Structural characterization of residual hemicelluloses from hydrothermal pretreated Eucalyptus fiber

•Mw of hemicelluloses decreased from 56520 to 7780 g/mol as temperature rose.•Hydrothermal and alkali treatments were combined to fractionate hemicelluloses.•Hemicelluloses showed narrow polydispersity (Mw/Mn = 1.11-2.17).

In this study, an environmental-friendly hydrothermal pretreatment of Eucalyptus fiber followed with alkali post-treatment was developed to produce bioethanol efficiently. This biorefinery process allowed all major components of biomass being converted into high value-added products. The chemical and structural features of the residual hemicelluloses isolated with alkali from the hydrothermal pretreated Eucalyptus fiber, were comparatively investigated. Sugar and spectral analyses indicated that the hemicelluloses were mainly composed of glucuronoxylans, and especially hemicelluloses prepared at higher temperature (180 °C) contained higher contents of glucomannans and α-glucan. Hydrothermal pretreatment resulted in a significant hydrolysis of the glycosidic linkages in xylan backbone, and thus the molecular weight of the hemicelluloses was significantly reduced from 56,520 to 7780 g/mol with the increase of temperature. This suggested that a combination of hydrothermal pretreatment at low temperatures (100–140 °C) and alkali post-treatment was an effective technique for isolating of hemicelluloses from Eucalyptus fiber.