Rhenium-based metathesis catalysts catalyst deactivation

The kinetics of the metathesis of propene over a rhenium oxide-alumina catalyst (5.8% Rea07) have been studied by Kapteijn and Mol. The data correlate with a model based on the carbene mechanism and are in agreement with infrared and adsorption studies. Hsu has developed a kinetic model to express the time-on-stream profile of activity during catalyst break-in and deactivation. 

Catalyst deactivation and regeneration. The activity of a rhenium-based catalyst in the metathesis of unsaturated esters is unavoidably limited by the complexation of the ester group to the active site [8]. Moreover, there are many routes that lead to deactivation of the catalyst. Polar compounds such as H2O or free acids, alcohols and peroxides, which might be present as an impurity in the substrate(s), can act as catalyst poisons. Other possible routes for the deactivation of rhenium-based catalysts include (i) reduction of the rhenium below its optimum oxidation state (ii) adsorption of (polymeric) product molecules on the surface of the catalyst, blocking the active sites (iii) reductive elimination of the metallacyclobutane intermediate [59]. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. 

Catalyst deactivation. The molybdenum-based catalysts deactivate faster than the rhenium-based ones. Studies concerning the stability of the catalyst during continuous metathesis of propene showed a loss of activity due to an intrinsic deactivation mechanism. Because of the high stability of both [Mo]=CH2 and [Mo]=CHCH3, the deactivation of the catalyst is assigned to isomerization of the intermediate metallacyclobutane complexes, leading to inactive 7i-complexes, in a way analogous to that depicted in Scheme 2. This hypothesis is supported by in situ UV/vis spectroscopic studies [67]. 

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