Selection of ionic liquids for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures

The separation of aromatic hydrocarbons (benzene, toluene, ethyl benzene and xylenes) from C4 to C10 aliphatic hydrocarbon mixtures is challenging since these hydrocarbons have boiling points in a close range and several combinations form azeotropes. In this work, we investigated the separation of toluene from heptane by extraction with ionic liquids. l ionic liquids are suitable for extraction of toluene from toluene/heptane mixtures. The toluene/heptane selectivities at 40 °C and 75 °C with several ionic liquids, [mebupy]BF4, [mebupy]CH3SO4, [bmim]BF4 (40 °C) and [emim] tosylate (75 °C), are a factor of 1.5–2.5 higher compared to those obtained with sulfolane (Stol/hept = 30.9, Dtol = 0.31 at 40 °C), which is the most industrially used solvent for the extraction of aromatic hydrocarbons from a mixed aromatic/aliphatic hydrocarbon stream. From these five ionic liquids, [mebupy]BF4 appeared to be the most suitable, because of a combination of a high toluene distribution coefficient (Dtol = 0.44) and a high toluene/heptane selectivity (Stol/hept = 53.6). Therefore, with [mebupy]BF4 also extraction experiments with other aromatic/aliphatic combinations (benzene/n-hexane, ethylbenzene/n-octane and m-xylene/n-octane) were carried out. The aromatic/aliphatic selectivities were all in the same range, from which it can be concluded that the toluene/heptane mixture is a representative model system for the aromatic/aliphatic separation.