The Nature of Lignin and Implications for Its Technical Use as a Source for Biogenic Aromatics—A Review

The composite material lignocellulose makes up the majority of biomass on earth and is characterized by a high biological and chemical resistance, which is essentially caused by the phenylpropanoid polymer lignin. Thus, the removal and depolymerization of lignin to produce aromatic chemicals can significantly enhance the material usability of all lignocellulose constituents. This review summarizes the current state of knowledge on the nature of lignin, including its biosynthesis, structure, chemistry and biodegradation. Second, it attempts to derive implications regarding the technical valorization of lignin from native biomass through depolymerization. Finally, the consequences of the findings for conventional, recently developed and future processes valorizing lignocellulose are assessed, and the associated technical and economic hurdles are discussed. It becomes clear that lignin depolymerizability is restricted in established pulping processes, primarily due to repolymerization reactions. Strategies avoiding lignin repolymerization involve an increased process complexity and additional economic expenditure but might enable an increased value creation from lignocellulosic biomass.