Olefin metathesis over heterogeneous catalysts:: Interfacial interaction between Mo species and a Hβ-Al2O3 composite support

The effect of molybdenum content on the structure and performance of Mo/H beta-Al2O3 catalysts in olefin metathesis was examined. The optimal performance was obtained with a catalyst of 4-6 wt % Mo on a composite support of 70% H beta zeolite and 30% Al2O3. The interfacial interaction between the Mo species and the H beta-Al2O3 composite support was carefully studied by X-ray diffraction (XRD), N-2 adsorption, and multinuclear magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. XRD and N-2 adsorption results indicate that some interactions may occur between the Mo species and the composite support during the sample preparation. Al-27 and Si-29 MAS NMR spectra show that this kind of interaction becomes more severe and leads to the dealumination of the framework and subsequent appearance of aluminum molybdate with increasing Mo loadings. Two-dimensional Al-27 MQ MAS NMR spectra further demonstrate that the framework Al on H beta zeolite at the specific T sites could be preferentially extracted upon Mo loading, which may result in the appropriate Bronsted acidity on the support as evidenced by the quantitative H-1 MAS NMR measurements. A moderate interaction of Mo species with the support and the proper acidity may be advantageous for the cross-metathesis of ethene and 2-butene to propene over heterogeneous catalysts.