Photocatalytic Synthesis of Hydrogen Peroxide by Rhenium Modified Metal-Organic Frameworks Incorporating Bianthracene Ligands.

Abstract

Photocatalytic synthesis of hydrogen peroxide (H₂O₂) from oxygen (O₂) is a challenging process. Metal-organic framework (MOF) materials are emerging photocatalysts with potential tunable light absorption properties. Herein, we report a rhenium (Re) modified Zr-based MOF, Re10-MFM-67, in which active Re sites are incorporated into MFM-67 by partial replacement of 9,9'-bianthracene-10,10'-dicarboxylic acid (H₂L¹) with a [(H₂L²)Re^I(CO)₃Cl] (H₂L² = 2,2'-bipyridine-5,5'-dicarboxylic acid) moiety. Re10-MFM-67 (10 refers to the molar percentage content of Re complex within the material) exhibits broadband light absorption with an exceptional rate of formation of H₂O₂ from O₂ of 8.50 mmol g_cat^-1 h^-1 and a record turnover frequency (TOF) of 28.7 h^-1 under visible light irradiation (λ > 400 nm). Synchrotron powder X-ray diffraction (SPXRD) and neutron powder diffraction (NPD) confirm the structure of Re10-MFM-67, and together with extended X-ray absorption fine structure (EXAFS) analysis establish the coordination environment and binding of the [Re^I(CO)₃Cl] moiety within the framework structure. In situ electron paramagnetic resonance (EPR) spectroscopy suggests that photocatalytic H₂O₂ generation on Re10-MFM-67 occurs via a two-step oxygen reduction reaction (ORR) pathway with the superoxide anion formed as an intermediate. This study promotes the design of MOF-based photocatalysts with conjugated ligands for efficient photosynthesis.