1. Introduction
The development of environmentally benign catalytic processes is one of the central challenges facing the modern chemical industry. Traditional oxidation reactions often rely on stoichiometric oxidants such as chromates, permanganates, or organic peracids, which generate large quantities of toxic waste. In this context, the discovery of titanium silicalite-1 (TS-1) in 1983 represented a paradigm shift. TS-1 belongs to the Pentasil family of zeolites and possesses an MFI-type three-dimensional microporous framework constructed from corner-sharing SiO₄ and TiO₄ tetrahedra.
What distinguishes TS-1 from conventional zeolites such as ZSM-5 is the absence of aluminum in its framework. Without Al³⁺, TS-1 lacks strong Brønsted acid sites and exhibits pronounced hydrophobicity. This property is critical: it prevents competitive adsorption of water on active sites during aqueous-phase oxidations with H₂O₂, thereby maintaining high catalytic activity even in water-rich environments.
2. Structural Characteristics
2.1 Framework Topology
TS-1 crystallizes in the orthorhombic crystal system and retains the MFI topology of its all-silica analogue, silicalite-1. The framework contains two intersecting sets of 10-membered-ring (10-MR) channels:
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Straight channels running parallel to the [010] direction, with dimensions of approximately 0.53 nm × 0.56 nm.
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Sinusoidal (zigzag) channels running parallel to the [100] direction, with dimensions of approximately 0.51 nm × 0.55 nm.
These intersecting channels create a three-dimensional pore network that imposes shape-selective constraints on reactant and product diffusion—a key factor underlying TS-1's remarkable selectivity in oxidation reactions.
2.2 Titanium Coordination and Active Sites
The catalytic activity of TS-1 is attributed to framework titanium species. For decades, the prevailing view held that isolated, tetrahedrally coordinated Ti(IV) sites (Ti–O–Si) were the sole active centers. However, a landmark 2020 study published in Nature by Copéret and colleagues at ETH Zurich challenged this assumption. Using a combination of solid-state ¹⁷O NMR spectroscopy, electron microscopy, and density functional theory (DFT) calculations, the researchers demonstrated that binuclear titanium sites—two Ti atoms bridged by a peroxo species—are responsible for the high efficiency of propylene epoxidation.
This finding has profound implications: it suggests that the spatial proximity of Ti atoms within the framework, rather than their mere presence as isolated centers, governs catalytic performance. Optimizing Ti–Ti distance and distribution may therefore be a more productive strategy for catalyst design than simply maximizing total Ti content.
3. Synthesis Methods
3.1 Conventional Hydrothermal Synthesis
The most widely used method for TS-1 preparation is hydrothermal synthesis, typically employing tetrapropylammonium hydroxide (TPAOH) as a structure-directing agent (SDA), a silicon source (e.g., tetraethyl orthosilicate, TEOS, or silica sol), and a titanium source (e.g., titanium tetrabutoxide, TBOT). The synthesis gel is sealed in an autoclave and crystallized at 130–200 °C for several days.
Advantages: Simple operation, good reproducibility, high product purity.
Limitations: Titanium precursors are prone to hydrolysis and aggregation, leading to the formation of extra-framework TiO₂ (anatase), which is catalytically inactive for selective oxidation. Moreover, the high cost and toxicity of TPAOH raise environmental and economic concerns.
3.2 Advanced Synthesis Strategies
To overcome these limitations, several innovative approaches have been developed:
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Dry-gel conversion (DGC): The synthesis mixture is dried before crystallization, reducing water consumption and improving Ti incorporation efficiency.
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Starch-assisted synthesis: Adding starch slows the crystallization rate, allowing the Ti insertion rate to match that of Si, thereby eliminating extra-framework TiO₂ formation.
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Inorganic precursor route: Zhang et al. (East China Normal University) developed a method in which Ti(SO₄)₂ is spontaneously dispersed on porous silica to form Si–O–Ti bonds before hydrothermal crystallization, achieving high framework Ti content without organic templates.
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Mesoporous TS-1: Introducing mesopores (2–50 nm) via surfactant templating or post-synthetic desilication alleviates diffusion limitations for bulky substrates, significantly enhancing activity in reactions involving large molecules.
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