Nitrogen-Functionalized Lignin: Current Status, Applications, and Challenges.

Lignin-a natural, abundant, and renewable biopolymer-has garnered increasing attention for its intrinsic sustainability and versatile chemical architecture. The abundance of reactive functional groups within lignin enables its modification to meet specific application requirements. Nitrogen-functionalized lignin (N-lignin), in particular, has emerged as a focal point in contemporary research, providing a pathway to introduce nitrogen functionality through chemical modifications, grafting copolymerization, or physical blending with nitrogen-containing materials. These strategies significantly enhance the performance of the lignin, making it suitable for diverse applications such as catalysts, antifouling agents, nitrogen-enriched fertilizers, and antibacterial materials. This review offers a comprehensive examination of current methodologies for the nitrogen functionalization of lignin. It also delves into the applications of these functionalized materials in diverse fields, including environmental remediation, biomedicine, energy, catalysis, and agriculture. The analysis highlights the potential of N-lignin to drive sustainable innovations while addressing practical challenges, such as scalability, structural heterogeneity, and process optimization. By synthesizing recent advancements and identifying ongoing challenges, this review provides a roadmap for lignin valorization, emphasizing its transformative role in fostering sustainable technologies and materials.