Selective separation of aliphatic and aromatic amines with CO2 switchable ionic liquids aqueous two-phase systems

Abstract

In many practical processes, separation of aliphatic amines from aromatic amines in water is imperative to obtain the desired products. In this work, a novel strategy was proposed for selective separation of aliphatic amines from aromatic amines with CO₂ switchable ionic liquids (ILs) aqueous two-phase systems (ATPSs). It was shown that upon introduction of CO₂ into an aqueous solution containing the IL, aliphatic amine and aromatic amine at 25 °C and atmospheric pressure, an ATPS could be formed with an IL-rich top phase and an ammonium-salt-rich bottom phase. Under these circumstances, the aromatic amine was largely partitioned in the IL-rich phase, while the aliphatic amine was transformed into the ammonium salt by the reaction with CO₂. Thus the aliphatic amine was well separated from the aromatic amine in aqueous solution. The distribution ratios of aromatic amines between the two phases were found to increase with increasing hydrophobicity of ILs and aromatic amines, pK_a values of aliphatic amines and tie-line length of the systems. Moreover, aliphatic amines retained in the ammonium-salt-rich phases were regenerated by bubbling N₂ under heating, while aromatic amines in the IL-rich phases were removed by simple steam distillation. The recovery efficiency for aliphatic amines was as high as 99.0%. The regenerated ILs could then be reused in the next separation process, and no decrease in the distribution ratios of aromatic amines was observed after six cycles.