Nitration of Hydrolysis Lignin with Nitric Acid in Dimethyl Sulfoxide and Water

Hydrolysis lignin is a by-product which is an insoluble residue formed after the hydrolysis of polysaccharides of lignocellulosic materials for further processing of the resulting monosaccharides into bioethanol or other products. Hydrolysis lignin, unlike other technical lignins, is insoluble in water and organic solvents, and also contains non-hydrolysed cellulose, extractive and mineral substances. Finding ways to rationalize the use of this by-product for the production of chemicals is an urgent task, the solution to which will increase the economic attractiveness of the cellulosic bioethanol technologies. In this work, the nitration of hydrolysis lignin with nitric acid in a mixture with and without dimethyl sulfoxide during heating has been investigated, which allows the lignin part to be converted into water-soluble products upon alkalization. Klason lignin has been used as a model of hydrolysis lignin that does not contain a cellulose component. Using electron spectroscopy, the formation of water-soluble products from lignin has been monitored, and the water-soluble products have been subject to degradation in the reaction mixture. For Klason lignin, a solvent-nitric acid mixture composition has been selected, which allows preserving the structure of lignin. Based on the mass values of residues during nitration of hydrolysis lignin, kinetic dependences of the course of its delignification have been constructed and the observed rate constants have been calculated by describing the process using 1st-order kinetic equations at 60...100 °C. The activation energies of the delignification of hydrolysis lignin in the case of using dimethyl sulfoxide as a co-solvent and without it have been 96 and 86 kJ/mol, respectively. Upon nitration of hydrolysis lignin, water-soluble products have been formed, containing 3 fractions with the following molecular mass values: less than 103, 103...104 and more than 104 g/mol. In this case, during the nitration process, the content of the last fraction has decreased due to the depolymerizing effect of nitric acid. An increase in the proportion of nitric acid in the mixture has led to a stronger depolymerization of lignin macromolecules, as a result of which the nitration products of hydrolysis lignin have lacked a fraction of more than 104 g/mol, and the 2 fractions with lower molecular mass values have shifted towards low molecular mass values.