Systematic Investigation of Nitrile Based Ionic Liquids for CO2 Capture: A Combination of Molecular Simulation and ab Initio 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-buty1-3-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 CO, 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.