The oxidation of liquid hydrocarbons. I. The chain formation of hydroperoxides and their decomposition

Abstract

The course of the low-temperature liquid-phase oxidation of hydrocarbons through hydroperoxide formation and subsequent decomposition has been confirmed. The function of a heavy metal catalyst is to increase both the rate of formation and decomposition of this hydroperoxide. The dependence of the stationary concentration of hydroperoxide on the oxidation rate and the catalyst concentration indicated that the hydroperoxide decomposition is unimolecular with the reaction velocity constant directly proportional to the catalyst concentration. This has been confirmed by independent decomposition experiments. It is suggested that the reaction involves the breakdown of a ‘heavy metal catalyst-hydroperoxide’ complex. From a study of the oxidation rates of hydrocarbons, alcohols and ketones, and the oxidation of mixtures, it was found that alcohols are the most resistant to oxidative attack and furthermore inhibit the catalysed oxidation of long-chain paraffins. The oxidation cannot therefore proceed through alcohols as primary intermediates, and the hydroxylation theory is thus inapplicable to these liquid-phase oxidations. It is suggested that the sequence hydrocarbon-hydroperoxide-ketone represents the major course for the reaction in the early stages of the oxidation. From the variation of the inhibited rate with time, a theoretical treatment has enabled the chain length of the uninhibited oxidation to be determined. For instance, at 120° C the chain length is 142 ± 10. The ‘activation energy’ of the chain length is — 13.3 ± 2 kcal.: thus at 20° C the chain length would have a value of 48,000. From this activation energy and that of the overall reaction, the activation energy of the chain initiation reaction has been found to be 28.2 ± 2.5 kcal. In the heavy-metal catalysed oxidation the rate is independent of the catalyst concentration above a certain value. This may be explained by the hypothesis that the heavy-metal catalyst both starts and stops reaction chains responsible for hydroperoxide formation. The phenomena of positive and negative catalysis follow, according to relative efficiency of the heavy-metal catalyst in these two reactions.