Kinetic study of the liquid-phase oxidation of cyclohexene catalyzed by manganese dioxide

Abstract

The oxidation of cyclohexene has been studied in the liquid phase over the temperature range 40 to 70 °C, using as catalysts three types of manganese dioxide with different specific surfaces. Short reaction periods (up to 60 min) were used to study the initial activity of the catalyst. The maximum rate of cyclohexene oxidation was 0.5 order with respect to the catalyst in the region of low catalyst weight to hydrocarbon volume ratios; a much lower order was found for high ratios. Above a critical amount of catalyst, the oxidation was completely inhibited due to the quantitative decomposition of hydroperoxide. The maximum oxidation rate proved to be first order with respect to the concentration of cyclohexene. The oxidation rate was zero order with respect to the initial hydroperoxide concentration in the range of 3 × 10 −3 to 0.5 mole/liter, giving support to the theory that free radicals were produced from the slow decomposition of a catalyst-hydroperoxide complex. Oxidation products such as cyclohexenol and cyclohexenone inhibited the oxidation rate when they were initially present in concentrations greater than 50 μmole/ml. Apparent activation energies for the oxidation ranged between 11.4 and 13.0 kcal/mole. Product distribution studies showed that cyclohexenol and cyclohexenone were produced in nearly equal amounts. A degenerate chain branching mechanism is proposed in which the catalyst plays an important role in the initiation and termination steps. Rate equations derived from this mechanism explained qualitatively the experimental observations.