Evaluation of chemical processing impact on E. globulus wood lignin and comparison with bark lignin

Abstract

In this work a comprehensive characterization of different Eucalyptus globulus lignins was performed in order to evaluate the influence of the industrial process on its structure: LOrg – lignin produced by organosolv process, LKraft – lignin isolated from black liquor (obtained by kraft process) and further purified, taking LEgwood – lignin produced by wood mild acidolysis, as representative of wood lignin. Lignin from bark, LEgbark, also isolated by mild acidolysis, was characterized and compared with LEgwood. Besides the contaminants content (inorganic material plus carbohydrates – 1–3%), lignins structure were studied with reference to yields obtained by nitrobenzene oxidation (NO), and with reference to functional groups and typical structures/linkages by quantitative 13C, 1H and 31P nuclear magnetic resonance (NMR). NO yield order was found to be LKraft (21%)<LOrg (26%)<LEgwood (31%)≈LEgbark (33%), demonstrating that the industrial processing, particularly the kraft one, induced unfavorable changes for a valorization route involving production of functionalized aldehydes by depolymerization. The main changes were the increase of degree of condensation (DC), cleavage of β-aryl ether linkages, and demethoxylation, in particular for the lignin resulting from the kraft process. The NO yield was successful correlated with DC and Syringyl/Guaiacyl (S/G) ratio from 31P NMR and also a good correlation was obtained between β-aryl ether content and DC, clearly ascertaining the structural features related with the potential of the E. globulus lignin as source of functionalized aldehydes. LEgwood and LEgbark have similar structures, whilst the presence of p-hydroxyphenyl (H) detected in the bark lignin leads to a S:G:H (73:23:4) different from wood lignin (80:20:0). The overall results showed that organosolv or a mild delignification process would be a preferable process to obtain lignin from E. globulus wood or bark for production of functionalized aldehydes.