Density Functional Theory Study on the Mechanism of Absorption of CO 2 by the Ionic Liquid 1-Butyl-3-Methylimidazolium Acetate

Abstract

The absorption of CO₂ on 1-butyl-3-methylimidazolium acetate ([Bmim] [Ac]) with different substituents are calculated systematically at the GGA/PW91 level. Three hydrogen bonds are formed between [Ac]^– and the cations of 1-n-[Bmim][Ac] ([NBmim]⁺) and 1-tert-[Bmim][Ac] ([TBmim]⁺). The interaction between CO₂ and [NBmim][Ac] via a C – O bond is much weaker than that with [TBmim][Ac] by its forming an O…O…C…C four-membered ring. The chemisorption of CO₂ on the ion pair [NBmim][Ac] is much weaker than that on [TBmim][Ac], as indicated from an absorption energy analysis. The frontier molecular orbitals show the electron density overlap between absorbed CO₂ and the [Ac]^– in CO₂ – [NBmim][Ac] is much weaker than that in [TBmim][Ac]. As a result, the chemisorption of CO₂ on [NBmim][Ac] is much weaker than that on [TBmim][Ac]. Ionic liquids based on [NBmim]⁺ can be used repeatedly, and the adsorbed CO₂ would be more easily desorbed.