Combined Supercritical and Subcritical Process for Cellulose Hydrolysis to Fermentable Hexoses

A combined supercritical and subcritical process for pretreatment and hydrolysis of lignocellulosic waste has been proposed. Batch experiments and kinetic analysis were performed to investigate the secondary hydrolysis of cellulose in subcritical water as well as the feasibility of combining the whole process. A 60 mg amount of microcrystalline cellulose and 2.5 mL of deionized water were mixed in each reactor and reacted at 380 degrees C and 16 s as the primary hydrolysis, followed by different subcritical conditions for secondary hydrolysis. The products were analyzed by HPLC, which showed that the primary hydrolysis produced 28.1% oligosaccharides and 26.3% hexoses. For the secondary hydrolysis in subcritical reactions, the highest yield of hexoses, obtained at 280 degrees C and 44 s, was 39.5%. At lower temperatures, higher maximum yield of hexoses was obtained, but the reaction times were longer. Kinetic analysis showed smaller reaction rate constants for hexoses decomposition compared to those for oligosaccharides hydrolysis, and the difference between these increased with the decreasing reaction temperature. This explains the observed hexose accumulations in the subcritical reaction and indicates the feasibility and advantage of combined supercritical and subcritical processes for biomass conversion.