Formation of Active Sites on WO3 Catalysts: A Density Functional Theory Study of Olefin Metathesis

Industrial demand for poly(propene) has spurred research on improved catalysts and mechanisms for propene production. Recently, tungsten trioxide (WO3) was reported to exhibit high activity for the metathesis of ethene and 2-butene to form propene. The whole process is divided into two stages: (1) Initiation (i.e., formation of W-carbene active sites on WO3 surfaces), and (2) Propagation (i.e., metathesis reaction on these active sites to yield propene). This study investigates the mechanism of W-carbene active site formation, using first-principles calculations based on density functional theory. For the WO3 orthorhombic crystal, the (001) surface has been found to be the most stable, since it contains four chemically distinguishable types of surface atoms: 5-fold coordinate W-50 6-fold coordinate W-60 2-fold bridging oxygen O-20 and singly coordinated oxygen O-10. Because of the different energies of adsorption for ethene and 2-butene at these four types of active sites, we find that 2-butene forms W-carbene active sites more readily, with lower activation energies, than ethene, because of a combination of electrophific and steric effects. These results reveal the relationship between WO3 catalyst surface structure and activity for metathesis, which ultimately will guide the development of more active and selective catalysts for propene production.