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Kraft pulping, organosolv process and acid hydrolysis were applied on an elm clone. The solubilized lignins were recovered and analyzed. Kraft pulping and acid hydrolysis led to lignins with higher phenolic OH content as result of extensive cleavage of β-O-4' linkages, as revealed by 13 C solid state and 13 C- 1 H heteronuclear single quantum coherence nuclear magnetic resonance. This depolymerization also yielded lower molecular weight lignins inferred by size exclusion chromatography. Contrarily, organosolv process gave rise to a lignin with a more preserved structure, maintaining a large number of β-O-4' linkages. Consequently, organosolv lignin presented lower phenolic OH content and higher molecular weight. Moreover, the high content of the labile native β-O-4' linkages in organosolv lignin resulted in a lower thermostability as compared to the kraft and acid lignins. On the other hand, the solubilized lignins from kraft and acid processes displayed an enrichment of S-units, whereas lignin from organosolv process was slightly enriched in G-units, containing all of them different native as well as pre-treatment derived units. These results could help to increase the inventory of lignin sources available for future lignin-based products, for which knowledge of the lignin properties versus application requirements is crucial., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Lignin streams produced in biorefineries are commonly used to obtain energy. In order to increase the competitiveness of this industry, new lignin valorization routes are necessary, for which a depth characterization of this biological macromolecule is essential. In this context, this study analyzed lignin streams of Robinia pseudoacacia L. generated during organosolv and acid hydrolysis pre-treatments and during the subsequent enzymatic hydrolysis. These lignins included dissolved lignins from pre-treatment liquors and saccharification lignins from pre-treated materials. Chemical composition and structural features were analyzed by analytical standard methods and Fourier Transform Infrared spectroscopy (FTIR), size exclusion chromatography (SEC), 13 C solid state nuclear magnetic resonance ( 13 C NMR) and 1 H- 13 C two-dimensional nuclear magnetic resonance (2D NMR); while thermal characterization included thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). In general, all studied lignins contained a predominance of β-O-4' aryl ether linkages, followed by resinol (β-β') and phenylcoumaran (β-5'), with a predominance of syringyl over guaiacyl and hydroxyphenyl units. Nevertheless, the dissolved lignins revealed a removal of linkages, especially β-O-4', leading to an enrichment of phenolic groups. Moreover, high thermal stability and good thermoplasticity were characteristics of these lignins. Contrary, the saccharification lignins exhibited a more intact structure, but with an important remaining carbohydrates content., (Copyright © 2019 Elsevier B.V. All rights reserved.)

The complexity and heterogeneity of lignin requires a detailed understanding in order to decide about more efficient lignin valorization approaches. This study deals with the characterization of lignins from Populus alba L. generated as by-products in different transformation processes: kraft pulping, organosolv and dilute acid hydrolysis. In addition to the composition, the chemical and structural features of the different lignins were investigated by Fourier Transform infrared spectroscopy (FTIR), solid-state 13 C nuclear magnetic resonance ( 13 C NMR), two-dimensional nuclear magnetic spectrometry (2D NMR), size exclusion chromatography (SEC), and thermal analysis. Organosolv lignin showed noticeably different characteristics compared to kraft and acid hydrolysis lignins; higher molar mass, higher amount of side-chain linkages (mainly aryl-β ether and resinol) together with lower phenolic content. On the contrary, kraft and acid hydrolysis lignins presented an extensive elimination of lateral chains and therefore a higher phenolic content, which suggests a much stronger lignin depolymerization (lower molar mass) during these processes. Moreover, thermal analysis results revealed that the thermal stability of kraft and acid hydrolysis lignins was higher than that of organosolv lignin, especially in the case of acid hydrolysis lignin. According to all these characteristics, several valorization pathways for studied lignin are discussed., (Copyright © 2018 Elsevier B.V. All rights reserved.)