Products and intermediates in propane hydrogenolysis on supported Pt

Abstract

Propane hydrogenolysis on Pt catalysts did not show serious deactivation over periods of 8 h below 630 K, but Arrhenius analyses were complicated. Selectivities to methane (S₁) and ethane (S₂) remained constant at 1.00 ± 0.02, with S₁/S₂ just above 1, when less than 10 mmol propane were converted per g catalyst per hour. However, S₁/S₂ rose as CH₄ was formed more selectively at higher conversions as the temperature increased. Thus at lower temperatures propane hydrogenolysis occurred by single C–C bond rupture, before switching to multiple bond rupture at higher temperatures. Transient analysis on Pt/SiO₂ using alternating H₂ and C₃H₈ pulses, similar in size to the adsorption capacity of the catalyst, enabled study of the surface at close to its working coverages of (θ_H) and H-deficient hydrocarbon species, such as _mn (θ_CmHn). C₃H₈ pulses at 573–773 K (to a catalytic surface that had just seen hydrogen) resulted in H₂ emergence as a result of dehydrogenation rather than displacement. H₂ pulses under the same conditions (onto the silica-supported Pt that had just seen C₃H₈) released C₃H₆ and C₂H₄ from previously accumulated surface _mn on the Pt, even though these are less favourable products than alkanes. Such observations may suggest a relationship between catalysed hydrogenolysis and dehydrogenation and a potential for catalytic fine-tuning through reaction coupling. Pulse work shows that hydrogenolysis activity at intermediate temperatures can involve multiple C–C scission initially, but that the build-up of carbonaceous deposits deactivates such sites only leaving those able to support single C–C bond rupture in propane. Such maturation effects on hydrogenolysis selectivity need to be better understood by a combination of pulse and steady-state experiments.