Polyethylenimine-Assisted Extraction of α‑Tocopherol from Tocopherol Homologues and CO2‑Triggered Fast Recovery of the Extractant

The separation of natural homologues provides a great challenge due to high similarities of their structures and properties. In this work, the use of the polymeric extractant polyethylenimine (PEI) for the separation of α-tocopherol from the tocopherol homologues was investigated. Various PEI–cosolvent solutions were used to extract tocopherols from their hexane solutions. The effects of PEI molecular weight, cosolvent type, and PEI–cosolvent composition were systematically studied. High distribution and selectivity coefficients of tocopherols were obtained with PEI–acrylonitrile (ACN) as the extractant. A clear advantage of PEI extractant is its stimuli-responsive property triggered by CO2. PEI–extracted tocopherols could be replaced and released from PEI chains with CO2 bubbling, with PEI–CO2 precipitated out from the extract phase, which greatly facilitates the back extraction of tocopherols and the recovery of PEI for reuse. With CO2 bubbling, it took only three back extractions to recover over 90% tocopherols, compared with 12 in theory without CO2 treatment. Precipitated PEI–CO2 could be redissolved in cosolvent for reuse by N2 bubbling and heating. The distribution coefficients decreased by approximately 10% after being recycled three times.