No abstract

The thermal decomposition of cresols, hydroxybenzaldehydes, nitrophenols, and benzenediols was studied in dilute aqueous solutions and in the absence of oxygen at 460 °C and 250 atm for residence times around 10 s. Thermolysis under these conditions produced conversions of less than 10% for o-, m-, and p-cresol, whereas hydroxybenzaldehydes and nitrophenols were much more reactive. Global rate expressions are reported for the thermolysis of each hydroxybenzaldehyde and nitrophenol isomer. Phenol was a major product from the decomposition of all of the substituted phenols studied. For a given substituent, ortho-substituted phenols reacted more rapidly than the other isomers. For a given substituted position, nitrophenols reacted more rapidly than hydroxybenzaldehydes, which in turn reacted more rapidly than cresols. These results demonstrate that the treatment of CHO- and NO2-substituted phenols by oxidation in supercritical water will involve the oxidation of thermal decomposition products in addition to the oxidation of the original compounds.

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.

Dilute aqueous solutions of o-cresol(2-methylphenol) were oxidized in a tubular flow reactor at near-critical and supercritical conditions. The power-law rate expression that best correlates the kinetics of o-cresol disappearance is rate = 105.7 exp(-29700/RT)[o-creso11°~57[0210~22[H~011~44.The power-law rate expression that best correlates the experimental results for the conversion of organic carbon to C02 is rate = 106.8 exp(-34000/RT)[TOClo~34[O~lo~'3[H~O11~18. All concentrations are in moles per liter, the activation energy is in calories per mole, and the rate is in moles per liter per second. The most abundant products from o-cresol oxidation were typically phenol, 2-hydroxybenzaldehyde, 1,3-benzodioxole, indanone, CO, and C02. 2-Hydroxybenzaldehyde was the major primary product. A reanalysis of published kinetics data for the oxidation of two other ring-containing compounds (pyridine and 4-chlorophenol) in supercritical water revealed that the rate laws previously reported for these two compounds do not provide the best correlation of the experimental data. We report the new rate laws, which are similar to those for o-cresol, 2-chlorophenol, and phenol in that the global reaction orders are between 0.55 and 0.9 for the organic compounds and between 0.2 and 0.5 for oxygen.

No abstract