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On the Carbon Dioxide Capture by Quaternary Ammonium-Based and Phosphonium-Based Ionic Liquids. The Role of Steric Hindrances and Transition States

Global warming is seen as a drastic environmental problem nowadays. Carbon dioxide (CO 2 ) concentration in the Earth's atmosphere is linked to the average temperature on the surface of the planet. Carbon capture and storage is an important technological endeavor aiming to improve the ecology. The present work investigates reaction paths that are responsible for CO 2 chemisorption by the ammonium- and phosphonium-based ionic liquids containing an aprotic heterocyclic anion 2-cyanopyrrolidine. We show that two moles of CO 2 per one mole 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 are moderately negative. The barriers of 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 is phosphonium ylide in the mechanism of the reaction is carefully characterized. The reported results foster a fundamental understanding of the outstanding CO 2 sorption performance of the quaternary ammonium and phosphonium-based 2-cyanopyrrolidines.