Mechanistic investigation of propylene epoxidation with H₂O₂ over TS-1: Active site formation, intermediate identification, and oxygen transfer pathway

Density functional theory (DFT) calculations were performed to investigate propylene epoxidation mechanisms with H₂O₂ over various TS-1 models in terms of active site formation, intermediate identification, and oxygen transfer pathway. Over tripodal sites of the hydrolyzed model, a bidentate Ti-OOH intermediate assembled in the 3-membered ring (3MR) configuration (Ti-(η²-OOH)-3MR) was identified through the stepwise mechanism for propylene epoxidation while a monodentate Ti-H₂O₂ intermediate stabilized in the 5MR configuration (Ti-(η¹-H₂O₂)-5MR) was obtained via the concerted mechanism. Both stepwise and concerted paths were found to be possible over the hydrolyzed models. On Ti/defect site of the Si-vacancy model, a new mechanism through a 5MR Ti-(η¹-OOH) intermediate was demonstrated to be energetically the most favorable candidate to represent the propylene epoxidation chemistry in H₂O₂/TS-1. The barriers (5.1 and 10.4 kcal/mol) for H₂O₂ dissociation and epoxidation are both relatively lower as compared to literature, which can be attributed to the steric advantage of the Si-vacancy model as well as the H-bond networks formed with surrounding silanol groups. The effect of co-produced H₂O on propylene epoxidation kinetics was also examined and the calculation results showed that H₂O coordination to the Ti center facilitates the formation of 5MR Ti-(η¹-OOH) intermediate and dramatically lowers the barriers for both H₂O₂ dissociation and epoxidation over the Ti/defect site of TS-1.