Comparative study of n-pentane isomerization over solid acid catalysts, heteropolyacid, sulfated zirconia, and mordenite:: dependence on hydrogen and platinum addition

Skeletal isomerization of pentane over solid acid catalysts (NH4)(2.5)H0.5PW12O40, sulfated zirconia, and H-mordenite has been studied. Hybrid bifunctional catalysts obtained by grinding equal weights of Pt/SiO2 and each of the above solid acids were also used for the reaction. Various mechanistic aspects for n-pentane isomerization over monofunctional and bifunctional catalysts, in the presence or absence of hydrogen in the feed, are discussed. Analysis of product distributions at 423 and 473 K indicated that over monofunctional heteropolyacids and sulfated zirconia catalysts, the main reaction path is the bimolecular one, involving conjunct polymerization and cracking. Under these operating conditions the selectivity to isopentane is low. Monofunctional H-mordenite shows a significant activity only at 498 K. HPA and ZrO2-SO4. although showing stronger acidity than H-mordenite, do not activate n-pentane through a monomolecular path as "superacid-type catalysts" are expected to. Bifunctional isomerization of pentane appears at a reaction temperature of 473 K and up since the equilibrium concentration of pentene is significant only when the reaction temperature is 473 K and up. A monomolecular bifunctional mechanism is the preferred route for reaching high isomerization selectivity. (C) 2003 Elsevier Inc. All rights reserved.