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-ch emical, as well as the surface and structural properties of the synthesized materials were strongly dependent on the Al/Mg atomic ratio. In solids wit h an Al/Mg atomic ratio higher than 2, a support with a mixture of Al2O3 an d MgAl2O4 phases was obtained. In all the supports a non stoichiometric mag nesium 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 re action at 550 degreesC and atmospheric pressure. On the Pt-Sn catalysts whi ch 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) de posited in both Al2O3 and MgAl2O4 phases. A stronger Pt-support interaction on MgAl2O4 phase than on Al2O3 phase was observed, The Pt-Sn catalyst impr egnated 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 final ly accelerated the catalyst deactivation. This is explained by the higher a cidity of alumina compared with magnesium aluminate. (C) 2001 Elsevier Scie nce B.V. All rights reserved.