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 CO 2 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 CO 2 /IL systems, CO 2 molecules are preferentially located at the CO 2 /IL interface, which is consistent with the observed minimum in the potential of mean force. The solubility and diffusivity of CO 2 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 CO 2 capture. CO 2 solubility is identified to be governed by the binding energy of cation–anion, rather than the binding energy of CO 2 –anion. The computational study provides quantitative microscopic insight into the role of −CN in CO 2 sorption and diffusion, and it suggests that [BMIM][B(CN) 4 ] might be an interesting candidate for CO 2 capture.