The oxidative dehydrogenation of ethane : gas-phase and catalytic

Abstract

The gas-phase and the catalytic oxidative dehydrogenation of ethane were studied. Lithium-promoted magnesium oxide was investigated as the catalyst. In the gas-phase oxidation study a perfectly mixed tubular reactor was used. The most dramatic effect in this study was an increase in the rate of ethane conversion when the concentration of water vapor was increased. The conversion for ethane and selectivity for ethylene with a C2H6:O2 ratio of 2 at 560 °C was 2.1 and 81%, respectively, while the addition of ca. 300 torr of water vapor increased the conversion and selectivity to 31 and 81%, respectively. This unusual and dramatic effect was attributed to water acting as a third body in the hydrogen peroxide decomposition to hydroxyl radicals. Lithium-promoted magnesium oxide has been found to be an effective catalyst in the conversion of ethane to ethylene. In this study a fixed-bed, plug flow reactor, operating at atmospheric pressure and in the range of 500-700 °C, was used. The activation of ethane in the gas-phase was inhibited by filling the empty space in the reactor with quartz chips. A selectivity of 75% was obtained at a 40% ethane conversion over 6.5 g of 3 wt % Li/MgO catalyst and a C2H6:O2 ratio of 2 at 600 °C. An apparent activation energy of 37.3 kcal mol^-1 was obtained for this process. The kinetic and spectroscopic data suggest that [Li+O-] centers are responsible for the initial activation of ethane via hydrogen atom abstraction to produced C2H5* radicals. These surface-generated radicals can be desorbed into the gas-phase to subsequently dehydrogenate to ethylene or they can be adsorbed on the surface and eventually decompose to carbon oxides. This mechanism is similar to the activation of methane over the same material, which may indicate that the activation of alkanes by [Li+O-] centers on the Li/MgO catalyst is a general phenomenon.