Skeletal rearrangements of hydrocarbons on metals

Abstract

AbstractThis paper is a report of some new investigations in the field of skeletal rearrangements on metals.The kinetics of isomerization are considered first. While in the case of pentanes and hexanes the orders vs hydrogen of the various elementary steps are negative, they are positive in the case of heptanes. However it is shown by using 13C‐labeled molecules that no scrambling of the label occurs during the isomerization process. The rate‐determining step then is in all cases the skeletal rearrangement of a highly dehydrogenated species. In order to account for the change of orders vs hydrogen, the multisite model proposed by Frennet and coworkers (J. Catal 53, 150, 1978) is adapted to skeletal isomerization.The particle size effects in isomerization of 2‐methylpentane to 3‐methylpentane and in methylcyclopentane hydrogenolysis are considered next. It is shown that both bond‐shift and cyclic‐type isomerizations take place one the same sites, presumably the edge atoms of the crystallites. Two limiting sizes are defined: one at 10 Å below which bond‐shift isomerization is converted into non‐selective cyclic type isomerization. The second one at 25 Å, below which the hydrogenolysis of methylcyclopentane changes from selective to nonselective. Possible crystallographic models are discussed, which could explain the results. None of them is satisfactory and it is suggested that the changes of mechanisms are due to changes in the electronic structure of the metal particles. This view is corroborated by a comparative study of the isomerization of 13C‐labeled hexanes on various group VIII metals.In the last section, one of the bond‐shift mechanisms and the two cyclic mechanisms, selective and non selective, are described in details. The bond‐shift isomerization is best described as a reversible dismutation of metallo‐cyclobutane into metallocarbene of internal CC bonds in isopentane and n‐pentane. On the other hand, the non‐selective and selective cyclic mechanisms for isomerization are represented as dicarbene and dicarbyne recombinations respectively. The importance of metallocarbenes and metallocarbynes for other types of skeletal rearrangement such as hydrogenolysis is also emphasized and a classification is given for the contact reactions of hydrocarbons on metal surfaces.