Integration of adsorption and photosensitivity capabilities into a cationic multivariate metal-organic framework for enhanced visible-light photoreduction reaction

The photoreduction of toxic Cr(VI) to environmental Cr(III) driven by visible-light is highly desired. Metal-organic frameworks (MOFs), as one class of outstanding porous materials, had been utilized for photoreduction of Cr(VI). Current methods modulated the photoreduction of Cr(VI) mainly by selection of suitable metal ions or organic ligands in single component MOFs. However, most of them still exhibit limited photoreduction performance due to low Cr(VI) adsorption rate/capacity, weak light harvesting efficiency, and/or poor electronic utilization efficiency. Multivariate metal-organic frameworks with highly visible-light photosensitive unit and strong Cr(VI) adsorption strut into one single phase is therefore expected to be an effective strategy to improve the photoreduction of Cr(VI), but remains unexplored. Herein, intense visible-light absorption porphyrin unit and strong toxic anions adsorption strut were integrated into one single MOF simultaneously via a sequential mixed-ligand and ionization method, which strongly improve the photoreduction performance of Cr(VI). The synergistic effect of strong adsorption of Cr2O72- and efficient utilization of light endowed H2TCPP subset of (I-)Meim-UiO-66 with highly efficient photoreduction activity toward toxic Cr2O72- 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). As far as we know, this is the best catalyst among all the reported MOF-based photocatalysts for Cr(VI) photoreduction, in which the I-in our imidazolium functionalized MOF acts as sacrificial agent. Based on the results from time-resolved photo-luminescence spectra, electron spin resonance, and theoretical calculation etc., a photoreduction mechanism of Cr2O72- over H2TCPP subset of (I-)Meim-UiO-66 was also well proposed. This general and facial strategy, combining the advantages of adsorption and photosensitivity in a multivariate MOF, paves the way to design of higher efficient photocatalytic materials.