Pt/Al2O3 catalysts and Pt-Sn/Al2O3 catalysts prepared by two different methods: Hydrogen pressure effects in the reactions of n-hexane

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)]Cl-2 as Pt precursors. Pt/Al2O3 ex [Pt(NH3)(4)]Cl-2 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 ina 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. (C) 1997 Academic Press.