**1. Introduction**

Natural zeolites are a kind of hydrated crystalline silica-aluminate with specific pore structure. The pore diameters of zeolites are similar to the sizes of molecules. Because they can sieve molecules, they are also known as molecular sieves. According to the pore diameter, IUPAC classified porous materials into microporous materials (<2 nm), mesoporous materials (2–50 nm), and macroporous materials (>50 nm) [1]. Zeolites are a microporous material. Due to their high hydrothermal stability, simple synthesis process, strong adsorption properties, adjustable acidity and alkalinity, and pore shape selectivity, they are widely used in petroleum refining, the chemical industry, and separations [2]. The most commonly used zeolites, such as type-A, faujasite, mordenite, and ZSM-5, have narrow pore size and more serious diffusion restrictions, which are disadvantageous. Mesoporous materials have larger surface areas and pore volumes, which is more favorable for larger molecular reactions, adsorption, and separation. However, the hydrothermal stability is not satisfactory due to the amorphous silica wall of mesopores.

Titanium silicalite-1 (TS-1) with MFI topology was first hydrothermally synthesized by Taramasso et al. [3]. After that, it has attracted much attention due to its excellent catalytic activity for selective oxidation with H2O2, such as alkene epoxidation [4–9], aromatics hydroxylation [10–12], ketone ammoximation [13–15], alkane oxidation [16, 17], and so on [18, 19]. Therefore, it is considered a milestone in the field of zeolitic catalysis. MFI topology contains two types of 10-membered ring channels, which are the straight channel (0.56 × 0.54 nm) and zigzag channel (0.55 × 0.51 nm). The substitution of titanium atoms for framework silicon or aluminum atoms generates a molecular sieve with tetrahedrally coordinated Ti. The isolated tetrahedrally coordinated titanium (also called framework Ti) in TS-1 is the main active center for catalytic oxidation. However, the amount of tetrahedrally coordinated Ti is limited (2.5 mol%), because the lattice expansion inhibits the insertion of Ti into the framework [20]. In the next 30 years, phenol hydroxylation to benzene diols, cyclohexanone ammoximation to cyclohexanone oxime, butanone ammoximation to diacetyl monoxime, and propene epoxidation to propene oxide catalyzed by TS-1 were commercialized successively. Nevertheless, there are still many problems in the synthesis and application of TS-1, such as the transformation of tetrahedrally coordinated Ti to octahedrally coordinated Ti or anatase TiO2 (loss of the active center) in the reaction, and the deactivation of the catalyst by blocking of channels [21]. Therefore, many researchers have made an effort to solve these problems, and so do we. To further improve the catalytic performance and expand the application of TS-1, it is necessary to summarize our current research achievement.

In this chapter, we describe our recent progress on controlling Ti coordination states, design of porosity, and applications of TS-1. We hope that this summary will help in understanding the developing process and our contribution to research on TS-1.
