A review on the active sites for titanium species in zeolites: coordination structure, synthetic strategies and activity

Titanium species in titanosilicate zeolites exist in three forms: framework titanium species, framework-associated titanium species and anatase TiO₂. They dominate the catalytic properties. Generally, the framework titanium species are considered as the active centers for catalytic reactions. However, the latest research has unveiled that additional titanium species within the framework, such as penta-coordinated and hexa-coordinate titanium species, can also exert their influence on catalytic processes. The catalytic activities of various titanium species, including penta- and hexa-coordinated titanium, exhibit superiority over traditional tetra-coordinated framework titanium species in some reactions. The urgent necessity lies in establishing a comprehensive understanding of the formation principles of various titanium species, characterization, and investigating their catalytic properties across diverse reactions. This review provides a comprehensive overview of contemporary advances in titanosilicate zeolites. The regulatory strategies, detection methods, and catalytic properties of titanium species are comprehensively summarized. Furthermore, a universal analysis is conducted on the mechanism of titanium species in the hydrogen peroxide catalytic system, offering valuable insights into both catalytic mechanism and precise regulation of microenvironmental conditions and spatial distribution of titanium species.