Cellulose hydrolysis in supercritical water to recover chemicals

Cellulose decomposition experiments were conducted in subcritical and supercritical water (25 MPa, 320 - 400 degreesC and 0.05 - 10.0 s). At 400 degreesC hydrolysis, products were mainly obtained, while in 320 - 350 degreesC water, pyrolysis products were main products. To understand this change of product distributions around the critical temperature, kinetic studies were conducted for reactions of cellulose, cellobiose and glucose in subcritical and supercritical water. Below 350 degreesC, the cellulose hydrolysis rate was slower than glucose or cellobiose decomposition rate. However, above 350 degreesC, the cellulose hydrolysis rate drastically increased and became higher than glucose or cellobiose decomposition rate. For understanding the change of cellulose hydrolysis rate around 350 degreesC, direct observation of the reaction field by using a diamond anvil cell (DAC) was conducted. Below 280 degreesC, cellulose particles became gradually smaller with time. However, the shrinking rate of the particles increased greatly around 300 degreesC. In the range of temperature from 300 to 320 degreesC, cellulose disappeared without changing its particle shape. After 2 hours cooling of the produced solutions at room temperature, white precipitates came out from the solutions, which was found to be cellulose-like materials which had been solubilized in high temperature water. These results suggest that cellulose can be dissolve in high temperature water, which is probably because of the cleavage of intramolecular and intermolecular hydrogen linkages in cellulose crystal. Thus, a homogeneous hydrolysis atmosphere is formed in high temperature water and this probably results in the drastic increase of the cellulose hydrolysis rate above 350 degreesC.