Amine functionalized ZIF-8 as a visible-light-driven photocatalyst for Cr^(VI) reduction

Hexavalent chromium (Cr^(VI)) is one of the most toxic and carcinogenic species known to living beings, the environment, and our eco-system. Thus, it is urgent to develop a facile and effective approach for Cr^(VI) removal. Zinc-based zeolitic imidazolate frameworks (ZIF-8), a typical metal organic framework, have high porosity, large specific surface area, high chemical stability, and abundant surface grafting sites. These sites can be easily modified with ethylenediamine (EDA) using a solvothermal process to generate a material that can serve as a potential candidate for photocatalytic Cr^(VI) reduction under visible light irradiation. Various EDA contents and synthetic conditions were adopted in an attempt to investigate the correlation between ZIF-8 amine-functionalization and photocatalytic Cr^(VI) reduction. The amine functionalization and the grafting sites on ZIF-8 were determined to be located at the –CH₃ site of the 2-methylimidazole chains via X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR). Under optimum conditions, amine-functionalized ZIF-8 exhibited a normalized rate constant (k/specific surface area, k_SSA), which was 9.8 times higher than that of unmodified ZIF-8 one for photocatalytic Cr^(VI) reduction. The increased catalytic activity and range of visible light absorption of amine-functionalized ZIF-8 can be attributed to the increase in electron density due to the lone pairs of the surface grafted amines. In summary, amine-functionalized ZIF-8 could serve as a promising visible-light-active photocatalyst for environmental remediation.