Design of composite based on UiO-66 and ionic liquid for the CO2 conversion into cyclocarbonate

Abstract

Exploiting an effective catalyst for CO2 conversion into value-added chemicals under mild circumstances still remains a challenge. Herein, UiO-66 decorated with ionic liquids (ILs) (denoted as UiO-66-ILs) were facilely synthesized by the mixed ligands of 1,4-benzene dicarboxylic acid and pyridyl-containing mono-carboxylic acid isomer (including nicotinic acid (NA), isonicotinic acid (INA), and 2-pyridine carboxylic acid (PCA)), followed by the further modification with different bromoalkanes. Among various UiO-66-ILs, UiO-66 derivative containing the ILs composed of PCA and bromoethane (named as UiO-66-ILs(PCA-EtBr)) displayed superior catalytic performance on CO2-epichlorohydrin cycloaddition into chloropropene carbonate, with a high yield of 90 % under 0.1 MPa CO2 at 110 °C without the addition of any solvent and cocatalyst. Whereas, UiO-66-ILs(NA-EtBr) and UiO-66-ILs(INA-EtBr) as analogues of UiO-66-ILs(PCA-EtBr) presented a low yield of 77 % and 67 %, respectively, unveiling that the orientation of carboxyl at monocarboxyl pyridine ligand played a great influence on the catalytic activity. The excellent activity of UiO-66-ILs(PCA-EtBr) was attributed to the synergetic effect of Lewis acidity from MOF nodes for activating C–O bond of epoxide, CO2 absorption by pyridyl moiety, and nucleophilicity of ILs decoration. In addition, UiO-66-ILs(PCA-EtBr) exhibited a broad scope of epoxide substrate, as well as a great recyclability. Moreover, a possible catalytic mechanism was proposed for CO2 conversion into cyclocarbonate.