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 Ts 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.