n-Butane isomerization on sulfated zirconia: the effect of nonspecific olefin addition

Butene has been suggested to be an intermediate in the proposed bimolecular mechanism for n-butane isomerization over sulfated zirconia (SZ). The addition of butene has been shown to significantly increase the rate of this reaction. The question can be asked as to whether other olefins would affect this reaction as well. In this study, the effects of propylene, 1-butene and 1-pentene addition on the reaction induction period and selectivity were studied and compared at 100 and 150 °C. Enhanced catalyst activity was found for all three olefins. There was an optimum concentration of added olefin for maximum activity. Catalyst activity increased and the induction period decreased with increased olefin concentration. Adding any of the three olefins for only the initial 2 min of reaction caused a long-term positive impact on the reaction rate and isobutane selectivity, reinforcing our earlier proposed concept of “olefin-modified sites.” The ratio of excess isobutane molecules formed per molecule of olefin added during the initial 2 min was around 14 at 100 °C and 692 at 150 °C for all three olefin additions. It is suggested that C8+ intermediates (part of the hypothesized bimolecular reaction pathway) are not the only oligomers leading to the selective product; the reaction appears to proceed through the formation of C7+ and C9+ oligomers as well. The results of this study clearly show that isobutane can be made equally well by a mechanism involving n-butane and olefins different from butene.