Effect of supercritical conditions on the oxidation of isobutane

The applications of oxidation products such as tert-butyl hydroperoxide (TBHP) and tert-butyl alcohol (TBA) in manufacturing important chemicals as propylene oxide and MTBE bring the focus on isobutane oxidation. Reaction since reaction conditions can be easily manipulated such that isobutane exists in the supercritical state. A study on the kinetics of isobutane oxidation in liquid phase and under supercritical conditions showed that temperature and wall material strongly influenced the reaction kinetics. Reactions were conducted in a stirred batch stainless-steel reactor lined with aluminum sheets and a glass-lined continuous plug flow reactor. Reactions in the continuous mode permitted kinetic studies without the intervention of gas-liquid mass transfer and eliminated the catalytic effect, if any, of the stainless-steel walls. The independent experiments indicated that TBHP tends to decompose and react with isobutane to form TBA. The reaction was autocatalytic and selectivity to TBHP decreased with an increase in overall conversion. Wall material influenced the decomposition, which was maximum in the unpassivated stainless-steel reactor. The rate was a weak function of oxygen concentration and agitation speed under the conditions studied. Under supercritical conditions, the rates and selectivity were enhanced over those obtained in liquid phase oxidation. A proposed kinetic model showed that this increase was due to the elevated temperature employed to bring the reaction mixture to the supercritical state. The rate expression used to predict the liquid-phase kinetics worked well under supercritical conditions, indicating that the substrate's ″supercritical″ state, as such, does not cause any significant effect on the reaction rate or the selectivity towards the desired products. In both subcritical and supercritical oxidations, temperature adversely affected the selectivity toward TBHP. The supercritical-phase oxidation exhibited the same features as seen in the liquid-phase oxidation.