Dependence of Dye Molecules Adsorption Behaviors on Pore Characteristics of Mesostructured MOFs Fabricated by Surfactant Template

In this work, mesostructured metal-organic frameworks (MOFs) of MIL-101-Crs with different specific surface areas were synthesized successfully under solvothermal conditions using cationic surfactant cetyltrimethyl ammonium bromide (CTAB) as a structural template. It was found that crystallinity degrees, specific surface areas, and pore size distributions strongly depended on the loading of CTAB. Nitrogen adsorption and positron annihilation lifetime spectroscopy (PALS) results showed that the mean mesopore size increased with loading more CTAB due to the formation of larger templated mesopores. Although Langmuir adsorption of both methylene blue (MB) and methyl orange (MO) was confirmed in MIL-101-Crs, the experimental results showed different adsorption behaviors for them depending on the dye molecular size, pore structure, and charge properties of dye molecules/MOFs in solution. The MB molecules were found to be mainly adsorbed in the interspaces between grains and the templated mesopores, whereas the MO molecules were adsorbed in the inherent pores as well as the templated ones in MOFs due to the unsaturated metal sites' electrostatic attraction on them. Remarkably, MO adsorption capacity was observed to be proportional to the specific surface area, which allowed one to get a good linear fitting of experimental data. Interestingly, the good consistence between the fitting experimental parameter, that is, the number of adsorbed MO-s per unit specific surface area, and the calculated one according to our rough estimation strongly suggests that MO-s are electrostatically attracted and rotating around the unsaturated metal sites on MOFs' inner surfaces, which exclude other MO-s to be adsorbed around due to the "hindering effect" of the rotating motion.