Tandem modification strategy on technical lignin realizing the balance of solubility and antioxidant activity in castor oil

In this study, demethylated-acylated enzymatic hydrolysis lignin (DAEHL) with excellent solubility in castor oil and antioxidant activities were prepared via the tandem modification strategy. First, iodocyclohexane simultaneously achieved β-O-4 breakage and hydrogenation, which enhanced the antioxidant activity of lignin. Furthermore, the acylation reaction by palmitoyl chloride increased the alkyl content in the lignin, which can improve the solubility of lignin in castor oil. The structural evolution of lignin showed the change in the hydroxyl group content, which induced a change in the solubility and antioxidant activity of the lignin. 1,1-diphenyl-2-picrylhydrazyl radical (DPPH), 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) ammonium salt (ABTS) radical scavenging, Fe³⁺ reduction capacity and oxidation induction time demonstrated the excellent antioxidant activity of the modified lignin DAEHL. In addition, the balance between antioxidant activity and solubility of lignin was realized by the tandem modification strategy. DAEHL shows good solubility over 1 wt% in castor oil. The OIT value of the castor oil was significantly increased from 162 s (pure castor oil) to 218 s with 0.3 wt% DAEHL. In addition, the wear volume of DAEHL-castor oil is only 61 % of that of castor oil. This study allows for the high-value utilization of technical lignin as a sustainable lubricant addition in castor oil. Synopsis: A tandem modification strategy for technical lignin dissolving in castor oil with antioxidant activity.