Deterpenation Of Citrus Essential Oil With Bio-based Choline Ionic Liquids

Deterpenation of citrus essential oil is important to improve the product quality and commercial value of citrus essential oils.Liquid-liquid extraction technology has been widely used in the deterpenation of citrus essential oil,but there still exists problem in the trade off between separation efficiency and safety.In this work,a series of bio-based ionic liquids(ILs)including[Ch][AA]([choline][amino acid])and[Ch][CA]([choline][carboxylic acid])were synthesized for the separation of limonene and linalool,based on the advantages of good biocompatibility,easy availability of raw materials,and strong molecular recognition ability.Through studying the liquid-liquid extraction equilibrium and separation mechanism,it will provide a theoretical basis for the design of high-efficiency deterpenation media.Herein,19 kinds of ionic liquids including[Ch][AA]ILs(11 kinds)and[Ch][CA]ILs(8 kinds)with the same cation and different anions were synthesized.Characterizations were carried through 1H-NMR,thermal gravimetric analysis,FT-IR,water content analysis,and the influence of anion species on the structure and properties of ionic liquids was discussed.The results show that the synthesized ionic liquid has good thermal stability,the thermal decomposition temperature is in the range of 168-213℃,and increases with the extension of the carbon chain of anions.In[Ch][AA],a strong interaction was formed between-NH2 and-COOH groups on the anion.The extraction of simulated essential oils close to the real composition in a series of[Ch][AA]ILs were studied,the linalool-limonene-ionic liquid ternary equilibrium phase diagram was obtained,and the influence of anion chain length,functional group types,temperature,and water contents were investigated.The results show that[Ch][Pro]and[Ch][Nle]based on proline and norleucine anions exhibit excellent separation performance,the distribution ratio reached 2.50 and 2.81 in the simulated essential oil with linalool content of 5 wt%respectively;and in the low concentration area of linalool,the highest selectivity coefficient reached 207 and 135,respectively.The growth of the alkyl side chain of the amino acid anion can increase the distribution ratio of linalool.For example,the distribution ratio of linalool increases by more than 8 times from[Ch][Ala]to[Ch][Nle].Surprisingly,the distribution ratio of limonene is much lower than expected when[Ch][Nle]is used as the extractant,resulting in higher selectivity,indicating the existence of a unique extraction mechanism.Lowering the extraction temperature can increase the distribution ratio without affecting the selectivity,but the presence of water and the introduction of diluents have an adverse effect on the extraction and separation process.The small-angle X-ray scattering experiment combined with molecular dynamics simulation was used to study the microstructure of[Ch][AA]ionic liquids and the mechanism of interaction with linalool and limonene.The results show that both[Ch][Nle]and[Ch][Pro]have microaggregation structures and alternating between polar regions and non-polar regions,which affect the interaction sites of anion and cation.At the same time,linalool can disperse well in[Ch][Nle]and[Ch][Pro],and it has a small self-diffusion coefficient,indicating a strong interaction with ionic liquids.Limonene has a self-diffusion coefficient which greater than other components,and it has a higher freedom degree.There exists an interaction between hydroxyl group of linalool and the carboxylate anion along with cation,but the specific situation is affected by the type of anion.