Photocatalytic Carbon Dioxide Reduction with Rhodium-based Catalysts in Solution and Heterogenized within Metal-Organic Frameworks

The first photosensitization of a rhodium-based catalytic system for CO₂ reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H₂-storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp∗Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10% molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H₂. There is no precedent for a MOF catalyzing the latter reaction so far.