Modification of the Lignin Structure during Alkaline Delignification of Eucalyptus Wood by Kraft, Soda-AQ, and Soda‑O<sub>2</sub> Cooking
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Abstract
The modification of the lignin structure
of an eucalyptus feedstock
during alkaline delignification by kraft, soda-AQ, and soda-O<sub>2</sub> cooking processes has been investigated by different analytical
techniques (size exclusion chromatography (SEC), pyrolysis gas chromatography–mass
spectroscopy (Py-GC/MS), <sup>1</sup>H–<sup>13</sup>C two-dimensional
nuclear magnetic resonance (2D-NMR), and <sup>31</sup>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<sub>2</sub> 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<sub>2</sub> process produced higher lignin degradation
and provided promising results as pretreatment for the deconstruction
of eucalyptus feedstocks for subsequent use in lignocellulose biorefineries