Supercritical Water Oxidation Kinetics, Products, and Pathways for CH₃- and CHO-Substituted Phenols

Abstract

Phenols bearing -CH 3 and -CHO substituents were oxidized in supercritical water at 460°C and 250 atm. Experiments with each compound explored the effects of the reactor residence time and the concentrations of the phenolic compound and oxygen on the reaction rate. These experimental data were fit to global, power-law rate expressions. The resulting rate laws showed that the reactivity of the different isomers at 460°C was in the order of ortho > para > meta for both compounds. Moreover, the CHO-substituted phenol was more reactive than the analogous CH 3 -substituted phenol, and all of these substituted phenols were more reactive than phenol itself under supercritical water oxidation conditions. Identifying and quantifying the products of incomplete oxidation allowed us to assemble a general reaction network for the oxidation of cresols in supercritical water. This network comprises three parallel primary paths. One path leads to phenol, a second path leads to a hydroxybenzaldehyde, and the third path leads to ring-opening products. The hydroxybenzaldehyde reacts through two parallel paths, which lead to phenol and to ring-opening products. Phenol also reacts via two parallel paths, but these lead to phenol dimers and ring-opening products. The dimers are eventually converted to ring-opening products, and the ring-opening products are ultimately converted to CO 2 . The relative rates of the different paths in the reaction network are strong functions of the location of the substituent on the phenolic ring.

Phase coexistence properties for the polarizable point charge model of water and the effects of applied electric field

“…[1][2][3][4] Water and aqueous solutions under these conditions have become a focus of intense research and speculation. Recent discussion about the microstructure of supercritical water is a particularly instructive example.…”

Experimental measurements of benzene oxidation in supercritical water

“…Ž ᎏCHO hydroxybenzaldehydes substituent groups Martino . et al, 1995;Martino and Savage, 1997 Krajnc and Levec, 1996 and catalytic Krajnc and . Levec, 1997 SCWO of phenol.…”

Water‐density effects on phenol oxidation in supercritical water

“…In yet another report, the oxidation rate was assumed to be independent of the water density Ž . Krajnc and Levec, 1996 . If the question of the effect of water concentration on SCWO rates is expanded to include other compounds, then the apparent conflict grows, for water has been reported to Ž increase Holgate and Tester, 1994;Li et al, 1993;Martino and Savage, 1997;Martino et al, 1995;Meyer et al, 1995; . Ž Rice et al, 1998;Smith, 1995 , decrease Dell' .…”