PT AL2O3 CATALYSTS AND PT-SN/AL2O3 CATALYSTS PREPARED BY 2 DIFFERENT METHODS - HYDROGEN PRESSURE EFFECTS IN THE REACTIONS OF N-HEXANE/

Pt-Sn/Al2O3 catalysts were prepared using two different methods, namel y, by ''traditional'' coimpregnation with H2PtCl6 and SnCl4 and by a ' 'new'' method in which the bimetallic complex precursor [Pt(NH3)(4)][S nCl6] is prepared on the support. Their catalytic activity and selecti vity in n-hexane reactions were studied as a function of the hydrogen pressure (60-480 Torr) and compared with those of monometallic Pt/Al2O 3 catalysts using H2PtCl6 or [Pt(NH3)(4)]Cl-2 as Pt precursors. Pt/Al2 O3 ex [Pt(NH3)(4)]Cl-2 showed very low dispersion and exhibited high s electivity in reactions attributed to multiatomic ensembles. The resul ts with bimetallic catalysts can be rationalized in terms of two phase s being present, a PtSn alloy phase plus Pt in fine distribution. The ''new'' Pt-Sn/Al2O3 from the bimetallic precursor contains the two met als in a better dispersion, resulting ina larger number of atomically dispersed surface Pt active sites. This catalyst gave more isomers (an d methylcyclopentane) and fewer fragments and less benzene than the '' traditional'' sample. The ''new'' Pt-Sn/Al2O3 sample possessed good lo ng-term stability, The ''traditional'' sample lost some of its activit y and its high hydrogenolysis selectivity during long use; i.e.,it app roached the catalytic properties of the ''new'' sample. Both samples a re potential candidates as catalysts with high isomerizing and low aro matic selectivities (up to 75% isohexanes plus methylcyclopentane as o pposed to a maximum of 20% benzene). The results could be explained su fficiently with a geometric model, electronic interactions playing a l ess important role in the catalytic phenomena observed. (C) 1997 Acade mic Press.