Catalysis and Inhibition by Water during the Liquid-Phase Oxidation of Hydrocarbons

In the abundant literature on the liquid-phase of hydrocarbons by dioxygen, many references can be found to the rate-influencing role of water (formed or added during the oxidation). Most of them relate to the inhibitory effect of water then present in high excess. For example, it has been reported that the autoxidation of different alkylbenzenes and [alpha]-phenylcarbinols is entirely prevented by an excess of water. On the other hand, it has also been stated that, of the polar molecules, water is able to form H-bonds with inhibitors such as phenols and aromatic amines; the inhibiting group is then blocked and the H-bonded inhibitor molecules do not react with the peroxy radicals. The inhibitory activity is therefore diminished, and the overall rate of the inhibited reaction will be increased by the presence of water. No references are in the literature with respect to real catalysis by water in the liquid-phase oxidation of hydrocarbons in the absence of metal ions. In the presence of metal ions, however, water may exert a rate-influencing effect, but this is always connected with the formation of aquo and hydroxo complexes, i.e., the catalytic species is modified. In this paper, the authors present examples to show that water is a real catalyst when applied in small quantities during the oxidation of hydrocarbons with dioxygen. 8 refs., 1 fig., 2 tabs.