## 2 Structural Complexity and Composition of Native Lignin
### Lignin as a Carbon Fiber Precursor  
L Lignin, a renewable and abundant biopolymer derived from plant biomass, represents a valuable feedstock for the development of sustainable chemicals and materials [63]. Among the most promising polarization pathways are the depolymerization of lignin into aromatic monomers and its use as a precursor for low-cost carbon fiber production (Figure 6) [64]. These routes support the transition toward a circular bioeconomy by converting lignin-rich waste streams into high-value products with reduced environmental impact. One key valorization strategy is the depolymerization of lignin to yield valuable aromatic compounds such as vanillin, syringol, and phenolic derivatives. These monomers, typically derived from petrochemical sources like benzene, serve as essential building blocks in the production of flavors, fragrances, adhesives, resins, and biofuels [65]. Depolymerization offers a renewable alternative, with typical aromatic yields ranging from 15% to 25% depending on feedstock and process conditions [66]. Technologies such as Rapid Thermal Processing (RTPTM), developed by Lignol Energy and now commercialized by Ensyn Corp (Ottawa, Canada) ([[https://www.ensyn.com/](https://www.ensyn.com/)]([https://www.ensyn.com/](https://www.ensyn.com/)) accessed on 10 May 2025) utilize high-temperature pyrolysis to convert lignin-rich biomass into Renewable Neutral Fuel (RNFTM), a versatile bio-oil feedstock for fuels and chemicals. Further applications of lignin-derived aromatics include the synthesis of renewable polymers, surfactants, and specialty chemicals. Despite these advancements, several challenges remain. Improving the selectivity and yield of desired aromatic monomers, minimizing energy consumption, and developing efficient separation and purification methods are critical for process optimization. Innovations in catalytic technologies, including heterogeneous catalysts, ionic liquids, and enzymatic systems, are actively addressing these limitations, enhancing the efficiency and sustainability of lignin depolymerization. Integrating these processes into existing biorefineries is essential for economic viability and maximizing resource utilization. The economic and environmental benefits of lignin depolymerization are driving increased investment and innovation, with both startups and established companies pursuing commercialization strategies. In parallel, lignin is also gaining momentum as a sustainable precursor for carbon fiber production, traditionally dominated by polyacrylamide (PAN) [67]. A significant breakthrough in this field is the melt-spinning process, which involves heating lignin to a molten state and extruding it to form fibers that are later stabilized and carbonized into high-performance carbon fibers. This method reduces energy consumption by up to 70% compared to PAN-based processes and is compatible with the existing manufacturing infrastructure facilitating industrial adoption [68]. In addition to cost and energy benefits, lignin-based carbon fibers contribute to lower lifecycle emissions and support sustainability goals by replacing fossil-derived materials with renewable alternatives. These materials are particularly promising for lightweight applications in aerospace, automotive, wind energy, and sporting goods, and align well with the growing demand for sustainable materials in emerging sectors such as electric vehicles and renewable energy. Overall, the dual valorization of lignin into aromatic monomers and carbon fibers highlights its versatility as a renewable feedstock and reinforces its role in advancing both green chemistry and sustainable materials development. The convergence of innovations in depolymerization and fiber processing technologies is accelerating commercial interest and investment in lignin-based products, paving the way for broader implementation across industrial sectors.