Graduate Thesis Or Dissertation
 

Studies in molybdenum oxide catalysts : I. Metal vapor reactions of molybdenum trioxide with various alkoxy silanes : II. Investigation of oxide supported molybdenum catalysts prepared from molybdenum(VI)dioxodiethoxide

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  • The cocondensation reactions of molybdenum trioxide vapor with tetraalkoxysilanes, trimethylalkoxysilanes, and hexamethyldisiloxane at -196°C were investigated. The products from the metal vapor reactions of tetramethoxysilane, tetraethoxysilane, and tetrapropoxysilane were characterized as molybdenum(VI)dioxodialkoxy compounds. The isolated compounds were found to be polymeric in the solid state by bridging through alkoxide groups. Metal vapor reaction of trimethylmethoxysilane gave an inseparable mixture of molybdenum(VI)alkoxysiloxy compounds which were also polymeric in nature. The metal vapor reactions of tetra-2-butoxysilane and trimethyl-t-butoxysilane gave butenes as a primary organic product and an uncharacterized blue molybdenum-containing solid. All products isolated in this study were sensitive to atmospheric moisture and light. The molybdenumdioxoalkoxy and molybdenumdioxoalkoxysiloxy compounds are good model compounds for the oxidation of alcohols. Decomposition studies of Mo(0)₂(OMe)₂ and Mo(0)₂(OMe)x(OSiMe³)y yielded oxidation products of methanol, including dimethylether, formaldehyde, methyl formate, and dimethoxymethane. The reactivity of Mo(0)₂(OEt)₂ towards acidic protons was used to attach the molybdenum(VI)dioxo group to the surface of alumina (Al₂0₃), silica (Si0₂), titania (Ti0₂), zirconia (Zr0₂), and HY-zeolite. Chemical analysis and infrared spectroscopic studies indicate that dioxomolybdenum is attached to the support through bridging oxides between the support metal and molybdenum. The +2, +4, and +6 oxidation states of the surface attached molybdenum could be accessed by oxidation or reduction with 0₂ or H₂, respectively. A survey of the catalytic nature for oxidation, metathesis, and Fischer- Tropsch synthesis of the supported molybdenum compounds in appropriate oxidation states was made. Dramatic differences in the activity and selectivity for oxidation of alcohols were observed depending on the support used. The Ti0₂ supported Mo(VI) showed the highest activity and selectivity for the oxidation of methanol to formaldehyde. The A1₂0₃ supported Mo(IV) system was the most active catalyst for propylene metathesis with the Si0₂ supported Mo(IV) system showing much less activity.
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