Extractive separation of citral and limonene with quaternary Ammonium/ Alkanediol deep eutectic Solvents: An experimental and mechanistic study

As an aldehyde derivative of monoterpene, citral is the main oxyterpene in various botanical essential oils which has been widely applied in food, cosmetics and medicines. The separation of oxyterpenes from terpenes (deterpenation) improves its stability and quality, which is an essential step for production of citral with high market value. Herein, the extractive separation of citral and limonene using deep eutectic solvents (DESs) of quaternary ammonium/alkanediol was explored. Influence of DESs components, including different quaternary ammonium salts as hydrogen bond acceptors (HBAs), various alkanediols as hydrogen bond donors (HBDs) and the changing of HBA to HBD ratios on citral extraction were studied. Distribution coefficient, selectivity and extraction yield of the citral of these DESs were experimentally evaluated. Among the HBAs investigated, tetrabutylammonium bromide (TBABr) was determined to be the suitable one, since halide Br- was better than Cl-, and quaternary ammonium cations with longer alkyl chain length was preferable. Meanwhile, 1,2-propanediol (1,2-PDO) outperformed other alkanediols, and higher HBD contents in DESs facilitated the separation. TBABr/1,2-PDO (molar ratio 1:10) exhibited the best performance, with citral distribution coefficient 1.28, selectivity 13.65 and extraction yield 64.26%. The type and strength of molecular interactions between citral solute and DESs were identified by employing quantum chemistry calculation and molecular dynamic simulations. The performance difference of these DESs can be ascribed to the difference of the direct hydrogen bond between the alpha-OH of alkanediols and the O from citral carbonyl group during extraction. The mechanisms exploration provides insights for rational fine-tuning the novel DESs with better deterpenation performance.