The design and preparation of NDI modified MOFs for high efficiency removal of MB and Cr(VI)

The naphthalene tetracarboxdiimide (NDI) modified NH2-UiO-66(Zr) and NH2-MIL-101(Cr) (NDI/UZr and NDI/MCr for short) metal organic frameworks (MOFs) materials with strong covalent bond O=C-N-C=O were designed and prepared by solvothermal method. The comprehensive physical and chemical properties of the materials were characterized, and the adsorption and photocatalytic properties of the materials were evaluated by using methylene blue (MB) and Cr(VI) in single and mixture system. When the initial concentration of MB was 50 mg·L−1, the photocatalytic degradation rate constant of NDI/MCr(2:1) was 10.3 and 13.8 times higher than that of NH2-MIL-101(Cr) and NDI/UZr(2:1), respectively. Additionally, NDI/MCr(2:1) exhibited 1.6 times higher reaction rate constant than that of NDI/UZr(2:1) for the removal of 10 mg∙L−1 Cr(VI). The enhanced adsorption and photocatalytic ability over NDI/MOFs were attributed to the MOFs larger specific surface area and wider spectral response, and NDI showed high electron affinity, superior charge carrier transport properties. Free radical and electron capture experiments indicated that h+, •O2−, and e− were the main active species in the NDI/MOFs system. Among them h+ and •O2− played an essential role in the removal of MB, while e− and •O2− contributed to the photocatalytic reduction of Cr(VI). The mechanism of the photocatalytic process was discussed in detail.