The enhancement of xylose monomer and xylotriose degradation by inorganic salts in aqueous solutions at 180 °C

Abstract

The inorganic salts KCl, NaCl, CaCl 2, MgCl 2, and FeCl 3, and especially the latter, significantly increased xylose monomer and xylotriose degradation in water heated to 180 °C with unaccountable losses of xylose amounting to as high as 65% and 78% for xylose and xylotriose, respectively, after 20 min incubation with 0.8% FeCl 3. Furthermore, losses of both xylose and xylotriose were well described by first order homogeneous kinetics, and the rate constants for xylose and xylotriose disappearance increased 6- and 49-fold, respectively, when treated with 0.8% FeCl 3 solution compared to treatment with just pressurized hot water at the same temperature. Although the addition of these inorganic salts produced a significant drop in pH, the degradation rates with salts were much faster than could be accounted for by a pH change. For example, the rate constants for the disappearance of xylose and xylotriose with 0.8% FeCl 3 were 3-fold and 7-fold greater, respectively, than for treatment with very dilute sulfuric acid at the same pH. In addition, xylose losses were greater than could be accounted for by just furfural production, suggesting that other degradation products were also formed, and xylose losses to unidentified compounds increased significantly with the addition of FeCl 3. The unidentified compounds could be formed through aqueous furfural resinification and condensation reactions that are accelerated by FeCl 3, but the actual mechanisms are still not clear.