The cornerstone of realizing lignin value-addition: Exploiting the native structure and properties of lignin by extraction methods

The complex interlacing structure among cellulose, hemicellulose and lignin and the unique properties of lignin itself limit the value-added utilization of the lignin. As the upstream process of biorefinery, lignin separation technology is very important for lignin conversion into high-value materials. However, it is not easy to extract lignin with relatively complete structure without affecting performance from biomass raw materials. Notably, most of reviews regarding lignin focus on its degradation. The extraction or separation methods are not introduced in detail, and the effects of these methods on raw materials or lignin have not been reported. Herein, this paper critically reviews the changes in physicochemical properties and structure of lignin caused by several mild organic solvent extraction methods and new ionic liquids which has been widely used in recent years. Furthermore, the application and potential economic benefits of these lignin are summarized. These methods promote the effective utilization of lignin, which is no longer limited to heat energy, but also alleviates the solid waste disposal problem caused by lignin as waste stream. Finally, perspectives on the future research on the lignin separation, depolymerization and application are highlighted. This review may provide an insight for the development of bio-refining technology.