Investigation of the degradation kinetic parameters and structure changes of microcrystalline cellulose in subcritical water

Cellulose is the most abundant nature polymer and the main component in all biomass. In this study, the degradation kinetic parameters and structure changes of microcrystalline cellulose (MCC), which served as fundamental studies for degradation of biomass in subcritical water, were investigated at the temperature range from 100 to 300 °C. The yield of the MCC residue began to decrease at 205 °C and reached the lowest value at 275 °C. However, it showed an increase at the temperature higher than 275 °C. The degradation area of MCC (205-275 °C) was separated into zones 1 and 2 with 245 °C as a boundary. The activation energy (E), pre-exponential factor (A), and reaction order (n) of MCC in each zone were 226.5 kJ mol^-1, 2.3 × 10²³ s^-1, and 0.6 (zone 1) and 423.1 kJ mol^-1, 9.0 × 10⁴⁰ s^-1, and 0.5 (zone 2), respectively. There showed a breaking point of 245 °C for the Arrhenius plot in the reaction area. The surface morphology of the MCC residue had no significant change below 260 °C, as indicated by scanning electron microscopy (SEM) images. However, it was completely destroyed at the temperature above 275 °C, and MCC residues with strong pore structures were obtained at higher temperatures. The structures of the crystalline region of the MCC residue below 275 °C had no significant change, although 89.2% MCC was degraded at 275 °C. However, they nearly disappeared at higher temperatures with the steeply reducing crystallinity index of MCC at the same time. As for the degradation mechanism of MCC, it was proposed that only the surface of MCC was degraded at 205-245 °C and the hydrogen and glycosidic bonds on the interior of MCC fibrils were destroyed at the temperature range from 245 to 275 °C. Finally, the remaining MCC as well as generated oligomer and monomer were further degraded at higher temperatures.