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

Abstract

The impact of replacing Zr⁴⁺ 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-NH₂ 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.