Molybdenum catalysts valence states

The catalyst is preliminarily oxidized to the state of the highest valence (vanadium to V5+ molybdenum to Mo6+). Only the complex of hydroperoxide with the metal in its highest valence state is catalytically active. Alcohol formed upon epoxidation is complexed with the catalyst. As a result, competitive inhibition appears, and the effective reaction rate constant, i.e., v/[olefin][ROOH], decreases in the course of the process due to the accumulation of alcohol. Water, which acts by the same mechanism, is still more efficient inhibitor. Several hypothetical variants were proposed for the detailed mechanism of epoxidation. 

A redox mechanism (Mars-van Krevelen) is generally accepted for the ammoxidation reaction of methyl aromatic compounds, thus most catalysts applied contain transition metal oxides (e. g. vanadium, molybdenum) readily enabling changes in valence states. 

Sited with chromium in the same group 6B in the Periodic Table of Elements, molybdenum attracts many efforts in the study of ethylene polymerization. As early as in 1950s, Indiana Standard Oil Company discovered supported molybdenum oxide catalyst was active for ethylene polymerization (Field and Feller, 1957). However, the catalyst was then abandoned because of its poor catalytic performance. Our preliminary experiments on the MoO /Si02 catalyst also showed very low reactivity with comparison to the Cr0 y/Si02 catalyst for ethylene polymerization. In order to improve the activity of Mo-based catalyst for ethylene polymerization, the active valence states of molybdenum sites, and the mechanism of the catalytic reaction should be first elucidated. We performed a detailed theoretical study combined with experiments to investigate the active oxidation states of molybdenum and the effects of surface hydroxyl on the polymerization activity of supported Mo-based catalysts (Cao et al., 2010). 

As reported in the Hterature, Mo species mainly existed in an isolated form in the low molybdenum content catalyst. In our work, four kinds of isolated molybdenum models of the active sites were built with a consideration of different valence states, as shown in Scheme 3.8. Models F and G represent molybdenum centers attached to two Al atoms that were bridged... 

Hu et al. (1995) reported extensive Raman experiments characterizing supported molybdenum oxide catalysts during methanol oxidation. In contrast to the stable oxidation state of vanadia, the valence of surface molybdenum species decreased during catalysis. The original band... 

Molybdenum normally has a valency of -i-6 but +4 and -h5 are also known. The +4 state is, as we know, predominant in nature in molybdenite MoSj. The capacity to change oxidation states makes molybdenum suitable as a catalyst for some reactions and an inhibitor for others. It catalyses different redox reactions. An important reduction that uses molybdenum as a catalyst is dehydrosulfurization, used to remove sulfur... 

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