Fabrication of hydrophobic Fe-based metal-organic framework through post-synthetic modification: Improvement of aqueous stability

Abstract

Synthesis of aqueous steady metal-organic frameworks is a critical issue to expand their photocatalyst applications in water treatment. In the current study, MIL-101(Fe)-NH2 was selected as the template, and grafting of 4-(trifluoromethyl)phenyl isocyanate was achieved by a postsynthetic modification method. As a result, a novel MIL-101(Fe)-1-(4-(trifluoromethyl)phenyl)urea (named MIL-101(Fe)-TfmPU) was fabricated. X-ray photoelectron spectroscopy confirmed the grafting of amino groups to form urea groups. Water contact angle of MIL-101(Fe)-TfmPU reached 151°, implying a superhydrophobicity. Introduction of carbamido-benzene-trifluoromethyl arm reduced the surface energy, and it may act as a shield for the coordination bonds between Fe core and ligands due to its steric hindrance. This raising hydrophobicity resulted in a significant improvement on water stability. After 7 d water exposure, MIL-101(Fe)-TfmPU possessed better stability with less Fe ion release and highly intact structure compared to its parent MIL-101(Fe)-NH2. In the photocatalysis reaction with visible light and oxidant donor (H2O2), MIL-101(Fe)-TfmPU demonstrated a high degradation efficiency of ciprofloxacin with a reaction rate at 0.0636 min−1. The dominant reaction mechanism was OH· oxidation. Acid condition was benefitcial for this photocatalysis reaction and its stability, while room temperature variation (278−318 K) had a negligible effect. In different actual water mediums, photocatalysis efficiency, crystal structure and chemical bonds of MIL-101(Fe)-TfmPU were all retained. Generally, the water stability of MIL-101(Fe)-TfmPU was successfully enhanced to ensure its photocatalysis applications in the aqueous mediums.