As shown in Figure , "Keggin" and "Wells-Dawson" are the basic structural types of POMs. Keggin-type polyanion is composed of four conformally connected {M3O13} wrapped by the {XO4} tetrahedron. Wells-Dawson-type polyanions are closed cluster cages connected by two {XM9O34} units. (where M= Mo, W, V, Nb, Ta, and X= Si, Ge, P, As, etc.). In fact, the above two types of POMs and their derivatives are the most common building blocks of POM-based materials, which are attributed to the following characteristics: (i) The preparation method is simple, and the yield is high. (ii) lacunary POMs provide a lot of active sites. In the presence of organic ligands (Scheme 2), they can construct high-nuclear metal clusters with transition metals. (iii) The structure is stable and can be self-assembled in the reaction system. Figure . The polyhedron model of Keggin and Wells-Dawson anions.