Nitrogen-containing switchable solvents for separation of hydrocarbons and their derivatives

Solvent extraction is commonly used to separate mixtures of hydrocarbons and their derivatives, and solvent choice is strongly influenced by the affinity to the target component, cost and safety. Nitrogen-containing switchable solvents can switch from water-immiscible form to water-miscible bicarbonate salts when CO₂ is injected and back to water-immiscible form when N₂ is injected. Switchable solvents, as a type of recyclable green solvent, can be used not only to separate such mixtures but also to reduce process costs. Herein, four switchable solvents, namely, dipropylamine, di-sec-butylamine, N,N-dimethylcyclohexylamine (CyNMe₂), and N,N,N',N'-tetraethyl-1,3-propanediamine (TEPDA), were employed to separate benzene/cyclohexane, ethyl acetate/acetonitrile, and ethyl acetate/n-heptane mixtures, and the corresponding partition and selectivity coefficients were determined. In all cases, separation selectivity increased in the order of dipropylamine < di-sec-butylamine < CyNMe₂ < TEPDA, i.e., TEPDA was best suited for the separation of hydrocarbons and their derivatives. In addition, cycling experiments revealed that the TEPDA can be re-used at least 15 times and was well suited for industrial applications. In the end, the mechanism of the separation was put forward.