N-Butane isomerization on sulfated zirconia: active site heterogeneity and deactivation

Abstract

The fast deactivation of sulfated zirconia (SZ) has limited its use in commercial processes such as n-butane isomerization. In order to investigate this deactivation, steady-state isotopic transient kinetic analysis (SSITKA) was utilized to study in situ changes in surface kinetic parameters for n-butane isomerization on a widely studied SZ at 150°C. Approximately 20% of the sulfate species was found to be n-butane adsorption sites, but only 1–2% of the sulfate species appeared to adsorb active surface reaction intermediates. The decrease in catalytic activity during deactivation could be attributed to the loss of active sites. The change in TOF ITK ∗ (TOF based on an average residence time of active surface intermediates) and the regeneration characteristics of the SZ catalyst suggest a possible active site heterogeneity. It appears that the high initial activity and the fast deactivation for TOS≤100 min were mainly due to the presence and deactivation of the more active sites, respectively. Following the loss of the more active sites, the less active sites provided the majority of the catalytic activity observed for TOS≥100 min . The less active sites appeared to be more easily regenerated than the more active sites as the catalytic activity at TOS≥100 min was recovered following regeneration at 315°C. Loss of active sites due to sulfur loss or migration seems unlikely. Site blockage by coke/oligomer formation appeared to be a significant contributor for catalyst deactivation for n-butane isomerization on SZ. The impact of sulfur reduction on catalyst deactivation cannot be ruled out at this point.