The Impact of Pt and H2 on n-Butane Isomerization over Sulfated Zirconia: Changes in Intermediates Coverage and Reactivity

Abstract

Previous steady-state isotopic transient kinetic analysis (SSITKA) studies in our laboratory showed that the deactivation of an unpromoted sulfated zirconia (SZ) catalyst during n-butane isomerization at 150°C is due to a loss of active sites as well as a small decrease in the average site activity (Kim, S. Y., Goodwin, Jr., J. G., and Galloway, D., Catal. Today63, 21 (2000)). The mechanism by which SZ deactivates is linked to coke/oligomer formation and, possibly, surface sulfur reduction. The use of Pt promotion of SZ and H2 addition to the reaction mixture has been shown to greatly decrease deactivation during n-butane isomerization. In this study, the mechanisms by which H2 addition and Pt promotion affect n-butane isomerization over SZ were studied using SSITKA. H2 probably inhibits n-butane isomerization activity of the unpromoted SZ catalyst through inhibition of butene formation. Little effect on the average residence time for reaction was observed. An improvement in catalyst stability was linked to a reduction in coke/oligomer formation. In the absence of H2 in the feed stream, the presence of Pt did not improve the activity or the stability of the catalyst. An enhanced catalytic activity and stability, however, was evident when both H2 addition and Pt promotion were used, but only at the higher reaction temperature of 250°C. This enhanced catalytic activity at 250°C upon both Pt promotion and H2 addition was a result of an increase in the concentration of active surface intermediates leading to isobutane. This concentration did not change greatly during reaction, thus showing the improvement also in catalyst stability. Although the concentration of active intermediates increased significantly, the average reaction rate (inverse seconds) of an intermediate actually decreased, but not enough to prevent an increase in overall activity. Only 4% (SZ)–16% (PtSZ) of the intermediate to strong acid sites appeared to be involved in the reaction after 5 min time-on-stream, suggesting a reason why NH3 adsorption characterization results do not correlate well with catalyst activity for SZ-based catalysts. The implications of the obtained results on the roles of Pt and H2 on SZ in enhanced catalyst activity and decreased deactivation are discussed.