Reactions of hexanes, unlabeled and Labeled with 13C, on alumina-supported palladium-gold and platinum-gold alloys

Abstract

The isomerization and hydrocracking of unlabeled and labeled hexanes have been investigated over palladium-gold and platinum-gold alloys (10% of metal on alumina). The palladium-gold catalysts are substantially less active than pure palladium, even after activation in air at 400 °C. However, no noticeable change in selectivity is observed when alloying gold and palladium. For both palladium and alloy catalysts, selective demethylation is the main reaction occurring and the mechanism of hexane isomerization is essentially “cyclic.” On the contrary, dramatic changes in reaction mechanisms are observed when alloying platinum with gold: while the mechanism of hexane isomerization on platinum is mostly “bond-shift,” on the 15% platinum-gold alloy, only the “cyclic” mechanism takes place. Similarly a nonselective ring opening of methylcyclopentane occurs on the platinum-gold alloy, while on platinum, only the CH 2CH 2 secondary-secondary bonds are broken. The 15% platinum-gold alloy is active only after pretreatment by air and is deactivated in the presence of hydrogen. Activation and deactivation are time-dependent, temperature-dependent and reversible. No change in selectivity and mechanism is observed during the activation and deactivation processes. It is therefore concluded that a high dispersion of the platinum atoms and the presence of oxygen ions in their immediate vicinity are both required to induce the cyclic mechanism of hexane isomerization and the nonselective hydrogenolysis of methylcyclopentane.