Mechanisms of n-butane isomerization over superacidic sulfated metal oxides

Superacidic sulfated metal oxides were prepared by impregnating different metal oxides (or hydroxides) with sulfuric acids. The superacidic strength of prepared samples were compared by temperature-programmed desorption of ammonia (NH₃-TPD) and showed a trend of HS/ZrO₂ > HS/MgAl₂O₄ > HS/Al₂O₃ > HS/Fe₂O₃. Catalytic properties of these superacidic samples towards isomerization of n-butane to isobutane were subsequently pursued. Both the activity and the isobutane selectivity increased with the superacidic strength of catalysts. Observed variation in the selectivity was interpreted with a difference in the reaction mechanism: A high selectivity was obtained from strong superacid sites that catalyze the isomerization through a revised monomolecular mechanism; while a low selectivity became dominated on weak superacid sites that favored a bimolecular mechanism. The relative importance of these two mechanisms was kinetically controlled by formation of different butyl carbenium ions (primary or secondary) while the n-butane reactant adsorbed on superacidic sites. (C) 2000 Elsevier Science B.V.