Structure and Characteristics of Lignin

Lignin contributes up to 30% of lignocellulosic biomass by weight and provides an alternative renewable source of aromatic chemicals. However, it has not been exploited effectively due to its natural complexity and high stability. Therefore, continuous and vigorous efforts should be paid to achieve a better understanding of the chemical structure and composition of lignin in order to design more efficient and greener degradation processes, and further to explore new value-added applications. Lignin is biosynthesized by radical coupling of the basic building blocks by enzymatically catalyzed random polymerization reactions. The chemical structure of lignin depends not only on the type of biomass material but also on the part of the plant, location of tissue, age, the growing conditions, and other natural conditions of living plants. In addition, in the elucidation of structural characteristics of lignin, the degradation processes, extraction techniques, and even the analytical methods used for characterization can also influence the results. This chapter presents a comprehensive overview of structural characteristics of lignin and related developments in the studies including experimental and computational assays, physicochemical properties investigation, isolation and separation methods, and the instrumental technologies. Monolignols/units and the inter-unit linkages as well as their formation and transformation are summarized in the processes of radical reactions and biosynthesis. Macroscopic physicochemical properties of lignin together with parameters are also listed. Different methods for generating lignins are compared. Various techniques applying on characterization of lignin structure are introduced and discussed, including spectroscopic, chromatographic, and spectrometric methods. New insight into structural characteristics of lignin and advanced separation methods and instrumental techniques is highlighted. Future investigations on the topic of lignin structure are also prospected and suggested.