Mechanisms of isomerization of hydrocarbons on metals. Part 11.—Isomerization and dehydrocyclization of 13C-labelled 3-methylhexanes on Pt–Al2O3 catalysts

Abstract

The isomerization, dehydrocyclization and hydrogenolysis of 3-methylhexane have been studied at 320–380°C over a series of Pt–Al₂O₃ catalysts with a metal dispersion extending from 0.05 to 1. The use of five labelled compounds, 3-methyl[1-¹³C], [2-¹³C], [3-¹³C], [6-¹³C]hexanes and 3-methyl-[¹³C]hexane, allowed distinction between the various parallel pathways.

On all catalysts the predominant reaction was the isomerization according to a cyclic mechanism involving either 1,3-dimethyl-, 1,2-dimethyl- or ethyl-cyclopentane intermediates with a relative contribution of 60, 40 and 20 %, respectively. These results are consistent with a dehydrocyclization scheme involving a metallocarbene as precursor and dicarbene or dicarbyne recombination as the rate-determining step.