Analytical Pyrolysis as a Tool to Assess Residual Lignin Content and Structure in Maritime Pine High-Yield Pulp

The residual lignin content of unbleached maritime pine (Pinus pinaster Aiton) Kraft pulps was assessed by analytical pyrolysis (Py-lignin) and the results were compared to the Klason lignin content and kappa number. Thirty samples, each from an individual tree, were delignified under identical conditions. The residual lignin content of the pulps varied widely as assessed by Py-lignin (5.9%–9.2%), Klason (8.2–15.1), and kappa number (59–112). Despite a systematic difference between Py-lignin and Klason, they were strongly correlated (R2 = 0.90). The H/G ratio of the residual pulp lignin ranged from 0.145 to 0.195, with a mean of 0.165, which is more than two times the average H/G ratio of Maritime pine wood lignin (0.064). The results show that Kraft pulping, which selectively degrades lignin with more labile inter-unit links, changed the pattern of pyrolysis products of pulp lignin considerably and, consequently, its structure. This pattern shows an enrichment in H-lignin-derived products, namely phenol, p-cresol, and m-cresol, and in some G-lignin-derived products such as guaiacol and 4-mthylguaiacol, and a decrease in coniferylaldehyde, homovanillin, and eugenol. Principal component analysis (PCA) of the G- and H-lignin-derived pyrolysis products shows that pulps are distributed along PC 1 based on their residual lignin content. The loadings plot shows that this separation is mainly due to a small number of G-lignin products, including 4-methyl guaiacol, 4-vinyl guaiacol, isoeugenol (trans), and guaiacol, which are more abundant in pulps with higher residual lignin content. The obtained results show that analytical pyrolysis is an appropriate method for quantifying the residual lignin content and H/G ratio of unbleached Kraft pulps and provide information regarding how lignin is degraded during the pulping process.