Lignin Valorization Problems (A Review).

This review considered two directions of lignin valorization: valorization of technical lignins as such, not subjected to preliminary depolymerization, and their valorization through monomeric compounds resulted from selective lignin degradation. The first direction includes production of lignin hydrogels and use of lignin in medicine, pharmacology, and 3D printing, as well as in preparation of carbon fibers and biofuels. Lignin hydrogels are distinguished by high sorption capacity with respect to heavy metals such as lead, iron, and copper, making ~25–50% of the lignin weight, depending on the content of acidic groups in lignin and on the molar mass of the sorbate, and therefore are suitable for treatment of chemical industry wastewater. Lignin possesses high biological activity toward various pathogens, including viruses, which makes this research area very topical, especially against the backdrop of the COVID-19 outbreak. Use of lignin in some composites for 3D printing allows increasing the mechanical strength of the finished products. Industrial implementation of the technology of carbon fibers production from lignin will provide a twofold reduction of the weight of vehicles. The second direction of lignin valorization, including hydrogenolysis and selective oxidation, affords monomeric compounds in close to theoretical yields. The review also considered economic aspects of valorization. In addition, based on comparison of the results of valorization of softwood and hardwood lignins, a hypothesis on the structure of native lignin was proposed. [ABSTRACT FROM AUTHOR]