Adsorbed carbocations as transition states in heterogeneous acid catalyzed transformations of hydrocarbons

Abstract

Ab initio quantum chemical calculations indicated that adsorbed carbenium and carbonium ion active intermediates of acid-catalyzed transformations of hydrocarbons on zeolites are not the really existing highly reactive species but the transition states of the corresponding elementary steps. Adsorbed carbenium ion-like activated complexes can be formed both via proton addition to the double bonds of olefins or as energetically excited unstable ion pairs resulting from partial dissociation of the carbonyl bonds in more stable alkoxy species. In contrast, the highly energetically excited adsorbed carbonium ion-like transition states result only from proton attack at the C–C or C–H bonds of paraffins. The quantum chemical calculations provided the information on geometry and electronic structure of these activated complexes which depend on the elementary reactions in which these transition states are involved. The calculated heat effects and activation energies for the main elementary steps in acid catalyzed transformations of hydrocarbons on zeolites, i.e. of double bond shift, skeletal isomerization and cracking of olefins or protolytic dehydrogenation, protolytic cracking of paraffins and hydride transfer from isoparaffins to carbenium ions are in a reasonable agreement with the experiment.