n-Butane isomerization on sulfated zirconia: How olefins affect surface intermediate concentration

Isotopic transient kinetic analysis (ITKA) was used to study the effect of various olefin additions (ethylene, propylene, 1-butene, and 1-pentene) on n-butane isomerization on sulfated zirconia (SZ) at 100 °C. Earlier work showed that the activity of this catalyst is enhanced not only by the addition of butene, but also by the addition of ethylene, propylene, and 1-pentene. This reaction activity enhancement observed for olefins other than butene was suggested to be due to the isomerization of n  -butane via different oligomeric species. In this work, we show that at the maximum reaction rate after the initial induction period, the activity of the reaction sites appeared to be similar, regardless of whether they were activated by the various added olefins or by butene produced during reaction. The addition of butene was able to lower the average surface residence time of the active intermediates leading to isobutane (τisoC4∗) at the very beginning of the reaction induction period, reinforcing an early suggestion that butene formation and accumulation of olefinic intermediates are required for the reaction. The improved isobutane formation rate caused by the addition of olefins was due to an increased concentration of surface intermediates leading to isobutane (NisoC4∗). Any type of added olefin led to the formation/utilization of additional active olefin-modified sites (i.e., adsorbed carbenium ions on acid sites). A larger NisoC4∗ was obtained compared with the reaction without olefin addition when the olefins were added only during the initial 2 min of reaction and then terminated. This suggests that most of the olefin-modified sites were probably formed by the addition of olefin at the early stages of the reaction. Those sites were able to integrate themselves into the reaction cycles and to persist for multiple turnovers.