Performance of PtSn catalysts supported on MAl2O4 (M: Mg or Zn) in n-butane dehydrogenation: characterization of the metallic phase.

Abstract

The Sn(0.3 or 0.5 wt.%) addition to Pt(0.3 wt.%)/ZnAl2O4 and Pt(0.3 wt.%)/MgAl2O4 catalysts leads to an increase of the activity and selectivity to olefins in the n-butane dehydrogenation reaction. Slight differences in the catalytic behaviour were found between 0.3 and 0.5 wt.% of Sn added to Pt in both catalyst series. Besides, the bimetallic catalysts also show a good stability through the five successive reaction-regeneration cycles, mainly the PtSn/MgAl2O4 one. The mono- and bimetallic catalysts were characterized by using different techniques: tests reactions of the metallic phase (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), temperature programmed reduction, X-ray photoelectron spectroscopy, H2 chemisorption, microcalorimetric measurements of the propylene adsorption and XRD. Results show that the nature of the metallic phase in monometallic samples appears to be different for the two supports. Thus, metallic particles with a very low concentration of hydrogenolytic sites (steps, corners and edges) would exist in the Pt/ZnAl2O4 catalyst in contrast with the structure of the metallic phase of the Pt/MgAl2O4 one, where the existence of an important concentration of hydrogenolytic sites is clearly observed (according to the cyclopentane hydrogenolysis results). The modification of Pt by the Sn addition clearly improves the catalytic behaviour in n-butane dehydrogenation due to important changes in the structure of the metallic phase. Thus, when Sn is added to Pt/ZnAl2O4 and Pt/MgAl2O4 catalysts the metallic surface structure seems to be more complex. In fact, results would indicate not only a partial formation of PtSn alloys or intermetallic compounds between Pt0 and a fraction of Sn(0), but also a surface enrichment in Sn, dilution effects as well as the presence of tin stabilized on the support, probably as Sn(II/IV) oxides and SnCl2 species.