Active sites of V2O5/gamma-Al2O3 catalysts in the oxidative dehydrogenation of ethane

The oxidative dehydrogenation (ODH) of ethane to ethylene was studied on V2O5/gamma-Al2O3 catalysts (1-20 wt% V2O5) The reducibility of the catalysts by hydrogen, ethane, and ethylene and their reoxidation by oxygen were determined by thermogravimetry linked to differential scanning calorimetry and by a pulse catalytic test linked to a microcalorimeter. Ammonia and sulphur dioxide microcalorimetry adsorptions were carried out on all catalysts to characterize their acid-base properties. In ethane ODH reaction at the steady state at 823 K, 60% ethylene selectivity was obtained at 28% ethane conversion. The improved catalytic properties for such reaction on vanadia/gamma-alumina catalysts compared to vanadia/silica and bulk vanadium pentoxide catalysts appear to be related in the first place to the dispersion and coordination degree of the vanadium cations of the active sites, and in the second to the development of their acidic properties. The active sites are suggested to be oxidized vanadate dimers composed of tetrahedrally coordinated and oxygen-bridged vanadium cations operating according to a redox mechanism involving very probably transient radicals generated via one-electron transfer. Such dimers undergo a relatively high rate of reduction by ethane. The population of vanadate dimers was estimated in the case of the sample with 20 wt% V2O5 loading. The number and strength of active sites are described. More than silica, gamma-alumina induces the dispersion of such active sites for ethane ODH. (C) 1996 Academic Press, Inc.