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Due to the enormous abundance of lignin and its unique aromatic nature, lignin has great potential for the production of industrially useful fuels, chemicals, and materials. However, the rigid and compact structure of the plant cell walls significantly blocks the separation of lignin. In this study, wheat straw was hydrothermally pretreated at different temperatures (120–200 °C) followed by post-treatment with 70% ethanol containing 1% NaOH to improve the isolation of lignin. Results demonstrated that the content of associated carbohydrates of the lignin fractions was gradually reduced with the increment of the hydrothermal severity. The structure of the lignins changed regularly with the increase of the pretreatment temperature from 120 to 200 °C. In particular, the contents of β-O-4′, β-β′, β-5′ linkages and aliphatic OH in the lignins showed a tendency of decrease, while the content of phenolic OH and thermal stability of the lignin fractions increased steadily as the increment of the pretreatment temperature.

An integrated pretreatment process based on hydrothermal pretreatment (HTP) followed by alkaline pretreatment has been applied to treat Eucalyptus. The chemical composition and structure changes of lignin during the pretreatment were comprehensively characterized. The surface morphology of the cell walls and lignin distribution of the pretreated Eucalyptus were detected by scanning electron and confocal Raman microscopies. It was found that the chemical bonds between lignin and hemicelluloses were cleaved during the pretreatment. The results also indicated that the contents of β-O-4′, β-β′, and β-5′ linkages were decreased with the increase of hydrothermal pretreatment temperature and the cleavage of β-O-4′ linkages in lignin was accompanied with repolymerization reactions. 31P NMR analysis showed that the content of aliphatic OH was reduced as the temperature increased and the total phenolic OH was elevated and then declined with the increase of temperature. Raman spectra analysis revealed that the dissolution rate of lignin in the secondary wall regions was faster than that in cell corner middle lamella regions during the pretreatment. These results will enhance the understanding of the cell wall deconstruction during the pretreatment and the mechanism of the integrated pretreatment process acting on Eucalyptus.

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