New insight into the role of gas phase reactions in the partial oxidation of butane

The partial oxidation of n-butane at high alkane to oxygen ratio was studied in the presence of a Pt–Rh gauze and in an empty tubular reactor under identical conditions. Temperature-programmed reaction (TPR) experiments with the metal gauze showed dramatic changes in the product distribution in the range 25–500 °C. Total oxidation of butane at low temperature is followed by selective conversion to olefins and oxygenates around 400 °C; a further increase in the oven temperature enhances selectively olefins formation. In situ infrared and visible imaging of the reacting gauze revealed remarkable ignition/extinction phenomena of the surface reactions. Fast ignition on the metal catalyst was observed around 200 °C; upon further raising the oven temperature, suppression of these reactions occurred at a temperature level corresponding to the transition to high selectivity conditions. These results indicate a shift from heterogeneous to homogeneous reaction mechanism, the latter being responsible for high selectivity to partial oxidation products (84% olefins+oxygenates). With the empty reactor, conversion and selectivity were similar to those obtained in the presence of the catalyst. The study shows that the gauze plays no significant role in butane partial oxidation, as reactions take place in the void upstream of the catalyst, presumably via an alkylperoxy intermediate.