The mechanisms of hydrogenolysis and isomerization of hydrocarbons on metals. VI. Isomerization of labeled hexanes on a 0.2% platinum-alumina catalyst

Abstract

The isomerization at low conversion of six methylpentanes (2-methyl- and 3-methyl-), labeled with carbon 13 in various positions, have been studied over a 0.2% PtAl 2O 3 catalyst. The nature of the carrier, an inert alumina, and the reaction temperature, 265–275 °C, were such that the skeletal rearrangement proceeds only on the metallic part of the catalyst. The position of the labeling atom in most of the isomeric reaction products, methylpentanes and n-hexanes, is consistent with a simple cyclic mechanism of isomerization. This mechanism consists of a dehydrocyclization to a methylcyclopentane intermediate followed by the opening of its ring. However, ten to twenty percent of the isomers referred to as “abnormal” products, cannot be explained by this mechanism. The abnormal n-hexanes are formed by the succession of a dehydrocyclization-ring opening process and an exchange of the two terminal carbon atoms in the resulting adsorbed n-hexane. The abnormal methylpentanes may be formed either by a simple methyl or ethyl shift, or by the succession of several rearrangements in the adsorbed phase, according to a bond shift or a cyclic mechanism. The simultaneous occurrence of a single dehydrocyclization-ring opening process, the major mechanism, and of a minor process involving several successive rearrangements before desorption, suggests a heterogeneity of the 0.2% PtAl 2O 3 catalyst.