Photochemical CO₂ Reduction by a Postsynthetically Modified Zr-MOF

Metal-organic frameworks are an excellent platform for photochemical CO₂ reduction into valuable chemicals. Herein, we report the synthesis and photocatalytic behavior of Ru@MOF-808, a Zr-based MOF, modified with a Ru-polypyridyl complex. The postsynthetic modification was achieved using solvent-assisted incorporation of bipyridine-carboxylate ligands onto the nodes of the MOF-808, followed by the coordination of Ru(II)-terpyridine moiety. A thorough characterization including ¹H NMR, diffuse reflectance UV/vis, X-ray absorption spectroscopy and gas adsorption studies, combined with DFT calculations, provides strong support for efficient incorporation of the molecular Ru-complex at the loading of one Ru center per node. In the presence of a strong sacrificial reductant BIH, Ru@MOF-808 was found to catalyze the photochemical reduction of CO₂ into a mixture of CO and formate ion. When compared to the homogeneous model catalyst Ru(tpy)(bpy)²⁺, Ru@MOF-808 was found to exhibit higher formate yields. To explain these formate enhancements, we propose a mechanism that involves CO₂ capture at the MOF nodes to form Zr-bicarbonate species, which further react in a hydride transfer reaction with photogenerated Ru-H donor, thereby outperforming molecular catalysts in HCOO^- production. Overall, the results presented in this work indicate the potential of Zr-based MOFs in integrating CO₂ capture with its photochemical conversion to desired products.