Computational Insights into Malononitrile-Based Carbanions for CO₂Capture

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.