The research progress on CO<sub>2</sub> absorption and conversion by metal coordination-based (chelate-based) ionic liquids

Abstract

<p indent=0mm>Metal coordination-based ionic liquids (ILs) are a type of low-temperature molten salts formed by the coordination of metal ions and organic or inorganic ligands. Among them, when the metal ion and the ligand have a multidentate coordination site, it is called metal chelate-based ILs. At present, metal coordination-based (chelate-based) ILs have been widely used in gas absorption and organic catalysis, showing excellent performance. In this paper, the research progresses on CO₂ absorption and conversion by metal coordination-based (chelate-based) ILs in recent years have been reviewed, focusing on the effects of metal ions and ligand types on CO₂ absorption and chemical conversion performance. Generally, the metal anion-coordination-based ILs absorb CO₂ through physical interaction, while the metal cation-chelate-based ILs with alcohol amine or amine ligand can react with CO₂ through chemical interaction. Therefore, the CO₂ absorption capacity of the former is lower than that of the latter. The stronger the chelation interaction between the central metal ion and the ligand, the greater the CO₂ absorption capacity of the metal cation-chelate-based ILs. The chelation interaction between the ligand and the central metal ion can be adjusted by changing the type of the central metal ion and the number of ligand coordination atoms. Increasing the central metal ion empty orbital and the number of ligand coordination atoms will enhance the chelation interaction of the metal cation-chelate-based ILs. In addition, the introduction of functionalized anion groups such as [Im]⁻, [Pyr]⁻, [Pro]⁻, [PhO]⁻, etc. is beneficial to improve the carbon capture ability of the metal cation-chelate-based ILs. In this way, the anion and cation of the metal cation-chelate-based ILs can react with CO₂, thereby increasing the CO₂ absorption capacity. Although the high viscosity of the metal cation-chelate-based ILs will affect its absorption performance, the mixed system obtained by mixing the metal cation-chelate-based ILs with other ILs can not only effectively reduce the viscosity of the system in the process of absorbing CO₂, but also can effectively improve the absorption performance of the mixed system. Considering the good thermal stability, excellent carbon capture ability and recycling performance of the metal cation-chelate-based ILs, it is a good solvent suitable for CO₂ absorption at high temperature. Besides the excellent carbon capture performance, the metal coordination (chelate-based) ILs can also effectively catalyze the reaction of CO₂ with the different substrates such as epoxy compounds (such as ethylene oxide, propylene oxide, styrene oxide, etc.), terminal alkynes, propargyl alcohol with mild reaction conditions and high product yields. The catalytic performance is mainly related to the types of central metal ions and ligands. Metal ions can effectively stabilize the reaction substrate and intermediates, thereby reducing the activation energy of the reaction. The chemical conversion of CO₂ catalyzed by the metal coordination (chelate-based) ILs can be achieved at room temperature and under atmospheric pressure, which provides a new idea for the resource utilization of carbon dioxide. Finally, present issues, challenges, and opportunities faced by metal coordination-based (chelate-based) ILs in the absorption and conversion of CO₂ are also discussed.