High-efficiency CO₂-epoxide cycloaddition using multifunctional ionic liquids grafted onto UiO-66-NH₂ under solvent /cocatalyst both-free conditions

Developing a highly efficient catalyst for the conversion of CO₂ into cyclic carbonate is crucial for achieving carbon neutrality. Incorporating target-specific active sites into solid catalysts represents a novel and promising strategy for enhancing the chemical conversion of CO₂. In this study, the original UiO-66-NH₂ was post-synthetically modified by replacing the H atom of –NH₂ functional group in UiO-66-NH₂ with the Br group in 1-bromobutylimidazole (BIm) under alkaline conditions. Subsequently, the imidazole group in UiO-66-NH₂-BIm was quaternized with bromoalkyls (including 3-bromopropionic acid, 2-bromoethanol, and 2-bromoethylamine hydrobromide), resulting in the synthesis of UiO-66-NH₂ modified by ionic liquids (ILs) containing hydrogen bond donors, denoted as IL-R@UiO-66-NH₂ (where R[dbnd6]OH, NH₂, COOH). The catalytic performance was assessed in the absence of cocatalyst and solvent at 100 °C, 5 h and 1.0 MPa CO₂. Notably, IL-COOH@UiO-66-NH₂ exhibited the highest catalytic performance, achieving propylene carbonate yield of 96.1 %. This outstanding performance was attributed to the synergistic interactions between the Lewis acidity of the MOF node, the strong nucleophilicity provided by the Br⁻ of the IL, and the hydrogen bonding interaction facilitated by the carboxyl group, Lewis basic N site from the IL cation. Additionally, IL-COOH@UiO-66-NH₂ demonstrated compatibility with a wide range of epoxy substrates and exhibited good recyclability.