Dissolution and hydrolysis of cellulose in subcritical and supercritical water

Decomposition experiments of microcrystalline cellulose were conducted in s ubcritical and supercritical water (25 MPa, 320-400 degrees C, and 0.05-10. 0 s). At 400 degrees C hydrolysis products were mainly obtained, while in 3 20-350 degrees C water, aqueous decomposition products of glucose were the main products. Kinetic studies of cellulose, cellobiose, and glucose at the se conditions showed that below 350 degrees C the cellulose decomposition r ate was slower than the glucose and cellobiose decomposition rates, while a bove 350 degrees C, the cellulose hydrolysis rate drastically increased and became higher than the glucose and cellobiose decomposition rates. Direct observation of the cellulose reaction in high-temperature water at high-pre ssure conditions by using a diamond anvil cell (DAC) showed that, below 280 degrees C, cellulose particles became gradually smaller with increasing re action time but, at high temperatures (300-320 degrees C), cellulose partic les disappeared with increasing transparency and much more rapidly than exp ected from the lower temperature results. These results suggest that cellul ose hydrolysis at high temperature takes place with dissolution in water. T his is probably because of the cleavage of intra- and intermolecular hydrog en linkages in the cellulose crystal. Thus, a homogeneous atmosphere is for med in supercritical water, and this results in the drastic increase of the cellulose decomposition rate above 350 degrees C.