Review of Recent Aromatic-Aliphatic-Ionic Liquid Ternary Liquid-Liquid Equilibria and Their Modeling by COSMO-RS

Separation of aromatics from aliphatics following catalytic reforming or steam cracking processes is difficult and energy intensive, even with highly optimized extraction solvents like sulfolane. As a result, ionic liquids (ILs) have been studied extensively for this application. Here we review the new liquid-liquid equilibrium (LLE) data that have appeared in the literature for aromatic/aliphatic/IL systems since 2012, when the last comprehensive review appeared. We model recent systems with the conductor-like screening model for real solvents (COSMO-RS) and discuss the ability of this model to predict trends in the aromatic selectivity (S) and aromatic distribution ratio (D). With some exceptions, COSMO-RS provides good performance in predicting trends in S and D with respect to changes in aliphatic, aromatic, IL cation and IL anion. However, COSMO-RS predictions generally do not show quantitative agreement with experimental data for aromatic/aliphatic/IL LLE. Previously known trends for increasing length of alkyl chains on the IL cation or anion, aliphatic length, alkyl chains on the aromatic, and cyclization were confirmed by both the new data and modeling. The new data with different anions show that D and S do not correlate with the anion solvatochromic basicity parameter, beta, as suggested previously. Rather, ILs with anions that have greater polarity or polarizability have higher S, whereas less-coordinating anions have greater capacity for the aromatic (higher D). Of all the trends, COSMO-RS has the greatest difficulty in predicting the effect of changing anion, and this is not limited to anions that interact more strongly with the cation. For new systems with mixed alkanes, one would expect S and D to vary between the values of the pure alkanes, as confirmed by the COSMO-RS predictions. On the basis of limited new mixed IL data, it appears that COSMO-RS may not be able to predict this effect well. Nonetheless, COSMO-RS is shown to be a useful tool in predicting many qualitative trends in S and D for the extraction of aromatics from aliphatics for pure IL systems.