Oxidation and thermolysis of methoxy-, nitro-, and hydroxy-substituted phenols in supercritical water

We have examined the oxidative decomposition of m- and p-methoxyphenol, m-, and p-nitrophenol, and resorcinol and the nonoxidative decomposition of o-, m- and p-methoxyphenol in dilute aqueous solutions at 460 °C and 25.3 MPa for residence times on the order of 5 s. The major products from methoxyphenol decomposition in the absence of added oxygen are phenol and hydroxyphenols. The thermolysis kinetics can be described by a power-law rate equation with a reaction order between 0.5 and 1.0, which is consistent with previous studies done at different reaction conditions. Comparing the thermolysis rates for methoxyphenols with those of other substituted phenols showed that the rates are sensitive to both the identity and the location of the substituent. For a given substituent location, NO₂-substituted phenols reacted more rapidly than either CHO- or OCH₃-substituted phenols. Additionally, the ortho isomer was always the most reactive. Phenol is a product of incomplete oxidation from the methoxyphenols and nitrophenols, but no phenol was observed when resorcinol was oxidized. The oxidation kinetics were correlated with power-law rate expressions. The experiments and subsequent kinetics analysis allowed us to separate and quantify the rates of thermolysis and oxidation individually. A comparison of these reactant disappearance rates shows that thermolysis accounts for about 5% of the total rate for m- and p-methoxyphenol during oxidation in supercritical water. Thermolysis accounts for up to 25% of the total rate for m- and p-nitrophenols.