Effect of tungsten oxide loading on metathesis activity of ethene and 2-butene over WO3/SiO2 catalysts

The metathesis of ethene and 2-butene to propene was studied over WO3/SiO2 catalysts with various WO3 loadings (2, 4, 8, 12, 16, and 24 wt%). The 2-butene conversion and propene selectivity increased greatly with WO3 loading increasing from 2 to 8 wt%, reached maximum at 8–12 wt% WO3 loading, and then decreased when the WO3 loading was higher than 12 wt%. From the above results and taking the economics into account, the optimal amount of WO3 loading was ~8 wt%. The catalysts were characterized by physico-chemical and spectroscopic techniques to elucidate the effect of different tungsten oxide loadings on the metathesis reactivity of ethene and 2-butene. The characterization data indicated that three types of tungsten species (i.e., surface tetrahedral tungsten species, surface octahedral polytungstate species, and WO3 crystallites) were present in the catalysts. It was found that WO3 was not the active centers, and surface tetrahedral tungsten species might be more active than octahedral polytungstate species in metathesis reaction. The reduced form of tungsten species [W+4, W+5, and W+(6−y) (0 < y < 1)] may be the suitable state of W species acting as metathesis active centers.