Amidst declining fossil-based resources and environmental challenges, the focus on biobased materials has intensified. Carboxymethylation is one way to introduce reactive functionality to enhance the reactivity of lignin for a specified application. This research investigates the carboxymethylation of four lignin sources: eucalyptus kraft lignin, spruce kraft lignin, birch cyclic extracted organosolv lignin, and spruce cyclic extracted organosolv lignin. Our aim is to elucidate the role of the lignin structure in its reactivity. Using the advanced analytical techniques NMR spectroscopy, Fourier transform infrared spectroscopy, density functional theory, and size-exclusion chromatography, we provide a comprehensive characterization of the modified lignin. The findings offer valuable insights into how the chemical and physical properties of molecular lignin affect the selectivity and efficiency of the carboxymethylation reaction. These fundamental findings hold great potential for guiding considerations on the selection of lignin sources for specific applications based on their molecular properties.

Research on lignin valorization has gained ground, driven by its potential to replace fossil-based phenolics in bio-based applications. Technical lignins are structurally complex and still poorly characterized, prompting the need for novel extraction processes for lignin of high analytical quality. In this context, a two-step cyclic extraction process for lignin was contrasted with a one-step cyclic extraction. The latter was shown to preserve the native structure of the spruce lignin product better and improved the yields of both the extracted lignin and residual fiber fraction. The application of the one-step cyclic extraction process to birchwood resulted in a similar protection of the lignin structure. Overall, a flexible physical protection (FPP) process for extraction of lignin with an abundance of native bonds is presented. The lignin product has a high abundance of ether bonds and hydroxyl functionalities, which are of interest in biochemical, polymer, and material applications.

There is need for well-defined lignin macromolecules for research related to their use in biomaterial and biochemical applications. Lignin biorefining efforts are therefore under investigation to meet these needs. The detailed knowledge of the molecular structure of the native lignin and of the biorefinery lignins is essential for understanding the extraction mechanisms as well as chemical properties of the molecules. The objective of this work was to study the reactivity of lignin during a cyclic organosolv extraction process adopting physical protection strategies. As references, synthetic lignins obtained by mimicking the chemistry of lignin polymerization were used. State-of-the-art nuclear magnetic resonance (NMR) analysis, a powerful tool for the elucidation of lignin inter-unit linkages and functionalities, is complemented with matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF MS), to gain insights into linkage sequences and structural populations. The study unraveled interesting fundamental aspects on lignin polymerization processes, such as identifications of molecular populations with high degrees of structural homogeneity and the emergence of branching points in lignin structure. Furthermore, a previously proposed intramolecular condensation reaction is substantiated and new insights into the selectivity of this reaction are introduced and supported by density functional theory (DFT) calculations, where the important role of intramolecular π-π stacking is emphasized. The combined NMR and MALDI-TOF MS analytical approach, together with computational modeling, is important for deeper fundamental lignin studies and will be further exploited.

The heterogeneous nature of lignin poses significant obstacles to its practical use in material applications. Common fractionation methods employ harsh processing conditions that further exacerbate lignin's structural complexity. We propose a microwave (MW)-assisted approach for a mild organosolv extraction of structurally-intact lignin from spruce wood. The efficient energy transfer enabled by microwave irradiation facilitates the rapid extraction of lignin in 5, 10, and 20 minutes, ensuring a low level of process severity. Comparison of the 10 minutes MW-extracted lignin products with a cyclic-extracted (CE) organosolv lignin revealed that equivalent amounts of β-O-4 linkages were preserved in both processes. This is indicative of the promising potential of MW-extraction as a biomass pretreatment method for the rapid extraction of more native-like lignin. Finally, we demonstrate the utilization of both MW- and CE-extracted lignins as property-enhancing fillers in a biobased photocurable resin for digital light processing (DLP). The more native-like structures of the mildly-extracted lignins proved beneficial for functionalization with reactive methacrylate moieties, enabling the mechanical reinforcement of DLP 3D printed thermosets with improved toughness after the incorporation of only 1 wt% of the lignins. Compared to the resin without lignin, the tensile strength was improved by 15 and 41% and elongation at break by 79 and 75% in the presence of methacrylated MW- and CE-lignins, respectively. This highlights the potential of MW and CE strategies to effectively process and modify lignin, thereby enhancing its utilization in targeted material applications.

Studies have shown that the size of LNP depends on the molecular weight (M-w) of lignin. There is however need for deeper understanding on the role of molecular structure on LNP formation and its properties, in order to build a solid foundation on structure-property relationships. In this study, we show, for similar M-w lignins, that the size and morphology of LNPs depends on the molecular structure of the lignin macromolecule. More specifically, the molecular structure determined the molecular conformations, which in turn affects the inter-molecular assembly to yield size- and morphological-differences between LNPs. This was supported by density functional theory (DFT) modelling of representative structural motifs of three lignins sourced from Kraft and Organosolv processes. The obtained conformational differences are clearly explained by intra-molecular sandwich and/or T-shaped pi-pi stacking, the stacking type determined by the precise lignin structure. Moreover, the experimentally identified structures were detected in the superficial layer of LNPs in aqueous solution, confirming the theoretically predicted self-assembly patterns. The present work demonstrates that LNP properties can be molecularly tailored, consequently creating an avenue for tailored applications.

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.

Potentialen av att använda lignin, som utgör 15-30% av biomassan, behöver utvärderas ytterligare. I övergången till en bioekonomi finns potential för lignin att ersätta fossila fenoler. I denna kontext försöker man hitta ett värde för de tillgängliga tekniska ligninerna, men det finns utmaningar associerade med den molekylära strukturella heterogeniteten. Följaktligen behövs nya processkoncept, såsom lignin-bioraffinaderier, där man kan ta fram ett lignin med olika önskade egenskaper samt med en bevarad molekylstruktur.

