Systematic Investigation of Nitrile Based Ionic Liquids for CO₂ Capture: A Combination of Molecular Simulation and <i>ab Initio</i> Calculation

Molecular simulation and ab initio calculation are performed to investigate CO2 capture in four nitrile (−CN) based ionic liquids (ILs), namely 1-n-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], 1-n-butyl-3-methylimidazolium dicyanamide [BMIM][N(CN)2], 1-n-butyl-3-methylimidazolium tricyanomethane [BMIM][C(CN)3], and 1-n-butyl-3-methylimidazolium tetracyanoborate [BMIM][B(CN)4]. In neat ILs, the simulated densities match well with experimental data, and the cation–anion interaction becomes weaker with increasing number of −CN. In CO2/IL systems, CO2 molecules are preferentially located at the CO2/IL interface, which is consistent with the observed minimum in the potential of mean force. The solubility and diffusivity of CO2 in the four ILs increase as [BMIM][SCN] < [BMIM][N(CN)2] < [BMIM][C(CN)3] < [BMIM][B(CN)4], thus increasing number of −CN is beneficial for CO2 capture. CO2 solubility is identified to be governed by the binding energy of cation–anion, rather than the binding energy of CO2–anion. The computational study provides quantitative microscopic insight into the role of −CN in CO2 sorption and diffusion, and it suggests that [BMIM][B(CN)4] might be an interesting candidate for CO2 capture.

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