Isobutane oxydehydrogenation on Al2O3-supported transition and rare-earth metal oxides

The Oxidative dehydrogenation of isobutane to isobutene was studied on transition (Cr, Mn, V, Ni and Zr), rare-earth and other (Ho, Sm, Pr, Ce, Tl, Yb and Tb) metal oxides supported on Al2O3 with metals loading varying from 2.5 to 10%. The reaction was carried out at atmospheric pressure and temperatures of 250-500degreesC. A 4:1 ratio of isobutane to oxygen at a total flow rate of 75 cm(3)/min was used. The tested metal oxides exhibited different activities and selectivities at different reaction temperatures. The catalysts are classified into two groups based on their activity over the reaction temperature range. Cr catalyst showed the best performance among the catalysts active below 400degreesC with 62% selectivity towards isobutene at 10% conversion. For the second group (active above 400degreesC) Ni showed the highest activity at 500degreesC (32% conversion) followed by Zr and Tb (both of 19.2% conversion). The BET surface areas of the catalysts were between 108 and 183 m(2)/g. Aqueous reduction potential of the metal cation was also used to explain the selectivity trend of these metal oxides. The obtained relation suggested that metals with low reduction potential are generally selective towards the production of olefins. (C) 2004 Published by Elsevier B.V.