I denna studie utvecklades ett nytt koncept för att extrahera lignin, där ligninets strukturella egenskaper kunde bevaras i hög grad med hjälp av en fysisk skyddsstrategi. Principerna för att bevara ligninstrukturen baserades på ett cykliskt organosolv-extraktions-koncept. Först utvärderades en två-stegs koncept, där en hydrotermisk extraktion, för att extrahera ut hemicellulosa, efterföljdes av en cyklisk organosolvextraktion för att extrahera ut lignin. Trendstudier utfördes för de individuella cyklerna för att få en fördjupad förståelse av hur ligninstrukturen påverkades av cyklerna. För att ytterligare undersöka metoden användes kemometri och experimentell design för att öka kunskapen om hur olika extraktionsförhållanden påverkade egenskaper hos ligninet samt hur egenskaperna hos ligninet kunde skräddarsys.Genom den erhållna kunskapen från den kemometriska studien och den studerade trenden av två-stegsmetoden, undersöktes en kortare en-stegsmetod för att kunna erhålla ett lignin med ytterligare förbättrad analytisk kvalité, där upp till 95% av bindningarna kunde bestämmas för granlignin, karaktäriserat med heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance spectroscopy (NMR), 13C NMR och size exclusion chromatography (SEC). Metodens universalitet undersöktes för träslag med olika lignin-egenskaper som gran och björk. Resultaten indikerade tillämpbarheten av konceptet för olika råvaror.

På grund av den komplexa naturen hos lignin finns ett behov av robusta analystekniker. För att möta detta kompletterades NMR-studier med matrix-assisted laser desorption/ionization (MALDI) time of flight (TOF) mass spectrometry (MS) analyser för att få nya insikter om olika molekylära ligninpopulationer samt för att öka kunskapen om ligninreaktiviteten under organosolv-extraktion.

Slutligen gjordes en undersökning av hur det cykliska ligninet förhöll sig mot en applikation. Det cykliska organosolv-extraherade ligninet användes i en fundamental studie om lignin-nano partiklar (LNP), tillsammans med benchmark-tekniska ligniner, för att öka insikter om de molekylära egenskapernas roll för LNP-egenskaperna. Det har visat sig att den molekylära strukturen spelar en betydande roll för att bestämma storleken och morfologin hos LNP, vilket öppnar upp för möjligheter att molekylärt skräddarsy LNP-egenskaper.

The interest in the bark and the attempt to add value to its utilization have increased over the last decade. By applying an integrated bark biorefinery approach, it is possible to investigate the recovery of compounds that can be used to develop green and sustainable alternatives to fossil-based materials. In this work, the focus is on extracting Norway spruce (Picea abies) bark lignin via organosolv extraction. Following the removal of the extractives and the subcritical water extraction to remove the polysaccharides, a novel cyclic organosolv extraction procedure was applied, which enabled the recovery of lignin with high quality and preserved structure. Main indicators for low degradation and preservation of the lignin structure were a high β-O-4' content and low amounts of condensed structures. Furthermore, high purity and low polydispersity of the lignin were observed. Thus, the obtained lignin exhibits high potential for use in the direct development of polymer precursors and other bio-based materials. During the extraction sequence, around 70% of the bark was extracted. Besides the lignin, the extractives as well as pectic polysaccharides and hemicelluloses were recovered with only minor degradation, which could potentially be used for the production of biofuel or other high-value products such as emulsifiers or adhesives.

Lignin is a renewable source of aromatics with great potential as a substitute for fossil-based phenolic compounds that are used in several material applications. However, the available technical lignins are heterogenic and still structurally not fully understood. This presents hurdles for studies focused on gaining fundamental insights into how materials properties are related to the molecular structure of lignin. In the present study, a novel cyclic extraction process for lignin is more deeply studied to investigate the potential to tailor the chemical and physical properties of lignin. For this purpose, a design of experiment (DoE) approach was adopted as a tool to investigate the effect on the lignin properties of the selected parameters by including linear, quadratic and interaction effects in a multiple linear regression (MLR) model. Molecular characterization techniques included 1D and 2D NMR, SEC and DSC. It was clearly demonstrated that the chemical and physical properties of lignin could be tuned for the cyclic process using the DoE approach, while preserving 66-82% of the commonly known lignin inter-units, substantiating that the cyclic extraction approach offered a decent to excellent level of protection to inter-units when compared to benchmark organosolv and kraft lignin. By manipulation of the extraction conditions, the β-O-4′ content can be tuned between 20 and 35% simultaneously with the content of phenolic and aliphatic hydroxyls. Finally, DSC studies showed Tgs in the range of 150-185 °C which are discussed with respect to the molecular properties of the analysed lignin. Overall, to advance efforts in lignin valorization, a green process to produce a library of well-characterized lignins, tailored with respect to chemical and physical properties by process conditions, is presented.

As part of the continuing efforts in lignin-first biorefinery concepts, this study concerns a consolidated green processing approach to obtain high yields of hemicelluloses and lignin with a close to native molecular structure, leaving a fiber fraction enriched in crystalline cellulose. This is done by subcritical water extraction of hemicelluloses followed by organosolv lignin extraction. This initial report focuses on a detailed characterization of the lignin component, with the aim of unravelling processing strategies for the preservation of the native linkages while still obtaining good yields and high purity. To this effect, a static cycle process is developed as a physical protection strategy for lignin, and advanced NMR analysis is applied to study structural changes in lignin. Chemical protection mechanisms in the cyclic method are also reported and contrasted with the mechanisms in a reference batch extraction process where the role of homolytic cleavage in subsequent repolymerization reactions is elucidated.