Effects of the addition of Nb2O5 to rhenium-based olefin metathesis catalysts

The effects of the addition of Nb2O5 to 6 wt.% Re2O7/Al2O3 metathesis catalysts were investigated. At least until 9 wt.% Nb2O5 loadings both tetrahedral and octahedral aluminum sites are evenly involved in the reaction between alumina and niobium oxide. Up to 12 wt.% Nb2O5 no significative influence was observed in the catalytic activity of the resulting systems. For higher niobia loadings, however, the catalytic activity decreased quickly. In such conditions a second rhenium species, probably [Nb]-OReO3 was detected by TPR. Sn-119 Mossbauer analyses showed that the reaction between the niobia-modified systems and SnBu(4) ([Sn]:[Re] = 0.55) led to the formation of only one kind of rhenium-tin site, with the tin atom attached to the surface by two Sn-O bonds. In contrast to what happens with the unmodified system, no physisorbed SnBu(4) was observed.