Efficient epoxidation over dinuclear sites in titanium silicalite-1.

Area of Science:

• Materials Science
• Catalysis
• Inorganic Chemistry

Background:

• Titanium silicalite-1 (TS-1) is crucial for industrial olefin epoxidation using hydrogen peroxide (H2O2).
• Its catalytic activity is traditionally linked to isolated Ti(IV) sites within the MFI framework.
• The precise structure of these active sites remains unconfirmed despite extensive research.

Purpose of the Study:

• To characterize the active titanium sites in TS-1 catalysts for propylene epoxidation.
• To elucidate the structural basis for TS-1's high catalytic efficiency and selectivity.
• To propose a revised model for the active sites in TS-1 catalysis.

Main Methods:

• Advanced spectroscopy (e.g., 17O NMR) and microscopy were employed.
• Detailed characterization of highly active and selective TS-1 catalysts.
• Density functional theory (DFT) calculations were performed to model reaction pathways.

Main Results:

• Spectroscopic analysis revealed the formation of bridging peroxo species on dinuclear titanium sites upon reaction with H217O2.
• DFT calculations confirmed a low-energy reaction pathway facilitated by cooperativity between two titanium atoms.
• A key oxygen-transfer transition state, similar to peracid epoxidation, was identified.

Conclusions:

• Dinuclear titanium sites, not isolated Ti(IV) atoms, are proposed as the active sites responsible for TS-1's high efficiency in propylene epoxidation.
• This revised understanding of active-site structure offers potential for further optimization of TS-1 catalysts and industrial epoxidation processes.