Rapid Characterization of Kraft Lignin Depolymerization Products by High-Resolution Mass Spectrometry with Visualization of Mass Spectrometric Data

Kraft (sulfate) lignin is a large-scale by-product of the pulp and paper industry and a promising renewable raw material for the production of a wide range of chemicals and materials with high added value. One of the main directions of valorization of technical lignin is conversion into low-molecular-weight phenolic products using various depolymerization methods. To solve the problems of operational control of ongoing processes and the molecular level characterization of complex mixtures of formed low-molecular-weight compounds, this study proposes the use of atmospheric pressure photoionization high-resolution mass spectrometry in combination with visualizing the resulting data sets based on elemental ratios (van Krevelen diagrams) and Kendrick mass defects. Using this methodology, the kraft lignin depolymerization products formed in a supercritical isopropanol medium were characterized, and the effect of additives of nitrogen-containing catalysts (hydroxylamine and diethylamine) on the ongoing processes and the composition of the resulting products were studied. The leading role of reduction processes in the transformation of lignin oligomers under the action of isopropanol was shown, and the characteristic differences between the products obtained in the absence and with the addition of nitrogen-containing catalysts were revealed.