The impact of Pt and H-2 on n-butane isomerization over sulfated zirconia:Changes in intermediates coverage and reactivity

Previous steady-state isotopic transient kinetic analysis (SSITKA) studies in our laboratory showed that the deactivation of an unpromoted sulfated zi rconia (SZ) catalyst during n-butane isomerization at 150 degreesC is due t o a loss of active sites as well as a small decrease in the average site ac tivity (Kim, S.P., Goodwin, Jc, J. G., and Galloway, D., Catal. Today 63, 2 1 (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 H-2 addition to the reaction mixture has been shown to greatly d ecrease deactivation during n-butane isomerization. In this study, the mech anisms by which H-2 addition and Pt promotion affect n-butane isomerization over SZ were studied using SSITKA. H-2 probably inhibits n-butane isomeriz ation activity of the unpromoted SZ catalyst through inhibition of butene f ormation. Little effect on the average residence time for reaction was obse rved. An improvement in catalyst stability was linked to a reduction in cok e/oligomer formation. In the absence of H-2 in the feed stream, the presenc e of Pt did not improve the activity or the stability of the catalyst. An e nhanced catalytic activity and stability, however, was evident when both H- 2 addition and Pt promotion were used, but only at the higher reaction temp erature of 250 degreesC. This enhanced catalytic activity at 250 degreesC u pon both Pt promotion and H-2 addition was a result of an increase in the c oncentration of active surface intermediates leading to isobutane. This con centration did not change greatly during reaction, thus showing the improve ment also in catalyst stability. Although the concentration bf active inter mediates increased significantly, the average reaction rate (inverse second s) of an intermediate actually decreased, but not enough to prevent an incr ease in overall activity. Only 4% (SZ)-16% (PtSZ) of the intermediate to st rong 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 impl ications of the obtained results on the roles of Pt and H-2 on SZ in enhanc ed catalyst activity and decreased deactivation are discussed. (C) 2001 Aca demic Press.