Chromium oxide alumina catalysts in oxidative dehydrogenation of isobutane

The chromium oxide-alumina catalysts of the Cr loading varying between 1 and 50 Cr nm(-2) have been characterized by different physicochemical techniques and probe reactions of acido-basic properties and of the catalyst oxygen reactivity. They have been tested in oxidative dehydrogenation, ODH of isobutane. The characterization of the catalysts with chemical analysis, XRD, XPS, and Raman spectroscopy has confirmed the general model of the chromium-oxide/alumina catalysts proposed in the literature. The Cr6+ species of two types: soluble and insoluble in water have been found at low Cr loading, and Cr2O3 in amorphous and crystalline forms have been observed at higher Cr content. The densities of the Cr6+ species increase with the increase in the total Cr content to constant values observed at 5-10 Cr nm(-2) for the water-soluble Cr6+ species and at 2 Cr nm(-2) for the water-insoluble Cr6+. The highest densities of the Cr6+ species are 1.6 Cr nm(-2) and 1.0 Cr nm(-2) for the water-soluble and insoluble species, respectively. The total initial activity in the ODH of isobutane and the selectivity to isobutene increase markedly with the increase in the Cr loading up to about 10 Cr nm(-2) and they change only slightly at higher loadings. TOF values referred to the content of Cr6+ species (particularly to water soluble Cr6+) do not differ considerably, however, in the wide range of the Cr loading from 2-50 Cr nm(-2). This suggests that the dispersed Cr6+ species are involved in the active centres of the ODH of IB. Unsupported chromia is more active and less selective (at comparable conversions) than the chromium oxide-alumina catalysts. The differences in catalytic performance between chromia and alumina supported chromium oxide catalysts are ascribed to lower acidity, higher oxygen bond energy, and lower rate of the oxygen chemisorption in the CrAl catalysts, as indicated by isopropanol decomposition probe reaction, oxygen TPD, TPR of hydrogen, and allyl iodide probe reaction. (C) 1998 Academic Press.