On the volatility of aromatic hydrocarbons in ionic liquids: Vapor-liquid equilibrium measurements and theoretical analysis

The use of ionic liquids (ILs) as solvent in the liquid-liquid extraction of aromatic compounds is one of their most studied applications. Nevertheless, the recovery of the extracted hydrocarbons has been much less investigated, being a required task to complete the global separation process. Taking into account the negligible vapor pressure of the ILs, this step could be easily carried out by flash distillation, which requires the study of vapor-liquid equilibrium (VLE). In order to study this topic deeper, in this work a systematic analysis of the VLE and vapor-liquid-liquid equilibrium (VLLE) data for {aromatic hydrocarbon + IL} binary mixtures was carried out, from both an experimental and computational point of view. For that, new experimental VLE and VLLE data of 24 {toluene + IL} binary mixtures were measured at 323.15 K using a technique based on the static headspace gas chromatography (HS-GC), providing relevant information on the toluene retained in the liquid depending on the cation/anion structure of the IL in the mixture. Furthermore, the quantum chemical Conductor-like Screening Model for Real Solvents (COSMO-RS) method was applied to better understand the structure-property relationship determining the phase behavior of {aromatic hydrocarbon + IL} binary systems. First, the suitability of COSMO-RS to predict VLE and VLLE data of {toluene + IL} binary mixtures was evaluated by comparison to 225 experimental data at 323.15 K, including 24 different ILs over the whole composition range. Valuable conclusions were achieved respect to the molecular model of IL needed to adequately predict VLE and VLLE data of the {aromatic hydrocarbon + IL} binary mixtures. Once the computational approach was stated, COSMO-RS methodology was used to analyze the influence of the intermolecular interactions between the toluene and the IL component on the phase behavior of their mixtures. As a result, COSMO-RS was demonstrated as a useful tool for the rational design of ILs with optimized properties for the separation of aromatic + aliphatic hydrocarbon binary mixtures, considering both liquid-liquid extraction and solvent regeneration steps.