n-Butane isomerization on sulfated zirconia: active site heterogeneity anddeactivation

The fast deactivation of sulfated zirconia (SZ) has limited its use in comm ercial processes such as rt-butane isomerization. In order to investigate t his 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 degreesC. Approximately 20% of the sulfate species was found to be n-butane adsorption sites, but o nly 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 TOFITK* (TOF base d on an average residence time of active surface intermediates) and the reg eneration characteristics of the SZ catalyst suggest a possible active site heterogeneity. It appears that the high initial activity and the fast deac tivation for TOS less than or equal to 100 min were mainly dub to the prese nce 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 greater than or equal to 100 min . The less active sites appeared to be more easily regenerated than the mor e active sites as the catalytic activity at TOS greater than or equal to 10 0 min was recovered following regeneration at 315 degreesC. 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 d eactivation for n-butane isomerization on SZ. The impact of sulfur reductio n on catalyst deactivation cannot be ruled out at this point. (C) 2000 Else vier Science B.V. All rights reserved.