Simon G. Kukes

Olefin metathesis catalyst

Olefins are converted into other olefins having different numbers of carbon atoms by contact with a catalyst comprising an inorganic refractory oxide support containing at least one of tungsten oxide and molybdenum oxide and a promoting amount of at least one methylating agent under conditions suitable for the methylating agent compounds to promote the activity of tungsten and molybdenum oxides for the disproportionation reaction.Abstract Model supports consisting of a thin layer of SiO2 on a Silicon single crystal have been used to study the [W]n+/SiO2/Si (100) model catalyst precursor prepared by a controlled reaction of π-C5H5W(CO)3Cl with the SiO2 surface. Characterization of the tungsten surface species has been performed by combination of conventional, angle-resolved and depth-profiling X-ray photoelectron spectroscopy (XPS). The silica surface consists of highly dispersed W-oxide units linked by two Si–O–W bonds. Further, compared with the powder analogues, a drastic increase in spectral resolution and detailed band structure is observed in the XPS spectra.

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 2 O 7 ·Al 2 O 3 and Re 2 O 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.