Quantum-chemical study of reaction routes and centers in the interaction of propylene with an oxide catalyst surface

Abstract

The MINDO/3 method was employed to study the interaction of propylene with the surface of gallium oxide for various orientations of the molecule with respect to the cluster. The heterolytic dissociation of the CH bond accompanied by the formation of an allyl complex was shown to proceed with the methylhydrogen oriented on the oxygen ion and with the methylene group turned aside from the cluster surface. The heat of the process (≈10 kcal/mol) was close to that observed for adsorption forms which lead to partial oxidation products. If the hydrogen is oriented on the gallium ion, stable bonds form between the hydrocarbon and the cluster oxygen. In this case, the heat of the process is relatively high (≈30 kcal/mol). This is close to the heat of formation of surface oxidized structures which are precursors of complete oxidation products. The diatomic energy contributions from the interaction of the molecule with each of the reaction centers (oxygen and gallium ions) have been analyzed. The energy barriers arise from the Coulomb repulsion of the molecule by the cation at large distances, which is displaced by a strong stabilizing covalent binding at small distances. Interaction between the molecule and the anion, in all cases, provides a stabilizing function. The requirements of the optimal catalyst for allyl activation of olefins have been formulated.