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The potential to utilize lignin, which constitutes as much as 15-30% of the biomass, needs further evaluation. In the transition towards a bioeconomy, lignin has the potential to replace fossil-based phenols. Attempts to valorize the available technical lignins are ongoing; however, that lignin suffers from molecular heterogeneity. Accordingly, new process concepts, coined in the term lignin biorefineries, are required to obtain less heterogenic lignin with different properties and preserved molecular structure.

In this study, a new lignin extraction concept was investigated, where the structural properties of lignin were preserved to a high degree using a physical protection strategy. The principle of preserving the lignin structure was based on a cyclic organosolv extraction concept. At first, a two-step concept was evaluated, where a hydrothermal extraction was performed to recover hemicellulose, followed by a cyclic organosolv extraction to obtain the lignin. Trend studies were performed for the individual cycles to gain a deeper understanding of how the lignin structure was affected by the cycles. To further investigate the method, chemometrics and design of experiment were used to gain knowledge about how different properties of lignin were affected by the extraction conditions and how the properties of lignin could be tailored. Based on the knowledge from the chemometric study and the observations from the two-step method, a refined one-step method was developed to obtain lignin with further improved analytical quality, i.e., up to 95% of the interunit linkages could be assigned for spruce lignin by heteronuclearsingle quantum coherence (HSQC) nuclear magnetic resonance (NMR)spectroscopy, 13C NMR and size exclusion chromatography (SEC). The universality of the method was investigated for different wood species, such as spruce and birch. The results indicate the applicability of the concept using different raw materials.

The complex nature of lignin substrates conveys the need for robust analytic techniques. Herein, NMR studies were complemented by matrix-assisted laser desorption/ionization (MALDI) time of flight (TOF) mass spectrometry (MS) to provide new insights into molecular lignin populations with respect to reactivity during the organosolv extraction.

Finally, a proof of concept for an application was investigated. The cyclic organosolv extracted lignin was used in a fundamental study on lignin nanoparticles (LNP), together with benchmark technical lignins, to gain knowledge about the role of the molecular structure in the LNP properties. It is suggested that the molecular structure of lignin plays an important role in determining the size and morphology of LNPs, opening possibilities to molecularly tailor LNP properties.

Professor Martin Lawoko

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