Structural changes in lignin of thermally treated eucalyptus wood

Abstract

Changes in the lignin structure of eight eucalypt species (E. botryoides, E. globulus, E. grandis, E. maculata, E. propinqua, E. rudis, E. saligna, and E. viminalis) upon mild thermal treatment, where the temperature was gradually raised from 160 to 230 °C over 3 h, were studied by solid-state carbon nuclear magnetic resonance (¹³C NMR) and after lignin isolation from the wood by acidolysis employing liquid-state 1D and 2D NMR techniques. The molecular weight of the isolated lignins was assessed by size exclusion chromatography (SEC) and their composition by analytic pyrolysis coupled with gas chromatography and mass spectrometry (Py-GC/MS). The thermal treatment induced in lignin partial demethylation (up to ca. 30% from total) and a remarkable reduction in β-O-4 structures (up to ca. 60%), in an extent depending on the ratio of syringyl and guaiacyl units of the specific lignins. The lignin molecular weight reduced only ca. 25% under the thermal treatment due to the likely occurrence of condensation reactions by homolytic and heterolytic mechanisms. The noticeable increase in phenylcoumaran (β-5) and pinoresinol/syringaresinol (β-β) structures in lignin after thermal treatment was tentatively explained by their relative thermal resistance and by newly formed structures as the result of radical coupling reactions occurred during homolytic dissociation of the β-O-4 linkages. Lignins in thermally treated wood contained more than twice phenolic hydroxyls and more aliphatic carboxyl groups than lignins in untreated wood.

Keywords:

• Thermal treatment
• lignin
• solid-state ¹³C NMR
• analytic pyrolysis
• eucalyptus wood

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Fundação para a Ciência e a Tecnologia (FCT) through funding of the Forest Research Centre (UID/AGR/00239/2019 & UIDB/00239/2020) and the CICECO-Aveiro Institute of Materials (UIDB/50011/2020 & UIDP/50011/2020) financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project N° 022161 (co-financed by FEDER through COMPETE 2020, POCI and PORL and by FCT through PIDDAC). Ana Lourenço and Solange Araújo acknowledge funding from FCT for their research contracts, respectively DL 57/2016/CP1382/CT0007 and DL 57/2016/CP1382/CT0018. The authors thank Inês Mendes for her help during the lignin isolation.