL-101-Al, 195 mg/g), suggesting electrostatic interactions between the amino groups of the MOF and the cationic MB.
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