Improving CO₂ Fixation with Epoxides by Replacing Zirconium by Hafnium in UiO66 MOFs

Abstract The impact of replacing Zr 4+ with Hf⁴⁺ as the metal site on CO₂ adsorption and catalytic activity in CO₂ fixation reaction with epoxide under mild conditions was investigated in UiO66‐NH₂ grafted with carbodiimides N,N ′‐dicyclohexylcarbodiimide (DCC) and N,N ′‐diisopropylcarbodiimide (DIC) ( UiO66(M)‐DCCBr and UiO66(M)‐DICBr ; M = Zr and Hf). Leveraging on Hf⁴⁺’s greater oxophilicity and stronger M—O bonds, Hf‐based UiO66‐NH 2 materials exhibited increased CO₂ adsorption capacity, influencing the catalytic performance in CO₂ fixation with epoxides. The materials were characterized by multiple techniques such as PXRD, FTIR, TGA, SEM, elemental analysis, as well as N₂ and CO₂ adsorption equilibrium measurements. UiO66(Hf)‐DCCBr and UiO66(Hf)‐DICBr demonstrated superior activity compared to their Zr‐based counterparts with high yield and TOF (14.6 and 11.9 h⁻¹, respectively) under milder conditions (0.1 MPa, 90 °C, 16 h, co‐catalyst‐free and solvent‐free). These findings underscore the pivotal role of unsaturated metal sites in enhancing the catalytic efficacy of guanidinium ionic UiO66‐NH₂ materials. Moreover, these catalysts exhibit excellent thermal stability and can be recycled and reused at least five times without a noticeable reduction in their catalytic efficiency.