Platinum-tin catalysts supported on silica highly selective for n-hexane dehydrogenation

Silica-supported Pt-Sn catalysts were prepared by two-step impregnation from [PtCl2(PPh(3))(2)] and SnCl2 solutions of appropriate concentrations to yield Pt/Sn atomic ratios ranging from 0.2 to 5.0. In these systems, the presence of true Pt-Sn alloys was confirmed by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray analysis and electron nanodiffraction. Pt and PtSn ahoy phases were found on catalysts with Pt/Sn > 1, PtSn alloy alone on the catalyst with Pt/Sn = 1 and PtSn and PtSn2 alloys, together with Sn in the catalysts with Pt/Sn < 1. All these catalysts were tested in the skeletal reactions of n-hexane at 753 K and atmospheric pressure. The selectivity of Pt changed significantly when alloyed with tin. For Sn-rich compositions a segregation of tin toward the catalyst surface was shown by photoelectron spectroscopy, and high hydrogenolysis selectivity and fast deactivation were observed. In contrast, Pt-rich catalysts, in which a well defined PtSn alloy was observed, were much more stable and exhibited high selectivity to dehydrogenation reaction while maintaining low conversions to benzene and hydrogenolysis products. This selectivity pattern can be interpreted in terms of a change in adsorption properties due to differences in the number of adjacent Pt atoms required for the various reaction pathways. (C) 1997 Academic Press.