The effect of supercritical water on the catalytic oxidation of 1,4-dichlorobenzene

Abstract

Catalytic oxidation of 1,4-dichlorobenzene has been accomplished in the gas phase and in supercritical water. Analysis of the gas-phase reaction has led to an understanding of the oxidation reaction in terms of the kinetics and mechanism. A mechanism consisting of irreversible adsorption of DCB followed by surface reaction (rate limiting) was consistent with the observed second order kinetics. Significant quantities of carbonaceous char were produced. The temporal variation of conversion during reaction in water (in the absence of catalyst or oxygen) was described by first-order kinetics. Additionally, in the presence of supercritical water, dichlorobenzene was partially converted to chlorobenze and benzene, supporting the hypothesis of a new mechanism for the decomposition of dichlorobenzene not observed in the absence of water. Catalytic oxidation in supercritical water was described by simultaneous reaction pathways consisting of a catalytic oxidation pathway and a pathway involving reaction with water leading to the sequential removal of chlorine atoms from the aromatic ring. Although the kinetics suggest simple addition of the two reaction pathways, the rate of disappearance was less than that observed during gas-phase oxidation indicating inhibition of the catalytic reaction by water.