Lignin : Structure, Production and Chemical Valorisation

Of all natural polymers, lignin is surely the most emblematic of the plant kingdom. Its biosynthesis and structure have long remained poorly understood, due to its resistance to chemical and biological agents. Nevertheless, research into lignin has intensified since the beginning of the third millennium, with the aim of improving paper-making processes and, above all, preparing the biorefinery that is set to overtake petrochemicals before the end of the century. xxi ^e century. The conversion of cellulose and hemicelluloses into simple sugars (C5 and C6) for the production of fuels or chemical precursors has now been mastered and industrial deployment has begun. While several million tonnes of lignin from the paper industry and bioethanol plants could be obtained and exploited each year (10-20% of the lignin generated), only a tiny fraction is currently recovered. A better understanding of lignin's chemical structure has enabled us to propose high-performance processes for separating it from polysaccharides. At the same time, adapting its biosynthesis using biotechnology tools will make it possible to offer plants that are better suited to the industrial processes used by biorefineries and in line with the principles of sustainable chemistry. This article first presents the main characteristics of lignin, followed by the different extraction processes from plant biomass. The different lignin recovery routes are described and discussed, with a particular focus on the enzymatic route, as this has the advantage of fully complying with the principles of green chemistry.