The mechanisms of hydrogenolysis and isomerization of hydrocarbons on metals: VII. Isomerization of labeled hexanes and hydrogenolysis of methyl ( 13C) cyclopentane on a 10% platinum-alumina catalyst

Abstract

The isomerization of 2-methyl-2- 13C, 3-methyl-3- 13C, and the hydrogenolysis of methyl-( 13C) cyclopentane were studied over a 10% platinum-alumina catalyst at 270 °C, Methylcyclopentane hydrogenolysis yielded mainly 2-methylpentane and 3-methylpentane, and isomerization was mostly restricted to an interconversion between both methylpentanes. The position of the carbon 13 in the various reaction products could only be explained by assuming that several successive rearrangements take place in the adsorbed phase, before desorption. A model was devised whereby acyclic adsorbed hydrocarbons may either be desorbed, or be rearranged according to a cyclic or a bond shift mechanism. The only bond shift reactions envisaged in this model were the three possible methyl- and ethyl- shifts involving a tertiary carbon atom, and the same rate was assumed for each of these three reactions. With these assumptions, the observed initial product concentrations (ten independent values) could be reproduced with two parameters only: the probability d of desorption of the adsorbed species, and the probability t of bond shift rearrangement. The mean number of rearrangements before desorption, equal to 1 d , was two for the experiments described, so that not less than seven trips had to be envisaged to account for all of the reaction products. The differences between these results on the 10% PtAl 2O 3 catalyst and the ones obtained on a highly dispersed catalyst, where most of the isotopic varieties are explained by a single dehydrocyclizationring opening process, are interpreted in terms of metallic particle sizes. It is believed that on the dispersed catalysts, the active sites consist, of a single atom, and the only possible path for isomerization is the cyclic mechanism. On catalysts with large crystallites, such as the 10% Pt-alumina catalyst, it is believed that sites involving several metal atoms are present on the surface, which allows the occurrence of bond shift reactions and of the repetitive process.