Variation of Lignin Monomeric Composition During Kraft Pulping of Eucalyptus globulus Heartwood and Sapwood

Abstract

Heartwood and sapwood samples from Eucalyptus globulus were characterized by Py-GC/MS and GC-FID in respect to composition and content of lignin. The pyrolysis lignin-derived compounds were assembled by groups: “syringol,” “S-aldehydes,” “S-ketones,” “S-alcohols,” and “C₁₁H₁₂O₃” (S-units); “guaiacol,” “eugenol,” “G-aldehydes,” “G-ketones,” “G-alcohols,” and “others” (G-units); “phenol” and “H-aldehydes” (H-units). Heartwood and sapwood had similar lignin content in an extractive-free basis (23.7% and 23.0%, respectively) and in lignin composition (S-units, 76.0% vs. 76.3%; G-units, 22.0% vs. 21.0%; H-units, 1.9% vs. 2.7%; S/G ratio 3.5 and 3.6). The wood samples were kraft pulped under isothermal conditions at 130°C, 150°C and 170°C and several cooking times. Heartwood and sapwood behaved similarly. At 130°C the delignification was slow with no significant selectivity in respect to lignin composition. At 150°C and 170°C the S-units were more susceptible to reaction and comparatively more removed, inducing a decrease of S/G ratio to 0.6. The main products to be extracted belong to “syringol” and “S-aldehydes,” while the residual lignin in pulps was enriched in “guaiacol,” “eugenol” (G-units), and “phenol” (H-units).

Keywords:

• Eucalyptus globulus
• heartwood
• pyrolysis
• S/G ratio
• lignin composition

Acknowledgments

This work was funded in part by the Portuguese Science Foundation (FCT), through a Ph.D. scholarship granted to the first author (SFRH/BD/40060/2007), an R&D project PTDC/AGR-CFL/110419/2009, through the base funding to the Forest Research Center (CEF) under the FEDER/POCI 2010 Programme, and by PEst-OE/AGR/UI0239/2011. The authors thank Clara Araújo and ALTRI, who supplied the wood material.