Selective Liquid–Liquid Extraction of Natural Phenolic Compounds Using Amino Acid Ionic Liquids: A Case of α-Tocopherol and Methyl Linoleate Separation

Abstract

Amino acid ionic liquids (AAILs) with different amino acid anions were investigated in the selective separation of a typical natural phenolic product, α-tocoperol, from its mixture with methyl linoleate by liquid–liquid extraction. A large separation selectivity, suitable distribution coefficient, and adequate extraction capacity were achieved with the AAIL/N,N-dimethylformamide (DMF) mixture as extractant. The selectivity of α-tocopherol to methyl linoleate reached up to 29 when using [emim]Ala and [emim]Lys as the extractant diluted by DMF with mole ratio of AAIL to DMF 15:85, at least 9 times larger than that using DMF or common ILs as the extractant. The presence of diluents, DMF, can not only reduce the viscosity of IL phase, but could also lead to much larger distribution coefficients. Back extraction of α-tocopherol using hexane and reuse of AAIL were both tested. Solvatochromic and infrared spectra measurements were used to investigate the mechanism of α-tocopherol extraction with ILs. A close linear relationship can be drawn between the distribution coefficients of α-tocopherol and the hydrogen-bond basicity (β) of the extraction solvents, and also between the selectivites and the β values.