(544c) Thermally Stable Ionic Liquid As Media for Separating Aliphatic and Aromatic Compounds

The process of separating aromatic and aliphatic hydrocarbons is energy intensive due to similar boiling points and formation of azeotropes. The use of ionic liquids (ILs) for these separations has the potential to lower these energy costs. We show that thermally-robust peraryl ILs especially, triphenyl-p-phenoxylphenylphosphonium bistriflimide [Ph₃P-p-POPNTf₂] have a high affinity towards aromatic compounds while remaining immiscible with aliphatic compounds. Furthermore, when mixed with excess aromatic the IL forms two distinct phases: an IL-phase saturated with aromatic and a nearly pure aromatic phase. The molecular-level interactions leading to this behavior, however, still remain unclear. Here, we use molecular dynamics (MD) simulations to provide a detailed molecular-level understanding of these systems. The fundamental interactions between ILs and model compounds such as ortho-, meta-, para- xylene, toluene, hexafluorobenzene, cyclohexane, naphthalene, ethylbenzene, pentylbenzene, bromobenzene, cyanobenzene and n-heptane are assessed. MD correctly predicts the formation of two phases and there is close quantitative agreement with the saturation limit of aromatics in the IL phase, while n-heptane, cyclohexane and pentylbenzene were completely immiscible with the ILs. The presence of aromatics in the ILs disrupt the IL network by forming cation-aromatic, anion-aromatic while weakening ion-ion repulsive interactions, meanwhile counter ions network remained intact. The change in IL-aromatic structure is further analyzed through radial/spatial distribution functions, nearest neighbors, pair-interaction energies between several atom pairs. The results showed formation of cage like structures (liquid clathrate) which are dynamic in nature. These efforts provide an improved insights into molecule-level understanding of the solvent environment provided by the cation and anion during separation and further help in tuning the ILs for efficient separation applications.