A brief review of the structure and extraction methods of lignin

One of the three basic components of lignocellulosic biomass (LB), lignin is the second most plenty of natural polymer on the planet after cellulose. It has the power to become the aromatic raw material of the future for industry. However, this potential has not been fully realized due to its structure has not yet been fully elucidated and its largely underexploited industrial availability. Lignin extraction methods are divided into two main processes are sulfur-containing and non-sulfur-containing. Sulfur processes contain sulfite and kraft (alkaline) pulping methods; the main types of lignin obtained by these methods are kraft lignin and lignosulfonates. Non-sulfur processes contain solvent-based pulping (organosolv) and soda pulping methods; the main types of lignin obtained by these methods are organosolv lignin and soda lignin. In addition to these methods, there are also other chemical-based delignification processes, such as acid hydrolysis from bio-ethanol production and vapor spraying methods with the application of high-temperature and high-pressure steam. In this context, understanding the structure and extraction of lignin is important to fully exploit its potential. This review provides an overview of the structure, chemical properties, and extraction methods of lignin.