Comparison of behaviour of rare earth containing catalysts in the oxidative dehydrogenation of ethane

Catalyst promotion by addition of either La and Sm to MgO or Na aluminate t o Sm2O3 and La2O3 has been investigated for the oxidative dehydrogenation o f ethane in the temperature range 550-700 degrees C. With all unpromoted an d promoted catalysts, the selectivity to ethylene is strongly enhanced by t he temperature, the highest values being obtained at 700 degrees C. Sm2O3 i s the most active among the bulk oxides. while samarium addition to MgO res ults in higher surface area, but does not enhance the catalytic activity. E thylene productivity on La2O3 promoted MgO samples is higher than with pure La2O3, Sm2O3 and MgO, not only due ro the stabilising effect of La on MgO surface area, but also due to a higher intrinsic activity. With both bulk o xides and rare earth promoted MgO, the selectivity to ethylene strongly inc reases by decreasing the O-2/C2H6 feed ratio, while it is quite unaffected by ethane conversion and catalyst composition. in agreement with the hypoth esis that the main role of catalyst in the experimental conditions investig ated is to produce ethyl radicals which are converted in the gas phase to C O and C2H4. When La2O3 is modified by the addition of sodium aluminate the catalytic behaviour significantly changes, Likely due to a different, mostl y heterogeneous reaction mechanism. On aluminate promoted lanthana, ethane is converted to ethylene with higher yields which do not depend on the feed ratio. Moreover. only CO2 is produced as by-product, the formation of CO b eing quite negligible. (C) 2000 Elsevier Science B.V. All rights reserved.