Computational Insights into Malononitrile-Based Carbanions for CO₂ Capture

Abstract

Although anionic N and O sites have been widely used in chemisorption of CO2, carbanions are much less explored for CO2 capture. Here we employ ab initio calculations and quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations to examine the interaction between CO2 and the malononitrile carbanion, [CH­(CN)2]−. We have explored the potential energy surface of CO2 binding by scanning the C–C distance between CO2 and the central C site of the carbanion. We find that CO2 prefers to bind to the nitrile group physically rather than to form a C–C bond via the carboxylation reaction at the sp2 C site. Moreover, the two −CN groups can attract two CO2 molecules at equal strength. The presence of an alkali metal ion enhances both physical and chemical interactions of CO2 with the malononitrile carbanion. QM/MM MD simulations further confirm the preference of physical interaction in the condensed ionic liquid phase with a phosphonium cation. Our findings suggest that ionic liquids based on the malononitrile carbanion may have a high CO2 solubility for carbon capture.

CO2 Chemisorption Behavior in Conjugated Carbanion-Derived Ionic Liquids via Carboxylic Acid Formation

“…In addition, the cations and the −CN groups have also been found to be helpful in interacting with CO 2 . 33 For STSILs in CO 2 chemisorption, besides the anioninvolved reaction pathway, CO 2 capture could also be achieved via nucleophilic attack of the basic anion on the alkyl chains in the cation through the ylide formation pathway, which has been demonstrated both experimentally and theoretically. 22,34−36 We examined the possibility of this reaction mechanism in the [MN]-derived STSILs upon reacting with CO 2 by DFT (Figure S18).…”

“…Of note, the carboxylic acid product between [MN] and CO 2 was recently reported for organic solvents such as DMSO, but our work shows that the IL provides a more concentrated and nonvolatile medium for CO 2 capture. In addition, the cations and the −CN groups have also been found to be helpful in interacting with CO 2 …”

High‐Performance CO2 Capture from Air by Harnessing the Power of CaO‐ and Superbase‐Ionic‐Liquid‐Engineered Sorbents

“…The unique features of ILs, including structure diversity, good stability, negligible volatility, and large tunability in the interaction strength with CO 2 , has trigger extensive research efforts and the development of diverse functionalized ILs towards efficient carbon capture [6,38–41] . Besides AILs, basic ILs with no active proton being involved in the structures, such as superbase‐derived ILs (SILs), represent another category of functionalized ILs capable of achieving efficient CO 2 capture via C−N, C−O, or C−C bond formation [8,41–43] . The fast carbon capture kinetics could be achieved by SILs without the extensive hydrogen‐bonding network formation, together with the benefits from the low viscosity of SILs (as low as 8.63 cP in the absence of any solvents) [44] .…”

“…[6,[38][39][40][41] Besides AILs, basic ILs with no active proton being involved in the structures, such as superbasederived ILs (SILs), represent another category of functionalized ILs capable of achieving efficient CO 2 capture via CÀ N, CÀ O, or CÀ C bond formation. [8,[41][42][43] The fast carbon capture kinetics could be achieved by SILs without the extensive hydrogenbonding network formation, together with the benefits from the low viscosity of SILs (as low as 8.63 cP in the absence of any solvents). [44] Notably, a sigmoid relationship between the reaction enthalpy and CO 2 uptake capacity of the ILs was demonstrated, [45] in which for a fixed CO 2 -involved reaction pathway, a criterion enthalpy value existed to ensure the equilibrium (maximum theoretical) capacity being achieved and CO 2 releasing with minimum energy input, and reaction enthalpy surpassing this value have no contribution to the uptake capacity, but higher energy input was required for the desorption procedure.…”