Dynamic properties of n-alkyl and s-alkyl intermediates in reactions of simple alkenes with hydrogen on MoS2 catalyst

Abstract

The hydrogen exchange between (Z)-[1-²H₁]propene, [graphic omitted], and [²H₆]propene taking place on MoS₂ in the presence of hydrogen yielded a variety of deuteropropene isomers in the relative proportions 7.5%[1,1-²H₂], 4.5%(Z)-[1,2-²H₂], 34.0%(E)-[1,2-²H₂], 51.0%(E)-[1-²H₁], 0%[2-²H₁] and 3%[²H₀]propene in the initial stage of the exchange reaction at room temperature. This distribution was in good agreement with calculations based on a relative contribution of 70% n-propyl and 30% isopropyl intermediates determined by a separate experiment, and the relative contributions of the n-alkyl and s-alkyl intermediates were found to be the same for the hydrogen exchange reaction of but-1-ene. The hydrogen addition and elimination processes in the isomerization reaction as well as in the hydrogen exchange of olefins are strictly controlled by a cis-stereo specificity of the site which has one coordination vacancy, [graphic omitted]. The s-alkyl species formed on this site are restricted from free rotation around the coordination bond by steric interaction with the surrounding sulphur ions. Such dynamic properties of the s-alkyl intermediates give unexpectedly low values for [3-²H₁]propene: [1-²H₁]propene of <1/15, and also cause two distinguishable conformations for the s-butyls, one from but-1-ene and the other from but-2-ene, depending on the orientation of the vacant coordination site. The alkyl species in the form of [graphic omitted] are intermediates for the isomerization and/or the hydrogen exchange but do not undergo consecutive hydrogenation, because dissociation of H₂ requires two vacancies.