Introduction of a secondary ligand into titanium-based metal-organic frameworks for visible-light-driven photocatalytic hydrogen peroxide production from dioxygen reduction

The introduction of multiple components with specific properties into metal-organic frameworks (MOFs) is an attractive strategy to modify their catalytic properties. Herein, through the introduction of the ligand 4,4 ',4 '',4 '''-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid (L2) into MIL-125 during its synthesis, four L2-functionalized titanium-based MOFs, MIL-125-xL2 (x = 0.035, 0.07, 0.14, and 0.21), were successfully prepared for the first time. Due to the introduction of the L2 ligand, the morphology of MIL-125-xL2 crystallites changed from a plate to an octahedron, and these MOFs contained more structural defects of missing ligands and possessed slightly larger BET surface areas and pore volumes. Most importantly, MIL-125-xL2 achieved a high photoactivity for H2O2 production from the dioxygen (O-2) reduction reaction that cannot be catalyzed by pristine MIL-125. The most active MIL-125-0.14L2 displayed a remarkable H2O2 production rate of 1654 mu mol L-1 h(-1) under visible-light irradiation (lambda > 400 nm) using triethanolamine as a sacrificial agent. Such high activity can be attributed to the unique visible light absorption ability of L2, which originates from the large aromatic ring consisting of an extended pi-electron system, making MIL-125-xL2 a visible-light-driven catalyst. This work provides an effective strategy for the design of multi-functional MOFs and enriches the application of MOFs in the field of new energy production.