New developments and concepts in enhancing activities of heterogeneous metathesis catalysts

Abstract

Our continuing investigations of heterogeneous metathesis catalysts have led to three findings: (1) Studies of MgO/WO 3·SiO 2 combinations suggest that MgO plays an unexpected beneficial role in olefin metathesis catalysis: that of generating gas-phase ‘excited species’ ( e.g. allyl or allyl-oxo radicals), which are initiators or precursors of metathesis sites, producing dramatic increases in metathesis activities. The proposed new concept is indirectly supported by findings that the activities of MoO 3 metathesis catalysts are not likewise enhanced by MgO; more significantly, enhancement of WO 3 catalysts does not occur when MoO 3, a radical scavenger, is present. (2) The catalytic activities of conventional heterogeneous metathesis catalysts are increased significantly by admixing minor amounts of elemental S, Si, Mg, Ba, Sn, Zn, Sb or W with the metathesis catalysts, and treating the admixtures at elevated temperature under an inert atmosphere. The enhanced activity is attributed to a partial reduction of the catalysts by the added reducing metals or elements. (3) An unusual and interesting catalytic behavior is exhibited by rhenium oxide supported on Th 3(PO 4) 4 : the metathesis activity increased by more than 50% when small amounts of oxygen were added to the olefin feed. Other heterogeneous metathesis catalysts, including Re 2O 7·Al 2O 3 and Re 2O 7·AlPO 4, do not show this behavior when oxygen is added; in most cases, traces of oxygen produce a decline in metathesis activity. This unusual behavior has fundamental implications: key roles for oxygen ligands have been claimed for both homogeneous and heterogeneous metathesis catalysts.