TS‑1 shares the MFI topology identical to ZSM‑5, belonging to pentasil family. It possesses a three-dimensional intersecting 10-membered-ring micropore system: straight channels of 0.53 nm × 0.56 nm and sinusoidal zigzag channels of 0.51 nm × 0.55 nm. The unit cell formula can be simplified as \(\ce{|Na_{n}(H_{2}O)_{m}|[Ti_{x}Si_{96-x}O_{192}]}\) with almost no framework aluminum in pure TS‑1.
Titanium atoms can be incorporated into the silica lattice during hydrothermal crystallization. Only framework tetrahedral Ti species serve as the real active centers for oxidation reactions, while extra-framework TiO₂ anatase is undesired because it accelerates ineffective decomposition of H₂O₂. The intersecting pore architecture provides confined microenvironment for oxidation reactions, bringing remarkable shape selectivity. Crystal size and intracrystalline diffusion resistance strongly affect substrate accessibility to internal Ti sites. Nano-sized TS‑1 shortens diffusion paths and suppresses product over-oxidation.
Tetrahedrally coordinated framework Ti is the core active site of TS‑1. Upon contact with H₂O₂, Ti sites form Ti-OOH hydroperoxo intermediates, which are responsible for selective oxygen transfer to organic substrates.
Key properties:
The coordination environment of Ti dominates catalytic behavior. Tetrahedral framework Ti gives high selective oxidation activity. Aggregated extra-framework TiO₂ promotes useless decomposition of hydrogen peroxide and reduces oxidant utilization efficiency. Controlling Ti loading within an appropriate range is essential: excessive Ti tends to form anatase TiO₂, while insufficient Ti leads to low activity.
The MFI intersecting channels determine substrate admission. Small molecules like propylene can easily enter micropores and react at internal Ti sites. Larger substrates only interact with active sites on the external crystal surface. Hierarchical TS‑1 with mesopores can be fabricated by desilication, which alleviates diffusion limitation and improves utilization of buried Ti sites. However, over-treatment may damage the framework and generate extra-framework Ti species.
Nano TS‑1 exhibits higher activity than large crystals because of shorter intracrystalline diffusion distance. Surface modification can passivate external amorphous Ti sites that trigger non-selective oxidation, raising target product selectivity.
Suitable surface hydrophobicity helps organic reactants compete with water to access Ti sites. Too strong hydrophilicity restricts organic molecule adsorption and reduces reaction rate.
This is the most iconic industrial application of TS‑1. TS‑1 catalyzes propylene epoxidation with H₂O₂ to produce propylene oxide. The mild reaction conditions and high atom efficiency make HPPO a clean route compared with traditional chlorohydrin process.
TS‑1 catalyzes phenol oxidation to hydroquinone and catechol, a classic aromatic hydroxylation reaction. The pore confinement controls product distribution.
TS‑1 serves as catalyst for cyclohexanone ammoximation to cyclohexanone oxime, the precursor for caprolactam (monomer of nylon 6). This route greatly reduces environmental pollution versus old processes.
It is also applied in alkane oxidation, alcohol oxidation and sulfide oxidation for removing organic sulfur compounds.
