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

Abstract

Selectivities for skeletal isomerizations of n-butane and n-pentane catalyzed by typical solid acids such as Cs 2.5H 0.5PW 12O 40 (Cs2.5), SO 4 2−/ZrO 2, WO 3/ZrO 2, 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 SO 4 2−/ZrO 2, while H-ZSM-5 was much less selective, and WO 3/ZrO 2 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- 13 C 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/SiO 2 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.