Purification of crude tea tree oil through enrichment of terpinen-4-ol by in-situ formation of deep eutectic solvent

As a key component in tea tree oil (TTO), terpinen-4-ol exhibits ideal physiological activities. However, the high content of terpenes in crude TTO makes it challenging to enrich terpineol-4-ol. In this study, molecular simulations combined with experimental methods were used to select the optimal hydrogen bond acceptor (HBA), tetrabutylammonium chloride (TBAC), which forms a deep eutectic solvent (DES) with terpinen-4-ol in situ. Combined with necessary post-treatment process, it provided a feasible way for purification of TTO. Related DESs were prepared and characterized, and their main properties were also investigated. When the amount of TBAC was three times that of terpinen-4-ol, the formation of hydrogen bonds became more pronounced. At this point, the DES achieved its highest distribution coefficient for terpinen-4-ol (D t  = 15.02), as well as superior selectivity for terpinen-4-ol (S t/c  = 24.59, S t/γ  = 57.69). Water as the back-extractant and n-hexane as the auxiliary extractant facilitated the deep purification of terpinen-4-ol. Response surface analysis and experimental results confirmed that the optimal enrichment conditions were 30 min of enrichment time, an enrichment temperature of 60℃, and 0.5 g/g of n-hexane dosage. The back-enrichment and recovery of TBAC also proved to be satisfactory, and when applied to real tea oil, the developed method achieved over 90 % enrichment efficiency for terpinen-4-ol.