A post-synthetic modified NH2-MIL-125 (Ti) catalyst for boosting photochemical Cr (VI) reduction

MOFs-based composites with visible light response have gradually attracted wide researchers' attention to water environmental remediation. Herein, a series of post-synthesis modified (PSM) NH2-MIL-125 (Ti) (R-BA-MIL-125 (Ti)) with phenyl-Schiff base-phenyl conjugate systems were successfully synthesized by a simple post-synthetic modification method. Under the optimization of catalyst structure designing, the photocatalytic reduction of Cr (VI) follows the order: -OH >-CH3 >-H >-Cl >-NO2-BA-MIL-125 (Ti) > NH2-MIL-125 (Ti). Among them, 4-hydroxybenzaldehyde-modified NH2-MIL-125 (Ti) (OH-BA-MIL-125 (Ti)) exhibited the highest photocatalytic removal efficiency of hexavalent chromium (Cr (VI)) at visible light conditions and the removal rate for Cr (VI) reached 98.3% under 90 min with only 5 mg photocatalyst, which displayed 2.07 times that of NH2-MIL-125 (Ti). The introduction of para-substituted benzaldehydes not only extended the conjugated chain length, but also subtly reduced the band-gap energy (Eg) of NH2-MIL-125 (Ti). Benefitting from broadening the range of photo -response and enhancing the ability of photoelectron generation, it could significantly enhance the ability of NH2- MIL-125 (Ti) to remove Cr (VI). ESR analyses indicate that e- and center dot O2- are the main active species. In addition, OH-BA-MIL-125 (Ti) exhibited good reusability and stability over 5 cycles. Therefore, this work will provide new insights into the fabrication of highly active catalysts by PSM for the remediation of chromium-containing wastewater.