Lignin Demethylated by Protic Ionic Liquid as a Novel and Sustainable Chrome-Free Tanning Agent for Eco-Leather Production

Sustainable and cost-effective tanning agents for chrome-free tanning are still highly desirable. Lignin-based polyphenols (LP) are more activity-added platform chemicals than lignin and can be widely used to manufacture eco-friendly biomaterials and antibacterial chemicals. Herein, we exploited the excellent performance of LP as a novel tanning agent to substitute for vegetable tanning agents for eco-leather production under chrome-free conditions for the first time. LP was prepared through a halogen-free demethylation protocol in low-cost, protic ionic liquids. Partial disruption of β-O-4′ linkages in raw lignin and of the C-O bonds on methoxy groups resulted in higher phenolic -OH content with more catechol structure in LP, resulting in significant tanning performance of LP. Furthermore, the combination of LP with Al³⁺/Zr⁴⁺ cotanning agents rendered the average T_s of tanned leather to 93 °C (48.2 °C for raw sheepskin) and a weight retention of 29.28% at 800 °C, which are well qualifying the standard of commercial tanning agents. This work provides new insights into developing vegetable tanning substitutes and achieving value-added utilization of industrial lignin resources.