FORMATION OF MALEIC-ANHYDRIDE ON A VANADYL PYROPHOSPHATE SURFACE - A THEORETICAL-STUDY OF THE MECHANISM

An analysis of the electronic structure of the vanadyl pyrophosphate surface and of the oxidation of 1,3-butadiene to maleic anhydride by molecular oxygen catalyzed by this surface is presented. The surface contains pairs of edge-sharing vanadium-oxygen octahedra. Each pair has two vanadyl groups, one pointing toward the bulk of the catalyst and the other one being free to interact with incoming molecules. The frontier orbitals of 1,3-butadiene are set up for an interaction with the oxygen in the vanadyl group in a [2 + 4]-like concerted mechanism forming a 2,5-dihydrofuran. The most favorable geometries of the adsorbed species are discussed from an analysis based on the extended Huckel approach. The activation of molecular oxygen on the surface is discussed. A comparison of two structures, eta-1-superoxo and eta-2-peroxo adsorbed dioxygen is presented. A mechanism for the oxygen transfer to 2,5-dihydrofuran is proposed involving an initial abstraction of a hydrogen in the 2-position from 2,5-dihydrofuran by the coadsorbed dioxygen species, leading to first a 2-hydroxy derivative followed by the formation of an asymmetric unsaturated lactone. The oxidation of the 5-position is suggested to take place in a similar way.