Homogeneous and heterogeneous pathways of ethane oxidative and non-oxidative dehydrogenation studied by temperature-programmed reaction

In this work, the temperature-programmed reaction (TP-reaction) technique was employed for the study of the mechanism of ethane oxidative dehydrogenation (ODH) and the identification of primary and secondary steps of the reaction. The reactions of ethane and ethene in an oxidizing and non-oxidizing atmosphere were investigated in the presence and absence of a 20 wt.% MoO3/Al2O3 catalyst, a highly active and selective catalytic material. Experiments were performed at steady flow conditions, with a 15 degreesC/min linear increase of temperature up to 700 degreesC, while the reaction effluent was monitored on-line by mass spectroscopy. Thermal reactions of ethane were found to be highly selective to ethene. Ethene seems to be primarily produced through a dehydrogenation mechanism, even in the presence of oxygen in the gas phase. However, oxygen was found to dramatically increase the overall rate of the reaction. Over the 20MoAl catalyst, ethane activation occurred at a significantly lower temperature, while the production of ethene was clearly a result of heterogeneous reactions via the oxidative dehydrogenation route. Surface pathways were found to have a significant role, even at temperatures where homogeneous reactions also occurred, indicating possible occurrence of a homogeneous-heterogeneous reaction scheme at high temperatures. In non-oxidative conditions, lattice oxygen activated ethane at the same temperature as in the presence of oxygen, leading mainly to ethene production, since lattice oxygen was found to be less reactive towards ethene. More labile forms of oxygen, resulting from adsorption of gas-phase oxygen on the surface, are probably responsible for the extensive ethene degradation observed at high conversion levels. (C) 2004 Elsevier B.V. All rights reserved.