Glycine, also known as aminoacetic acid, is the amino acid with the simplest structure and is widely used in food, medicine and pesticides. It is an important chemical raw material and intermediate. Chloroacetic acid ammoniation is one of the main approaches to produce glycine. However, it needs a large amount of toxic and volatile methanol to separate glycine from the ammonium chloride byproduct, with low separation efficiency and the formation of a large amount of waste liquor which is environmental non-benign. Ionic liquids, as a novel green media, have shown great potential in the field of extraction and separation owing to their advantages of low vapor pressure, good thermal stability, adjustable structures and properties. In this work, designable ionic liquids (ILs) were proposed as the extractant to separate glycine. The effects of the ILs structure on the extraction efficiency were studied using imidazolium and quaternary ammonium ILs. The effects of pH, extraction temperature, time, and the initial concentrations of glycine and dicyclohexyl-18-crown-6 (DCH18C6) on the partition coefficient and extraction efficiency were investigated. The IL system was recycled and reused for glycine separation. The extraction mechanism was revealed by FT-IR and quantum chemical calculation. The results showed that choline bis(trifluoromethylsulfonyl)imide ([N1112(OH)][NTf2]) had best extraction efficiency. The addition of DCH18C6 to the system could enhance the extraction efficiency, and that of the [N1112(OH)][NTf2]–DCH18C6 system could reach 85.4%. Under the optimal conditions, the partition coefficient and extraction efficiency reached 10.9 and 94.4%, respectively. The extraction efficiency remained 90% after the IL was recycled 5 times. The strong hydrogen bonds between [N1112(OH)][NTf2], DCH18C6 and glycine played an important role in the extraction of glycine. Thus, the system of [N1112(OH)][NTf2]–DCH18C6 was effective in separating glycine from water and could be an alternative extractant in the industrial glycine purification process.
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