Comprehensive characterization of lignins in the context of their valorization: A combination of NMR/2D-COS IR spectroscopy.

The widespread development of lignin valorization is hindered by a number of challenges. In particular, efficient valorization necessitates comprehensive characterization of initial lignins. In this work, the structural features of lignins from birch wood (Bétula péndula), obtained by various methods of hard and mild fractionation of biomass: hydrolysis (Hyd-L), kraft (Kraft-L), soda (Soda-L), and soda-ethanol (SodaEt-L) processes, as well as organosolv processes with dioxane (MWL, DL) and dimethyl sulfoxide (DMSO-L) have been comprehensively studied. The main novelty of this study is the first-time application of Two-Dimensional Correlation Infrared Spectroscopy (2D-COS IR) for rapidly and comprehensively assessing structural transformations of lignins, in combination with P NMR. It has been established that changing delignification conditions primarily affects the processes of demethoxylation, dehydration and polymerization/condensation of lignin, and then the breaking of lignin-carbohydrates linkages. An approach to evaluating the potential of different types of lignin for valorization (valorization index), based on their structural characteristics, has been proposed. The applied approach allowed for the arrangement of the studied lignins in order of decreasing valorization index: DMSO-L, MWL, DL, SodaEt-L, Soda-L, Kraft-L.