Theoretical investigations of isolated Mo(VI) and Mo(IV) centers of a molybdena-silica catalyst for olefin metathesis

A molybdena-silica catalyst for olefin metathesis is theoretically studied using the two-layer ONIOM method and the conventional cluster approach. Various possible structures and locations of Mo methylidene sites are examined. Both the fully hydroxylated and partially dehydroxylated silica surface is considered. It is predicted that four-coordinate Mo(VI) alkylidene species are active in olefin metathesis, while five-coordinate Mo(VI) alkylidene centers as well as surface Mo(IV) alkylidene complexes are not the metathesis active sites. The theoretical wavenumbers of the C-H stretching vibrations for the Mo methylidene, Mo ethylidene, and molybdacyclobutane centers are compared with the reported IR data. It is concluded that the four-coordinate Mo(VI) alkylidene species and the corresponding square pyramidal molybdacyclobutane forms are most likely detected on the real catalyst by the IR measurements. The theoretical frequencies for the five-coordinate Mo(VI) methylidene species and the related molybdacyclobutane structure are also satisfactorily consistent with the experimental data. The comparison of the calculated energies of ethene adsorption on the Mo(IV) oxide precursors with the reported experimental value suggests that mono-oxo Mo(IV) species are present in the reduced catalyst.