A comparative study of zirconia and alumina supported Pt and Pt–Sn catalysts used for dehydrocyclization of n-octane Original Research Article

Alumina and zirconia supported Pt and Pt–Sn catalysts have been tested in the dehydrocyclization (DHC) of n-octane at 823 K to alkylaromatics as desired products using pure hydrogen (H2) and water vapour containing hydrogen (WVH2) atmospheres. The materials were characterized by nitrogen adsorption, NH3-TPD, TPR, XRD, XPS, TEM and EDX. The characterization results reveal the formation of PtSn alloys on alumina support after a catalyst precursor reduction in H2 at 873 K. This observation enables an interpretation of the higher on-stream stability of Pt–Sn/Al2O3 catalyst in H2 as well as in WVH2 compared to the monometallic sample, using the well-known concept of the ensemble effect of Sn by alloying to Pt. The alumina supported Pt and Pt–Sn catalysts act bifunctionally in H2 atmosphere, producing a mixture of benzene, mono- and dialkylaromatics, near the chemical equilibrium. In WVH2 atmosphere, however, o-xylene and ethylbenzene are predominant among the aromatic products, indicating a monofunctional action mode of the catalyst. Independent of the reaction atmosphere, the zirconia supported Pt and Pt–Sn catalysts act monofunctionally with o-xylene and ethylbenzene as predominating products among the aromatics produced. The zirconia supported samples are more stable in WVH2 than in H2 atmosphere. The addition of Sn significantly increases the activity of that catalyst but does not retard its deactivation in H2. The formation of PtSn alloys is not evidenced in the Pt–Sn/ZrO2 catalyst after hydrogen treatment. The behaviour of the zirconia supported catalysts can be interpreted considering the weak-acidic character of the support and its interactions with Pt as well as with SnO2.