Isopentane dehydrogenation on Pt-Sn catalysts supported on AL-Mg-O mixed oxides:: effect of Al/Mg atomic ratio

A series of Mg-Al-O supports was prepared by the coprecipitation method at constant pH. The Al/Mg atomic ratio ranged from 2.06 to 4.9. The physico-chemical, as well as the surface and structural properties of the synthesized materials were strongly dependent on the Al/Mg atomic ratio. In solids with an Al/Mg atomic ratio higher than 2, a support with a mixture of Al2O3 and MgAl2O4 phases was obtained. In all the supports a non stoichiometric magnesium aluminate spinel structure (MgAl2+xO4+(3/2)x) was not detected. The supports were impregnated with Pt and Sn (0.6 and 1.2 wt.%, respectively). The resulting catalysts were evaluated in the isopentane dehydrogenation reaction at 550 degreesC and atmospheric pressure. On the Pt-Sn catalysts which were prepared with aluminum rich supports (Al/Mg = 2.4 and 4.9), TPD-H-2 results showed the presence of Pt particles (unalloyed and tin alloyed) deposited in both Al2O3 and MgAl2O4 phases. A stronger Pt-support interaction on MgAl2O4 phase than on Al2O3 phase was observed, The Pt-Sn catalyst impregnated on pure MgAl2O4 showed the maximum selectivity to isoamilenes. The high Al2O3 content supports (Al/Mg > 3) bring about a higher light olefins (C-2-C-4) production, which promotes the coke formation. This process finally accelerated the catalyst deactivation. This is explained by the higher acidity of alumina compared with magnesium aluminate. (C) 2001 Elsevier Science B.V. All rights reserved.