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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. [ABSTRACT FROM AUTHOR]

Correction for 'Protected lignin biorefining through cyclic extraction: gaining fundamental insights into the tuneable properties of lignin by chemometrics' by Maria Karlsson et al., Green Chem., 2022, 24, 1211–1223, DOI: 10.1039/D1GC04171A. [ABSTRACT FROM AUTHOR]

Dissolving pulps are technically produced by prehydrolysis kraft, one-stage or two-stage acid sulfite pulping. Like other pulping methods, the delignification process is incomplete and bleaching is required for complete lignin removal. Here, we explored the molecular aspects of lignin recalcitrance during the pulping, in order to gain insights that could inform future pulping efforts. For this purpose, we adopted a protocol for the controlled fractionation of pulp into soluble fractions that could be analyzed by spectroscopic methods including size exclusion chromatography (SEC) and 2D NMR methods. In addition, lignosulfonates (i.e. technical lignin) was analysed as a reference to gain insights on the structural basis for dissolution. Overall, the results identify a sequence of reactions responsible for the dissolution of lignin. In the first stage, the sulfonation of lignin begins and occurs at the α-carbon of β-O-4 and β-5 sub-structures. In the second stage the cleavage of lignin carbohydrate bonds (LCC) of benzyl ether, gamma ester and phenylglycosides types, all of which were detected in the residual lignin of the earlier phases, occurs and enhances lignin dissolution. Finally, condensation reactions of benzylic cations with activated positions on aromatic ring were detected in lignosulfonates. This suggest that a competing reaction mode to the sulfonation at C-α position in lignin was occurring at prolonged pulping conditions, here considered to be unproductive. [Display omitted] • 100% Pine heartwood was subjected to acidic sulfite pulping to produce dissolving grade pulp. • In pulp samples sulfonated lignin structures like α-sulfonated β-O-4 and β-5 units were identified. • Phenyl glycosides, gamma esters and benzyl ethers type LCC bonds were detected in pulp sample with NMR studies. • Condensation reactions between activated aromatic rings and benzylic cation on lignin during acid conditions were observed. • Cleavage of LCC bonds is probably required to dissolve sulfonated lignin as substantiated by NMR studies. [ABSTRACT FROM AUTHOR]