The reduction of isomerisation activity on a WO3/SiO2 metathesis catalyst Original Research Article

By increasing the WO3 loading on WO3/SiO2 metathesis catalysts it was observed that metathesis activity is closely related to the tungsten surface compound and not to the crystalline WO3. It was observed that metathesis activity reached a maximum while WO3 crystalline material continued to increase with increasing WO3 loading. At high WO3 loadings the formation of crystalline material also protected the active tungsten surface compound from deactivation by being the preferred species for over-reduction. The occurrence of branched metathesis products was attributed to an increase in Brønsted acidity indicating an alkoxide (carbenium ion) mechanism responsible for skeletal isomerisation. Reduction of branched metathesis products were achieved with the addition of small amounts of an alkali-metal ion. Large amounts of the alkali-metal ion resulted in a sharp decline in conversion indicating destruction of the Lewis acidity necessary for metathesis activity. Preparation of WO3/SiO2 catalysts at a pH above the iso-electric point of silica also had a significant effect on catalyst selectivity.