New solutions for biomass fractionation by pressurized hot water extraction, supercritical water treatment and delignification

Pressurized hot water extraction (PHWE) and subsequent delignification processes were examined for their ability to separate hemicelluloses, lignin and cellulose from wood biomass. In addition, the separation and purification of PHWE extracts and high-temperature hydrothermal treatment of microcrystalline cellulose were also studied.

PHW extraction parameters were successfully tailored to enable the extraction of relatively large amounts of hemicellulose of relatively high molar mass from wood. If molar mass is not considered a critical factor, nearly all hemicelluloses – constituting approximately one third of wood biomass – were extractable. Additionally, the pH of the PHWE process was monitored and adjusted as desired by the addition of dilute alkali.

The hemicellulose-rich extracts obtained by PHWE contain mixtures of hemicelluloses of varying chain length. A variety of impurities, such as wood extractives and lignin-derived compounds, are also present. In addition, the dry solids content of the extracts is usually relatively low. It was possible to concentrate, purify and fractionate the extracts according to molecular size by combining appropriate pre- and/or post-treatments with membrane filtration, while maintaining sufficient filtration capacity.

Crystalline cellulose residues were successfully hydrolyzed and dissolved by rapid near- and supercritical water treatments to produce narrowly distributed, low-molar-mass celluloses and cello-oligosaccharides.

The fibrous fraction after PHWE was further isolated as a cellulose-rich fraction by sulfur-free delignification processes. Hardwoods defibrated more readily than softwoods due to differences in lignin structure. The cellulose-rich fraction can be further processed into various products, such as regenerated cellulose.