Cascade CO₂ Insertion in Carbanion Ionic Liquids Driven by Structure Rearrangement

The CO₂ chemisorption in state-of-the-art sorbents based on oxide/hydroxide/amine moieties is driven by strong chemical bonding formation in the carbonate/bicarbonate/carbamate products, which in turn leads to high energy input in sorbent regeneration. In addition, the CO₂ uptake capacity was limited by the active sites’ utilization efficiency, with each active site incorporating one CO₂ molecule or less. In this work, a new concept and generation of sorbent was developed to achieve cascade insertion of multiple CO₂ molecules by leveraging structure rearrangement as the driving force, leading to in situ generation of extra CO₂-binding sites and significantly reduced energy input for CO₂ release. The designed ionic liquids (ILs) containing carbanions with conjugated and asymmetric structure, deprotonated (methylsulfonyl)acetonitrile ([MSA]) anion, allowed the cascade insertion of two CO₂ molecules via consecutive C-C and O-C bond formations. The proton transfer and structure rearrangement of the carboxylic acid intermediates played critical roles in stabilizing the first integrated CO₂ and generating extra electron-rich oxygen sites for the insertion of the second CO₂. The structure variation and reaction pathway were confirmed by operando spectroscopy, magnetic resonance spectroscopy (NMR), mass spectroscopy, and computational chemistry. The energy input in sorbent regeneration could be further reduced by harnessing the phase-changing behavior of the carbanion salts in ether solutions upon reacting with CO₂, avoiding the energy consumption in heating the solvent. The fundamental insights obtained herein provide a promising approach to greatly improve the CO₂ sorption performance via sophisticated molecular-scale structural engineering of the sorbents.