Title: A study of MoO3/SiO2 metathesis catalyst break-in: active site formation and characterisation
Abstract: Metathesis catalyst break-in, or extended induction period, for a series of silica-supported Mo catalysts during propene metathesis has been studied in a conventional flow reactor as well as by following the generation of Mov paramagnetic species by ESR spectroscopy. It is shown that two kinds of Mov centers, having entirely different metathesis reactivities, coexist in this transient period. Catalytically, only the distorted square pyramidal Mov site shows a fairly linear correlation between relative intensity and increasing metathesis conversion. By assuming the propene conversion to be proportional to the active site concentration, a first-order rate expression was deduced and verified experimentally. Hence, the intrinsic nature of the break-in phenomenon has been explored. A model is postulated to explain the roles of both Mov sites in catalyst break-in. It is apparent that the oxidation state (via the electronic effect) and coordination structure (via the geometric effect) are both critical in the creation of a site which is active for metathesis. Finally, the active site is characterised not only by its oxidation state and configurational modification, but also by assigned MO bonding coefficients on a molecular level. In summary, the stronger π-bonding covalency in the out-of-plane Mo species is related to the optimal activity of the Mov site in metathesis catalysis.
Publication Year: 1988
Publication Date: 1988-07-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 17
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