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

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 degrees C, 150 degrees C and 170 degrees C and several cooking times. Heartwood and sapwood behaved similarly. At 130 degrees C the delignification was slow with no significant selectivity in respect to lignin composition. At 150 degrees C and 170 degrees 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).