Interface Behaviors of Acetylene and Ethylene Molecules with 1-Butyl-3-methylimidazolium Acetate Ionic Liquid: A Combined Quantum Chemistry Calculation and Molecular Dynamics Simulation Study

Although imidazolium-based ionic liquids (ILs) combined with oxygen-containing anions were proposed as the potential solvents for the selective separation of acetylene (C₂H₂) and ethylene (C₂H₄), the detailed mechanism at the molecular level is still not well understood. The present work focuses on a most effective IL for removing C₂H₂ from a C₂H₄ stream, 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]), aiming at understanding the first steps of the adsorption process of the molecules at the IL surface. We present a combined quantum mechanical (QM) calculation and molecular dynamics (MD) simulation study on the structure and property of the IL as well as its interaction with C₂H₂ and C₂H₄ molecules. The calculated results indicate that C₂H₂ presents a stronger interaction with the IL than C₂H₄ and the anion of the IL is mainly responsible for the stronger interaction. QM calculations show a stronger hydrogen-binding linkage between an acidic proton of C₂H₂/C₂H₄ and the basic oxygen atom in [OAc]⁻ anion, in contrast to the relative weaker association via the C–H···π interaction between C₂H₂/C₂H₄ and the cation. From MD simulations, it is observed that in the interfacial region, the butyl chain of cations and methyl of anions point into the vapor phase. The coming molecules on the IL surface may be initially wrapped by the extensive butyl chain and then devolved to the interface or caught into the bulk by the anion of IL. The introduction of guest molecules significantly influences the anion distribution and orientation on the interface, but the cations are not disturbed because of their larger volume and relatively weaker interaction with the changes in the guest molecules. The theoretical results provide insight into the molecular mechanism of the observed selective separation of C₂H₂ form a C₂H₄ stream by ILs.