Visible-light-driven sonophotocatalysis for enhanced Cr(vi) reduction over mixed-linker zirconium-porphyrin MOFs

In this study, we synthesized and characterized two mixed-linker zirconium-porphyrin metal-organic frameworks, PCN-134 and PCN-138, constructed from tetratopic light harvesting TCPP ligands and tritopic BTB/TBTB ligands (TCPP = trakis(4-carboxyphenyl)porphyrin), BTB = benzene tribenzoate, TBTB = 4,4 ',4 ''-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoate). In consideration of good chemical stability, ligand-based photogenerated charge generation and broad visible light absorption, PCN-134 and PCN-138 were employed as catalysts for reducing Cr(vi) to Cr(iii) in an established sonophotocatalytic system. The average reaction rates of Cr(vi) reduction in sonophotocatalysis (20 kHz, 120 W) were 0.237 mg L-1 min(-1) and 0.266 mg L-1 min(-1) for PCN-134 and PCN-138, respectively. By comparison, the average reaction rates of Cr(vi) reduction in photocatalysis were 0.074 mg L-1 min(-1) and 0.115 mg L-1 min(-1) for PCN-134 and PCN-138. In addition, there are no catalytic activities over the synthesized MOFs in sonocatalytic Cr(vi) reduction. The two mixed-linker MOFs in the sonophotocatalytic Cr(vi) reduction system exhibited 2-3 times higher reactivity than that in the individual photocatalysis. Notably, no electron sacrificial agents were used in the established sonophotocatalytic process. A mechanism study demonstrated that the synergistic effect of sonocatalysis and photocatalysis promoted the photoinduced electron transfer from the MOFs to Cr(vi) and enhanced the catalytic activity. This work might provide a novel strategy for improving the efficiency of MOF-based photocatalytic systems.