DEACTIVATION OF RHENIUM-BASED CATALYSTS FOR THE METATHESIS OF PROPENE

The stability of supported Re2O7 metathesis catalysts during the metathesis of propene has been studied in a microcatalytic flow reactor. Promoting the metathesis catalysts with alkyltin compounds led to a large increase in activity, but the stability of the catalyst decreased considerably. The stability of the catalysts appeared to be influenced to a large degree by the choice of support: a 3 wt.% Re2O7/SiO2.Al2O3 catalyst promoted with tetraethyltin proved to be much more stable than a corresponding gamma-alumina-supported one. However, the stability of an alumina-supported catalyst can be improved by increasing its rhenium content: a Re2O7/Al2O3 catalyst containing 12 wt.% Re2O7 turned out to be the most stable. This is not the case for catalysts on silica-alumina. This can be explained on the basis of the different types of hydroxyl groups of the support that can react with ReO4- ions. The use of a gamma-alumina guard bed in the reactor can increase the stability of a 12 wt.% Re2O7/Al2O3 catalyst. This is probably not only due to purification of the feed but also to the gradual release of the promoter from the guard bed onto which it was originally adsorbed. Besides the poisoning of the catalyst, which occurs due to impurities in the feed, an intrinsic deactivation mechanism is also present. This deactivation mechanism is strongly temperature dependent: the lower the reaction temperature the more stable the system. It could be shown by means of temperature-programmed oxidation (TPO), differential thermogravimetry (DTG) and energy dispersive X-ray analysis (EDAX) that the promoter is bonded to the catalyst and that regeneration of a spent catalyst results in formation of SnO2 which remains behind on the catalyst surface. A mechanism is proposed for the first alkylation by the promoter of the oxidic catalyst, in which the tin atom of the promoter becomes bonded to rhenium via an oxygen bridge.