Mechanistic Insight into Propylene Epoxidation with HO over Titanium Silicalite-1: Effects of Zeolite Confinement and Solvent.

Density functional theory (DFT) calculations were performed to investigate the effects of zeolite confinement and solvent on propylene epoxidation with HO over the titanium silicalite-1 (TS-1) catalyst. The 144T and 143T cluster models containing typical 10MR channels of TS-1 were constructed to represent the tripodal(2I) and Ti/defect sites. It was found that the confinement of the zeolite pore channel not only impacts the adsorption stability of guest molecules but also alters reaction barriers, as compared to the results obtained based on small cluster models. When dispersion corrections were considered, an enhancement of the adsorption stability of guest molecules was observed because of the important contribution from van der Waals interactions, especially for propylene adsorption. An explicit protic methanol molecule was introduced into the catalytic system to probe the influence of the solvent on propylene epoxidation, based on which a significant enhancement of CHOH-HO co-adsorption was obtained owing to H-bond formation. More importantly, the energy barrier for HO dissociation was largely reduced by ∼13 kcal/mol because of the participation of the methanol in the H-transfer process and the formation of H-bond network, resulting in an alteration of the rate-limiting step. By comparison, adding an aprotic acetonitrile solvent did not have substantial effect on reaction path and kinetics. The calculation results clearly demonstrate the important role of the protic methanol solvent, which not only strengthens the adsorption of guest molecules but also promotes the kinetics for propylene epoxidation with HO over TS-1 catalyst.