Issues of the Structural Organization of Lignin and Prospects for Its Processing (A Review)

This article presents a review of the literature on studies of lignin, which is one of the most abundant biopolymers on Earth, basing on publications in the world’s most-cited sources. It is noted that, already nowadays, lignin is considered as a powerful renewable source of valuable organic raw materials, but the potential for lignin valorization is much broader than the presently exploited opportunities. Particular focus is placed on the structure and molecular organization of birch lignin; birch wood is actively used in chemical industry, while the literature on hardwood lignins is much scarcer compared to softwood lignins. The basic methods of structural investigation of such a complex and irregular polymer as lignin are discussed, and their advantages, disadvantages, and application prospects are analyzed. It is pointed out that, nowadays, the most effective methods for studying the lignin structural units are pyrolysis-gas chromatography and nuclear magnetic resonance spectroscopy. Various methods of lignin isolation from wood are described, and the influence of the isolation method on the structure of the isolated substance was evaluated. Also, the structural features of birch lignin and its distinctions from lignins of other species are shown.