Mechanisms of n-butane isomerization over superacidic sulfated metal oxides

Superacidic sulfated metal oxides were prepared by impregnating different m etal oxides (or hydroxides) with sulfuric acids. The superacidic strength o f prepared samples were compared by temperature-programmed desorption of am monia (NH3-TPD) and showed a trend of HS/ZrO2 > HS/MgAl2O4 > HS/Al2O3 > HS/ Fe2O3. Catalytic properties of these superacidic samples towards isomerizat ion of n-butane to isobutane were subsequently pursued. Both the activity a nd the isobutane selectivity increased with the superacidic strength of cat alysts. Observed variation in the selectivity was interpreted with a differ ence in the reaction mechanism: A high selectivity was obtained from strong superacid sites that catalyze the isomerization through a revised monomole cular mechanism; while a low selectivity became dominated on weak superacid sites that favored a bimolecular mechanism. The relative importance of the se two mechanisms was kinetically controlled by formation of different buty l carbenium ions (primary or secondary) while the n-butane reactant adsorbe d on superacidic sites. (C) 2000 Elsevier Science B.V. All rights reserved.