Mechanism of isomerization of hydrocarbons on metals. Part 9.—Isomerization and dehydrocyclization of 2,3-dimethyl[2-13C]pentane on a 10 % Pt–Al2O3 catalyst

Abstract

The isomerization, dehydrocyclization and hydrogenolysis of 2,3-dimethyl[2-¹³C]pentane have been studied at 260°C over a 10 % Pt–Al₂O₃ catalyst of low dispersion, under various hydrocarbon and hydrogen pressures. Most of the isomerization products are accounted for either by a bondshift mechanism or by a cyclic mechanism involving 1,2-dimethylcyclopentane intermediate. The absence of significant scrambling of the label suggests that the rate-determining step in isomerization is the skeletal rearrangement of highly dehydrogenated species. The positive order as a function of hydrogen which is found (0.8–1.2) cannot then be taken as evidence that desorption is rate-determining. It is best explained by assuming multisite adsorption of hydrocarbon and competition with hydrogen for chemisorption on the same sites. The results provide argument in favour of a reactive rather than dissociative-type adsorption step.