*Review* **Recent Advances of Ti/Zr-Substituted Polyoxometalates: From Structural Diversity to Functional Applications**

**Zhihui Ni 1,\*, Hongjin Lv 2,\* and Guoyu Yang <sup>2</sup>**


**Abstract:** Polyoxometalates (POMs), a large family of anionic polynuclear metal–oxo clusters, have received considerable research attention due to their structural versatility and diverse physicochemical properties. Lacunary POMs are key building blocks for the syntheses of functional POMs due to their highly active multidentate O-donor sites. In this review, we have addressed the structural diversities of Ti/Zr-substituted POMs based on the polymerization number of POM building blocks and the number of Ti and Zr centers. The synthetic strategies and relevant catalytic applications of some representative Ti/Zr-substituted POMs have been discussed in detail. Finally, the outlook on the future development of this area is also prospected.

**Keywords:** polyoxometalates; titanium; zirconium; transition metal substitution
