Kinetics of Glucose Epimerization and Decomposition in Subcritical and Supercritical Water

Glucose decomposition kinetics in subcritical and supercritical water were studied for the temperatures 573, 623, and 673 K, pressures between 25 and 40 MPa, and residence times between 0.02 and 2 s. Glucose decomposition products were fructose, saccharinic acids, erythrose, glyceraldehyde, 1,6-anhydroglucose, dihydroxyacetone, pyruvaldehyde, and small amounts of 5-hydroxymethylfurfural. Fructose was also studied and found to decompose to products similar to those of glucose, except that its epimerization to glucose was negligibly low and no formation of 1,6-anhydroglucose was detected. We concluded that only the forward epimerization of glucose to fructose was important. The glucose decomposition pathway could be described in terms of a forward epimerization rate, r_gf, a fructose to decomposition products rate, r_f, and a glucose to decomposition products rate, r_g. A kinetic model based on this pathway gave good correlation of the experimental data. In the subcritical region, r_g, r_f, and r_gf showed only small changes with pressure at a given temperature. In the supercritical region, the rate of glucose decomposition decreased with pressure at a given temperature. The reason for this decrease was mainly due to the decrease in r_gf. The pressure effect in the supercritical region shows that there is a shift among the kinetic rates, which can lead to higher selectivity for glucose when decomposing cellulosic materials.