Ionic Liquids for Aromatics Extraction. Present Status and Future Outlook

Ionic liquids (ILs) can be used to replace conventional solvents in liquid liquid extractions of aromatic hydrocarbons. An IL-based extraction process requires fewer process steps and less energy consumption, provided that the mass-based aromatic distribution coefficient and/or the aromatic/aliphatic selectivity are higher than those of the current state-of-the-art solvents such as sulfolane. Only a small number of ionic liquids are able to combine higher mass-based distribution coefficients with selectivities comparable to or higher than those of sulfolane. The most suitable ILs from our analysis are [bmim]C(CN)(3), [3-mebupy]N(CN)(2), [3-mebupy]C(CN)(3), and [3-mebupy]B(CN)(4). The mass-based distribution coefficients with these four ILs for benzene, toluene, and p-xylene are factors of 1.2-2.5 higher than those with sulfolane, and the aromatic/aliphatic selectivities are up to a factor of 1.9 higher than with sulfolane. Based on the performed analysis, it can be concluded that industrial application of ILs for aromatics extraction has not yet materialized because only four of the total of 121 investigated ILs are considered suitable for aromatic/aliphatic separation. Most of the reported ILs do not provide higher mass-based aromatic distribution coefficients and/or higher aromatic/aliphatic selectivities than those achieved by conventional solvents such as sulfolane.