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Layered zirconium hydrogenophosphates such as α and β Zr(HPO4)2 have been studied with the idea to introduce redox-type transition metal cations as chromium either in the interlayer void space (d = 0.76 and 0.95 nm, respectively) by cationic exchange or deposited on the crystallites by impregnation. Pure CrPO4 and Cr2O3 and Cr2O3 impregnated on different supports as SiO2, ZrO2, ZrO2O7 have also been studied for comparison. It has been observed for the oxidative dehydrogenation of ethane into ethylene at 550°C that Cr impregnated on ° Zr(HPO4)2 calcined at 500°C and CrPO4 are the most efficient catalysts with 20 to 30% conversion and 50 to 60% selectivity in ethylene. All other exchanged and/or impregnated samples were shown to be less active and to yield total oxidation. A comparative and complementary study by UV-vis and ESR techniques indicates that isolated Cr3+ cations and/or small Cr3+ clusters are responsible for the better catalytic behavior while Cr2O3 as bulk material or as large crystallites deposited on any of the supports studied lead much more to total oxidation or at least to COx. This is a new example of the sensitivity of partial oxidation reactions to the structure of the oxide catalyst.

Summary Two batches of V 2 O 5 grains with different morphologies have been used to prepare two families of VPO catalysts according to the “organic medium” procedure. The morphological differences observed along the chain V 2 O 5 - VOHPO 4 , 0.5 H 2 O - VPO catalysts have been studied. The catalytic differences are compared through two catalysts representative of the two families. They are explained by differences in the repartition of VOPO 4 domains on specific faces of (VO) 2 P 2 O 7 crystals.

Bulk V 2 O 5 and V 2 O 5 /SiO 2 catalysts have been studied in the ethane oxidative dehydrogenation reaction. The surface characterization and the reactivity of these catalysts have been investigated using microcalorimetry linked to other techniques, such as volumetry or thermogravimetry. The number of acid sites of the supported catalysts and the initial heats of ammonia adsorption were found to increase with vanadium loading. The interaction with the support was enhanced after catalytic reaction as evidenced by the detection of new strong Lewis acid sites by DRIFT spectroscopy measurements and by higher heats of ammonia adsorption.