As the most abundant natural renewable aromatic polymers, lignins can be used to produce bio-fuels and value-added chemicals. Lignins attract more and more attention in energy and chemical industrials, and have the potential to substitute petroleum as raw materials to some extent. To realize the application of lignins, the chemical composition and structural characteristics should be fully understood. However, the ubiquitous crosslinks among lignins, cellulose and hemicelluloses are complex, resulting in the incomplete separation of lignin from biomass.In addition, diverse sources and complex structures embarrass the development of investigation and application.With revealing the chemical compositions of lignins as the starting point, methodologies and technologies, such as pretreatment, separation, transformation and analysis, for the structural investigation of lignins are compared, and the up-to-date advances, including monolignols and linkages of lignins, biosynthetic processes, the reactions of model compounds and lignification theories are importantly reviewed. The applications of lignins and their derivatives in the fields of polymer blends, resins, carbon fiber, activated carbon and other chemicals along with related difficult problems are pointed out. Contents
1 Introduction
2 Methods of structural investigation
2.1 Methods of pretreatment and separation of lignins from biomass
2.2 Analysis methods
3 Progress in structural investigation of lignins
3.1 Molecular mass and molecular mass distribution
3.2 Monolignols
3.3 Linkages
3.4 Lignin/phenolic carbohydrate complexes
3.5 Macromolecules and stereochemistry of lignins
3.6 Biosynthesis
3.7 Lignin-related model compounds
3.8 Models of lignins
3.9 Computer-added designing and simulation
3.10 Theories of lignification
4 Applications
4.1 Additives and blends
4.2 Lignin-based resins
4.3 Carbon fiber and activated carbon
4.4 Value-added chemicals
5 Conclusions and prospectives