The removal of toxic Cr(VI) ions from industry wastewater is necessary because they can cause severe damage to organisms and environment.

The photoreduction of Cr(VI) to Cr(III) driven by visible-light is a more energy-efficient and environmentally friendly approach compared with other techniques including ion exchange, chemical precipitation, membrane filtration, reverse osmosis and adsorption. However, most of photocatalysts show relative low activities due to low Cr(VI) adsorption rate/capacity, weak light harvesting efficiency, and/or poor electronic utilization efficiency. 

In a study published in Applied Catalysis B: Environmental, a research group led by Prof. CAO Rong from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences utilized a sequential mixed-ligand and ionization strategy to fabricate a multivariate metal-organic framework(MOF).

The framework integrates highly visible-light photosensitive units and strong Cr₂O₇^2- adsorptive cationic struts into one single phase to improve the photoreduction of Cr₂O₇^2-. 

Researchers firstly incorporated porphyrin as the intense visible-light sensitization unit into a Zr-MOF via mixed-ligand strategy, which effectively expanded the spectral response from 425 nm to 750 nm.  

They found that the porphyrin incorporated MOF was further ionized by the reaction with methyl iodide to afford a cationic H₂TCPPì(I^-)Meim-UiO-66, which could enhance the adsorption of Cr₂O₇^2- and desorption of the Cr³⁺ due to the electrostatic effect. 

The synergistic effect of strong adsorption of Cr₂O₇^2- and efficient utilization of light endowed H₂TCPPì(I^-)Meim-UiO-66 with highly efficient photoreduction activity toward toxic Cr₂O₇^2- under visible light in a rate of 13.3 mg_Cr(VI)/g_catalyst/min, much higher than the reported MOF-based photocatalysts including typical NH₂-UiO-66 (0.2 mg_Cr(VI)/g_catalyst/min) and NH₂-MIL-125 (1.6 mg_Cr(VI)/g_catalyst/min). 

Based on the results from time-resolved photoluminescence spectra, electron spin resonance, and theoretical calculation etc., researchers proposed the photoreduction mechanism of Cr₂O₇^2- over H₂TCPPì(I^-)Meim-UiO-66. 

The brief fabrication process of cationic multivariate metal-organic framework.(Image by Prof. CAO’s Group) 

Contact: 

Prof. CAO Rong  

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

E-mail: rcao@fjirsm.ac.cn