The swelling and dissolution of cellulose crystallites in subcritical and supercritical water

Supercritical water treatment for cello-oligosaccharide production from microcrystalline cellulose

“…The prepared cellulose suspension was treated with a bench-scale tubular flow reactor system described earlier. 13 Three experiments were conducted at the temperature of 380°C employing treatment times of 0.20 s, 0.40 s, and 0.60 s. The pressure was held at 25.0 MPa in all experiments. The concentration of the cellulose suspension was 0.50 wt % in the feeding tank and 0.20 wt % in the beginning of the supercritical water treatment after dilution by supercritical heating water.…”

“…In order to avoid the earlier reported problem with the re-deposition of precipitated cellulose, the time required for the removal of the undissolved residue was reduced to approximately 15 s. 13 The solution from the reactor's outlet was taken into a 10 mL syringe and immediately pressed through a syringe filter (Acrodisk, PN4523T). In total 50 mL of reaction product solution was filtered in this way.…”

“…This precipitating fraction has been earlier identified as cellulose II allomorph, showing that it originates from completely dissolved cellulose chains. 12,13,17,22 The yield and molar mass of the cellulose precipitate depended on the treatment time: a longer treatment time resulted in a lower yield of precipitate (Tables 1 and 2). This is explained by the depolymerization of the dissolved chain, which was observed as the shifted molar mass distributions, reducing the degree of polymerization below the solubility limit of cellulose therefore rendering the chains watersoluble ( Table 2).…”

“…11 This transformation results in a rapid destruction of the crystallites in near critical and supercritical water. [12][13][14] When the temperature reaches the critical point (374°C and 22.1 MPa) at a pressure of 25 MPa, the internal energy of the system increases almost stepwise and the physical properties of water are drastically changed. These factors were attributed to a stepwise acceleration of cellulose dissolution.…”

Hydroxyl accessibility in wood cell walls as affected by drying and re-wetting procedures

“…The dominant sugar-based macromolecule of wood cell walls, cellulose is known for its ability to form very stable aggregated structures which are only accessible for water in harsh physical conditions of elevated temperature and pressure (above 200°C and 25 MPa;Deguchi et al 2008). Although cellulose is also thermally degraded under such conditions (Tolonen et al 2011(Tolonen et al , 2013, the crystals are seen to completely dissolve in water under very short reaction times (less than 10 s) forming amorphous cellulose fragments (Deguchi et al 2008;Tolonen et al 2013), which nonetheless re-crystallises upon drying (Tolonen et al 2011). From this it is seen that cellulose has an innate tendency to form aggregated structures which are partly inaccessible for water.…”