The mechanisms of isomerization and exchange of olefins over metal catalysts: III. Isomerization of butenes in the presence of perdeuteropropene on iron films

Abstract

The initial deuterium distributions of the reaction products obtained by isomerization of but-1-enes, cis- and trans-butenes in the presence of perdeuteropropene on iron films at −37 °C are examined in this paper, together with the product distributions obtained by co-isomerization of cis-d 0- and d 8-butenes. While but-1-ene yields poorly exchanged cis and trans isomers, best interpreted by assuming an intramolecular hydrogen shift, highly deuterated but-1-enes with a maximum in d 8 are obtained from cis- and trans-butenes. This result can only be explained if two different types of site for double bond migration are present on the surface and α- and β-olefins are adsorbed on each type of site with very different strengths. Cis-trans isomerization, on the other hand, occurs mostly by a Horiuti-Polanyi mechanism restricted to a single interconversion between mono- and diadsorbed species (formation of the d 1 isomer), but also by a direct process which does not involve the breaking or the formation of any CH bond and leads to d 0 -butenes. The results obtained in the co-isomerization experiment confirm these views and suggest moreover that the replacement of the hydrogen atoms of an olefin during the exchange reaction is fast when compared with the mobility of the adsorbed hydrogen or deuterium atoms.