Structural characterization of Kraft lignins from different spent cooking liquors by 1D and 2D Nuclear Magnetic Resonance spectroscopy

• Lignins from three Kraft spent liquors were obtained by acid precipitation.
• Structural characterization of the dissolved lignins was performed by NMR.
• Wood source and pulping conditions determine the lignin characteristics.
• Kraft process implies cleavage of β–O–4 linkages and survival of resinol linkages.
• Comparison of the samples would aid decisions on its future revalorization.

Three Kraft lignins isolated from black liquors of several paper pulp mills of the North of Spain and Portugal were structurally characterized by using monodimensional (1H and 13C) and bidimensional Nuclear Magnetic Resonance (NMR) spectrometry. From the latter, 13C–1H heteronuclear single quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) were employed. Lignins from black liquors are usually burned for power generation. Nevertheless, they could become high value added products within a biorefinery context. In that case, a good understanding of their structure is a prior step to transform them. From all the NMR techniques studied, HSQC has risen as the most powerful tool in lignin characterization. Kraft cooking conditions and the type of wood seem to be the main factors that determine the differences observed in the lignins. All the samples have shown an important decrease in the number of β–O–4′ linkages, due to the Kraft process, and resinol has become the most resistant linkage to the process. Moreover, all samples seem to be mainly linked to a one polysaccharide: xylan. Several parameters like S/G ratio, portion of phenolic and aliphatic hydroxyls, amount of aromatic protons and other structural aspects were also estimated.

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