Mechanism and Kinetics of Cellobiose Decomposition in Sub- and Supercritical Water

Cellobiose decomposition kinetics and products in sub- and supercritical water were studied with a flow apparatus at temperatures from 300 to 400°C at pressures from 25 to 40 MPa, and at short residence times (0.04-2 s). Cellobiose was found to decompose via hydrolysis of the glycosidic bond and via pyrolysis of the reducing end. Pyrolysis products were glycosylerythrose (GE) and glycosylglycolaldehyde (GG) which were confirmed by FAB-MS. Hydrolysis products were glucose, erythrose, and glycolaldehyde from cellobiose, GE, and GG, respectively, as well as glucose decomposition products. The kinetics from glucose decomposition were used to fit the experimental results and evaluate rate constants of hydrolysis (k_H) and pyrolysis rate constants (k₁ and k₂). The activation energy for the hydrolysis of cellobiose and pyrolysis products GG and GE was found to be 108.6, 110.5, and 106.1 kJ/mol, respectively. In the supercritical region, there was a decrease in the pyrolysis rates k₁ and k₂ and a corresponding increase in hydrolysis selectivity from 85% to 95% as the pressure increased from 30 to 40 MPa.