Regulating Copper Nuclearity in Metal-Organic Frameworks for the Hydrogenation of Carbon Dioxide

Cu-metalated MOFs, particularly Zr-based frameworks, are promising CO₂ hydrogenation catalysts. Tuning Cu nuclearity toward small clusters or isolated atoms is crucial for controlling product selectivity, yet very challenging. One way to achieve this is by adjusting the number of OH/H₂O pairs on Zr₆ nodes. While the UiO-66 framework allows only one anchoring pair per node, the MOF-808 can host up to six defects via formate substitution, enabling much higher Cu loadings (Cu/Zr₆ up to 5.2). In addition, we found that the Cu precursor influences Cu nuclearity in MOFs: CuCl₂ forms mainly isolated sites (Cu₁), whereas Cu(NO₃)₂ promotes clusters (Cu_x). Cu_x/MOF catalysts were examined for CO₂ hydrogenation at the gas/solid and at liquid(gas)/solid interfaces. At the gas/solid interface, Cu_x/MOF catalysts showed higher activity and selectivity toward methanol, with Cu_x/UiO-66 outperforming the Cu_x/MOF-808 catalyst, while Cu₁/MOFs were barely active toward CH₄ and CO formation. In the liquid phase, both Cu_x/MOF catalysts were active only for methanol formation. These results, together with detailed structural characterizations using X-ray absorption and diffuse reflectance FTIR spectroscopy during pretreatment and CO₂ reduction, electron microscopy, and a number of basic characterizations (N₂ adsorption, PXRD, DR-UV-vis, thermal analyses, and NMR spectroscopy), are discussed in relation to the structure-reactivity relationships, and are supported by DFT modeling of the Cu_x ensembles in both frameworks.