A study of MoO 3/SiO 2 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 Mo v paramagnetic species by ESR spectroscopy. It is shown that two kinds of Mo v centers, having entirely different metathesis reactivities, coexist in this transient period. Catalytically, only the distorted square pyramidal Mo v 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 Mo v 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 Mo v site in metathesis catalysis.