Reaction kinetics of D-xylose in sub- and supercritical water

Reactions of D-xylose were investigated with a flow apparatus in water at high temperatures (350 and 400 degrees C) and high pressures (40-100 MPa) to elucidate the reaction pathway and reaction kinetics. The products obtained from the reaction of D-xylose were furfural, D-xylulose, glyceraldehyde, glycolaldehyde, dihydroxyacetone, pyruvaldehyde, lactic acid and formaldehyde. Experimental results showed evidence of a dehydration reaction pathway, a retro-aldol reaction pathway and a Lobry de Bruyn-Alberta van Ekenstein (LBET) pathway from D-xylulose. The proposed reaction pathway and kinetic model were in accord with the experimental results. The kinetic constants showed dependence with water density (pressure). At 400 degrees C and water density of 0.52 g/cm(3) at 40 MPa, the reaction from D-xylose to D-xylulose occurred by the LBET pathway with the reverse reaction being negligible. At 400 degrees C, increasing the water density from 0.52 to 0.69 g/cm3 decreased the kinetic rate constant of the forward LBET pathway and increased that of the reverse LBET pathway. The kinetic rate constant of the dehydration of D-xylulose to furfural increased with increasing water density at constant temperature. The kinetic rate constant of the retro-aldol reaction of D-xylose increased, and the retro-aldol reaction of D-xylulose decreased with increasing water density at 400 degrees C. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.