| Summary: | 博士 === 國立中央大學 === 化學工程與材料工程學系 === 107 === Two kinds of ZIF-8 crystals have been prepared from the ultrasound-assisted hydrothermal reaction of a stoichiometric metal-ligand precursor solution. The addition of surfactant promoted the formation of nanocrystals having a much larger external surface area compared to the non-promoted one. The two samples were then employed to examine the effects of particle size on dye adsorption. The adsorption capacity of Rhodamine B (RB) and methyl orange (MO) was found to be directly proportional to the external surface area of the ZIF-8 particles, while that of methylene blue (MB) did not vanish with decreasing external area. Our calculations suggested that each RB and MO molecule occupied 4.7 and 7.9 nm2 of the external surface, respectively. In contrast, MB adsorption occurs both inside the ZIF-8 structure, at a loading of 0.11 molecules per unit cell, and on the external surface with 2.3 nm2/molecule.
In the second part of this thesis, the effect of electrostatic attraction mechanism in dye adsorption on UiO-66 and their modified samples was studied. Two samples of Co/UiO-66 and H+/UiO-66 were prepared under normal stirring and heating conditions to increase the positive charge on the surface. Characterization with XRD, SEM, and FTIR indicated the existence of proton H+ on UiO-66, while Raman spectroscopy, EPR, and ICP-MS confirmed the successful doping of cobalt into the UiO-66 structure at about 0.6 wt%. Results from methyl orange (MO) adsorption indicate surface enhanced adsorption capacity for both Co-UiO-66 and H+/UiO-66 compared to parent UiO-66. Moreover, highly selective removal of anionic dye compared to cationic dye were performed in detail for mixed dye solutions of MO with MB, indicating the important role of electrostatic attraction mechanism.
In general, the electrostatic attraction mechanism and external surface area play important roles in dye adsorption on MOFs, highlighting differences compared to traditional adsorbents in this field due to the special structure of MOFs. ZIF-8 is a flexible framework which showed selectivity for dye entering the structure as commented above. Also, the electrostatic attraction plays as a major contribution to dye adsorption on both UiO-66 and ZIF-8.
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