Lignin to value-added chemicals and advanced materials: extraction, degradation, and functionalization

Lignin is a green and sustainable feedstock for the production of chemicals and materials; however, its utilization is still low. Over the past decade, remarkable progress has been achieved in the extraction, degradation, and functionalization of lignin and the fabrication of lignin-containing materials. In this review, we provide a critical examination of the in-depth insights into recently developed strategies for the green and efficient extraction, selective degradation and diverse functionalization of lignin. Its green extraction involves the judicious selection of solvent, and mild enzymatic conditions, leading to different technical lignin products. The degradation of lignin can be achieved via thermochemical, electrochemical, photocatalytic and biological processes. Lignin can be functionalized via the modification of its aromatic rings, modification of its hydroxyl groups, modification of its methoxy groups, and graft copolymerization. Based on the top-down degradation and bottom-up functionalization strategies, lignin can be converted into a variety of value-added chemicals and polymers and can be engineered into many types of lignin-based materials. These materials hold great promise in energy storage, targeted drug delivery, efficient adsorption and catalysis, functional packaging and slow-release fertilizers. The correlation among the sources, degradation, and functionalization of lignin and lignin-based materials is discussed. Also, we present a techno-economic analysis on the production of chemicals and materials from lignin. Finally, an outlook for lignin valorization is presented.