Pt-Sn/Al2O3 catalysts were prepared using two different methods, namely, by "traditional" coimpregnation with H2PtCl6 and SnCl4 and by a "new" method in which the bimetallic complex precursor [Pt(NH3)4][SnCl6] is prepared on the support. Their catalytic activity and selectivity in n-hexane reactions were studied as a function of the hydrogen pressure (60-480 Torr) and compared with those of monometallic Pt/Al2O3 catalysts using H2PtCl6 or [Pt(NH3)4]Cl2 as Pt precursors. Pt/Al2O3 ex [Pt(NH3)4]Cl2 showed very low dispersion and exhibited high selectivity in reactions attributed to multiatomic ensembles. The results with bimetallic catalysts can be rationalized in terms of two phases being present, a PtSn alloy phase plus Pt in fine distribution. The "new" Pt-Sn/Al2O3 from the bimetallic precursor contains the two metals in a better dispersion, resulting in a larger number of atomically dispersed surface Pt active sites. This catalyst gave more isomers (and methylcyclopentane) and fewer fragments and less benzene than the "traditional" sample. The "new" Pt-Sn/Al2O3 sample possessed good long-term stability. The "traditional" sample lost some of its activity and its high hydrogenolysis selectivity during long use; i.e., it approached the catalytic properties of the "new" sample. Both samples are potential candidates as catalysts with high isomerizing and low aromatic selectivities (up to 75% isohexanes plus methylcyclopentane as opposed to a maximum of 20% benzene). The results could be explained sufficiently with a geometric model, electronic interactions playing a less important role in the catalytic phenomena observed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry