Hydrogenolysis of alkanes. Part 4. —Hydrogenolysis of propane, n-butane and isobutane over Pt/ Al2O3 and Pt–Re/Al2O3 catalysts

Abstract

The hydrogenolysis of propane, n-butane and isobutane on Pt/Al₂O₃(0.3 and 0.6 % Pt) and on Pt–Re/Al₂O₃(0.3 % Pt, 0.3 % Re) has been investigated by a thermal cycling technique that permits evaluation of initial rates of deactivation and of consequential changes in product distributions. Less deactivation is found with propane than with the butanes; with n-butane on Pt/Al₂O₃ catalysts, two types of sites are apparent, a one of which, having a higher probability of giving central C—C bond fission, is selectively deactivated. However, the relative chances of desorption and of further hydrogenolysis of the adsorbed intermediates are the same at both types of site.

With the Pt–Re/Al₂O₃ catalyst, further bond-breaking in the adsorbed species formed from all three alkanes is relatively more favoured than with the Pt/Al₂O₃ catalysts, and with n-butane the chance of central bond-fission is enhanced. No changes in product distribution result from initial deactivation, which is comparable to that found with Pt/Al₂O₃ catalysts. Isomerisation of n-butane is slight (> 3 %) on all three catalysts.

Multiply adsorbed species are suggested as possible intermediates, and differences in product selectivities are attributed chiefly to variations in electron density at the active site.