Effect of the presence or absence of a gas–liquid interface on the initiation of liquid phase hydrocarbon oxidation

The kinetic description often overlooks the initiation of liquid-phase hydrocarbon autoxidation because of the induction period. The start of oxidation, or oxidation at a low conversion rate, occurs largely in the bulk liquid, which remains near air-saturated, maintaining a minimal Hatta-number. At low conversion rates, the oxidation rate still depended on the gas–liquid interface area per unit liquid hydrocarbon volume under similar conditions. Mixing under otherwise similar conditions impacted product selectivity to oxygen and radical–radical additions at the same conversion level. These observations indicated that in an equilibrated reaction system, there was not an equal probability for initiation by O₂ at the gas–liquid interface and in the bulk liquid. The purpose of the study was to determine if the presence of a gas–liquid interface affected the initiation of hydrocarbon autoxidation. Reactions were performed using air-saturated tetralin and indan at 130 and 140°C in systems with and without gas–liquid interface for 1 and 7 days. It was found that at the same conditions, the initiation rate was faster when a gas–liquid interface was present. It was further found that initial product selectivity to oxygen versus radical addition was lower when a gas–liquid interface was present. These results and those from previous studies can be explained if autoxidation initiates faster at the gas–liquid interface than in the bulk liquid at equilibrated conditions. In conclusion, oxygen at the gas–liquid interface has different initiation kinetics, but it does not affect the liquid-phase hydrocarbon oxidation mechanism.