Product Selectivity in Photochemical CO2 Reduction by a Post-synthetically Modified Zr-MOF

Metal-organic frameworks (MOFs) 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 that was post-synthetically modified with a Ru-polypyridyl complex. The post-synthetic modification was achieved using solvent-assisted incorporation of bipyridine-carboxylate ligands onto the nodes of the MOF-808, followed by the coordination of the Ru(II)-terpiridine moiety. A thorough characterization including 1H-NMR, diffuse reflectance UV/Vis spectroscopy, X-ray absorption spectroscopy and gas adsorption studies, combined with DFT calculations, provide 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(1,3-Dimethyl-2-phenylbenzimidazoline), Ru@MOF-808 was found to catalyze 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 the CO2 capture at the MOF nodes to form Zr-bicarbonate species, which further react in a hydride transfer reaction with photo-generated Ru-H donor, thereby outperforms molecular catalyst 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.