Comparative studies of methyl orange adsorption in various metal-organic Cheddar frameworks by nitrogen adsorption and positron annihilation lifetime spectroscopy

In this work, typical metal-organic frameworks (MOFs) MIL-101-Cr, MIL-53-Al and ZIF-8 with high specific surface areas were synthesized and applied in the removal of anionic dye methyl orange (MO) from water. Nitrogen adsorption and positron annihilation lifetime spectroscopy (PALS) were applied to study the pore characteristics of MOFs and their adsorption of MO, respectively. The specific surface areas of the synthesized MOFs are in the order MIL-101-Cr > ZIF-8>MIL-53-Al. The pore sizes of MOFs obtained from PALS are essentially the same as those from their crystal structure, suggesting that PALS is a useful probe to characterize the pore properties of MOFs. After MO adsorption in MOFs, the nominal mean pore sizes for MIL-101-Cr-MO and MIL-53-Al-MO from nitrogen adsorption are mainly attributed to the interspaces between grains because their inherent pores were occupied by MO molecules as revealed by PALS, while that of ZIF-8-MO represents the inherent pores and interspaces between grains. Langmuir adsorption of MO was found to occur in the MOFs, whereas the MO adsorption behaviors are different depending on the unique pore structures of specific MOFs. MIL-101-Cr and MIL-53-Al show the MO adsorption capacities of 196.08 and 100.0 mg/g due to their large specific surface areas and accessible pore windows, which can provide more accessible unsaturated metal sites. However, despite that the specific surface area of ZIF-8 is larger than that of MIL-53-A1, MO adsorption capacity in ZIF-8 is rather low because the size of its pore windows is smaller than MO dimension, which results in MO molecules inaccessible to the pores. Results showed that methyl orange anions (MO-) were electrostatically attracted by unsaturated metal sites in the cages/channel pores of MIL-101-Cr and MIL-53-Al in aqueous solutions, whereas MO was only adsorbed in the interspaces between grains in ZIF-8.