Organosolv pretreatment represents one of the most promising biomass valorization strategies for renewable carbon-based products; meanwhile, there is an overall lack of holistic approach to how extraction conditions affect the suitable end-usages. In this context, lignin extracted from silver birch (Betula pendula L.) by a novel hybrid organosolv/steam-explosion treatment at varying process conditions (EtOH %; time; catalyst %) was analyzed by quantitative NMR (¹H–¹³C HSQC; ¹³C NMR; ³¹P NMR), gel permeation chromatography, Fourier transform infrared (FT-IR), Pyr-gas chromatography–mass spectroscopy (GC/MS), and thermogravimetric analysis, and the physicochemical characteristics of the lignins were discussed regarding their potential usages. Characteristic lignin interunit bonding motifs, such as β-O-4′, β-β′, and β-5′, were found to dominate in the extracted lignins, with their abundance varying with treatment conditions. Low-molecular-weight lignins with fairly unaltered characteristics were generated via extraction with the highest ethanol content potentially suitable for subsequent production of free phenolics. Furthermore, β-β′ and β-5′ structures were predominant at higher acid catalyst contents and prolonged treatment times. Higher acid catalyst content led to oxidation and ethoxylation of side-chains, with the concomitant gradual disappearance of p-hydroxycinnamyl alcohol and cinnamaldehyde. This said, the increasing application of acid generated a broad set of lignin characteristics with potential applications such as antioxidants, carbon fiber, nanoparticles, and water remediation purposes.

Organosolv lignins (OSLs) are important byproducts of the cellulose-centred biorefinery that need to be converted in high value-added products for economic viability. Yet, OSLs occasionally display characteristics that are unexpected looking at the lignin motifs present. Applying advanced NMR, GPC, and thermal analyses, isolated spruce lignins were analysed to correlate organosolv process severity to the structural details for delineating potential valorisations. Very mild conditions were found to not fractionate the biomass, causing a mix of sugars, lignin-carbohydrate complexes (LCCs), and corresponding dehydration/degradation products and including pseudo-lignins. Employing only slightly harsher conditions promote fractionation, but also formation of sugar degradation structures that covalently incorporate into the oligomeric and polymeric lignin structures, causing the isolated organosolv lignins to contain lignin-humin hybrid (HLH) structures not yet evidenced as such in organosolv lignins. These structures effortlessly explain observed unexpected solubility issues and unusual thermal responses, and their presence might have to be acknowledged in downstream lignin valorisation.

Lignocellulosic biomass is one of the most important renewable materials to replace carbon-based fossil resources. Solvent-based fractionation is a promising route for fractionation of biomass into its major components. Processing is governed by the employed solvent-systems properties. This review sheds light on the factors governing both dissolution and potential reactivities of the chemical structures present in lignocellulose, highlighting how proper understanding of the underlying mechanisms and interactions between solute and solvent help to choose proper systems for specific fractionation needs. Structural and chemical differences between the carbohydrate-based structural polymers and lignin require very different solvents capabilities in terms of causing and eventually stabilizing conformational changes and consequent activation of bonds to be cleaved by other active components in the. A consideration of potential depolymerization events during dissolution and energetic aspects of the dissolution process considering the contribution of polymer functionalities allow for a mapping of solvent suitability for biomass fractionation.

Beech sawdust was treated with a ternary solvent system based on binary aqueous ethanol with partial substitution of ethanol by acetone at four different water contents (60, 50, 40, and 30%v/v). In addition to standard, i.e., noncatalyzed treatments, the application of inorganic acid in the form of 20 mm H2SO4 was evaluated. The various solvent systems were applied at 180 °C for 60 min. The obtained biomass fractions were characterized by standard biomass compositional methods, i.e., sugar monomer and oligomer contents, dehydration product contents of the aqueous product, and lignin, cellulose, and hemicellulose contents in isolated solid fractions. More advanced analyses were performed on the lignin fractions, including quantitative 13C NMR analyses, 1H–13C HSQC analysis, size exclusion chromatography, and pyrolysis-GC/MS, and the aqueous product, in the form of size exclusion chromatography and determination of total phenol contents. The picture emerging from the thorough analytical investigation performed on the lignin fractions is consistent with that resulting from the characterization of the other fractions: results point toward greater deconstruction of the lignocellulosic recalcitrance upon higher organic solvent content, replacing ethanol with acetone during the extraction, and upon addition of mineral acid. A pulp with cellulose content of 94.23 wt % and 95% delignification was obtained for the treatment employing a 55/30/15 EtOH/water/acetone mixture alongside 20 mm H2SO4. Furthermore, the results indicate the formation of two types of organosolv furan families during treatment, which differ in the substitution of their C1 and C5. While the traditional lignin aryl–ether linkages present themselves as indicators for process severity for the nonacid catalyzed systems, the distribution of these furan types can be applied as a severity indicator upon employment of H2SO4, including their presence in the isolated lignin fractions.

Fractionation of lignocellulose by applying hot, pressurized aqueous organic solvents, or organosolv processing, has provided efficient separation of compositional constituents present in various types of woody and herbaceous biomasses. In the present work, novel ternary solvents (aqueous ethanol/acetone, aqueous acetone/acetic acid) in absence of, and by applying both acidic (H2SO4) and alkaline catalyst (NaOH) was used to fractionate wheat straw. The isolated fractions were described through structural analysis (contents of cellulose, hemicellulose, lignin, ash, sugar monomers, sugar oligomers, inhibitors/sugar dehydration products), size distribution (lignin gel permeation chromatography; hemicellulose size exclusion chromatography), and functional group analysis. The data presented herein are indicative of an intricate interrelation occurring between the major components where pulp residual lignin content, and lignin extraction is largely correlated with ash extraction, whereas the aqueous product and size distribution of the polysaccharide fraction is influenced by ash co-extraction. Regarding the lignin characteristics – the data indicate a preference of lignin side-chain oxidation into carboxylic acid functionalities when extraction is conducted with acetic acid and/or H2SO4, whereas in absence of acidic catalyst, or presence of alkaline catalyst, the isolated lignins are described as less modified relative their original structure.    

The viability of second generation biorefineries relies on value-added utilization of all the components present in lignocellulose. Till today, lignin presents the largest room for improvement in such sense. Meanwhile, the lignin structural characteristics are complex, disperse, and largely dependent on extraction conditions and factors such as severity. Herein, two beechwood lignins best described as largely native and non-native are isolated through organosolv processing in absence and presence of H2SO4. The behavior of the two parental lignins are further investigated through post-organosolv water-acetone fractionation with accompanied functional group analysis, size distribution characterization, and solubility trials. The original parental lignin chemistry alongside the fraction size-distribution determines properties such as solubility in aqueous acetone solutions. Finally, to correlate the lignin extract chemistry with value-added down-stream properties, the DPPH radical scavenging properties of the parent and fractionated lignins were evaluated alongside their UV absorption in solution. It was found that the preference of the lignins to generate solution aggregates is a result of extraction conditions, which eventually influences not only solubility, but also their display of antioxidant properties.