Resolving the Reaction Mechanism and Active Sites for Oxidation–Hydration of Ethylene toward Ethylene Glycol Synthesis over Bifunctional Al/TS-1 Catalysts

Due to the complex internal structure of titanium silicalite-1 (TS-1), the active sites and mechanistic details of catalytic reactions using TS-1-based catalysts have not been well understood. In this work, density functional theory (DFT) calculations were conducted to provide fundamental insights into the reaction mechanism of oxidation–hydration of ethylene toward ethylene glycol production over the TS-1 catalyst model with perfect titanium(IV) active sites and the Al/TS-1 model with both Brønsted acid and titanium(IV) sites. The computational results revealed that the introduction of Al into TS-1 altered the reaction pathway, facilitated the H2O2 activation, and significantly reduced the energy barrier of the rate-limiting step for ethylene glycol generation, demonstrating a synergistic action of Ti–Al(H) sites. Over Al/TS-1, the titanium(IV) site participated in the ethylene epoxidation, while the Brønsted acid site was responsible for the hydration of ethylene oxide to ethylene glycol. The new insights into the catalytic mechanism and the identified role of bifunctional active sites offer a useful reference for the future design of Al/TS-1 catalysts with tailored catalytic performance for this important reaction.