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| - Theoretical Investigations of Isolated Mo(VI) and Mo(IV) Centers of a Molybdena−SilicaCatalyst for Olefin Metathesis
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| - A molybdena−silica catalyst for olefin metathesis is theoretically studied using the two-layer ONIOM methodand the conventional cluster approach. Various possible structures and locations of Mo methylidene sites areexamined. Both the fully hydroxylated and partially dehydroxylated silica surface is considered. It is predictedthat 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. Thetheoretical wavenumbers of the C−H stretching vibrations for the Mo methylidene, Mo ethylidene, andmolybdacyclobutane centers are compared with the reported IR data. It is concluded that the four-coordinateMo(VI) alkylidene species and the corresponding square pyramidal molybdacyclobutane forms are most likelydetected on the real catalyst by the IR measurements. The theoretical frequencies for the five-coordinateMo(VI) methylidene species and the related molybdacyclobutane structure are also satisfactorily consistentwith 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 inthe reduced catalyst.
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