Modern multidimensional NMR spectroscopic methods were applied to investigate the effects of kraft pulping and oxygen delignification on lignin side-chain structures. In addition to the two-dimensional HSQC measurements, the three-dimensional HSQC-TOCSY technique was utilized to elucidate the (1)H-(1)H and (1)H-(13)C correlations of individual spin systems and thus indicate a certain lignin side-chain structure. Unlike earlier, nonlabeled samples were used for 3D measurements. According to 2D and 3D NMR spectra, most of the structures identified in milled wood lignin (MWL) are still present in technical lignins after kraft pulping and oxygen delignification. Although the main reaction during kraft pulping is the cleavage of beta-O-4 linkages, these structures are still left in spent liquor lignin as well as in residual lignin. The amount of coniferyl alcohol and dihydroconiferyl alcohol end groups, as well as some unidentified saturated end groups, is higher in technical lignins than in MWL. Contrary to our earlier observations, no diphenylmethane structures were observed in any technical lignins. Vinyl aryl ether structures could not be detected in technical lignins either.

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Technical lignins are structurally heterogeneous and polydisperse. This work describes the use of a simple and green method for lignin fractionation, using different proportions of acetone (40 and 60%) in water. Lignins from three different sources (wheat straw organosolv lignin, wheat straw soda lignin and softwood kraft lignin) were used in this fractionation protocol. The obtained fractions showed different molar mass and functional groups. The lower molar mass fractions showed more phenolic hydroxyl groups and carboxylic acid moieties than higher molar mass fractions, which also possessed much higher amounts of carbohydrates. The chemical characterization of these fractionated lignins showed that the PREC fraction was exceptionally pure and homogeneous lignin. Its total lignin content was >96% for all three lignins and it was practically free from carbohydrates and inorganics (ash). Furthermore, PREC fraction possessed the highest carbon content for the three lignin samples (63.05-69.26%). These results illustrate that the proposed aqueous acetone fractionation protocol could indeed produce pure and uniform lignin fraction and it was applicable for lignins from different sources.

The modification of the lignin structure of an eucalyptus feedstock during alkaline delignification by kraft, soda-AQ, and soda-O 2 cooking processes has been investigated by different analytical techniques (size exclusion chromatography (SEC), pyrolysis gas chromatography−mass spectroscopy (Py-GC/MS), 1 H− 13 C two-dimensional nuclear magnetic resonance (2D-NMR), and 31 P NMR). The characteristics of the lignins were compared at different pulp kappa levels, and with the native lignin isolated from the wood. The structural differences between the kraft, soda-AQ, and soda-O 2 residual lignins were more significant at earlier pulping stages. At the final stages, all the lignin characteristics were similar, with the exception of their phenolic content. Strong differences between lignins from pulps and cooking liquors were observed, including enrichment in guaiacyl units in pulp residual lignin and enrichment in syringyl units in black liquor lignin. A comparison of the alkaline cookings indicate that soda-O 2 process produced higher lignin degradation and provided promising results as pretreatment for the deconstruction of eucalyptus feedstocks for subsequent use in lignocellulose biorefineries.