n-butane isomerization on sulfated zirconia: isotopic transient kinetic analysis of reaction at the site level

By using SSITKA (steady-state isotopic transient kinetic analysis), n-butan e (n-C-4) isomerization on sulfated zirconia (SZ) has been studied for the first time at the site level. Accurate measures of the average residence ti me and the concentration of the most active surface intermediates leading t o isobutane (iso-C-4) were able to be determined. As has previously been ob served, a fast initial deactivation of the catalyst followed by a slow stea dy-state deactivation was observed over 400 min time-on-stream (TOS). It wa s shown that even though a large amount (similar to 100 mu mol/g) of n-C-4 (Nn-C4) was adsorbed on the catalyst, the concentration of active surface i ntermediates leading to iso-C-4 (Niso-C4*) was only similar to 10 mu mol/g at 30 min TOS. The continuous decrease in Niso-C4* indicated that the decre ase in catalytic activity was due to loss of active sites. An increase in t he average residence time of active surface intermediates leading to iso-C- 4 (tau(iso-C4)*) was observed only between 30 and 100 min TOS. This suggest s a possible presence of two types of active sites for n-C-4 isomerization on SZ under the reaction condition studied. It is suggested that the more-a ctive sites contributed to the high initial activity, while the majority of active sites after 100 min TOS were the less active sites.