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

Abstract

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 10.4kcal/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.