Selectivity and mechanism for skeletal isomerization of alkanes over typical solid acids and their Pt-promoted catalysts

Selectivities for skeletal isomerizations of n-butane and n-pentane catalyzed by typical solid acids such as CS2.5H0.5PW12O40 (Cs2.5), SO42-/ZrO2, WO3/ZrO2, and H-ZSM-5 and their Pt-promoted catalysts were compared. High selectivities for n-butane and low selectivity for n-pentane were observed over Cs2.5 and SO42-/ZrO2, while H-ZSM-5 was much less selective, and WO3/ZrO2 was highly selective for both reactions. The Pt-promoted solid acids were usually selective for these reactions in the presence of H-2 except for Pt-H-ZSM-5 for n-butane isomerization. Both the acid strength and pore structure would be factors influencing the selectivity. Mechanism of skeletal isomerization of n-butane was investigated by using 1,4-C-13(2)-n-butane over Cs2.5 and Pt-Cs2.5. It was concluded that n-butane isomerization proceeded mainly via monomolecular pathway with intramolecular rearrangement on Pt-Cs2.5, while it occurred through bimolecular pathway with intermolecular rearrangement on Cs2.5. The higher selectivity on Pt-Cs2.5 would be brought about by the monomolecular mechanism. In the skeletal isomerization of cyclohexane, Pt-Cs2.5/SiO2 was highly active and selective, while Pt-Cs2.5 was less selective. Control in the acid strength of Cs2.5 by the supporting would be responsible for the high selectivity. (C) 2002 Elsevier Science B.V. All rights reserved.