Carbon Dioxide Chemisorption by Ammonium and Phosphonium Ionic Liquids: Quantum Chemistry Calculations

Carbon capture and storage is an important technological endeavor aiming to improve the ecology by combating global warming. The present work investigates reaction paths that are responsible for CO₂ chemisorption by the ammonium- and phosphonium-based ionic liquids containing an aprotic heterocyclic anion 2-cyanopyrrolide. We exemplify that 2 mol of CO₂ per 1 mol of the gas scavenger can be theoretically fixed by such ionic liquids. Both the cation and anion participate in the chemisorption. The corresponding standard enthalpies and potential energies are moderately negative. The chemisorption reaction, as revealed by the simulations of competing pathways, is started by the donation of the proton from the cation to the anion. The double covalent bond in the cation's structure emerges. The barriers to all reactions involving the phosphonium-based cation are relatively small and favor practical applications of the considered sorbents. The performance of the ammonium-based cation is less favorable due to the inherent instability of the tetraalkylammonium ylide. The role of phosphonium ylide in the mechanism of the reaction is carefully characterized. The performance of the aprotic anion as a CO₂ scavenger is unaffected by the chemical identity of the counterion. The essential heights of the identified steric barriers underline the necessity to simulate the entire structures of the reacting species to obtain a reliable description of chemisorption. The reported results foster a fundamental understanding of the outstanding CO₂ sorption performance of the quaternary ammonium- and phosphonium-based 2-cyanopyrrolides.