Highly Selective Separation of Levulinic Acid from Bamboo Pulp Hydrolysates by Forming a Hydrophobic Deep Eutectic Solvent with Terpenoids

To address the challenges of impurities (humins and formic acid), this work evaluated terpenoids, including thymol, geraniol, and menthol, as extractants to extract levulinic acid (LA) from bamboo pulp hydrolysate by forming a hydrophobic deep eutectic solvent (DES). Among these, thymol exhibited the best extraction performance for LA, with an extraction efficiency of 66.54 ± 0.09% and a maximum selectivity αLA/FA of 31.09 ± 6.30. The extraction efficiency for LA increased to 96.82% after four stages of extraction simultaneously using thymol. The extraction mechanism revealed noncovalent interactions, such as π–π and C–H···π interactions, between thymol and LA, resulting in high extraction efficiency and selectivity. Furthermore, thermodynamic analysis indicated that the extraction process using thymol was a spontaneous, exothermic process with a reduction in entropy. The kinetic data showed that equilibrium was achieved at around 60 min at an extraction temperature of 50 °C, with an equilibrium extraction capacity of 9.43 g L–1. More importantly, thymol showed stable recycling performance for LA, and the extraction efficiency remained at 62–64% during five times of recycling.