1. Introduction
Titanium silicalite molecular sieves are a class of environmentally friendly heteroatom-substituted zeolite materials, typically represented by TS-1 (MFI topology) and TS-2 (MEL topology). First developed in the 1980s, these materials replace partial framework silicon atoms with tetravalent titanium atoms, forming an all-silicon skeleton doped with isolated framework titanium active sites. Unlike conventional acidic zeolites such as ZSM-5 and ZSM-11, pure titanium silicalite contains no framework aluminum, possesses negligible Brønsted acidity, and exhibits excellent hydrophobicity after high-temperature calcination. These unique structural and surface properties endow titanium silicalite with superior selective catalytic oxidation performance, making it the core heterogeneous catalyst for green oxidation processes using hydrogen peroxide as a clean oxidant. With the global advancement of green chemical industry and carbon neutrality goals, titanium silicalite molecular sieves have become indispensable functional materials in fine chemical synthesis, petrochemical upgrading, environmental governance and other fields.
2. Structural and Fundamental Properties
TS-1, the most mature and widely used titanium silicalite, possesses a three-dimensional cross-linked ten-membered ring pore structure consistent with ZSM-5 zeolite, with pore sizes of 0.54 × 0.56 nm (straight channel) and 0.51 × 0.55 nm (sinusoidal channel). TS-2 corresponds to the MEL topology of ZSM-11, featuring two groups of mutually perpendicular straight pores with lower molecular diffusion resistance. The adjustable silicon-titanium ratio (SiO₂/TiO₂ ranging from 30 to 100 in commercial grades) directly determines catalytic performance: framework tetra-coordinated titanium serves as the dominant active site for activating H₂O₂, while non-framework anatase TiO₂ is an impurity that causes invalid decomposition of hydrogen peroxide and reduces reaction selectivity.
A distinct characteristic of calcined pure titanium silicalite is strong surface hydrophobicity. The absence of aluminum-induced polar hydroxyl groups and the compact Si-O-Ti/Si-O-Si skeleton greatly weaken water adsorption capacity. This hydrophobic effect enables organic reaction substrates to preferentially adsorb on the catalyst surface while excluding water by-products, effectively promoting forward oxidation reactions and inhibiting side reactions such as product hydrolysis and ring-opening. In addition, titanium silicalite exhibits excellent thermal and hydrothermal stability, with a service temperature up to 550 °C, supporting long-cycle industrial catalytic operation and high-temperature regeneration.
3. Core Industrial Applications
3.1 Propylene Epoxidation (HPPO Process)
The hydrogen peroxide to propylene oxide (HPPO) process is the largest industrial application of TS-1 catalysts. Traditional propylene oxide production via the chlorohydrin method suffers from severe pollution, high energy consumption and massive salt wastewater discharge. In contrast, the TS-1 catalyzed HPPO reaction proceeds under mild temperature and pressure conditions, with H₂O₂ as the oxidant and water as the only by-product. The framework Ti sites activate hydrogen peroxide to form Ti-OOH active intermediates, which selectively transfer oxygen to propylene molecules to generate propylene oxide. Benefiting from the hydrophobic surface and size-selective pore structure of TS-1, the process achieves high propylene oxide selectivity and hydrogen peroxide utilization efficiency. At present, the HPPO process has completely replaced traditional polluting processes in industrial production and become the mainstream green production route for propylene oxide.
3.2 Cyclohexanone Ammoximation
Cyclohexanone ammoximation catalyzed by TS-1 is a key green reaction for the production of cyclohexanone oxime, an important intermediate of caprolactam and nylon 6. Relying on the selective oxidation performance of TS-1, cyclohexanone reacts with ammonia and hydrogen peroxide to generate cyclohexanone oxime in one step. Compared with the traditional hydroxylamine method, this route features simple process flow, high atom utilization, few by-products and low environmental pollution. The non-acidic characteristic of TS-1 effectively avoids acid-induced side reactions, ensuring high conversion and selectivity of the target product. This technology has been fully industrialized and supports the large-scale production of nylon chemical raw materials worldwide.
