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 degrees C and 2 5.3 MPa for residence times on the order of 5 s. The major products from me thoxyphenol decomposition in the absence of added oxygen are phenol and hyd roxyphenols. 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 wit h previous studies done at different reaction conditions. Comparing the the rmolysis rates for methoxyphenols with those of other substituted phenols s howed 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. Additio nally, the ortho isomer was always the most reactive. Phenol is a product o f incomplete oxidation from the methoxyphenols and nitrophenols, but no phe nol 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 thermoly sis and oxidation individually. A comparison of these reactant disappearanc e 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.