(147e) Active Sites for Olefin Metathesis On WO3 Catalysts : A Density Functional Theory Study

Industrial demand for polypropylene has spurred research on improved catalysts and mechanisms for propylene production. Recently, WO₃was reported to exhibit high activity for the metathesis of ethylene and 2-butylene to form propylene. This study investigates active sites for this metathesis reaction on WO₃surfaces, 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 with sixfold coordinated tungsten sites W_6cand fivefold coordinated tungsten sites W_5c. The whole process is divided into two stages: 1. Initiation - the formation of active W-carbene species from the W-oxo precursor, and 2. Propagation - the metathesis reaction on these active sites to yield propylene. The oxidation states of W on the surface have a great influence on the adsorption of ethylene and 2-butylene. It is more favorable for W5c sites to adsorb ethylene while it is more favorable for W6c sites to adsorb 2-butylene, which were the compromising results of electrophilic and steric effects. The formations of the W-carbene species from ethylene and 2-butylene were found to follow different pathways. These study results reveal the relationship between WO₃surface structure and metathesis activities, which is significant in guiding the preparation of more suitable WO3 catalysts for the production of propylene.